Gold and its Complexes in Anticancer Chemotherapy 9813363134, 9789813363137

Citation preview

Mohmmad Younus Wani Manzoor Ahmad Malik

Gold and its Complexes in Anticancer Chemotherapy

Gold and its Complexes in Anticancer Chemotherapy

Mohmmad Younus Wani • Manzoor Ahmad Malik

Gold and its Complexes in Anticancer Chemotherapy

Mohmmad Younus Wani Department of Chemistry, College of Science University of Jeddah Jeddah, Saudi Arabia

Manzoor Ahmad Malik Department of Chemistry Jamia Millia Islamia New Delhi, Delhi, India

ISBN 978-981-33-6313-7 ISBN 978-981-33-6314-4 https://doi.org/10.1007/978-981-33-6314-4

(eBook)

# The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Among the different types of human diseases known, cancer is the most dreadful and challenging disease. It is the second leading cause of death globally, accounting for an estimated 9.6 million deaths, or one in six deaths. The new cancer cases worldwide are estimated to increase to 19.3 million per year by 2025 due to the changing lifestyle and increasing longevity. The accidental exploration of the anticancer properties of cisplatin, a platinum metal complex, persuaded scientists for the design and development of metal complexes as anticancer drugs. This resulted in the research and development of a large reservoir of structurally diverse platinum containing metal complexes with interesting anticancer properties. Obviously, different platinum containing metal complexes are currently being used for the treatment of different types of cancers. However, the scientific interest in the development of anticancer metal complexes saw a paradigm shift from platinum complexes to the non-platinum complexes due to some tenacious hitches of the platinum containing anticancer complexes. A need for the development of novel anticancer metal complexes with different pharmacological profiles as compared to that of the platinum containing complexes was felt by researchers, the world over. This resulted in the development of metal complexes with non-platinum d- and f-block metals with interesting anticancer properties. Among the different non-platinum metal ions, gold is one of the most sought after metal that has been used for the development of anticancer drugs. Despite the recognition of the medicinal and therapeutic values of gold a long time ago, its use in medicine began only in the early 1920s. The isoelectronic nature like that of platinum (II) complexes and tetracoordinate geometries of gold(III) complexes served as stimulation for researchers to explore the anticancer properties of gold(III) complexes. Gold complexes (in both +1 and +3 oxidation states) have shown considerable antiproliferative activities, and in many cases, such effects have been attributed to their ability to target different proteins and enzymes, rather than DNA, making gold complexes exciting candidates compared to the platinum containing complexes with DNA as the main target. Presently, a large reservoir of gold complexes has been developed with interesting anticancer profiles, and some of the gold complexes have entered clinical trials as well. This book has been written to

v

vi

Preface

provide the basic understanding of the complex disease—cancer and the use of chemotherapy as a treatment modality. Although a brief has been made about the platinum and some non-platinum based therapies, the main focus has been the use of gold and its complexes in anticancer chemotherapy. Jeddah, Saudi Arabia

Mohmmad Younus Wani

Contents

1

Cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Types of Cancers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Cancer or Tumor Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Invasion and Metastasis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 How Cancer Arises? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 3 5 6 7 7 9

2

Epidemiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Economic Burden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 14 16

3

Treatment Modalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Chemotherapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Alkylating Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Antimetabolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 Antibiotics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.4 Microtubule Inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.5 Steroid Hormones and Antagonists . . . . . . . . . . . . . . . . 3.1.6 Immunomodulators . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.7 Growth Factor Receptor Inhibitors . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . .

17 17 26 27 27 28 29 29 29 33

4

Anticancer Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Properties of Metals and Metal Complexes . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

35 36 39

5

Platinum-Based Anticancer Agents . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Chemistry of Cisplatin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Mechanism of Action of Platinum Drugs . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41 43 45 47

. . . .

vii

viii

Contents

6

Non-platinum Anticancer Agents . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Ruthenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Gallium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .

51 51 54 55 56 58 59 60 62

7

Chemistry of Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Gold and Its Complexes in Medicine . . . . . . . . . . . . . . . . . . . . 7.2 Auranofin and Its Analogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . .

69 71 73 76

8

Classes of Gold Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Complexes with Nitrogen Donor Ligands . . . . . . . . . . . . . . . . . 8.1.1 Complexes with Sulfur Donar Ligands . . . . . . . . . . . . . 8.2 Complexes with Phosphorus Donar Ligands . . . . . . . . . . . . . . . 8.3 Complexes with Sulfur-Phosphorus Donor Ligands . . . . . . . . . . 8.4 Organometallic Gold Complexes . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . .

81 81 99 105 116 129 152

9

Mechanisms of Action of Anticancer Gold Complexes . . . . . . . . . . 9.1 Thioredoxin System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Cysteine Proteases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Protein Tyrosine Phosphatases (PTP) . . . . . . . . . . . . . . . . . . . . 9.4 Thiol/Selenol Containing Enzymes . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

159 160 163 164 165 168

10

Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

About the Authors

Mohmmad Younus Wani Graduated from Jamia Millia Islamia, New Delhi, India, in 2013. He received a senior research fellowship from CSIR-India and FCT postdoctoral fellowship from Portugal. He worked as a postdoctoral fellow with Prof. A. Sobral group at the University of Coimbra, Coimbra, Portugal from 2013 to 2016 on the development of fungal cell wall targeted antifungal therapies. In fall 2016, he moved to the United States, and worked with Dr. K. Tsuchikama’s group at Texas Therapeutics Institute, Brown Foundation Institute of Molecular Medicine, UTHealth on the development of nontraditional antimicrobial agents and peptidebased cleavable linkers for ADCs (Antibody Drug Conjugates). In fall 2017, he was appointed as assistant professor in the Department of Chemistry, College of Science, University of Jeddah, KSA. He continues to work on the development of new small molecules, metal complexes, and novel strategies to combat microbial infections and cancer at the interface of chemistry and biology. He has many international publications, book chapters, and two books, besides many international and national honors and awards to his credit. He is working hard to advance the medicinal chemistry and drug discovery field with new questions and pertinent issues of the twentieth-first century. Manzoor Ahmad Malik Born and raised in the Pulwama district of Jammu and Kashmir, India. He did his BSc. from the University of Kashmir and received his Master’s in Chemistry from the Department of Chemistry, Jamia Millia Islamia, New Delhi, India. He received M. Phil in Chemistry from Jiwaji University and Ph. D. from Jamia Millia Islamia, New Delhi, India. He received a Senior Research Fellowship from CSIR, India, in 2019. He has several international publications including research articles and book chapters to his credit. His research interests include design and synthesis of small-molecule inhibitors and metal complexes for microbial infections and cancer. He is currently working on the development of non-platinum metal complexes that could be used for anticancer chemotherapy with better efficacy and less toxicity compared to the platinum drugs used in cancer therapy.

ix

1

Cancer

Cancers are defined by the National Cancer Institute as a collection of diseases in which abnormal cells can divide and spread to nearby tissues (https://www.cancer. gov/publications/dictionaries/cancer-terms/def/cancer). Cancer cells can spread to other parts of the body through the blood and lymph systems. There are several main types of cancer. Carcinoma is a cancer that begins in the skin or in tissues that line or cover internal organs. Sarcoma is a cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue. Leukemia is a cancer that starts in blood-forming tissue, such as the bone marrow, and causes large numbers of abnormal blood cells to be produced and enter the blood. Lymphoma and multiple myeloma are cancers that begin in the cells of the immune system. Central nervous system cancers are cancers that begin in the tissues of the brain and spinal cord, also called malignancy. The origin of the word cancer is credited to the Greek physician Hippocrates (460–370 BC), who is considered the “Father of Medicine.” Hippocrates used the terms carcinos and carcinoma to describe non-ulcer forming and ulcer-forming tumors, thus calling cancer “karkinos.” [1] In Greek, these words refer to a crab, most likely applied to the disease because the finger-like spreading projections from a cancer called to mind the shape of a crab. The Roman physician, Celsus (28–50 BC), later translated the Greek term into cancer, the Latin word for crab. Galen (130–200 AD), another Greek physician, used the word oncos (Greek for swelling) to describe tumors. Galen also used the suffix -oma to indicate cancerous lesions. It is from Galen’s usage that we derive the modern word oncology [2]. Although Hippocrates, Celsus, or Galen may have named the disease “cancer,” they were certainly not the first to discover the disease. The history of cancer actually begins much earlier. The world’s oldest documented case of cancer hails from ancient Egypt in 1500 BC [3]. The details were recorded on papyrus, documenting eight cases of tumors occurring on the breast. It was treated by cauterization, which destroyed tissue with a hot instrument called “the fire drill.” It was also recorded that there was no treatment for the disease, only palliative treatment. There is evidence # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_1

1

2

1

Cancer

that the ancient Egyptians were able to tell the difference between malignant and benign tumors [4]. According to inscriptions, surface tumors were surgically removed in a similar manner as they are removed today. In Ancient Greece, much less was known about the human body than what is known today, of course. For example, Hippocrates believed that the body was composed of four fluids: blood, phlegm, yellow bile, and black bile, often known as four humors. He believed that an excess of black bile in any given site in the body caused cancer, thus hypothesizing that cancer was caused by “an imbalance of the four humors.” This was the general thought of the cause of cancer for the next 1400 years [5]. In ancient Egypt, it was believed that cancer was caused by the Gods. The lymph theory developed in the seventeenth century, replaced Hippocrates’ black bile theory on the cause of cancer. The discovery of the lymphatic system gave new insight into what may cause cancer. It was believed that abnormalities in the lymphatic system were the cause [6]. It was not until the late nineteenth century that Rudolf Virchow recognized that cells, even cancerous cells, are derived from other cells [7]. Other theories surfaced, such as cancer being caused by trauma, parasites, and it was thought that cancer may spread “like a liquid.” It was later concluded that cancer spreads through malignant cells by German surgeon, Karl Thiersch. In 1926, a Nobel Prize was wrongfully awarded for the discovery of the cause of stomach cancer, a worm. The twentieth century saw the greatest progression in cancer research. Research identifying carcinogens, chemotherapy, radiation therapy, and better means of diagnosis was discovered.

1.1

Types of Cancers

Cancer is probably the most complex class of human diseases and is actually considered to be a family of diseases that share a common set of characteristics such as reprogrammed energy metabolism, uncontrolled cell growth, tumor angiogenesis, and avoidance of immune destruction, referred to as cancer hallmarks. The complexity of cancer lies in the abnormal nature of functional state at the molecular, epigenomic, and genomic levels. It is also added due to its growth and expansion to encroach and replace normal tissue cells along with its abilities to resist both endogenous and exogenous measures for stopping or slowing down its growth. Many cancers form solid tumors, which are masses of tissue. Cancers of the blood, such as leukemia, generally do not form solid tumors. Cancerous tumors are malignant, which means they can spread into, or invade, nearby tissues. In addition, as these tumors grow, some cancer cells can break off and travel to distant places in the body through the blood or the lymph system and form new tumors far from the original tumor. However, unlike malignant tumors, benign tumors do not spread into, or invade, nearby tissues. Benign tumors can sometimes be quite large, however. When removed, they usually do not grow back, whereas malignant tumors sometimes do. Unlike most benign tumors elsewhere in the body, benign brain tumors can be life threatening.

1.2 Classification

3

There are various types of cancers. Based on their original cell types, cancers are classified into five classes: 1. Carcinoma, which begins in epithelial cells and represents the majority of the human cancer cases. 2. Sarcoma, derived from mesenchymal cells, e.g., connective tissue cells such as fibroblasts. 3. Lymphoma, leukemia, and myeloma, originating in hematopoietic or bloodforming cells. 4. Germ cell tumors, developing, as the name implies, from germ cells. 5. Neuroblastoma, glioma, glioblastoma, and others derived from cells of the central and peripheral nervous system and denoted as neuroectodermal tumors because of their beginning in the early embryo. Each class may consist of cancers of different types. For example, carcinoma comprises adenocarcinoma, basal-cell carcinoma, small cell carcinoma, and squamous cell carcinoma, independent of their underlying tissue types. Cancers of the same type and developing in the same tissue may have distinct properties in terms of their growth patterns, malignancy levels, survival rates, and possibly even different underlying mechanisms. They may respond differently to the same drug treatment and hence have different mortality rates. As of now, over 200 types of human cancers have been identified and characterized, the majority of which are determined based on the location, the originating cell type, and cell morphology. It is now becoming evident that this type of classification, in large part subjective, is not adequate for developing personalized treatment plans, which are becoming increasingly desirable and clearly represent the future of cancer medicine.

1.2

Classification

The earliest description of cancer can be traced back to 2500 BC by Egyptian physician Imhotep. Evidence exists suggesting that Egyptian physicians at the time could distinguish between benign and malignant tumors. The study of cancer as a scientific discipline came in the nineteenth century when microscopes became widely available to physicians and surgeons. Microscopic pathology, pioneered by German doctor Rudolf Virchow, laid the foundation for the development of cancer surgery as practiced now. Since then, cancer tissues removed from patients are microscopically examined and classified based on their morphological characteristics. Scientific oncology was born out of the debate concerning a few competing hypotheses regarding the possible causes of cancer in the late 1800s through the early 1900s. It developed based on findings that linked microscopic observations made on cancer tissues to clinical data during the course of the disease development. Early classification of cancers was based on a cancer’s location, such as lung cancer, skin cancer, or blood cancer (e.g., leukemia) (see Fig. 1.1). Over time,

4

1

Cancer

Fig. 1.1 Classification of Cancers based on the site of location

oncologists began to realize that different types of cancers can develop from the same organ. The earliest classification of cancers from the same organ, in this case bone marrow which houses the hematopoietic stem cells, can be traced back to the early 1900s when it was found that there were at least four types of leukemia, namely ALL (acute lymphoblastic leukemia), AML (acute myelogenous leukemia), CLL (chronic lymphoblastic leukemia), and CML (chronic myelogenous leukemia). This realization occurred about 50 years after the diagnosis of the first documented leukemia case. For other cancers, recognition of multiple cancer types originating from the same organ came rather late. For example, small cell lung cancer was not considered as a separate type of lung cancer from the more prevalent and less aggressive non-small cell lung cancer until the 1960s. Gastric cancers were found to have at least two subtypes, intestinal and diffuse, in 1965. It is worth noting that correct diagnosis of a cancer type has significant implications to designing the most effective treatment protocols and prognosis. For example, statistics show that the current 5-year survival rates for adult ALL, AML, CLL, and CML patients are 50%, 40%, 75%, and 90%, respectively, and the treatment plan for each of them is quite different. ALL is typically treated using chemotherapy followed by anti-metabolite

1.3 Cancer or Tumor Nomenclature

5

drugs; AML is generally treated using chemotherapy; CLL, while incurable, is often being controlled with chemotherapy using a combination of fludarabine and alkylating agents; and CML is, in most cases, successfully treated using the so-called miracle drug Gleevec, or else newer and improved drugs. The multistage nature of a cancer was first discovered by Japanese researchers Yamagiwa and Ichikawa in the beginning of the twentieth century [8, 9]. Basically, for most cancer types, the histological stage refers to the extent the cancer has spread, which is typically numbered from stage I through stage IV, with IV representing the most advanced stage. The stage of a cancer is an important predictor for survival, with the treatment plan often determined based on staging. Currently the stage of a cancer is generally determined by pathological analysis from a biopsied specimen of the cancer tissue, including lymph nodes, as well as analysis by imaging techniques with the results interpreted by radiologists; only limited molecular level information such as the expression levels of a few marker genes are determined by immunedetection. In addition to type and stage, cancer grade is another important parameter that has been used by pathologists to represent the level of malignancy of a given cancer, determined based on surgical specimens. This parameter is largely independent of the type and the stage of a cancer. A popular grading system uses four grades: (1) G1 (highly differentiated), (2) G2 (moderately differentiated), (3) G3 (poorly differentiated), and (4) G4 (undifferentiated), with G4 representing the most malignant.

1.3

Cancer or Tumor Nomenclature

Malignant and benign are important distinctions, but they are broad categories that comprise many different forms of cancer. In the majority of cases, benign tumors are named by attaching the suffix -oma to the name of the tissue or cell from which the cancer arose. For example, a tumor that is composed of cells related to bone cells and has the structural and biochemical properties of bone substance (osteoid) is classified as an osteoma. That rule is followed with a few exceptions for tumors that arise from mesenchymal cells (the precursors of bone and muscle). Benign tumors arising from epithelial cells (cells that form sheets that line the skin and internal organs) are classified in a number of ways and thus have a variety of names. Sometimes classification is based on the cell of origin, whereas in other cases it is based on the tumor’s microscopic architectural pattern or gross appearance. The term adenoma, for instance, designates a benign epithelial tumor that either arises in endocrine glands or forms a glandular structure. Tumors of the ovarian epithelium that contains large cysts are called cystadenomas. When a tumor gives rise to a mass that projects into a lumen (a cavity or channel within a tubular organ), it is called a polyp. Most polyps are epithelial in origin. Strictly speaking, the term polyp refers only to benign growths; a malignant polyp is referred to as a polypoid cancer in order to avoid confusion. Benign tumors built up of finger like projections from the skin or mucous membranes are called papillomas.

6

1

Cancer

For the naming of malignant tumors, the rules for using prefixes and suffixes are similar to those used to designate benign neoplasms. The suffix -sarcoma indicates neoplasms that arise in mesenchymal tissues—for instance, in supportive or connective tissue such as muscle or bone. The suffix -carcinoma, on the other hand, indicates an epithelial origin. As with benign tumors, a prefix indicates the predominant cell type in the tumor. Thus, a liposarcoma arises from a precursor to a fat cell called a lipoblastic cell; a myosarcoma is derived from precursor muscle cells (myogenic cells); and squamous cell carcinoma arises from the outer layers of mucous membranes or the skin (composed primarily of squamous, or scale-like, cells). Just as adenoma designates a benign tumor of epithelial origin that takes on a gland-like structure, so adenocarcinoma designates a malignant epithelial tumor with a similar growth pattern. Usually the term is followed by the organ of origin—for instance, adenocarcinoma of the lung. Malignant tumors of the blood-forming tissue are designated by the suffix -emia (Greek: “blood”). Thus, leukemia refers to a cancerous proliferation of white blood cells (leukocytes). Cancerous tumors that arise in lymphoid organs, such as the spleen, the thymus, or the lymph glands, are described as malignant lymphomas. The term lymphoma is often used without the qualifier malignant to denote cancerous lymphoid tumors; however, this usage can be confusing, since the suffix -oma, as mentioned above, more properly designates a benign neoplasm. The suffix -oma is also used to designate other malignancies, such as seminoma, which is a malignant tumor that arises from the germ cells of the testis. In a similar manner, malignant tumors of melanocytes (the skin cells that produce the pigment melanin) should be called melanocarcinomas, but for historical reasons the term melanoma persists. In some instances, a neoplasm is named for the physician who first described it. For example, the malignant lymphoma called Hodgkin disease was described in 1832 by English physician Thomas Hodgkin. Burkitt lymphoma is named after British surgeon Denis Parsons Burkitt; Ewing sarcoma of bone was described by James Ewing; and nephroblastoma, a malignant tumor of the kidney in children, is commonly called Wilms’ tumor, for German surgeon Max Wilms.

1.4

Heterogeneity

Pathologists and clinicians have known for years that solid tumors are heterogeneous with regard to cellular morphology and patient responses to treatment. Cancer heterogeneity and different genetic backgrounds even within the same type of cancer are a major challenging issue that clinical oncologists have to deal with [10–12]. It is also one of the major reasons for the loss of efficacy of effective medicines in a short span of time. Thus, cancer heterogeneity, first proposed several decades ago, is an important aspect of cancer. It is now appreciated that considerable heterogeneity exists in any given cancer, both at the molecular and cellular level. Cancer is clearly many diseases, and even individual tumors within similar types of cancer may be unique. A given tumor is likely composed of a dominant clone and several subclones, each of which may grow at different rates and respond differently to

1.6 How Cancer Arises?

7

treatment(s). This intratumor heterogeneity impacts on the evolution of cancer and the natural selection of clones more favorable for sustained growth, survival, and ability to colonize distant sites (extravasation and metastasis). Cancer heterogeneity, evolution, and natural selection are emerging as significant features in our understanding of cancer growth and control.

1.5

Invasion and Metastasis

Distant metastases cause 90% of cancer deaths. Invasion and metastasis involve sequential orchestration of complex biological processes: (1) detachment of the tumor cell from immediate neighbors and stroma at the local site; (2) enzymatic degradation of the extracellular matrix followed by specific directional motility (as single cells or in groups); (3) penetration (intravasation) of blood or lymphatic vessels and tumor embolization; (4) survival in the circulation until arrival at the metastatic site that may be selected on the basis of provision of a favorable supply of appropriate growth factors; (5) attachment to the endothelium of blood vessels at its destination and extravasation from the vessel; and (6) proliferation and invasion of its new location and recruitment of a new blood supply. One of the key processes underlying invasion and metastasis of epithelial tumors is the epithelial-to-mesenchymal transition. This multifaceted programme can be engaged transiently or stably by invading cancer cells. In addition, when the tumor reaches the end-organ in which it will form a metastatic deposit, a mesenchymal-to-epithelial transition frequently occurs. The patterns of metastasis of different cancers to specific organs (e.g. breast cancer to liver, bone and brain; lung cancer to brain and adrenal gland) are not random, but appear to be driven by expression of chemokine receptors by tumor cells that allow them to “seek” a suitable environment in which to establish a colony.

1.6

How Cancer Arises?

In the sixteenth and seventeenth centuries, it became more acceptable for doctors to dissect bodies to discover the cause of death. The German professor Wilhelm Fabry believed that breast cancer was caused by a milk clot in a mammary duct. The Dutch professor Francois de la Boe Sylvius, a follower of Descartes, believed that all diseases were the outcome of chemical processes, and that acidic lymph fluid was the cause of cancer. His contemporary Nicolaes Tulp believed that cancer was a poison that slowly spreads, and concluded that it was contagious [13]. The first cause of cancer was identified by British surgeon Percivall Pott, who discovered in 1775 that cancer of the scrotum was a common disease among chimney sweeps. The work of other individual physicians led to various insights, but when physicians started working together, they could draw firmer conclusions. With the widespread use of the microscope in the eighteenth century, it was discovered that the “cancer poison” eventually spreads from the primary tumor through the lymph nodes to other sites (“metastasis”). This view of the disease

8

1

Cancer

was first formulated by the English surgeon Campbell De Morgan between 1871 and 1874 [14]. The use of surgery to treat cancer had poor results due to problems with hygiene. The renowned Scottish surgeon Alexander Monro saw only 2 breast tumor patients out of 60 surviving surgery for 2 years. In the nineteenth century, asepsis improved surgical hygiene and as the survival statistics went up, surgical removal of the tumor became the primary treatment for cancer. With the exception of William Coley who in the late nineteenth century felt that the rate of cure after surgery had been higher before asepsis (and who injected bacteria into tumors with mixed results), cancer treatment became dependent on the individual art of the surgeon at removing a tumor. The underlying cause of his results might be that infection stimulates the immune system to destroy left tumor cells. During the same period, the idea that the body was made up of various tissues, that in turn were made up of millions of cells, laid rest the humor-theories about chemical imbalances in the body. Today, it is believed that cancer is a heterogeneous disease which can display different origins and lead to diverse prognosis [15]. Many factors can contribute to its development; some are genetic inheritance, age, epigenetic, or genetic mutations occurring in cancer-related genes, such as tumor suppressor genes, genome stability genes, and oncogenes. Other factors contributing to the insurgence of cancer are linked to living habits, such as smoking, alcohol abuse, diet or environmental and occupational exposures [16]. Very aggressive types of cancer are being characterized by high tendency to metastasize from the primary site and invade other vital organs, spreading through the circulatory and lymphatic systems, as well as having the capacity to develop resistance to treatments. The genetic basis of cancer was recognized in 1902 by the German zoologist Theodor Boveri, professor of zoology at Munich and later in Würzburg [17]. In genetic terms, cancer occurs when the information in cellular DNA is corrupted, leading to abnormal patterns of gene expression. Consequently, the effects of normal genes that control normal cellular functions, such as growth, survival, and invasion/ motility, are enhanced and those of genes that suppress these effects are repressed. The main mechanism of alteration is through the accumulation of mutations although non-mutational (epigenetic) changes are increasingly being seen as central to the process. Aberrant gene expression causes fundamental changes to biological processes within cancer cells. The genetic changes that contribute to cancer tend to affect three main types of genes-proto-oncogenes, tumor suppressor genes, and DNA repair genes. These changes are sometimes called “drivers” of cancer. Proto-oncogenes are involved in normal cell growth and division. However, when these genes are altered in certain ways or are more active than normal, they may become cancer-causing genes (or oncogenes), allowing cells to grow and survive when they should not. Tumor suppressor genes are also involved in controlling cell growth and division. Cells with certain alterations in tumor suppressor genes may divide in an uncontrolled manner. DNA repair genes are involved in fixing damaged DNA. Cells with mutations in these genes tend to develop additional mutations in other genes. Together, these mutations may cause the cells to become cancerous.

References

9

Frequently, cancer cells can break away from this original mass of cells, travel through the blood and lymph systems, and lodge in other organs where they can again repeat the uncontrolled growth cycle. This process of cancer cells leaving an area and growing in another body area is termed metastatic spread or metastasis. For example, if breast cancer cells spread to a bone, it means that the individual has metastatic breast cancer to bone. This is not the same as “bone cancer,” which would mean the cancer had started in the bone.

References 1. Papavramidou N, Papavramidis T, Demetriou T (2010) Ancient Greek and Greco-Roman methods in modern surgical treatment of cancer. Ann Surg Oncol 17(3):665–667 2. Karpozilos A, Pavlidis N (2004) The treatment of cancer in Greek antiquity. Eur J Cancer 40 (14):2033–2040 3. Lonardo AD, Nasi S, Pulciani S (2015) Cancer: we should not forget the past. J Cancer 6 (1):29–39 4. Faguet GB (2015) A brief history of cancer: age-old milestones underlying our current knowledge database. Int J Cancer 136(9):2022–2036 5. American Cancer Society (2014) Early theories about cancer causes 6. Padera TP, Meijer EFJ, Munn LL (2016) The lymphatic system in disease processes and cancer progression. Annu Rev Biomed Eng 11(18):125–158 7. Walter E, Scott M (2017) The life and work of Rudolf Virchow 1821-1902: cell theory, thrombosis and the sausage duel. J Intensive Care Soc 18(3):234–235 8. Weiss RA (2004) Multistage carcinogenesis. Br J Cancer 91:1981–1982 9. Armitage P (1985) Multistage models of carcinogenesis. Environ Health Perspect 63:195–201 10. Fisher R, Pusztai L, Swanton C (2013) Cancer heterogeneity: implications for targeted therapeutics. Br J Cancer 108(3):479–485 11. Dagogo-Jack I, Shaw AT (2018) Tumour heterogeneity and resistance to cancer therapies. Nat Rev Clin Oncol 15:81–94 12. de Lartigue J (2018) Tumor heterogeneity: a central foe in the war on cancer. JCSO 16(3):e167– e174 13. Yalom M (1997) A history of the breast. Alfred A. Knopf, New York 14. Grange JM, Stanford JL, Stanford CA (2002) Campbell De Morgan’s ‘observations on cancer’, and their relevance today. J R Soc Med 95(6):296–299 15. Kwabi-Addo B, Lindstrom TL. Cancer causes and controversies: understanding risk reduction and prevention, ABC-CLIO, CA, USA 16. McMullin J (2016) Cancer. Annu Rev Anthropol 45(1):251–266 17. Boveri T (2008) Concerning the origin of malignant tumours. J Cell Sci 121(Supplement 1):1–84

2

Epidemiology

The eighteenth century saw the birth of cancer epidemiology when three insightful observations were made. Firstly, the high incidence of breast cancer in nuns compared to non-celibate women gave the first hint that hormones may play a role in cancer. Secondly, descriptions of cancer of the scrotum prevalent in London’s chimney sweeps led to public health measures to reduce a person’s cancer risk at work. And thirdly, a book was published linking tobacco use with cancer, which laid the foundations for the U.S. Surgeon General’s 1964 warning that smoking caused lung cancer. Today cancer is the second leading cause of death worldwide, surpassed only by cardiovascular diseases [1]. Of the more than 18.1 million cancer cases worldwide, 48.4% were reported from Asia, 23.4% from Europe, 13.2% from North America, 7.8% from Latin America and Caribbean (LAC), 5.8% from Africa, and 1.4% from Oceania. The mortality rates were 57.3% from Asia, 20.3% from Europe, 7.3% from North America, 7% from Latin America and Caribbean (LAC), 7.3% from Africa, and 0.7% from Oceania (Fig. 2.1). The Global Burden of Disease estimates that 9.6 million people died prematurely as a result of cancer in 2018 [2] (including non-melanoma skin cancers), with one in four men and one in five women developing the disease, and one in eight men and one in eleven women dying from it. In addition, there were 43.8 million persons living with cancer in 2018 who were diagnosed within the last 5 years. Half of the new cancer cases and cancer deaths in the world occur in Asia. Based on projected population aging and growth, the global burden of cancer is set to increase by more than 60% by 2040, from 18.1 million new cases in 2018 to a predicted 29.4 million cases in the year 2040. To add to the existing burden, the number of cancer cases and deaths is expected to grow rapidly as populations grow, age, and adopt lifestyle behaviors that increase cancer risk. The adoption of lifestyle behaviors that are known to increase cancer risk, such as smoking, poor diet, physical inactivity, and reproductive changes (including lower parity and later age at first birth), has further increased the cancer burden in less economically developed countries. Based on # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_2

11

12

2

Epidemiology

Fig. 2.1 Cancer incidences and mortality rates worldwide in 2018. (NA is North America and LAC is Latin America and Caribbean)

GLOBOCAN estimates, about 18.1 million new cancer cases and 9.5 million deaths occurred in 2018 worldwide [3]. Over the years, the burden has shifted to less developed countries, which currently account for about 57% of cases and 65% of cancer deaths worldwide (see Fig. 2.2). Lung cancer is the leading cause of cancer death among males in both more and less developed countries, and has surpassed breast cancer as the leading cause of cancer death among females in more developed countries; breast cancer remains the leading cause of cancer death among females in less developed countries. Other leading causes of cancer death in more developed countries include colorectal cancer among males and females and prostate cancer among males. In less developed countries, liver and stomach cancer among males and cervical cancer among females are also leading causes of cancer death. Although incidence rates for all cancers combined are nearly twice as high in more developed than in less developed countries in both males and females, mortality rates are only 8% to 15% higher in more developed countries. This disparity reflects regional differences in the mix of cancers, which is affected by risk factors and detection practices, and/or the availability of treatment. Risk factors associated with the leading causes of cancer death include tobacco use (lung, colorectal, stomach, and liver cancer), overweight/obesity, and physical inactivity (breast and colorectal cancer), and infection (liver, stomach, and cervical cancer). According to GLOBOCAN 2018, lung cancer is the most commonly diagnosed cancer among males (14.5%) followed by prostate (13.5%), colorectum (10.9%), stomach (7.2%), bladder (4.5), esophageal (4.4%), and other cancers together (38.9%). In females breast cancer is the most common (24.2%), followed by

2

Epidemiology

13

Fig. 2.2 Estimated age-standardized cancer incidence and mortality rates (World) in 2018, all cancers, both sexes, all ages. Source: International Agency for Research on Cancer (IARC) WHO

MALE

Other cancers 3 681 464 (38.9%)

Prostate 1 276 106 (13.5%)

Colorectum 1 026 215 (10.9%) Stomach 683 754 (7.2%)

Oesophagus 399 699 (4.2%) Bladder 424 082 (4.5%)

FEMALE

Lung 1 368 524 (14.5%)

Liver 596 574 (6.3%)

Total:9 456 418

Breast 2 088 849 (24.2%)

Other cancers 3 246 828 (37.7%)

Colorectum 823 303 (9.5%)

Stomach 349 947 (4.1%) Corpus uteri 382 069 (4.4%) Thyroid 436 344 (5.1%)

Lung 725 352 (8.4%)

Cervix uteri 569 847 (6.6%) Total:8 622 539

Fig. 2.3 Different types of cancer incidences in males and females, all ages, worldwide (estimated in 2018)

colorectum (9.5%), lung (8.4%), cervix uteri (6.6%), thyroid (5.1%), corpus uteri (4.4%), stomach (4.1%), and other cancers together (38.9%) as shown in Fig. 2.3. Using a formula accepted by public health researchers and economists to measure the global burden of disease, there were 83 million years of “healthy life” lost due to death and disability from cancer in 2008. The top three cancers that account for the highest number of healthy life years lost were lung cancer (15.5%), stomach cancer (9.6%), and liver cancer (8.6%).

14

2.1

2

Epidemiology

Economic Burden

Cancer is one of the critical issues causing economic and financial burden in the world today. It results in economic burden for patients, healthcare systems, and countries due to healthcare spending, and productivity losses from morbidity and premature mortality [4] [5]. The global cancer burden is dominated by Europe, China, and Northern America. China, with the largest population size in the region and worldwide—1.4 billion inhabitants, representing 19% of the global population in 2019—has the greatest global proportion of new cases (4.3 million cases, 24% of the total) and deaths (2.9 million deaths, 30%). Northern America is second in terms of new cases (2.4 million, 13%), and fourth for cancer deaths (0.7 million, 7%). Close to one fourth of all new cases globally (4.2 million) and one fifth of deaths (1.9 million) occur in Europe, despite the region representing less than one tenth of the global population. The global economic burden of cancer is unknown although data are available in some countries [6]. The Agency for Healthcare research and Quality (AHRQ) estimates that the direct medical costs (total of all health care costs) for cancer in the USA in 2015 were $80.2 billion. In the USA in 2017, estimated cancer healthcare spending was US$161.2 billion; productivity loss from morbidity, US$30.3 billion; and premature mortality, US$150.7 billion [7]. The economic burden of cancer in the USA is approximately 1.8% of gross domestic product (GDP). In the European Union, healthcare spending was €57.3 billion, and productivity losses due to morbidity and premature death were €10.6 billion and €47.9 billion, respectively. With informal care costs of €26.1 billion, total burden rose to €141.8 billion, 1.07% of GDP. Productivity losses due to premature deaths vary in transitioning countries (Fig. 2.4). Cancer causes the highest economic loss of all of the 15 leading causes of death worldwide. The economic toll from cancer is nearly 20% higher than heart disease, the second leading cause of economic loss ($895 billion and $753 billion, respectively, see Fig. 2.5). The top three cancers that caused the most economic impact globally were lung cancer ($188 billion), colon/rectum cancer ($99 billion), and breast cancer ($88 billion). Although major advances in cancer diagnostics and treatment have led to significant improvements in clinical outcomes during the last few decades, the cost of therapy, including chemotherapy, targeted agents, and more recently, immunotherapy, is substantial. Approximately 5% of the U.S. population, or 15.5 million people, are cancer survivors, incurring not only initial diagnostic and treatment costs, but also long-term costs throughout survivorship [8]. Cancer is now the second most expensive disease in the USA, with an estimated health care cost of $124 billion in 2010 that is expected to rise to $157 billion at the end of 2020.

2.1 Economic Burden

15

Fig. 2.4 Productivity losses due to premature deaths from cancer in transitioning economies of the world (Source: canceratlas.cancer.org)

895.2

Cancer Heart diseases Cerebrovascular disease

753.2 298.2 204.4 204.4 203.1

Diabetes mellitus Road traffic accidents COPD HIV/AIDS Perinatal conditions Suicides

193.3 192.8 140.8

LRI 125.8 Cirrhosis of the liver 92.8 Diarrhoeal diseases 70.1 Tuberculosis 45.4 Malaria 24.8 Measles 8.1 0 COPD= Chronic obstructive pulmonary disease LRI= Lower respiratory infections

200 400 600 800 Economic Value Lost (US $ Billion ) in 2008

Fig. 2.5 Economic Loss from the top 15 global causes of death

16

2

Epidemiology

References 1. Torre LA, Siegel RL, Ward EM, Jemal A (2016) Global Cancer incidence and mortality rates and trends—an update. Cancer Epidemiol Biomark Prev 25(1):16–26 2. Global Burden of Diseases (2018) Lancet Global Health Metrics 392(10159):P1736–P1788. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(18)32203-7/fulltext 3. https://gco.iarc.fr/today/data/factsheets/populations/900-world-fact-sheets.pdf. Accessed 1 Jul 2020 4. American Society of Clinical Oncology (2017) The state of cancer care in America, 2017: A report by the American Society of Clinical Oncology. J Oncol Pract 13(4):e353–e394 5. Mariotto AB, Yabroff KR, Shao Y, Feuer EJ, Brown ML (2011) Projections of the cost of cancer care in the United States: 2010–2020. J Natl Cancer Inst 103(2):117–128 6. Pisu M, Henrikson NB, Banegas MP, Yabroff KR (2018) Costs of cancer along the care continuum: what we can expect based on recent literature. Cancer 124(21):4181–4191 7. Guy GP, Ekwueme DU, Yabroff KR et al (2013) Economic burden of cancer survivorship among adults in the United States. J Clin Oncol 31(30):3749–3757 8. Yabroff KR, Lund J, Kepka D, Mariotto A (2011) Economic burden of cancer in the United States: estimates, projections, and future research. Cancer Epidemiol Biomark Prev 20 (10):2006–2014

3

Treatment Modalities

The treatment of cancer has undergone evolutionary changes as understanding of the underlying biological processes has increased. Tumor removal surgeries have been documented in ancient Egypt, hormone therapy and radiation therapy were developed in the late nineteenth century. Chemotherapy, immunotherapy, and newer targeted therapies are products of the twentieth century (Fig. 3.1). Surgery and radiotherapy dominated the field of cancer therapy into the 1960s until it became clear that cure rates after ever more radical local treatments had plateaued at about 33% due to the presence of heretofore-unappreciated micrometastases and new data showed that combination chemotherapy could cure patients with various advanced cancers. The landscape of therapeutic options for treating cancer has dramatically changed over recent years resulting in more treatment options, higher response rates, improvement in survival, and changing toxicity profiles [1]. Although a variety of high-tech treatment modalities are currently available to treat different cancers, chemotherapy, which involves the use of small-molecule drugs (synthetic or natural), remains one of the most preferred methods for the treatment of cancer. Chemotherapy takes into account the metastatic nature of majority of the cancers. Besides, the anticancer drugs used during chemotherapy reach all the cells and tissues of the human body where other treatment procedures cannot be used. Additionally, the availability of a huge reservoir of anticancer drugs with diverse mechanisms of action and drug combination procedures makes chemotherapy a standout performer in cancer treatment procedures [2, 3].

3.1

Chemotherapy

Chemotherapy, or “chemical treatment,” has been around since the days of the ancient Greeks. In the early 1900s, the famous German chemist Paul Ehrlich set about developing drugs to treat infectious diseases [4]. He was the one who coined the term “chemotherapy” and defined it as the use of chemicals to treat diseases. # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_3

17

18

3 Treatment Modalities

Fig. 3.1 Cancer treatment modalities

However, chemotherapy for cancer treatment began in the 1940s with the use of nitrogen mustard [5]. Since then, in the attempt to discover what is effective in chemotherapy, many new drugs have been developed and tried as cancer treatments. Today’s therapy uses more than 100 drugs for cancer treatment [6, 7]. They are often divided into groups based on how they work and what they are made of (Table 3.1). Each group of drugs destroys or shrinks cancer cells in a different way. • Some drugs damage the DNA of cancer cells to keep them from making more copies. They are called alkylating agents, the oldest type of chemotherapy. They treat many different types of cancer, such as leukemia, lymphoma, Hodgkin’s disease, multiple myeloma, and sarcoma, as well as breast, lung, and ovarian

(continued)

Alkylating agents Nitrogen mustards Nitrosoureas Alkyl sulfonates Ethylenimines Mechlorethamine, Carmustine, Busulfan Triethylenemelamine, Triethylenethio-phosphoramide Cyclophosphamide, Lomustine, melphalan, chlorambucil, Semustine Ifosfamide Alkylating agents work by DNA alkylation. That is, they form alkyl bonds between the nucleotides of the DNA (N7 Guanine). Since cancer cells divide all the time their DNA succumbs to multiple errors during replication due to the alkylation and thus the cells undergo apoptosis Abnormal base pairing, cross linking of bases and DNA strand breakage Antimetabolites Purine antagonists Pyrimidine Folic acid analogs antagonists Mercaptopurine (6-MP), Thioguanine (6-TG), Fluorouracil (5-FU), Methotrexate (MTX) also known as Amethopterin, Phototrexate Fludarabine Phosphate, Cladribine (Leustatin), Cytarabine (ARA-C) (Photoactivated version) Pentostatin (Nipent) Azacitidine Purine analogs block DNA synthesis by blocking conversion of AMP to ADP. They also block the first in purine synthesis by feedback inhibition Pyrimidine analogs work by interfering with DNA replication, they get utilized instead of natural nucleotides and thus cause halting of cell division in S phase of cell cycle Methotrexate is an antimetabolite of the antifolate type. It inhibits formation of tetrahydrofolate (FH4) from folic acid by inhibiting the enzyme dihydrofolate reductase (DHFR). Since FH4 transfers methyl groups essential to DNA synthesis, it causes inhibition of DNA synthesis. Methotrexate also inhibits the synthesis of RNA, thymidylates, and proteins Anticancer antibiotics Anthracycline antibiotics Non-anthracycline antibiotics Daunorubicin (DaunoXome) Dactinomycin (Cosmegen) Doxorubicin (Adriamycin, Rubex, Doxil) Idarubincin (Idamycin) Epirubicin Plicamycin (Mithramycin) Idarubicin Mitomycin (Mutamycin) Bleomycin (Blenoxane)

Table 3.1 Different types of anticancer chemotherapeutic agents used in medicine

3.1 Chemotherapy 19

Alkylating agents Popularly referred to as stacking or intercalating agents. These drugs bind to DNA and inhibit RNA synthesis, causing impaired mRNA production, and protein synthesis. They also work by inhibition of the enzyme topoisomerase. This prevents the DNA from closing itself after opening and thus halts replication. Often the intercalation will cause histone eviction of the DNA. This applies mainly to anthracycline antibiotics Bleomycins work by incorporating a break in the DNA strands in an oxygen-dependent manner. These are a group of compounds, unlike other agents Microtubule Inhibitors Tubulin binding Antimitotic drugs Epipodophyllotoxins Vinblastine, Vincristine Paclitaxel (Taxol) Etoposide, Teniposide Vinorelbine, Vindesine Docetaxel Tubulin binding drugs cause inhibition of mitotic spindle formation by binding to tubulin. Vinblastine methylates DNA and inhibits DNA synthesis and function Antimitotic drugs bind tubulin, promote microtubule formation, and retard disassembly, resulting in mitotic arrest Epipodophyllotoxins bind to and inhibit topoisomerase II and its function. They also cause fragmentation of DNA, leading to cell death, apoptosis Steroid hormones and antagonists Estrogen and Androgen Gonadotropin-Releasing Hormone Aromatase Inhibitors Inhibitors Agonists Tamoxifen and Flutamide Leuprolide, Goserelin, Nafarelin Aminoglutethimide, Letrozole, Anastrozole Cell-cycle-non-specific, they work by making the cancer cells unable to use the hormone they need to grow, or by preventing the body from making the hormone Act in a variety of ways. Mechanism of action not fully understood May have “direct lytic action” on cells in certain diseases (e.g. corticosteroids in leukemia, lymphoma) Interfere with protein synthesis Immunomodulators Immunosuppressants Immunostimulants Glucocorticoids Cytotoxic Drugs acting on Monoclonal Biological Angiogenesis inhibitors agents immunophilins antibodies response modifiers Prednisolone Alkylating Cyclosporin Trastuzumab Cytokines, Thalidomide Dexamethasone agents Sirolimus Avelumab Interleukins, LenalidomidePomalidomideApremilast Antimetabolites Cemiplimab

Table 3.1 (continued)

20 3 Treatment Modalities

(continued)

Interferons, Chemokines Glucocorticoids exert a majority of their effects by altering gene expression. The majority of effects produced by glucocorticoids result from initial steroid binding to intracellular glucocorticoid receptors followed by translocation to the nucleus and changes in gene transcription Monoclonal antibodies work by recognizing the protein on the surface of the cancer cell and then locking onto it. This triggers the body’s immune system to attack the cancer cell causing the cell to kill itself Interleukins and interferons stimulate the body’s own immune system to fight some kinds of cancer, encouraging the body’s immune system to kill cancer cells Angiogenesis inhibitors interfere with the development of blood vessels, meaning the cancer is unable to receive the oxygen and nutrients it requires to survive. e.g. Thalidomide Growth factor receptor inhibitors VEGF/VEGFR# HER2#/neu HDACs# EGFR# # Monoclonal antibodies TKI Sunitinib Lapatinib Vorinostat (SAHA) Sorafenib Trastuzumab Panobinostat Gefitinib, Cetuximab Nilotinib, Ramucirumab Imatinib Dasatinib EGFR inhibitors suppress cell growth and metastasis, inhibition of EGFR signaling (down regulates active EGFRVII) and ADCC Monoclonal antibodies (mAb) bind to the extracellular component of the HER2, prevent the actual substrates from binding to the receptors, and stop the receptor activation TKIs act by inhibiting particular tyrosine kinase enzymes, instead of non-specifically inhibiting rapidly dividing cells VEGFR inhibits tumor angiogenesis and growth; Blocks VEGF binding to the receptor and thus VEGF-signaling and subsequently angiogenesis HDAC inhibitors block the action of a group of enzymes that remove chemicals called acetyl groups from particular proteins. This can stop cancer cells from growing and dividing and sometimes kills them completely # EGFR¼ Epidermal Growth Factor Receptor # VEGF/VEGFR¼ Vascular Endothelial Growth Factor Receptor # HER2¼ Human Epidermal Growth Factor Receptor 2 # TKI¼ Tyrosine Kinase Inhibitors # HDACs ¼ Histone deacetylase inhibitors

3.1 Chemotherapy 21

Alkylating agents Anticancer metal complexes Platinum-based anticancer agents Non-platinum anticancer agents Cisplatin, Carboplatin, Oxaliplatin, Nedaplatin, Metal complexes other than platinum Lobaplatin, and Heptaplatin Platinum-based drugs act by binding to guanine base of DNA or RNA DNA intercalating agents Cellular processing leading to apoptosis Non-platinum agents target of action are enzymes and proteins like thioredoxin reductase, glutathione, proteasome, etc.

Table 3.1 (continued)

22 3 Treatment Modalities

3.1 Chemotherapy

•

•

•

•

• •

23

cancers. Some examples of alkylating agents are cyclophosphamide, melphalan, and temozolomide. One type of alkylating agent involves the platinum drugs like carboplatin, cisplatin, or oxaliplatin and many more. One type of chemo drug interferes with the normal metabolism of cells, which makes them stop growing. These drugs are called antimetabolites and are used to treat leukemia and cancer in the breasts, ovaries, and intestines. Drugs in this group include 5-fluorouracil, 6-mercaptopurine, cytarabine, gemcitabine, and methotrexate, among many others. Anthracycline chemotherapy attacks the enzymes inside cancer cells’ DNA that help them divide and grow. They work for many types of cancer. Some of these drugs are actinomycin D, bleomycin, daunorubicin, and doxorubicin, among others. Drugs called mitotic inhibitors stop cancer cells from further growing and making copies. They can also stop the body from making the proteins that cancer cells need to grow. Mitotic inhibitors include docetaxel, estramustine, paclitaxel, and vinblastine. Another type of medicine, called topoisomerase inhibitors, also attack enzymes that help cancer cells divide and grow. They treat some types of leukemia and cancer of the lung, ovaries, and intestines, among other types. This group of medicine includes etoposide, irinotecan, teniposide, and topotecan. Steroids are drugs that act like the body’s own hormones. Some of the steroids that are used in anticancer treatment are prednisone, methylprednisolone, and dexamethasone. Other cancer treatments such as targeted therapies, hormone therapy, and immunotherapy are currently being used. Unlike chemo, these types of medicine are better at attacking only cancer cells and leaving healthy cells alone, causing milder side effects.

Chemotherapy is considered to be the most effective therapeutic approach. It relieves painful symptoms, prolongs life, and/or even heals the disease. This treatment modality is predominantly used in the treatment of cancer, with metastases and dissemination. The aim of chemotherapy can be curative or palliative depending on the therapeutic scheme; chemotherapy can use a single therapeutic agent, a combination of substances or it can be an adjuvant to surgical and/or radiation therapy. In the majority of cases, chemotherapy leads to the prolongation of survival, and in other cases it results in the eradication of the disease. One of the first drugs that were used clinically in modern medicine for the treatment of cancer was the alkylating agent mechlorethamine, a nitrogen mustard that in the 1940s was found to be effective in treating lymphomas [8]. In 1956 the antimetabolite methotrexate became the first drug to cure a solid tumor, and the following year 5-fluorouracil was introduced as the first of a new class of tumor-fighting compounds known as pyrimidine analogs. Since then many anticancer drugs have been developed and used with much success. They include alkylating agents, antimetabolites, natural products, and hormones, as well as a variety of other chemicals that do not fall within

24

3 Treatment Modalities

these discrete classes but are capable of preventing the replication of cancer cells and thus are used in the treatment of cancer. Hormones are used primarily in the treatment of cancers of the breast and sex organs. These tissues require hormones such as androgens, progestins, or estrogens for growth and development. By countering these hormones with an antagonizing hormone, the growth of that tissue is inhibited. A selected history and timeline of events related to the development of cancer chemotherapy are shown in Fig. 3.2. Drugs are chemicals, and like other chemicals, they can be organized by their molecular structure. Another is by origin, and this method is often used for natural products such as the vinca alkaloids. Mechanism of action (biological target) is one way we group medicines. For instance, inhibitors of cyclin-dependent kinases could be a class, or angiogenesis inhibitors could be a class. Further, we tend to identify drugs made as monoclonal antibodies. Many drugs could fall into more than one class. Some monoclonal antibodies work by antiangiogenesis. Some metabolic drugs inhibit more than one type of kinase. Some chemotherapy agents work in specific parts of the cell’s reproductive cycle, inhibiting the S phase or the M phase. Others are non-phase specific. Chemotherapeutic agents are therefore, generally, classified according to their structure or cell cycle activity as either cell cycle phase specific or cell cycle phase non-specific. Cell cycle phase specific agents act on the cells in a specific phase (Fig. 3.3). They are most effective against tumors that have a large proportion of cells actively moving through the cell cycle and cycling at a fast rate. Rapid cycling ensures that the cell passes through the phase in which it is vulnerable to the effect of the drug. Cell cycle phase non-specific agents are not dependent on the cell being in a particular phase of the cell cycle for them to work— they affect cells in all phases of the cell cycle. Resting cells (phase G0) are as vulnerable as dividing cells to the cytotoxic effects of these agents. As a result, phase non-specific agents have been found to be some of the most effective drugs against slow-growing tumors. Cell cycle specific Most effective during a specific phase of the cell cycle Greatest effect on actively dividing cells Not active in G0 phase Greatest action in divided doses or as continuous infusion Cytotoxic effects occur when cell repair or division attempted

Cell cycle non-specific Effective regardless of the cell cycle phase Work in any phase of the cell cycle Active in G0 phase Greatest action when administered as a bolus Cytotoxic effects occur when cell division attempted

Chemotherapeutic agents are also traditionally divided by their origin or mechanism of action. The main groups include:

Fig. 3.2 Key advances in the history of cancer chemotherapy

3.1 Chemotherapy 25

26

3 Treatment Modalities

Fig. 3.3 Cell cycle phases and action of chemotherapeutic agents

3.1.1

Alkylating Agents

Classic alkylating agents interfere with DNA replication by crosslinking DNA strands, DNA strand breaking, and abnormal pairing of base pairs. They exert their lethal effects on cells throughout the cell cycle but tend to be more effective against rapidly dividing cells. Direct DNA damage by alkylating agents stops division of cancer cells and is efficacious in all stages of the cell cycle. Many cancers are treated with alkylating agents such as lymphoma, leukemia, multiple myeloma, Hodgkin’s disease, and sarcomas. Also included are several cancers of the ovary, breast, and lungs. The nitrosoureas are a subgroup of the alkylating agents. They also interfere with DNA replication and repair. They are highly lipid soluble and readily cross the blood–brain barrier. An example is carmustine. Another subgroup of alkylators called platinum-containing compounds includes agents such as cisplatin, carboplatin, and oxaliplatin. Their cytotoxic properties also extend to alteration of the cell membrane transport systems and suppression of mitochondrial function. On the downside of alkylating agents, they can cause damage to bone marrow as they damage DNA. Long-term damage can result in acute leukemia, depending on dosages used, although rarely. Leukemia from alkylating agents arises after 5 to 10 years of treatment. Families of alkylating agents are given in Table 3.1. Based on similar mode of action of alkylating agents and platinum drugs, i.e., cisplatin, carboplatin, and oxaliplatin, they are sometimes grouped together. These drugs have a reduced tendency to cause post-treatment leukemia.

3.1 Chemotherapy

3.1.2

27

Antimetabolites

Antimetabolites interfere with DNA and RNA synthesis by acting as false metabolites, which are incorporated into the DNA strand or block essential enzymes, so that DNA synthesis is prevented. These agents are most effective when used against rapidly cycling cell populations and are consequently more effective against fast-growing tumors than slow-growing tumors. Major toxicities occur in the hematopoietic and gastrointestinal systems. Examples include methotrexate, which is a folic acid analog (Fig. 3.4), 5-fluorourocil, and cytosine arabinoside. These drugs are analogs for the units of DNA and RNA, and hence by incorporation, they stop the growth of DNA and RNA. Such drugs particularly affect the S phase of the cell and are used for the treatment of leukemia, cancers of ovary, breast, intestinal tract, and various others. Examples of antimetabolites are given in Table 3.1.

3.1.3

Antibiotics

There are two types of antibiotics which are classified as anthracycline and non-anthracycline type. The anthracycline type of antibiotics target DNA replication enzymes, affecting cells in all phases of the cell cycle. Various cancers lie in the scope of these drug treatments. A big limitation of these drugs is that exceeding a critical limit can permanently damage the heart. Therefore, dose limits for a lifetime are determined for these drugs. There are some antitumor antibiotics that do not belong to anthracyclines and are known as non-anthracycline antibiotics, including actinomycin D, bleomycin, and mitomycin C. Another anticancerous antibiotic is mitoxantrone, comparable in many ways to doxorubicin, both of which can damage the heart at high dosage. Their mode of action is also the same, i.e., inhibiting the topoisomerase II, and thus can lead to Fig. 3.4 Folic acid analog methotrexate

O

N H

O N

HN H2N

N

H CO2H

N H

N Folic acid O NH2

N H2N

CO2H

N

N

H CO2H N H

N

N Methotrexate

CO2H

28

3 Treatment Modalities

posttreatment acute myelogenous leukemia, after 2–3 years in most cases. Prostate and breast cancers, lymphoma, and leukemia are also treated with mitoxantrone.

3.1.4

Microtubule Inhibitors

Microtubules are composed of heterodimeric chains of α-tubulin and β-tubulin molecules. There is a third subtype termed γ-tubulin in eukaryotes, which is needed for the nucleation of the α/β -tubulin polymerization [9]. The various interactions between the tubulin dimer in the microtubules are responsible for maintaining its tubular form as well as trafficking proteins and organelles. While the microtubule is a key regulator of cell division, its dysregulation may contribute to the development of tumorous cells, as evidenced by the fact that the majority of tumor cells display aneuploidy [10, 11]. Furthermore, tubulin mutations can also contribute to the development of chemo-resistance and tumor propagation through altered responses to cell microenvironment. Some of these mutations have been identified through notable differences between tubulin isomers, tubulin post-translational modifications, and differences in the tubulin associated molecular patterns. While a specific oncogenic tubulin isotype is yet to be discovered, many studies have shown that oncogenic pathways such as the AKT and ERK pathways function through the microtubule. There are many different anticancer agents targeting various components of cell regulation, especially the mitotic stage of tumor cell proliferation. Although the microtubule is an effective target at different stages of cell regulation, many microtubule inhibitors are especially effective at the mitotic stage. Microtubule inhibitors (MTI) such as taxanes, vinca alkaloids, and epothilones stabilize or destabilize microtubules, thereby suppressing microtubule dynamics required for proper mitotic function, effectively blocking cell cycle progression and resulting in apoptosis. Microtubules are composed of tubulin, a heterodimer composed of α-tubulin and β-tubulin. The polymerization of tubulin leads to formation of microtubules. Microtubule inhibitors are divided into two groups: stabilizing and destabilizing agents. Stabilizing agents, which include the taxanes and epothilones, promote polymerization and destabilizing agents, which include the vinca alkaloids, lead to depolymerization. As a microtubule inhibitor, paclitaxel acts to stabilize polymerized microtubules during mitosis, thus leading to cell cycle arrest in the G2 and M phases. Vinca alkaloids are also microtubule inhibitors, and they also bind to β-tubulin; the difference is that they prevent polymerization of β-tubulin into microtubules and thus destabilize the microtubule. Although MTIs have significant clinical activity in multiple tumor types, their effectiveness is reduced by drug resistance mechanisms. As a result, there is an ongoing effort to develop new agents within this class to improve efficacy and circumvent drug resistance.

3.1 Chemotherapy

3.1.5

29

Steroid Hormones and Antagonists

Steroid hormones are frequently classified into broad categories such as “progestins,” “androgens,” and “glucocorticoids,” but this classification is often misleading or just incomplete since it only accounts for their known primary biological activity and does not adequately describe the multiplicity of actions that many of them display. Steroid hormones function by interacting with specific intracellular receptors. The hormone antagonist tamoxifen is used for the treatment of breast cancer. It is an estrogen antagonist, structurally related to diethylstilbestrol. It is known to compete with estrogen by binding to estrogen receptors on the membrane of target cells, thus limiting the effect of estrogen on breast tissue. It makes tamoxifen an excellent therapeutic agent against breast cancer. Gonadotropinreleasing hormone agonists such as leuprolide, goserelin, nafarelin, and buserelin can under certain circumstances inhibit gonadotropin release used to treat advanced breast and prostate cancer. Enzyme inhibitors such as aminoglutethimide are also used for the therapy of adrenocortical adenomas and carcinomas. They inhibit the synthesis of pregnenolone (a precursor of estrogen) from cholesterol and inhibit the synthesis of estrone and estradiol by inhibiting aromatase enzyme that converts androstenedione to estrone. The selective non-steroidal aromatase inhibitor letrozole is used to treat estrogen positive breast cancer that is no longer responsive to tamoxifen.

3.1.6

Immunomodulators

Immunomodulators are a group of drugs that mainly target the pathways that treat multiple myeloma and a few other cancers. They have many ways to work, including working on the immune system directly by turning down some proteins and turning up others. A successfully coordinated immune response is dependent on secreted chemical signals (cytokines), protein–protein interactions between adjacent cells, and the activity of various enzymes that alter the microenvironment. Tumor cells must learn how to hijack this signaling system in order to evade the immune system and metastasize throughout the host. Many immunotherapy agents work by activating an effective antitumor response or reversing tumor-mediated immunosuppression through manipulation of key regulatory pathways.

3.1.7

Growth Factor Receptor Inhibitors

Growth factors are chemicals produced by the body that control cell growth. Growth factor receptors are transmembrane proteins which bind to specific growth factors and transmit the instructions conveyed by the factors to intracellular space. The growth factor receptors on cell surface are very common, and cells mainly contain receptors for several growth factors.

30

3 Treatment Modalities

The heterogeneous nature of cancer is characterized by continuous clonal expansion and uncontrolled growth of mutated cells, intravasation, and extravasation of blood and lymphatic vessels, dissemination, and finally metastasis into distant organs. In the tumor microenvironment, cells are supplied with nutrients by the formation of disorganized blood vessels with leaky vasculature by the process of angiogenesis. Genetic, epigenetic, and somatic changes deregulate the expression of growth factor receptors (GFRs), leading to cancer initiation and progression. Growth factor receptors (GFRs), expressed on cell membranes or in the cytoplasm, have profound roles in cell growth, survival, angiogenesis, and metastasis. Amplification of GFRs generates inherent and acquired resistance to classical chemotherapies and targeted molecules, thus causing problem in treatment. Escalated growth signals cross-talk differently with death signals to inhibit apoptosis. Accordingly, signals mediated by GFRs function in collaboration to enhance the complexity of the tumor microenvironment. There are many different types of growth factors and they all work in different ways. These include: • Epidermal growth factor (EGF)—controls cell growth and proliferation. • Vascular endothelial growth factor (VEGF)—controls blood vessel development. • Platelet derived endothelial growth factor (PDGF)—controls blood vessel development and cell growth. • Transforming growth factor-TGF-β—control of cell growth, cell proliferation, cell differentiation, and apoptosis. • Fibroblast growth factor (FGF)—controls cell growth. • Insulin-like growth factors—plays an important role in childhood growth and anabolic effects in adults. A cancer growth blocker also known as growth factor receptor inhibitor blocks the growth factors that trigger the cancer cells to divide and grow.

3.1.7.1 Epidermal Growth Factor Receptor (EGFR) The epidermal growth factor receptor (EGFR) family encompasses four receptor proteins, namely ErbB-1/EGFR-1 to -4 (also called HER 1-4) that are expressed on cell surface and exhibit tyrosine kinase activities. These proteins have similar structures and comprise three domains: an extracellular domain with ligand binding site, a transmembrane domain, and an intracellular domain with kinase activity. There are 11 different growth factors, each possessing a conserved EGF domain that can bind with those four receptors. Upon ligand binding, the receptors form homo-or heterodimers, promoting activation, relaying signals for proliferation, survival, migration, and differentiation and thus playing major roles in cancer progression. However, EGFR overexpression and/or gene amplification are also found in several cancers. Along with autocrine loops involving an EGF-like ligand, mutation, amplification, or dysregulation of at least one of the ErbB family members has been identified in 20% of solid tumors. For example, 50% of glial tumors harbor EGFR gene amplification, and a large fraction of these also present EGFRvIII, a mutant lacking

3.1 Chemotherapy

31

a portion of the extracellular domain. Despite its inability to bind soluble ligands, EGFRvIII exhibits constitutive tyrosine phosphorylation and multiple downstream signaling pathways. Stimulatory EGFR mutations that concentrate at regulatory regions within the kinase domain are found in at least 10% of non-small cell lung cancer patients. ErbB-2/HER2 rarely presents tumorigenic point mutations in tumors, but amplification of the corresponding portion of chromosome 17 is a feature of 20–25% of metastatic breast cancers, and it associates with worse prognosis. Overexpression of ErbB-2 has also been observed in ovarian cancer, stomach cancer, and aggressive forms of uterine cancer. ErbB-3 is frequently expressed in human mammary tumors along with ErbB-2, and overexpression of neuregulins, the natural ligands for ErbB-3 and ErbB-4, leads to increased tumorigenicity. Recent work demonstrated ErbB-4 mutations in 19% of individuals with melanoma. Seven missense mutations were identified, and they resulted in increased kinase activity and transformation ability.

3.1.7.2 Vascular Endothelial Growth Factor Receptor (VEGFR) VEGFs regulate both vasculogenesis and angiogenesis. This GF family consists of five glycoproteins, VEGFA (VEGF), VEGFB, VEGFC, VEGFD, and PlGF (placenta growth factor). The VEGF family members bind to at least one of the three known VEGFRs, namely VEGFR1-3. The VEGFR family of receptors is predominantly expressed on endothelial cells and a few additional cell types. VEGFRs are single pass proteins with seven immunoglobulin (Ig)-like domains on the extracellular site and two split tyrosine kinase domains in the intracellular site. VEGFRs are thought to be responsible for blood and lymph vessel formation in tumor microenvironment and thus promote tumor growth and progression. High expression of VEGFR gene is observed in many different types of malignancies. VEGFR-2 seems to mediate most known cellular responses to VEGF and has much higher intracellular signaling intermediates than VEGFR-1. Unlike VEGFR-3, which is largely restricted to lymphatic endothelial cells, both VEGFR-1 and VEGFR-2 are expressed in vascular endothelial cells, as well as monocytes, macrophages (VEGFR-1), and hematopoietic stem cells (VEGFR-2). Importantly, expression of VEGFR-1 and VEGFR-2, as well as the co-receptors NP1 and NP2, has been detected on subsets of solid tumor cells, and according to a recent study activation of VEGFR-1 in breast cancer cells supports their growth and survival. 3.1.7.3 Fibroblast Growth Factor Receptor (FGFR) The fibroblast growth factor receptor (FGFR) family consists of four closely related transmembrane proteins (FGFR1–4) and their different isoforms with altered ligand specificity due to differential splicing of FGFR mRNA. These single chain receptors contain one extracellular domain with three immunoglobulin repeats (Ig I-III) with ligand binding capacity, one transmembrane domain, and one intracellular domain with kinase activity at the carboxy-terminus. There are 18 different FGF ligands that can bind to different FGF receptors. Upon binding, dimerization of FGFR leads to auto-phosphorylation and kinase activation.

32

3 Treatment Modalities

FGFR expression causes tumor cells to acquire resistance to several drugs, especially inhibitors targeting other growth factor receptors (EGFR, PDGFR, and VEGFR) because of their extensive cross-talks. Amplification and mutations in FGFR genes that lead to constitutive activation/upregulation of receptors are found in different types of malignancies, including breast, ovarian, gastric, and lung cancers.

3.1.7.4 Transforming Growth Factor-Beta Receptor (TGF-bR) The TGF-βR family comprises three membrane receptors (TβRI, TβRII, and TβRIII) which are expressed in diverse types of cells and regulate distinct cellular functions by the signals transduced upon TGF-β ligand binding. TβR and TβRII are single pass serine/threonine kinases with N-terminal ectodomains and C-terminal kinase domains. TβRIII (also known as beta-glycan) is a cell surface proteoglycan >300 kDa in molecular mass and does not possess an intracellular kinase domain. TβIII binds with TGF-β ligands and presents them to TβRII or the ligands bind directly with TβRII depending on cell types. After binding, TβRII recruits and transphosphorylates TβRI, which in turn activates SMAD proteins. SMAD complexes translocate into the nucleus and function as transcription factors for TGF-β responsive genes and thus regulate cell proliferation, survival, migration, and differentiation. TGF-βR-mediated signals play context-dependent dual roles in cell growth. Under physiological conditions, TGF-β prevents cell growth, stimulates apoptosis or differentiation. During tumorigenesis, TGF-βR-mediated signals promote cell growth due to genetic and epigenetic changes. Mutations and dis-regulation of TGF-βR genes were observed in different cancers, for example, downregulation of TGF-βRII gene in breast and lung cancer and different mutations in colon and pancreatic cancer. 3.1.7.5 TGF-b-Signaling The functional duality of TGF-β in tumor progression requires inhibition in advanced metastatic cancer while retaining the growth inhibitory abilities exhibited in early stages of tumorigenesis. One approach employs the antisense oligodeoxynucleotide AP 12009, a synthetic 18-mer phosphorothioate oligodeoxynucleotide, which is complementary to the sequence of the TGF-β2 mRNA. Promising results were obtained in phase I clinical trials of AP 12009 performed in recurrent or refractory glioma patients. In laboratory studies, administration of a TGF-β neutralizing antibody has been demonstrated to restore natural killer cell activity and to reduce metastasis. Potentially, CAT-192 (Metelimumab), a human antibody that neutralizes TGF-β is a candidate agent. Likewise, preclinical studies employing a dominant-negative mutant approach identified TGF-βR2 as a target. Hence, it is likely that future attempts to inhibit the pathway will involve, in addition to antisense oligonucleotides and antibodies, also kinase inhibitors specific to TGF-βR2.

References

33

3.1.7.6 Insulin-Like Growth Factors The insulin-like growth factor (IGF) axis consists of two cell surface receptors (IGF1R and IGF2R), two ligands (IGF1 and IGF2), a family of six high-affinity IGF binding proteins (IGFBP1–6), as well as associated IGFBP degrading enzymes. IGF1 is produced primarily in the liver under the control of the growth hormone. IGF1 plays an important role in childhood growth, and it exerts anabolic effects in adults. Almost all circulating IGF1 molecules are constitutively bound to an IGFBP, which attenuates the bioactivity of these GFs. The action of IGF1 is mediated by binding to two receptor tyrosine kinases, IGF1R and the insulin receptor (at low affinity), as well as their heterodimers. On the other hand, IGF2 can bind IGF1R, and it is the sole ligand for the IGF2R/mannose 6-phosphate receptor. It is notable that the receptors of the IGF axis are expressed on most types of tumors. Amplification of the IGF1R gene has also been reported in a small number of breast and melanoma cases. In addition, a strong positive association was observed between plasma IGF1 levels and prostate cancer risk. Similar to IGF1, IGF2 is also linked to tumor development, including Wilms’ tumors and colorectal cancer. 3.1.7.7 Histone Deacetylase Inhibitors (HDACS) Histone acetyltransferases (HATs) and histone deacetylases (HDACs) are two protein classes with antagonistic roles that regulate acetylation of histones as well as various cytoplasmic non-histone proteins. HATs catalyze the transfer of acetyl groups to the ε-amino substituent of specific Lys residues, whereas classical HDACs reverse this process using a metal co-factor to catalyze hydrolysis of the acetyl group. There are 18 HDACs in the human proteome grouped into four classes. Classes I, II, and IV are metal-dependent with corresponding HDAC inhibitors typically coordinating to the metal co-factor rendering the protein inactive. Class III HDACs (sirtuins) operate in conjunction with an NAD+ co-factor. To date, four small-molecule HDAC inhibitors have received FDA approval for cancer treatment: Vorinostat (SAHA, 1), Belinostat (PXD101, 2), Panobinostat (LBH-589, 3), and Romidepsin (depsipeptide- FK228, 4). All compounds are approved specifically for hematological malignancies although multiple clinical trials are ongoing with these compounds in combination studies against various cancers, including gliomas, solid tumors, and AML. In contrast to the FDA approved drugs, Ricolinostat and Citarinostat are the first HDAC6 selective inhibitors in clinical trials. HDAC6 is unique among the HDACs in that it is predominantly cytosolic and facilitates microtubule deacetylation as well as regulation of PDL1 and other important targets related to cancer immunotherapy. As such, HDAC6 has been implicated in oncogenesis and metastasis, with the emergence of selective inhibitors as viable cancer therapeutics.

References 1. Usborne CM, Mullard AP (2018) A review of systemic anticancer therapy in disease palliation. Br Med Bull 125(1):43–53

34

3 Treatment Modalities

2. DeVita VT Jr, Chu E (2008) A history of cancer chemotherapy. Cancer Res 68(21):8643–8653 3. Shakeri-Zadeh A, Khoei S, Khoee S, Sharifi AM, Shiran M-B (2013) Targeted, monitored, and controlled chemotherapy: a multimodal nanotechnology-based approach against cancer. ISRN Nanotechnol:5 4. Boscha F, Rosicha L (2008) The contributions of Paul Ehrlich to pharmacology: a tribute on the occasion of the centenary of his Nobel prize. Pharmacology 82(3):171–179 5. Jabbour R, Salem H, Sidell FR (2014) Nitrogen mustards. In: Wexler P (ed) Encyclopedia of toxicology, 3rd edn. Academic Press, pp 560–566 6. Hosseinzadeha E, Banaeeb N, Nedaie HA (2017) Cancer and treatment modalities. Curr Cancer Ther Rev 13:17–27 7. Lind MJ (2008) Principles of cytotoxic chemotherapy. Medicine 36(1):19–23 8. Silverman RB, Holladay MW (2014) DNA-interactive agents. In: The organic chemistry of drug design and drug action, 3rd edn, pp 275–331 9. Luduena RF (1998) Multiple forms of tubulin: different gene products and covalent modifications. Int Rev Cytol 178:207–275 10. Pasquier E, Kavallaris M (2016) Microtubules: a dynamic target in cancer therapy. Biotechnol Biochem:165–170 11. Jordan MA, Wilson L (2004) Microtubules as a target for anticancer drugs. Nat Rev Cancer 4:253–265

4

Anticancer Metal Complexes

Therapeutic potential of metal-based compounds dates back to ancient time. During this period, the ancient Assyrians, Egyptians, and Chinese knew about the importance of using metal-based compounds in the treatment of diseases, such as the use of cinnabar (mercury sulfide) in the treatment of ailments [1]. The advent of “theoretical science,” by Greek philosophers (Empedocles and Aristotle) in the fifth and fourth century BC, boosted the knowledge of metal-based compounds as therapeutic agents. This was supported by the information handed down by Pliny and Aulus Cornelius Celsus (Roman physicians) on the use of cinnabar in the treatment of trachoma and venereal diseases. In the ninth and eleventh century BC, the contributions of ancient scientists such as Rhazes (Abu Bakr Muhammad ibn Zakariya al-Razi) who in his masterpiece entitled: “The comprehensive book on medicine,” known also as “The large comprehensive or Continens Liber,” had thoroughly studied cancer, discussing its diagnosis and treatment [2]. He was among the pioneers to introduce the notion of chemotherapy by combining alchemical, chemical, medical, and pharmaceutical knowledge. Ibn Sina (980–1037 AD), known as Avicenna in the west, was one of the leading pioneers of medical science in the Islamic Golden Age. He clarifies his surgical approach to early removal of a tumoral growth in his eminent work “Al-Qanun-fi-al-Tibb” (The Canon of Medicine): “All diseased tissue should be removed with radical excision, which could utilize amputation and removal of veins surrounding the growth, or catheterization if necessary.” [3] Both were applauded, sequel to the discovery of toxicological effects of mercury in the animals and the use of mercury (quicksilver ointment) for skin diseases, respectively. Arsenic trioxide (ATO) was used as an antiseptic and in the treatment of rheumatoid diseases, syphilis, and psoriasis by traditional Chinese medical practitioners [4]. Certainly, ATO was among the first compounds suggested for use in the treatment of leukemia during eighteenth and nineteenth centuries, until in the early twentieth century when its use was replaced by radiation and cytotoxic chemotherapy [5]. Therapeutic use of gold and copper can be traced to the history of # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_4

35

36

4 Anticancer Metal Complexes

civilization [6], where the Egyptians and Chinese were famous users in the treatment of certain disease conditions, such as syphilis [7]. The discovery of platinum compound (cisplatin) by Barnett Rosenberg in 1960s was a milestone in the history of metal-based compounds used in the treatment of cancer [8, 9]. This forms the foundation for the modern era of the metal-based anticancer drugs [10–12]. Despite the wide use of the metal-based compounds, the lack of clear distinction between the therapeutic and toxic doses was a major challenge. This was so because practitioners of ancient time lack adequate knowledge of dose-related biological response [13]. The advent of molecular biology and combinatorial chemistry paves the way for the rational design of chemical compounds to target specific molecules [14–16].

4.1

Properties of Metals and Metal Complexes

Transition metals are member elements of the “d” block and are included in groups III–XII of the periodic table [17]. They possess unique properties that include: Charge Variation In aqueous solution, metal ions exist as positively charged species. Depending on the existing coordination environment, the charge can be modified to generate species that can be cationic, anionic, or neutral [18]. Most importantly, they form positively charged ions in aqueous solution that can bind to negatively charged biological molecules [19]. Structure and Bonding Relative to organic molecules, metal complexes can aggregate to a wide range of coordination geometries that give them unique shapes. The bond length, bond angle, and coordination site vary depending on the metal and its oxidation state. In addition to this, metal-based complexes can be structurally modified to a variety of distinct molecular species that confer a wide spectrum of coordination numbers and geometries, as well as kinetic properties that cannot be realized by conventional carbon-based compounds. Metal–Ligand Interaction Different forms of metal–ligand interaction exist; however, these interactions usually lead to the formation of complexes that are unique from those of individual ligands or metals. The thermodynamic and kinetic properties of metal–ligand interactions influence ligand exchange reactions. The ability of metals to undergo this reaction offers a wide range of advantages to the metals to interact and coordinate with biological molecules. Lewis Acid Properties Characterized by high electron affinity, most metal ions can easily polarize groups that are coordinated to them, thus facilitating their hydrolysis. Partially Filled D Shell For transition metals, the variable number of electrons in the d shell or f shell (for lanthanides) influences the electronic and magnetic properties of transition metal complexes.

4.1 Properties of Metals and Metal Complexes

37

Redox Activity Many transition metals have a tendency to undergo oxidation and reduction reactions. The oxidation state of these metals is an important consideration in the design of coordination compounds. In biochemical redox catalysis, metal ions often serve to activate coordinated substrates and to participate in redox-active sites for charge accumulation. Metal complexes and metal-based compounds possess the ability to coordinate ligands in a three-dimensional configuration, thereby allowing functionalization of groups that can be shaped to defined molecular targets. Despite the majority of chemotherapeutics currently employed in anticancer therapy being organic molecules, an important and promising class of drugs is nowadays represented by metal-based compounds [20, 21]. After the approval of cisplatin [PtIICl2(NH3)2] by the Food and Drug Administration (FDA) in 1978 and its great success in treating genitourinary cancer, a great effort was put into the development of second- and third-generation analogs to overcome the well-known limitations related to this therapy (1). Cisplatin has become one of the best-selling anticancer drugs in the world although its administration induces the onset of severe side effects (mainly represented by nephro-, oto-, and neurotoxicity, as a consequence of the high affinity of platinum center for sulfur-containing biomolecules) and is poorly selective towards cancerous cells. This drug is currently used for the treatment of testicular cancer, is one of the most effective drugs against melanoma and non-small cell lung carcinoma, and administered in combination with other therapeutics, it is considerably active against ovarian cancer (2). Moreover, the narrow spectrum of action and the induction of resistance in some cancer types (intrinsic or acquired after few cycles of therapy) encouraged researchers to design new metal complexes, either based on platinum, or on other transition metals, aimed at improving the selectivity toward cancerous cells and reducing the side-effects (3). Therefore, in the past decade, a large number of metal-based compounds were synthesized and tested, each one characterized by different physicochemical properties and biological behavior, achieved by exploiting the peculiarities of various metal oxidation states and different ligands. The metal ions such as the ones belonging to first transition series (scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc), second transition series (yttrium, zirconium, nobelium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium), third transition series (tungsten, rhenium, osmium, iridium, gold), boron family (gallium), carbon family (tin), nitrogen family (antimony, bismuth), lanthanides (lanthanum, cerium, neodymium, samarium, europium, gadolinium, terbium, dysprosium), and actinides (uranium) (Fig. 4.1) have been frequently used for metal-based anticancer drug design [22]. Out of the various metals/metal ions used for anticancer drug design, some complexes based on palladium, ruthenium, copper, gold, iron, and tin have shown exciting anticancer properties.

Fig. 4.1 Metals other than platinum (Pt) involved in the development of anticancer non-platinum metal complexes. The most important elements are highlighted

38 4 Anticancer Metal Complexes

References

39

References 1. Norn S, Permin H, Kruse E, Kruse PR (2008) Mercury a major agent in the history of medicine and alchemy. Dan Med Årboq 36:21–40 2. Tsoucalas G, Karamanou M, Laios K, Androutsos G (2019) Rhazes’ views on cancer and the introduction of chemotherapy (864-925). J BUON 24(2):868–871 3. Kardeh S, Kardeh B (2019) Avicenna’s concepts on Cancer metastasis from the 11th century. Gal Med J 8:e1292 4. Ravandi F (2004) Arsenic trioxide: expanding roles for an ancient drug? Leukemia 18:1457–1459 5. Baskar R, Lee KA, Yeo R, Yeoh K-W (2012) Cancer and radiation therapy: current advances and future directions. Int J Med Sci 9(3):193–199 6. Galib MB, Mashru M, Jagtap C, Patgiri BJ, Prajapati PK (2011) Therapeutic potentials of metals in ancient India: a review through Charaka Samhita. J Ayurveda Integr Med 2(2):55–63 7. Frith J (2012) Syphilis - its early history and treatment until penicillin, and the debate on its origins. History 20:49–58 8. Trimmer EE, Essigmann JM (1999) Cisplatin. Essays Biochem 34:191–211 9. Ghosh S (2019) Cisplatin: the first metal based anticancer drug. Bioorg Chem 88:102925 10. Ndagi U, Mhlongo N, Soliman ME (2017) Metal complexes in cancer therapy – an update from drug design perspective. Drug Des Devel Ther 11:599–616 11. Chen D, Milacic V, Frezza M, Dou QP (2009) Metal complexes, their cellular targets and potential for cancer therapy. Curr Pharm Des 15(7):777–791 12. Benjamin K, Singh G, Grant MP, Harper BW, Krause-Heuer AM, Manohar M, Orkey N, Aldrich-Wright JR (2011) Transition metal based anticancer drugs. Curr Top Med Chem 11 (5):521–542 13. Calabrese EJ (2016) The emergence of the dose–response concept in biology and medicine. Int J Mol Sci 17(12):2034 14. Liu R, Li X, La KS (2017) Combinatorial chemistry in drug discovery. Curr Opin Chem Biol 38:117–126 15. Imran A, Qamar HY, Ali Q, Naeem H, Riaz M, Amin S, Kanwal N, Ali F, Sabar MF, Nasir IA (2017) Role of molecular biology in cancer treatment: a review article. Iran J Public Health 46 (11):1475–1485 16. Pirrung M (2004) Molecular diversity and combinatorial chemistry, vol 24, 1st edn. Elsevier Science 17. Pattan SR, Pawar SB, Vetal SS, Gharate UD, Bhawar SB (2012) The scope of metal complexes in drug design – a review. Indian Drugs 49(11):5–12 18. Haas KL, Franz KJ (2010) Application of metal coordination chemistry to explore and manipulate cell biology. Chem Rev 109(10):4921–4960 19. Frezza M, Hindo S, Chen D, Davenport A, Schmitt S, Tomco D, Dou QP (2010) Novel metals and metal complexes as platforms for cancer therapy. Curr Pharm Des 16(16):1813–1825 20. Desoize B (2004) Metals and metal compounds in cancer treatment. Anticancer Res 24:1529–1544 21. Johnstone TC, Suntharalingam K, Lippard SJ (2015) Third row transition metals for the treatment of cancer. Philos Trans R Soc A373:20140185 22. Trudu F, Amatoa F, Vaňharac P, Pivettab T, Peña-Méndezd EM, Havel J (2015) Coordination compounds in cancer: past, present and perspectives. J Appl Biomed 13:79–103

5

Platinum-Based Anticancer Agents

A platinum-based anticancer drug is any agent that contains one or more platinum atoms in the oxidation state of II or IV, contains mono- or multidentate non-labile am (m)ine carrier ligands and labile chlorido or bidentate carboxylate ligands, and which acts as a prodrug; it undergoes aquation to yield a reactive complex capable of forming coordinate bonds with DNA bases. Its binding to DNA prevents replication and transcription which causes cell death through apoptosis. Platinum-based drugs have become a mainstay of cancer therapy; approximately half of all patients undergoing chemotherapeutic treatment receive a platinum drug [1]. The widespread use of platinum agents in the treatment of cancer began with the discovery of the antineoplastic activity of cisplatin by Barnett Rosenberg in the 1960s [2].The preparation of a coordination complex with the simple formula cis-[Pt (NH3)2Cl2] was first described by Peyrone in the mid-nineteenth century and as was often the custom at the time, the compound came to bear his name as Peyrone’s chloride [3]. The discovery of the antineoplastic properties of this complex by Barnett Rosenberg is a great example of the role that serendipity can play in science [4]. During the course of investigating the effect of electric fields on bacterial cell division, platinum electrodes that had been chosen for their inertness began to leach platinum ions into the ammonia-containing growth medium. The bacteria incubated in this growth medium continued to grow but did not divide. Rigorous control experiments revealed that the most potent agent to recapitulate this effect in bacteria was Peyrone’s chloride. In a deductive leap, Rosenberg proposed that if this platinum complex could inhibit bacterial cell division, then it might be able to stop the uncontrolled cell growth that characterizes cancer. In 1969, Rosenberg published the results of a study showing that cis-[Pt(NH3)2Cl2] was effective in treating sarcoma 180 and leukemia L1210 in mice. When Rosenberg carried out his investigations, all anticancer drugs approved for use in the USA were organic compounds, either natural or synthetic. Anything which contained heavy metal (platinum is the second neighbor of mercury) was treated as a toxic compound that should be kept away from humans. For this reason, Rosenberg convinced the National Cancer Institute that # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_5

41

42

5

Platinum-Based Anticancer Agents

they carried out more extensive animal tests on the platinum complexes. The compound was very effective in those human cancers where other forms of chemotherapy resulted in no improvements and he patented his new discovery—the use of cisplatin. He could not patent the compound since it had been synthesized 100 years earlier by Peyrone. In 1978, only nine years after the initial publication describing its anticancer activity, this compound which came to be known as cisplatin was approved by the US Food and Drug Administration (FDA) for clinical treatment of genitourinary tumors [5]. Later in 1979, the Bristol-Myers (now Bristol-Myers Squibb), who intensively researched anticancer drugs, carried out additional investigations to provide information about the safety and the efficacy for the Food and Drug Administration (FDA). There are currently six platinum drugs with marketing approval in various regions throughout the world: cisplatin, carboplatin, oxaliplatin, nedaplatin, lobaplatin, and heptaplatin (Fig. 5.1). Platinum-based drugs are used for the treatment of human and animal tumors and are currently indicated for bladder, testicular, ovarian, non-small cell lung, small cell lung, and colorectal cancers [6]. Following the initial reports of the anticancer activity of cisplatin, inorganic chemists began preparing a variety of platinum complexes with different ligands and testing their antineoplastic effects. The collective result of these many separate studies was the emergence of a set of rules governing molecular structure that appeared to be required in order for a platinum complex to have activity [7]. These SARs specified that the platinum complex has to have a square-planar geometry, be charge neutral, contain two cis am(m)ine ligands, and two cis anionic ligands. The anionic ligands should not bind the platinum too tightly, or activity would be reduced. If these ligands were too labile, however, the compounds would exhibit prohibitively high levels of toxicity. Moreover, the two am(m)ine ligands or two anionic ligands could be replaced by a chelating diamine or chelating dicarboxylate, respectively. Extensive drug discovery programs were initiated that relied on systematic variation of ligands according to these rules. As a result of these programs,

H3N

Cl

H3N

Cl

H3N

Pt H3N

Cisplatin

O

_ O O _

2+ NH3 Pt NH3

Nedaplatin

Pt

O

O O

O

Carboplatin

O

_ O O _

4+ Pt

O

O

O

O

Pt N H2

Oxaliplatin

H2 N

O

_ O

H2N

O

O _

Lobaplatin

H2 N

2+ Pt

H2 N

O

H2N

O

Heptaplatin

Fig. 5.1 Platinum drugs with marketing approval in various regions throughout the world

5.1 Chemistry of Cisplatin

43

Table 5.1 Timeline of the events in platinum-based anticancer drug development 1845 1965 1968 1971 1976 1978 1989 1989 1993 1995 1997 1999

2002 2003 2006 2007 2010 2011 onwards

Cis-Pt (NH3)2 discovered by M. Peyrone Barnett Rosenberg discovered that the electrolysis of platinum electrode produces cisplatin Cisplatin shows anticancer properties in mouse models First patient treated with cisplatin Oxaliplatin discovered at Nagoya city University by Prof. Yoshinori Kidani Cisplatin approved for testicular and bladder cancer Carboplatin discovered at Michigan State University Carboplatin approved for ovarian cancer First patient treated with an orally administered platinum drug, Satraplatin (JM216) Nedaplatin or Aqupla® a cis-diammine-glycolato platinum approved in Japan since 1995 First patient treated with Picoplatin (JM473) Heptaplatin was developed by Sunkyong Industry Research Center in Korea under the name SKI 2053R. Received approval from the Korean Food and Drug Administration Oxaliplatin gets initial approval from USA FDA for colorectal cancer treatment Lobaplatin received regulatory approval Bevacizumab gets approval for treatment in non-small cell lung cancer in combination with carboplatin and paclitaxel Satraplatin taken into consideration for approval by FDA for prostate cancer A new light activated platinum complex Pt (N3)2(OH)2(Py)2 discovered with 80 times more efficacy than cisplatin Liposomal cisplatin or lipoplatin is under a phase III randomized clinical trial for patients suffering from small cell lung cancer, whereas polymer-based drug, Prolindac™ is currently under investigation for pretreated ovarian cancers in up to eight European centers Aroplatin (Liposomal Oxaliplatin) Other platinum-based complexes in clinical trials Nano delivery Non-platinum anticancer agents

two other platinum agents, cis-diamminecyclobutane-dicarboxylatoplatinum(II) and R,R-cyclohexane-1,2-diamineoxalatoplatinum(II) were approved by the FDA for clinical use in the USA [8]. The former is commonly referred to as carboplatin and the latter as oxaliplatin. These two compounds obey the classical SARs and were thought to operate by a mechanism of action similar to that of cisplatin. A timeline of the events in platinum-based anticancer drug development is given in Table 5.1.

5.1

Chemistry of Cisplatin

The chemistry of cisplatin is completely different from all other chemistries of typical organic anticancer drugs [9]. The behavior of cisplatin in aqueous solution is presented in Fig. 5.2. In blood, the concentration of the chloride ions is about 96 to

44

5 H2O

pK1= 4.29 o (3.74 at 35.5 C)

(NH3)2PtCl2 (1.1)

pK-1= 2.11

-

Cl

H2O pK2= 4.56

monoaqua

(NH3)2PtCl(H2O)

Platinum-Based Anticancer Agents

(1.2)

diaqua 2+

+

pK-2= 1.03

(NH3)2Pt(H2O)2 -

(1.3)

Cl

pKa2= 5.35

pKa1= 6.41

H+

H+

(NH3)2PtCl(OH) (1.4) monohydroxo

(NH3)2Pt(H2O)(OH)

+

monohydroxo (1.5) monoaqua pKa3= 7.21 H+

(NH3)2PtCl(OH)2 dihydroxo

(1.6)

Fig. 5.2 The behavior of cisplatin in aqueous solution. Cisplatin (1.1) is a bright yellow solid; when dissolved in water it is attacked by water molecules and as a result one of the chloride ions is eliminated and monoaqua (1.2) species is formed. Diaqua species (1.3) is formed when the second water molecule replaces the chloride ion. Water replaces chloride ions because the metal and nitrogen form stronger bond than metal and the chloride ion. Bound water became very acidic and at physiologic pH became completely deprotonated—as a monohydroxo form (1.4), and the product of the dissociation of the second proton from the diaqua form isdihydroxo species (1.6). Logarithms of rate constants (pK1, pK 1, pK2, and pK 2) are given for 25  C and of dissociation constants (pKa1, pKa2, and pKa3) for 27  C

106 milliequivalents (mEq) per liter and, according to Le Chatelier’s principle, the loss of the chloride ion from cisplatin is suppressed by the chloride ion in solution; the reaction shown in Fig. 5.2 does not progress very far to the right (from 1.1 to 1.2). According to the first order kinetics for conversion from 1.1 to 1.2, the half-life for cisplatin at 35.5  C is 1.05 h. The binding of a water molecule to the Pt2+ ion makes water very acidic and monoaqua species 1.3 is dissociated in the monohydroxo complex 1.4. So, in an aqueous solution with the high chloride concentration the forms 1.1, 1.2, and 1.4 predominate. In the cytoplasm, where chloride ion concentration is only 4 mM, the equilibria are shifted to the right and form 1.3, 1.5, and 1.6 predominate. A common route of cisplatin administration is the infusion of the solutions such as Platinol® and Plationol®AQ, which contain 3.3 mM cisplatin (1 mg/ml) and 154 mM sodium chloride, NaCl (normal saline solution). Since the pH is adjusted to pH about 4 solution in Platinol contains mainly (95%) of species 1.1 and only smaller amounts of 1.3 and 1.4.

5.2 Mechanism of Action of Platinum Drugs

5.2

45

Mechanism of Action of Platinum Drugs

In years following the initial clinical implementation of cisplatin, much research by laboratories worldwide was conducted to determine the mechanism by which this drug carries out its anticancer action. As a result, a relatively clear picture has emerged of the steps involved in this process [10]. Although some details continue to be refined, the four main steps in the mechanism of action are (1) cellular uptake, (2) aquation/activation, (3) DNA platination, and (4) cellular processing of Pt–DNA lesions, leading to cell survival or apoptosis. Passive diffusion was initially thought to play a significant role in the uptake of cisplatin [11]. The importance of passive diffusion was embedded in the SARs through the requirement of charge neutrality. In recent years, however, active transport via the copper transporters CTR1 and CTR2 has been implicated as a major route of platinum access into the cell [12]. The matter has not been unambiguously resolved, however, and new iconoclastic data continue to surface [13]. Studies of overexpression of the organic cation transporters (OCTs) revealed that these proteins help facilitate entry of oxaliplatin into cells, and the propensity of colorectal cancer cells to overexpress these transporters may explain the efficacy of this drug in the treatment of this particular malignancy [14]. As discussed below, a study of the ability of these OCTs to transport cationic monofunctional platinum compounds ultimately led to the discovery of phenanthriplatin. Once cisplatin has entered the cell, a lower chloride ion concentration of approximately 3–20 mM, as compared to 100 mM in the extracellular fluid, favors the substitution of the chloride ligands for water molecules [15]. The chelating dicarboxylate of carboplatin exchanges for water much more slowly and it has been proposed that activation by carbonate may be important in permitting this compound to bind to DNA [16]. This mechanism, however, does not occur with cisplatin [17]. The cellular target of the three FDA-approved platinum drugs, as well as many related compounds that have been investigated, is nuclear DNA. The aquated/activated platinum complexes can react with nucleophilic centers on purine bases of DNA, particularly the N7 positions of guanosine and adenosine residues. The two labile coordination sites on the platinum center permit cross-linking of adjacent guanine bases. To a lesser extent, the platinum center can coordinate to guanine bases from different DNA strands to form interstrand cross-links. The major intrastrand dGpG cross-link induces a significant distortion in the DNA double helix [18]. The DNA lesion is then recognized by cellular machinery that repairs the lesion, bypasses it, or initiates apoptosis. The most significant mechanism by which classical platinum complexes are believed to induce apoptotic cell death is inhibition of transcription. When RNA polymerases transcribe DNA, they stall at the platinum cross-link and recruit the transcription-coupled repair machinery (see Fig. 5.3). If this machinery is unable to repair the lesion, then the cell evokes a programmed cell death pathway. The development of platinum-based anticancer compounds has long been focused on the synthesis and evaluation of complexes that obey the SARs set forth in the 1970s. These pursuits have produced carboplatin and oxaliplatin, two widely

5

Fig. 5.3 Mechanism of action of Platinum drugs and interaction of Cisplatin with DNA; formation of DNA-cisplatin adduct

46 Platinum-Based Anticancer Agents

References

47

employed anticancer drugs. The prevalence of inherent and acquired resistance to platinum treatment, however, requires the development of new complexes that operate via different mechanisms. Although initially thought to be ineffective, the discovery of phenanthriplatin has revealed that monofunctional compounds can indeed be potent anticancer agents. They distort DNA significantly less than cisplatin, but their efficacy tracks with transcription inhibition, corroborating the fact that DNA is their major target. The spectrum of activity of these compounds is highly differentiated from that of classical platinum complexes, giving rise to the hope that they might form a class of clinically relevant drug candidates. In a more general sense, these results also validate the exploration of other metal complexes that can only interact with DNA in a monofunctional manner as anticancer drug candidates. Analogs of cisplatin have been prepared in the thousands, with the two-ammine ligands being the focus of substitution. Although this approach has yielded other active compounds, toxicity remains a problem [8, 19, 20]. Taking advantage of the various oxidation states of platinum, a more recent approach has been the use of platinum(IV) compounds, which contain cis-dichloro-diammine ligands in the equatorial plane and hemilabile O-donor axial ligands [21–23]. These complexes, which are generally nontoxic, release their axial ligands upon photoexcitation or reduction of the platinum center, thus forming the highly active platinum(II) species in situ [24]. Furthermore, incorporation of a targeting vector into the axial ligands of the platinum(IV) species can lead to accumulation at the cancer site, leading to less side effects. Despite the promise of new anticancer platinum compounds with better efficacy and lower inherent toxicity than cisplatin, only 6 compounds from >10,000 synthesized have made it to clinical use. An emerging field, and possibly the future of platinum-based anticancer drugs is the delivery of platinum drugs using nanomaterial carriers, such as upconversion nanoparticles (UCNPs) [25, 26], metal organic frameworks [27, 28], polymers [29] or micelles [30]. By taking advantage of the enhanced permeation and retention effect [31], nanomaterials accumulate in the tumor interstitial space and once there, they can release the active platinum species. Indeed, the drug lipoplatin developed by Regulon, composed of cisplatin encased in a liposomal vesicle and used in combination with pemetrexed is presently in Phase III clinical trials conducted on non-squamous NSCLC patients, has shown encouraging results in comparison to cisplatin and pemetrexed combination. However, there are increasing reports that platinum-based anticancer drugs have severe side effects including myelotoxicity, peripheral neuropathy among others and it is inevitable to search for other non-platinum therapies without these side effects [32, 33].

References 1. Galanski M, Jakupec MA, Keppler BK (2005) Update of the preclinical situation of anticancer platinum complexes: novel design strategies and innovative analytical approaches. Curr Med Chem 12:2075–2094

48

5

Platinum-Based Anticancer Agents

2. Rosenberg B, VanCamp L, Trosko JE, Mansour VH (1969) Platinum compounds: a new class of potent antitumour agents. Nature 222:385–386 3. Peyrone M (1844) Ueber die Einwirkung des Ammoniaks auf Platinchlorür. Liebigs Ann 51:1–29 4. Rosenberg B (1999) Platinum complexes for the treatment of cancer: why the search goes on. In: Lippert B (ed) Cisplatin: chemistry and biochemistry of a leading anticancer drug. Verlag Helvetica Chimica Acta, Zürich, pp 1–27 5. Smith GH (1979) New drugs released in 1978. Nurse Pract 4:35–41 6. Johnstone TC, Park GY, Lippard SJ (2014) Understanding and improving platinum anticancer drugs – Phenanthriplatin. Anticancer Res 34(1):471–476 7. Cleare MJ, Hoeschele JD (1973) Studies on the antitumor activity of group VIII transition metal complexes. Part I. Platinum(II) complexes. Bioinorg Chem 2:187–210 8. Wheate NJ, Walker S, Craig GE, Oun R (2010) The status of platinum anticancer drugs in the clinic and in clinical trials. Dalton Trans 39:8113–8127 9. Makovec T (2019) Cisplatin and beyond: molecular mechanisms of action and drug resistance development in cancer chemotherapy. Radiol Oncol 53(2):148–158 10. Wang D, Lippard SJ (2005) Cellular processing of platinum anticancer drugs. Nat Rev Drug Discov 4:307–320 11. Gately DP, Howell SB (1993) Cellular accumulation of the anticancer agent cisplatin - a review. Br J Cancer 67:1171–1176 12. Howell SB, Safaei R, Larson CA, Sailor MJ (2010) Copper transporters and the cellular pharmacology of the platinum-containing cancer drugs. Mol Pharmacol 77:887–894 13. Ivy KD, Kaplan JH (2013) A re-evaluation of the role of hCTR1, the human high-affinity copper transporter, in platinum-drug entry into human cells. Mol Pharmacol 83:1237–1246 14. Zhang S, Lovejoy KS, Shima JE, Lagpacan LL, Shu Y, Lapuk A, Chen Y, Komori T, Gray JW, Chen X, Lippard SJ, Giacomini KM (2006) Organic cation transporters are determinants of oxaliplatin cytotoxicity. Cancer Res 66:8847–8857 15. Pil P, Lippard SJ (2002) Cisplatin and related drugs. In: Joseph RB (ed) Encyclopedia of cancer. Academic Press, New York, pp 525–543 16. Di Pasqua AJ, Goodisman J, Kerwood DJ, Toms BB, Dubowy RL, Dabrowiak JC (2006) Activation of carboplatin by carbonate. Chem Res Toxicol 19:139–149 17. Todd RC, Lovejoy KS, Lippard SJ (2007) Understanding the effect of carbonate ion on cisplatin binding to DNA. J Am Chem Soc 129:6370–6371 18. Takahara PM, Rosenzweig AC, Frederick CA, Lippard SJ (1995) Crystal-structure of doublestranded DNA containing the major adduct of the anticancer drug cisplatin. Nature 377:649–652 19. Dilruba S, Kalayda GV (2016) Platinum-based drugs: past, present and future. Cancer Chemother Pharmacol 77(6):1103–1124 20. Johnstone TC, Suntharalingam K, Lippard SJ (2016) The next generation of platinum drugs: targeted Pt(II) agents, nanoparticle delivery, andPt(IV) prodrugs. Chem Rev 116(5):3436–3486 21. Han X, Sun J, Wang Y, He Z (2015) Recent advances in platinum (IV) complex-based delivery systems to improve platinum (II) anticancertherapy. Med Res Rev 35(6):1268–1299 22. Tolan D, Gandin V, Morrison L et al (2016) Oxidative stress induced by Pt(IV) pro-drugs based on the cisplatin scaffold and indole carboxylicacids in axial position. Sci Rep 6:29367 23. Gibson D (2016) Platinum(IV) anticancer prodrugs–hypotheses and facts. Dalton Trans 45 (33):12983–12991 24. Wilson JJ, Lippard SJ (2014) Synthetic methods for the preparation of platinum anticancer complexes. Chem Rev 114(8):4470–4495 25. Ruggiero E, Hernandez-Gil J, Mareque-Rivas JC, Salassa L (2015) Near infrared activation of an anticancer PtIV complex by Tm-doped ´upconversion nanoparticles. Chem Commun 51 (11):2091–2094

References

49

26. Min Y, Li J, Liu F, Yeow EK, Xing B (2014) Near-infrared light-mediated photoactivation of a platinum antitumor prodrug and simultaneous cellular apoptosis imaging by upconversionluminescent nanoparticles. Angew Chem Int Ed 53(4):1012–1016 27. He C, Lu K, Liu D, Lin W (2014) Nanoscale metal–organic frameworks for the co-delivery of cisplatin and pooled siRNAs to enhance therapeutic efficacy in drug-resistant ovarian cancer cells. J Am Chem Soc 136(14):5181–5184 28. Huxford RC, Rocca JD, Lin W (2010) Metal–organic frameworks as potential drug carriers. Curr Opin Chem Biol 14(2):262–268 29. He C, Liu D, Lin W (2015) Self-assembled nanoscale coordination polymers carrying siRNAs and cisplatin for effective treatment of resistant ovarian cancer. Biomaterials 36:124–133 30. He S, Li C, Zhang Q et al (2018) Tailoring platinum(IV) amphiphiles for self-targeting all-inone assemblies as precise multimodal theranostic nanomedicine. ACS Nano 12(7):7272–7281 31. Jhaveri AM, Torchilin VP (2014) Multifunctional polymeric micelles for delivery of drugs and siRNA. Front Pharmacol 5:77 32. Galanski M (2006) Recent developments in the field of anticancer platinum complexes. Recent Pat Anticancer Drug Discov 1:285–295 33. Samimi G, Kishimoto S, Manorek G, Breaux JK, Howell SB (2007) Novel mechanisms of platinum drug resistance identified in cells selected for resistance to JM118 the active metabolite of satraplatin. Cancer Chemother Pharmacol 59:301–312

6

Non-platinum Anticancer Agents

The limitations correlated to the clinical use of platinum-based therapies, systemic toxicity, and inherent or acquired resistance prompted the scientific world to investigate other possible metal-based anticancer candidates displaying different mechanisms of action. Nevertheless, compared with platinum-based compounds, only a few clinical studies have been conducted on non-platinum drugs. The first non-platinum complex to enter clinical trials was budotitane (Fig. 6.1) although its applications were limited due to its low solubility and liver toxicity [1, 2]. There has however been an upsurge in the synthesis of non-platinum anticancer metal complexes to address the issues encountered with platinum-based therapy. Here are some of the important classes of non-platinum anticancer metal complexes that are believed to either replace or augment the current platinum-based therapy.

6.1

Ruthenium

Ruthenium is a transition metal in group 8, the same chemical group as iron. Ruthenium has two main oxidation states, Ru(II) and Ru(III). Ruthenium (IV) compounds are also possible, but they are generally unstable due to their higher oxidation states [3]. The ruthenium ion is typically hexa- coordinated with octahedral coordination geometries. Generally, the thermodynamic and kinetic stability of Ru(III)complexes are lower than that of Ru(II) complexes, and the kinetics of the hydration of Ru(II/III) compounds depends significantly on the nature of their ligands and net charge [4]. Many Ru(III) compounds contain exchangeable ligands and require activation by the tumor microenvironment [5]. The antitumor properties of the Ru(III) complexes occur when they are reduced to their corresponding Ru (II) counterparts in vivo. Under biological circumstances of low oxygen concentration, acidic pH, and high levels of glutathione, the Ru(II/III)redox potential can be altered, and thus, Ru(III) complexes can be readily reduced to Ru(II) complexes [6– 8]. Development of new chemotherapeutic agents based on ruthenium metal has # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_6

51

52

6 Non-platinum Anticancer Agents

Fig. 6.1 Structure of the first non-platinum complex, budotitane

O O O Ti O O O

received fascinating interest due to their ability to strongly bind nucleic acids and proteins, ligand exchange kinetics similar to those of their platinum counterparts, the prevalence of two main oxidation states (II and III), and the iron mimicking property when bound to biological molecules [9, 10]. Ruthenium is less toxic than platinum and it is believed that the remarkable anticancer activity of ruthenium resides in its ability to mimic iron in binding to several biomolecules, including serum transferrin and albumin. Ruthenium complexes including [ImH][trans-Ru(DMSO)(Im)Cl4] (NAMIA, Im ¼ imidazole), [IndH][trans-Ru(Ind)2Cl4] (KP1019, Ind ¼ indazole), [(C6H5Ph) Ru(en)Cl][PF6] (RM175, en ¼ ethylenediamine) and [(pi PrC6H4Me)Ru (pta)Cl2] (RAPTA-C, pta ¼ 1,3,5-triaza-7- phosphatricyclo[3.3.1.1]decane (Fig. 6.2) have shown great therapeutic promises with NKP-1339 [11], the first ruthenium-based anticancer drug on the edge to clinical application and more ruthenium complexes waiting in the pipeline [12, 13]. Ruthenium complexes show multiple targets and diverse mechanisms for its antitumor properties (Fig. 6.3). Some ruthenium complexes act on telomere DNA, some interfere with replication and transcription of DNA, and others inhibit related enzymes [14, 15]. Furthermore, ruthenium complexes can block the cell cycle [16– 18] and induce the formation of DNA photocrosslinking products to prevent RNA polymerization enzymes or exonucleases from binding to DNA, thereby causing tumor cell apoptosis [19, 20]. Studies have found that some dinuclear and polynuclear Ru(II) polypyridyl complexes bind stably to the G-quadruplex (G4-DNA) structure of telomere DNA [21], inhibiting telomerase activity and blocking the function of DNA replication, thus preventing normal cells from developing into immortalized tumor cells [22, 23]. Ruthenium complexes have good topoisomerase (Topo) inhibitory activity; however, some studies have found that inhibition of one type of Topo increases the activity of others [24]. To solve this problem, studies have been conducted to synthesize a ruthenium complex with dual inhibitory property on Topo I and Topo II, which significantly inhibits tumor cell proliferation [25, 26]. Researchers have also designed a ruthenium complex with dual inhibitory effects on G4-DNA and Topo [27], achieving multitarget synergy with strong apoptosis promoting effects on tumor cells. In addition, Hurley and coworkers reported a ruthenium complex with dual stabilizing effects on Topo and G4-DNA, which also inhibited some drug resistant tumor cells [28]. In addition, it was found that ruthenium complexes accumulate more in organelles, such as mitochondria, endoplasmic reticulum, and lysosome, than in nucleus [29]. A number of studies

6.1 Ruthenium

53

−

HN Cl Cl

N Ru N

Cl

N

H N

+ NH

N Cl Cl Ru Cl Cl S H3C O

Cl NH

−

+

H N N H

CH3 KP1019

NAMI-A +

Cl Ru H P Cl

N

Cl

Ru

-

PF6

NH2

H2N

N N RAPTA-C

RM175

Fig. 6.2 Structures of some important Ruthenium(III) and Ruthenium(II)complexes showing remarkable therapeutic potential

have revealed that mitochondria is a key target of ruthenium complexes [30–32], because ruthenium complexes can quickly decrease the membrane potential of mitochondria, leading to mitochondrial dysfunction or activating mitochondrial apoptosis pathways. Furthermore, this effect promoted the expression of pro-apoptotic members of the B-cell lymphoma-2 (Bcl-2) family, releasing cytochrome c (Cyto C), and activating cascade reactions of the caspase family members to induce tumor cell apoptosis. Ruthenium complexes can target the endoplasmic reticulum, cause oxidative stress or endoplasmic reticulum stress (ERS), and induce tumor cell apoptosis by activating caspase family members [33, 34]. In addition, ruthenium complexes can target another significant participant in autophagy, the lysosomes, inducing autolysosome production and hydrolase release [35, 36]. Thereby, they increase apoptosis of tumor cells [37]. Although the nucleus is reported to be the key target for ruthenium complexes, many studies have demonstrated that the accumulation of some ruthenium complexes in the nucleus is far lower than that in other subcellular regions [38, 39]. Some nonnuclear targets, such as the cell surface, and especially mitochondria, have also been reported to be targets for the anticancer activity of some Ru(II) complexes. Mitochondria plays a significant role in cellular metabolism

54

6 Non-platinum Anticancer Agents

Fig. 6.3 General representation of the main targets and proposed mechanisms of action of ruthenium compounds as anticancer drugs (Adopted with permission from the publisher)

and under certain cellular conditions, release molecules that can activate the extrinsic and intrinsic apoptotic pathways [40]. Another exciting area within the field of ruthenium-based anticancer compounds is the use of ruthenium photosensitizers (PS) to reactive oxygen species (ROS) that causes cancer cell death upon one or two photon light activation [41]. This allows precise spatial and temporal control of where cytotoxicity will occur, leading to fewer side effects. In this form of photodynamic therapy, the excited PS will react with oxygen at the site of photoactivation in the tissue to form radicals that damage the surrounding tissue. One such PS that has shown promise in this area is TLD1433 (Fig. 6.4), a mononuclear tris(polypyridyl) ruthenium(II) complex [42]. TLD1433 exhibits a near 100% singlet oxygen quantum yield when excited at 530 nm. The compound displayed a strong photodynamic effect against colon and glioma cancer cell lines in vitro, with minimal dark toxicity. TLD1433 is currently in Phase Ib clinical trials for patients with nonmuscle invasive bladder cancer.

6.2

Palladium

The notable analogy between the coordination chemistry of Pt(II) and Pd (II) compounds has advocated studies of Pd(II) complexes as antitumor drugs [43, 44]. A key factor that might explain it is the ligand exchange kinetics. The

6.3 Titanium

55

S

N N

Ru

N

N

N

H N

N

N

S

2+

S

TLD-1433 Fig. 6.4 Structure of TLD-1433, a PDT Ru-complex

H3N Cl Pd H3N Cl (1.7)

H3N Cl Pd Cl NH3 (1.8)

Cl Pd Cl (1.9)

Cl Pd Pd Cl (1.10)

Pd

Cl Cl

(1.11)

Fig. 6.5 Palladium(II) complexes as anticancer agents

hydrolysis of leaving ligands in palladium complexes is too rapid, 105 times faster than their corresponding platinum analogues. They dissociate readily in solution leading very reactive species that are unable to reach their pharmacological targets. In addition, some of them undergo an inactive trans-conformation. This considerably higher activity of palladium complexes implies that if an antitumor palladium drug is to be developed, it must somehow be stabilized by a strongly coordinated nitrogen ligand and a suitable leaving group. If this group is reasonably non-labile, the drug can maintain its structural integrity in vivo, long enough. Various simple Pd (II) compounds with interesting biological properties have been previously reported such as [cis-(NH3)2PdCl2] [trans-(NH3)2PdCl2], [(1,5-COD)PdCl2], [(π-C3H5) PdCl2]2, and [(cyclopentyl)2PdCl2] (1.7–1.11; Fig. 6.5). Recent advances in this field have also focused on Pd(II) compounds bearing bidentate ligands as a way to prevent any possible cis-trans isomerism [45–47].

6.3

Titanium

Titanium complexes such as the octahedral species budotitane or the metallocene titanocene dichloride (Figs. 6.1 and 6.6) have already been investigated in clinical trials. Preclinical trials had shown activity in a broad variety of cancerous tissues [48, 49]. For example, budotitane was investigated in a clinical phase I trial and pharmacokinetic study administered as i.v. infusion twice weekly with a starting dose of 100 mg/m2. Unfortunately, no objective tumor response was observed. The

56

6 Non-platinum Anticancer Agents

Fig. 6.6 Structure of Titanocene dichloride

Ti

Cl Cl

dose-limiting toxicity was cardiac arrhythmia. However, it must be stated that 17 of the 18 patients enrolled in this study may have been resistant as they had received prior chemotherapy [50]. Titanocene dichloride showed promising results in phase I trials and was further investigated in clinical phase II studies. In a phase I trial on 40 patients with refractory solid malignancies a lyophilized formulation of titanocene dichloride afforded two minor responses (in bladder carcinoma and in non-small cell lung cancer), the dose-limiting side effect was nephrotoxicity. In another phase I study one patient with adenocarcinoma (total: 20 patients) experienced a minor response [51, 52]. In phase II, 14 patients with metastatic renal-cell carcinoma were administered the complex at a dose of 270 mg/m2 every 3 weeks but no responses could be noted [53]. Two minor responses and five patients with stable disease but no objective response were observed in another phase II study in 15 metastatic breast cancer patients (dosage: 270 mg/m2 every 3 week) [54]. The main disadvantages of titanocenium compounds, which may have led to the low activities in the clinical phase II trials, are their poor solubility in aqueous media and their hydrolytic instability under physiological conditions [55]. Furthermore, 70–80% plasma protein binding for titanocene dichloride was determined in phase I. It was found that titanocene dichloride inhibited DNA synthesis, enriched in areas near the nuclear chromatin, bound covalently to the DNA, and induced apoptosis [56, 57]. Most interestingly, the binding to the DNA occurred rather via the phosphate backbone than via the nucleobases [58]. Binding studies suggested that the cellular uptake of titanocene dichloride may be mediated by the iron transport protein transferrin [59]. The mechanism of drug action is not fully understood up to now. However, an active Ti(IV) species formed by titanocene dichloride under physiological conditions is considered to be involved significantly. In vitro studies showed that titanocene dichloride and other metallocenes were able to inhibit the enzyme topoisomerase, which may also contribute to the biological activity of these compounds [60].

6.4

Iron

The salts ferrocenium picrate and ferrocenium trichloroacetate were the first iron complexes that showed antitumor activity [61]. Unsubstituted ferrocene itself is not active as it is not soluble in water. However, appropriately substituted active ferrocenes could interconvert inside the tumor cells between the oxidation state +II (ferrocene) and the oxidation state +III (ferrocenium ions). It was shown that the cytotoxic activity of ferrocenium salts is not based on their direct linking to the DNA

6.4 Iron

57

OH

+ − BF4

Fe

Fe OH Ferrocene derivative of Tamoxifene (1.13)

DEMFc+ (1.12) NH2 N N O

TDSO

H3CCN N

O

N

Fe(CO)3 O O

Iron-nucleoside derivative (1.14)

N

Fe

N N

NCCH3 Iron complex with pyridyl ligands (1.15)

Fig. 6.7 Structures of several iron complexes

but on their ability to form reactive oxygen species leading to an oxidative DNA damage. A high stability and a comparable high lipophilicity were required for obtaining more efficient compounds out of this class (1.12; Fig. 6.7) [62–64]. Ferrocene derivatives of the antiestrogen tamoxifen (1.13) or ferrocene pyrazole conjugates with various transition metals have shown interesting antiproliferative activity. The efficacy of these compounds also depended on their redox behavior [65–67]. Another interesting approach for the development of antitumor iron complexes was taken by preparing iron carbonyl nucleosides, which were able to induce apoptotic effects. Interestingly, the cytotoxic activity of such metal complexes was dependent on the presence of the iron carbonyl moiety [68]. The concept of preparing apoptosis-inducing iron complexes of nucleosides has also been successfully applied to ferrocene compounds (1.14) [69]. Apoptosis-inducing effects could also be noted with other iron species. For example, iron (II) complexes containing pentadentate pyridyl ligands (1.15) displayed high cytotoxic activities and induced apoptosis. These compounds were stable under physiological conditions and were able to cleave supercoiled plasmid DNA in vitro [70]. However,

58

6 Non-platinum Anticancer Agents

to date, no iron-based metal complex has been approved or reported in clinical trials as cancer treatment.

6.5

Copper

The ability of copper to form complexes with different coordination numbers, geometries, oxidation states, and ligand classes has meant that many compounds have been prepared in the context of cancer therapy. Many of these complexes have been found to target DNA by intercalation or groove binding or inhibiting topoisomerase or proteasome enzymes [71] while some others can be used in photodynamic therapy [72]. A Copper(II) cyclen complex (1.16) has been found to be an efficient inhibitor of DNA/RNA synthesis as well as cytotoxic due to bisintercalation into DNA. By possessing a modest binding affinity, polymerase inhibition and bulky groups in the minor groove, the compound fulfils the requirements for a transcription inhibitor. Copper(II) complexes containing phenanthroline and indomethacin ligands, the latter of which is an NSAID that is a potent inhibitor of COX isoenzymes, COX-1 and COX-2 [73]. These compounds were tested against breast cancer stem cells (CSCs), a subpopulation of breast cancer cells that are found in larger proportions in breast tumors associated with the lowest life expectancies [74]. Another series of copper(II) metallopeptides containing a mitochondrionpenetrating peptide displayed selectivity for breast CSCs and induced cytotoxicity through mitochondrial-mediated apoptosis [75]. Photo active copper(II)–borondipyrromethene (BODIPY) conjugates have shown promising results against HeLa and MCF-7 cancer cells [76]. The mitochondria-targeting complexes induced apoptosis by generation of ROS. One of the complexes (1.17; Fig. 6.8) displayed an excellent photoselectivity index, with nanomolar IC50 values when irradiated with light, far superior to that of the clinically used photodynamic therapy drug, Photofrin®.

N O

I N

O

H

N

Cu

(1.16)

N N

H

O

N

HO N O

Cu O

N

(1.17)

Fig. 6.8 Copper complexes having shown promising anticancer activities

N B F N F

N

I

6.6 Gallium

6.6

59

Gallium

Toxicities and antitumor activities of gallium salts were described as early as 1971 [77, 78] and numerous biological properties have been reported so far [79]. Gallium showed effects on DNA structure, biosynthesis of DNA and proteins, inhibited various enzymes including ATPases or DNA polymerases, and showed effects on mitochondrial function [80]. Based on the similar behavior of Ga(III) to Fe(II) it was transported by transferring and found mainly in the lysosomes within the cell. Accordingly, gallium toxicity was enhanced strongly by transferrin based on an increased uptake [81, 82]. Gallium was effective not only in the exponential growth phase of tumor cells but also during the stationary plateau phase and the effect depended strongly on the exposure time [83, 84]. Clinical phase I and phase II studies were performed on gallium nitrate, gallium chloride, and gallium maltolate. Gallium nitrate given intravenously was not effective in various malignancies such as melanoma or breast cancer but showed better results in others. For example, in a phase II study on metastatic urothelial carcinomas gallium nitrate in combination with vinblastine and ifosfamide was very effective (response rate 67%). However, strong toxic effects (main toxicity: granulocytopenia) were noted and the median duration of response was only 20 week. Another active gallium nitrate containing combination regimen was VIG (vinblastine, ifosfamide, and gallium nitrate) in heavily pre-treated patients with ovarian cancer. This regimen yielded five partial responses (total: 14 patients) with a median response duration of 14 week. The toxicity was primarily hematologic. In androgen-independent prostate cancer patients gallium nitrate showed only moderate activity and was poorly tolerated [85–87]. Orally given gallium chloride yielded partial responses in ovarian cancer. Interestingly, gallium seemed to potentiate the action of cisplatin and etoposide. Oral gallium maltolate had higher bioavailability in a pharmacokinetic study than gallium chloride. The synergistic effects of gallium with other chemotherapeutic agents demonstrated in the clinical investigations have also been observed in the preclinical stage (e. g. for the combinations with paclitaxel, gemcitabine, or vinorelbine). A novel gallium compound currently undergoing clinical phase I trials is KP46 (Fig. 6.9). Similar to the gallium species described above, KP46 has shown synergistic effects in vitro (in combination with platinum compounds) [88]. Due to its lipophilic ligands bioavailability after oral administration and antitumor activity Fig. 6.9 Structure of tris (8-quinolinolato)gallium (III) KP46

O N

O Ga O

N N

60

6 Non-platinum Anticancer Agents

were found to be improved. Concerning the mode of action, the enzyme ribonucleotide reductase is under discussion [89].

6.7

Gold

Throughout history, gold has been treasured for its natural beauty and radiance. For this reason, many cultures have imagined gold to represent the sun. Gold is the first metal ever known to man (see Fig. 6.10). It was revered from the earliest times, as precious. Golden items already existed in the 5–4 millennium BC. This precious metal has been exploited for its putative medical properties throughout the history of civilization [90–92]. The earliest medical use of gold can be traced back to the Chinese in 2500 BC. In medieval Europe alchemists had numerous recipes for an elixir known as aurum potabile, many of which contained little gold. The great alchemist Paracelsus coined the name “Aurum Potabile” for a colloidal gold solution, which he believed could cure any number of mental, spiritual, and physical ailments. A gold cordial could be found in the new pharmacopoeias of the seventeenth century and was advocated by Nicholas Culpepper for the treatment of ailments caused by a decrease in the vital spirits, such as melancholy, fainting, fevers, and falling sickness. Later in the nineteenth century a mixture of gold chloride and sodium chloride, “muriate of gold and soda,” Na[AuCI4] was used to treat syphilis. The use of gold compounds in modern, twentieth century medicine began with the discovery in 1890 by the German bacteriologist Robert Koch that gold cyanide K [Au(CN)2] was bacteriostatic towards the tubercle bacillus. Gold therapy for tuberculosis was subsequently introduced in the 1920s. The suggestion that the tubercle bacillus was a causative agent for rheumatoid arthritis led to the use of gold therapy for this disease. Gold therapy soon proved to be ineffective for tuberculosis but, after a 30 year debate a clinical study sponsored by the Empire Rheumatism Council confirmed the effectiveness of gold compounds against rheumatoid arthritis. Since that time gold drugs have also been used to treat a variety of other rheumatic diseases including psoriatic arthritis, a form of arthritis associated with psoriasis, juvenile arthritis, palindromic rheumatism, and discoid lupus erythematosus [93]. Encouraging results have also been obtained with gold therapy as a treatment for various inflammatory skin disorders such as pemphigus, urticaria, and psoriasis.

Fig. 6.10 Gold, the king of metals

6.7 Gold

61

Chrysotherapy treatment with gold-based drugs (from the Greek word for gold, chrysos) is now an accepted part of modern medicine. In 1929, the French scientist Jacques Forestier became the first one who used injections of gold sodium aurothiomalate as the treatment of patients with rheumatoid arthritis. Aurotherapy (treatment with gold compounds) made a highly positive effect. The injections alleviated pain in the joints and removed edema. However, such treatments were too expensive. Injections had to be performed for several months, until the result was achieved. From history, it is also known that colloidal gold solutions were used to cure various kinds of infections [94]. With the advances in the use of nanotechnology in medicine gold nanoparticles or surface functionalized gold nanoparticles are being investigated for their use in anticancer chemotherapy [95, 96]. Gold nanoparticles due to their considerable proven biocompatibility and unique optical properties [97] induced by surface plasmon resonance of the gold nanoparticles surface are unique candidates for cancer treatment and diagnosis [98]. In addition, given the strong binding affinity of gold to thiol and amine groups, the surface of gold nanoparticles can be easily functionalized with biomolecules such as DNA, siRNA, peptides, antibodies, and receptors. The ability to functionalize the gold nanoparticles by multiple ligands (chemical and biomolecules) with high loading capacity makes them excellent candidates for applications such as in drug delivery, labeling, sensing, and imaging [99–101]. The functionalization of nanoparticles with biological targeting moieties to bind to specific biomarkers on the surface of cancerous cells, enabling drug delivery for tumor specific applications is termed as active targeting or ligand-based targeting while as passive targeting represents to the nanocarrier mediated delivery of therapeutics within the tumor through their permeable vasculature primarily by passive diffusion [102]. Therefore, gold nanoparticles have been explored as drug carriers due to the following advantages: (1) the large surface area provides high loading capacity for drug loading and improves the hydrophilicity and stability of drugs; (2) the ability to modify surface with targeting ligands to enhance the tumor selective accumulation compared to free drugs; (3) the passive targeting ability to tumor site due to their leaky neovessels, which is called enhanced permeability and retention (EPR) effect; and (4) the controlled release of loaded drugs in response to internal or external stimulus. With the recent developments and advances in nanotechnology it has become possible to precisely control the surface properties of nanoparticles, their size, shape, and stiffness (composition) during synthesis and these properties have been found to play an important and significant role in the design and delivery of nanoparticlebased drug carriers with high efficiency [103, 104] as illustrated in Fig. 6.11. The size of gold nanoparticles has been found to affect their intracellular localization, which may have significant consequences in drug delivery efficacy [105– 108]. Of particular interest, Hu and Liang showed that only nanoparticles with a hydrodynamic size smaller than the 9 nm diameter of the nuclear pore complex can be transported to the cell nucleus. For PEGylated nanoparticles attached to a fluorescent ligand, this size restriction meant that only small nanoparticles with a 2 nm core diameter could reach the nucleus. Gold nanoparticles linked to

62

6 Non-platinum Anticancer Agents

Fig. 6.11 Design of nanoparticle (NP)-based drug delivery platform, according to the size, shape, stiffness (composition), and surface properties of NPs. The figure is adapted with permission of the publisher

doxorubicin, a DNA intercalating agent, by a nonexcisable amide bond, were shown to reach the nucleus only when their size was smaller than 2.7 nm [109]. Recently, stimuli (such as pH or temperature) and redox-responsive gold nanoparticles have been developed for the treatment of cancer [110]. Surface functionalized gold nanoparticles and gold nanoparticles conjugated with certain bioactive agents or approved drugs have resulted in the development of target specific anticancer agents [111–117]. This is an active area of research with tremendous scope and is beyond the scope of this book. Gold nanoparticles have, however, brought about a new direction and new ideas to build improved and more potent diagnostic and therapeutic agents for different biomedical-based applications in the field of medicine. Currently, two gold-based nanomaterials (Aurimmune and AuroShell) are under investigation in clinical trials with United States Food and Drug Administration approval.

References 1. Schilling T, Keppler KB, Heim ME, Niebch G, Dietzfelbinger H, Rastetter J, Hanauske AR (1996) Clinical phase I and pharmacokinetic trial of the new Titanium complex Budotitane. Investig New Drugs 13(4):327–332

References

63

2. Heim ME, Flechtner H, Keppler BK (1989) Clinical studies with Budotitane—a new non-platinum metal complex for cancer therapy. In: Baulieu E et al (eds) Ruthenium and other non-platinum metal complexes in cancer chemotherapy, Progress in clinical biochemistry and medicine, vol 10. Springer, Berlin 3. Duan L, Fischer A, Xu Y, Sun L (2009) Isolated seven-coordinate Ru(IV) dimer complex with [HOHOH] bridging ligand as an intermediate for catalytic water oxidation. J Am Chem Soc 131:10397–10399 4. Abid M, Shamsi F, Azam A (2016) Ruthenium complexes: an emerging ground to the development of metallopharmaceuticals for cancer therapy. Mini-Rev Med Chem 10:772–786 5. Dragutan I, Dragutan V, Demonceau A (2015) Ruthenium complex: the expanding chemistry of the ruthenium complexes. Molecules 20:17244–17274 6. Hartinger CG, Groessl M, Meier SM, Casini A, Dyson PJ (2013) Application of mass spectrometric techniques to delineate the modes-of-action of anticancer metallodrugs. Chem Soc Rev 42:6186–6199 7. Antonarakis ES, Emadi A (2010) Ruthenium-based chemotherapeutics: are they ready for prime time? Cancer Chemother Pharmacol 66(1):1–9 8. Minchinton AI, Tannock IF (2006) Drug penetration in solid tumours. Nat Rev Cancer 6:583–592 9. Li F, Collins JG, Keene FR (2015) Ruthenium complexes as antimicrobial agents. Chem Soc Rev 44:2529 10. L. Zeng, P. Gupta, Y. Chen, E. Wang, L. Ji, H. Chao and Z-S. Chen, (2017) The development of anticancer ruthenium(II) complexes: from single molecule compounds to nanomaterials, Chem Soc Rev 46(19): 5771–5804 11. Trondl R, Heffeter P, Kowol CR, Jakupec MA, Bergerbd W, Keppler BK (2014) NKP-1339, the first ruthenium-based anticancer drug on the edge to clinical application. Chem Sci 5:2925–2932 12. Smith GS, Therrien B (2011) Targeted and multifunctional areneruthenium chemotherapeutics. Dalt Trans 40:10793 13. Kilpin KJ, Cammack SM, Clavel CM, Dyson PJ (2013) Ruthenium(II) arene PTA (RAPTA) complexes: impact of enantiomerically pure chiral ligands. Dalt Trans 42:2008–2014 14. Kurzwernhart A, Kandioller W, Bartel C, Bachler S, Trondl R, Muhlgassner G et al (2012) Targeting the DNA-topoisomerase complex in a double-strike approach with a topoisomerase inhibiting moiety and covalent DNA binder. Chem Commun 48:4839–4841 15. Jain SS, Anderson CM, Sapse IA, Lundgren SH, Freer AK, Hoang H et al (2018) A rutheniumplatinum metal complex that binds to sarcin ricin loop RNA and lowers mRNA expression. Chem Commun 54:8987–8990 16. Kou JF, Qian C, Wang JQ, Chen X, Wang LL, Chao H et al (2012) Chiral ruthenium (II) anthraquinone complexes as dual inhibitors of topoisomerases I and II. J Biol Inorg Chem 17:81–96 17. Wang J-Q, Zhao Z-Z, Bo H-B, Chen Q-Z (2016) Synthesis, characterization, and antitumor properties of ruthenium(II) anthraquinone complexes. J Coord Chem 69:177–189 18. De Carvalho NC, Neves SP, Dias RB, Valverde LF, Sales CBS, Rocha CAG et al (2018) A novel ruthenium complex with xanthoxylin induces S-phase arrest and causes ERK1/2mediated apoptosis in HepG2 cells through a p53-independent pathway. Cell Death Dis 9:79 19. Le Gac S, Rickling S, Gerbaux P, Defrancq E, Moucheron C, Kirsch-De Mesmaeker A (2009) A photoreactive ruthenium(II) complex tethered to a guanine-containing oligonucleotide: a biomolecular tool that behaves as a “seppuku molecule”. Angew Chem Int Ed Engl 48:1122–1125 20. Rickling S, Ghisdavu L, Pierard F, Gerbaux P, Surin M, Murat P et al (2010) A rigid dinuclear ruthenium(II) complex as an efficient photoactive agent for bridging two guanine bases of a duplex or quadruplex oligonucleotide. Chemistry 16:3951–3961

64

6 Non-platinum Anticancer Agents

21. Ambrus A, Chen D, Dai J, Bialis T, Jones RA, Yang D (2006) Human telomeric sequence forms a hybrid-type intramolecular G-quadruplex structure with mixed parallel/antiparallel strands in potassium solution. Nucleic Acids Res 34:2723–2735 22. Rajput C, Rutkaite R, Swanson L, Haq I, Thomas JA (2006) Dinuclear monointercalating RuII complexes that display high affinity binding to duplex and quadruplex DNA. Chemistry 12:4611–4619 23. Shi S, Liu J, Yao T, Geng X, Jiang L, Yang Q et al (2008) Promoting the formation and stabilization of G-quadruplex by dinuclear RuII complex Ru2(obip)L4. Inorg Chem 47:2910–2912 24. Crump M, Lipton J, Hedley D, Sutton D, Shepherd F, Minden M et al (1999) Phase I trial of sequential topotecan followed by etoposide in adults with myeloid leukemia: a national cancer Institute of Canada clinical trials group study. Leukemia 13:343–347 25. Du KJ, Wang JQ, Kou JF, Li GY, Wang LL, Chao H et al (2011) Synthesis, DNA-binding and topoisomerase inhibitory activity of ruthenium(II) polypyridyl complexes. Eur J Med Chem 46:1056–1065 26. Zhang P, Wang J, Huang H, Qiao L, Ji L, Chao H (2013) Chiral ruthenium(II) complexes with phenolic hydroxyl groups as dual poisons of topoisomerases I and IIalpha. Dalton Trans 42:8907–8917 27. Liao G, Chen X, Wu J, Qian C, Wang Y, Ji L et al (2015) Ruthenium(II) polypyridyl complexes as dual inhibitors of telomerase and topoisomerase. Dalton Trans 44:15145–15156 28. Kim MY, Duan W, Gleason-Guzman M, Hurley LH (2003) Design, synthesis, and biological evaluation of a series of fluoroquinoanthroxazines with contrasting dual mechanisms of action against topoisomerase II and G-quadruplexes. J Med Chem 46(571):583 29. Groessl M, Zava O, Dyson PJ (2011) Cellular uptake and subcellular distribution of ruthenium-based metallodrugs under clinical investigation versus cisplatin. Metallomics 3:591–599 30. Wang JQ, Zhang PY, Qian C, Hou XJ, Ji LN, Chao H (2014) Mitochondria are the primary target in the induction of apoptosis by chiral ruthenium(II) polypyridyl complexes in cancer cells. J Biol Inorg Chem 19:335–348 31. Liu J, Chen Y, Li G, Zhang P, Jin C, Zeng L et al (2015) Ruthenium(II) polypyridyl complexes as mitochondria-targeted two-photon photodynamic anticancer agents. Biomaterials 56:140–153 32. Wan D, Tang B, Wang YJ, Guo BH, Yin H, Yi YQ et al (2017) Synthesis and anticancer properties of ruthenium (II) complexes as potent apoptosis inducers through mitochondrial disruption. Eur J Med Chem 139:180–190 33. Gill MR, Cecchin D, Walker MG, Mulla RS, Battaglia G, Smythe C et al (2013) Targeting the endoplasmic reticulum with a membrane-interactive luminescent ruthenium(ii) polypyridyl complex. Chem Sci 4:4512–4519 34. Sano R, Reed JC (2013) ER stress-induced cell death mechanisms. Biochim Biophys Acta 1833:3460–3470 35. Tan C, Lai S, Wu S, Hu S, Zhou L, Chen Y et al (2010) Nuclear permeable ruthenium(II) betacarboline complexes induce autophagy to antagonize mitochondrial-mediated apoptosis. J Med Chem 53:7613–7624 36. Chen L, Li G, Peng F, Jie X, Dongye G, Cai K et al (2016) The induction of autophagy against mitochondria-mediated apoptosis in lung cancer cells by a ruthenium (II) imidazole complex. Oncotarget 7:80716–80734 37. Yuan J, Lei Z, Wang X, Zhu F, Chen D (2015) Ruthenium complex lambda-WH0402 induces hepatocellular carcinoma LM6 (HCCLM6) cell death by triggering the Beclin-1-dependent autophagy pathway. Metallomics 7:896–907 38. Puckett CA, Barton JK (2007) Methods to explore cellular uptake of ruthenium complexes. J Am Chem Soc 129(1):46–47

References

65

39. Groessl M, Zava O, Dyson PJ (2011) Cellular uptake and subcellular distribution of ruthenium-based Metallodrugs under clinical investigation versus Cisplatin. Metallomics 3 (6):591–599 40. Green DR, Reed JC (1998) Mitochondria and apoptosis. Science 281(5381):1309–1312 41. Zeng L, Gupta P, Chen Y et al (2017) The development of anticancer ruthenium (II) complexes: from single molecule compounds to nanomaterials. Chem Soc Rev 46 (19):5771–5804 42. Fong J, Kasimova K, Arenas Y et al (2015) A novel class of ruthenium-based photosensitizers effectively kills in vitro cancer cells and in vivo tumors. Photochem Photobiol Sci 14 (11):2014–2023 43. Maitlis P (1971) In: Maitlis P, Stone F, West R (eds) The organic chemistry of palladium in organometallic chemistry, a series of monographs, vol 1. Academic Press, New York 44. Rau T, van Eldik R (1996) In: Sigel H, Sigel A (eds) In metal ions in biological systems, vol 31. Marcel Dekker, New York, p 339 45. Ghaham R, Williams D (1979) The synthesis and screening for anti-bacterial, cancer, fungicidal and -viral activities of some complexes of palladium and nickel. J Inorg Nucl Chem 41(8):1245–1249 46. Butour J, Wimmer S, Wimmer F, Castan P (1997) Palladium(II) compounds with potential antitumour properties and their platinum analogues: a comparative study of the reaction of some orotic acid derivatives with DNA in vitro. Chem Biol Interact 104(2–3):165–178 47. Mansuri-Torshizi H, Srivastava T, Parekh H, Chitnis M (1992) Synthesis, spectroscopic, cytotoxic, and DNA binding studies of binuclear 2,20 -bipyridine-platinum(II) and -palladium (II) complexes of meso-alpha,alpha'-diaminoadipic and meso-alpha,alpha'-diaminosuberic acids. J Inorg Biochem 45(2):135–148 48. Caruso F, Rossi M (2004) Antitumor titanium compounds. Mini Rev Med Chem 4(1):49–60 49. Harding MH, Mokdsi G (2000) Antitumour metallocenes: structure-activity studies and interactions with biomolecules. Curr Med Chem 7(12):1289–1303 50. Schilling T, Keppler KB, Heim ME, Niebch G, Dietzfelbinger H, Rastetter J, Hanauske AR (1996) Clinical phase I and pharmacokinetic trial of the new titanium complex budotitane. Invest New Drugs 13(4):327–332 51. Korfel A, Scheulen ME, Schmoll HJ, Grundel O et al (1998) Phase I clinical and pharmacokinetic study of titanocene dichloride in adults with advanced solid tumors. Clin Cancer Res 4 (11):2701–2708 52. Christodoulou CV, Ferry DR, Fyve DW, Young A et al (1998) Phase I trial of weekly scheduling and pharmacokinetics of titanocene dichloride in patients with advanced cancer. J Clin Oncol 16:2761–2769 53. Lummen G, Sperling H, Luboldt H, Otto T, Rubben H (1998) Phase II trial of titanocene dichloride in advanced renal-cell carcinoma. Cancer Chemother Pharmacol 42:415–417 54. Kroger N, Kleeberg UR, Mross K, Edler L, Hossfeld DK (2000) Phase II clinical trial of titanocene dichloride in patients with metastatic breast cancer. Onkologie 23:60–62 55. Melendez E (2002) Titanium complexes in cancer treatment. Crit Rev Oncol Hematol 42 (3):309–315 56. Kopf-Maier P (1994) Complexes of metals other than platinum as antitumour agents. Eur J Clin Pharmacol 47(1):1–16 57. Cristododoulou CV, Eliopoulos AG, Young LS, Hodgkins L, Ferry DR, Kerr DJ (1998) Antiproliferative activity and mechanism of action of titanocene dichloride. Br J Cancer 77:2088–2097 58. Guo M, Guo Z, Sadler PJ (2001) Titanium(IV) targets phosphoesters on nucleotides: implications for the mechanism of action of the anticancer drug titanocene dichloride. J Biol Inorg Chem 6:698–707 59. Timerbaev AR, Hartinger CG, Aleksenko SS, Keppler BK (2006) Interactions of antitumor metallodrugs with serum proteins: advances in characterization using modern analytical methodology. Chem Rev 106(6):2224–2248

66

6 Non-platinum Anticancer Agents

60. Mokdsi G, Harding MM (2001) Inhibition of human topoisomerase II by the antitumor metallocenes. J Inorg Biochem 83(2–3):205–209 61. Kopf-Maier P, Kopf H, Neuse EW (1984) Ferricenium complexes: a new type of water-soluble antitumor agent. J Cancer Res Clin Oncol 108(3):336–340 62. A. M. Joy, D. M. Goodgame and I. J. Stratford, (1989) High efficiency of ferricenium salts as radiosensitizers of V79 cells in vitro and the kht tumor in vivo, Int J Radiat Oncol Biol Phys, 16(4): P1053–1056 63. Osella D, Ferrali M, Zanello P, Laschi F, Fontani M, Nervi C, Cavigiolio G (2000) On the mechanism of the antitumor activity of ferrocenium derivatives. Inorg Chim Act 306(1):42–48 64. Tabbi G, Cassino C, Cavigiolio G, Colangelo D, Ghiglia A, Viano I, Osella D (2002) Water stability and cytotoxic activity relationship of a series of Ferrocenium derivatives. ESR insights on the radical production during the degradation process. J Med Chem 45 (26):5786–5796 65. Hillard E, Vessieres A, Thouin L, Jaouen G, Amatore C (2006) Ferrocene-mediated protoncoupled electron transfer in a series of ferrocifen-type breast-cancer drug candidates. Angew Chem 118(2):291–296 66. Hillard E, Vessieres A, Le Bideau F, Plazuk D, Spera D, Huche M, Jaouen G (2006) A series of unconjugated ferrocenyl phenols: prospects as anticancer agents. ChemMedChem 1 (5):551–559 67. Duivenvoorden WCM, Liu Y, Schatte G, Kraatz HB (2005) Synthesis of redox-active ferrocene pyrazole conjugates and their cytotoxicity in human mammary adenocarcinoma MCF-7 cells. Inorg Chim Acta 358:3183–3189 68. Schlawe D, Majdalani A, Velcicky J, Heßler E, Wieder T, Prokop A, Schmalz HG (2004) Eisenhaltige Nucleosidanaloga mit apoptoseinduzierender Wirksamkeit. Angew Chem 116 (13):1763–1766 69. James P, Neudrfl J, Eissmann M, Jesse P, Prokop A, Schmalz H (2006) Enantioselective synthesis of ferrocenyl nucleoside analogues with apoptosis-inducing activity. Org Lett 8 (13):2763–2766 70. Wong ELM, Fang GS, Che CM, Zhu N (2005) Highly cytotoxic iron(ii) complexes with pentadentate pyridyl ligands as a new class of anti-tumor agents. Chem Commun 36:4578–4580 71. Santini C, Pellei M, Gandin V, Porchia M, Tisato F, Marzano C (2014) Advances in copper complexes as anticancer agents. Chem Rev 114(1):815–862 72. Bhattacharyya A, Dixit A, Banerjee S et al (2016) BODIPY appended copper(II) complexes for cellular imaging and singlet oxygen mediated anticancer activity in visible light. RSC Adv 6(106):104474–104482 73. Boodram JN, McGregor IJ, Bruno PM, Cressey PB, Hemann MT, Suntharalingam K (2016) Breast cancer stem cell potent copper(II)–non-steroidal anti-inflammatory drug complexes. Angew Chem Int Ed. 55(8):2845–2850 74. Owens TW, Naylor MJ (2013) Breast cancer stem cells. Front Physiol 4(225):1–10 75. Laws K, Bineva-Todd G, Eskandari A, Lu C, O’Reilly N, Suntharalingam K (2018) A copper (II) phenanthroline metallopeptide that targets and disrupts mitochondrial function in breast cancer stem cells. Angew Chem Int Ed 57(1):287–291 76. Mukherjee N, Podder S, Mitra K, Majumdar S, Nandi D, Chakravarty AR (2018) Targeted photodynamic therapy in visible light using BODIPY-appended copper(II) complexes of a vitamin B6 Schiff base. Dalton Trans 47(3):823–835 77. Hart MM, Adamson RH (1971) Antitumor activity and toxicity of salts of inorganic group 3a metals: aluminum, gallium, indium, and thallium. Proc NatI Acad Sci USA 68(7):1623–1626 78. Hart MM, Smith CF, Yancey ST, Adamson RH (1971) Toxicity and antitumor activity of gallium nitrate and periodically related metal salts. J NatI Cancer Inst 47:1121–1127 79. Bernstein LR (1998) Mechanisms of therapeutic activity for gallium. Pharm Rev 50 (4):665–682

References

67

80. Collery P, Keppler B, Madoulet C, Desoize B (2002) Gallium in cancer treatment. Crit Rev Oncol Hematol 42(3):283–296 81. Rasey JS, Nelson NJ, Larson SM (1982) Tumor cell toxicity of stable gallium nitrate: enhancement by transferrin and protection by iron. Eur J Cancer Clin Oncol 18(7):661–668 82. Head JF, Wang F, Elliott RL (1997) Antineoplastic drugs that interfere with Iron metabolism in cancer cells. Adv Enzym Regul 37:147–169 83. Perchellet EM, Ladesich JB, Collery P (1999) Microtubule-disrupting effects of gallium chloride in vitro. Anti-Cancer Drugs 10(5):477–488 84. Rasey JS, Nelson NJ, Larson SM (1981) Relationship of iron metabolism to tumor cell toxicity of stable gallium salts. Int J Nucl Med Biol 8(4):303–313 85. Einhorn LH, Roth BJ, Ansari R, Dreicer R, Gonin R, Loehrer PJ (1994) Phase II trial of vinblastine, ifosfamide and gallium combination chemotherapy in metastatic urothelial cancer. J Clin Oncol 12:2271–2276 86. Dreicer R, Lallas TA, Joyce JK, Anderson B, Sorosky JI, Buller REL (1998) Vinblastine, Ifosfamide, gallium nitrate, and filgrastim in platinum- and paclitaxel-resistant ovarian Cancer: a phase II study, Am. J Clin Oncol 21(3):287–290 87. Senderowicz AM, Reid R, Headlee D, Abornathy T, Horti J, Lush RM, Reed E, Figg WD, Sausville EA (1999) A phase II trial of gallium nitrate in patients with androgen-metastatic prostate cancer. Urol Int 63:120–125 88. Hofheinz RD, Dittrich C, Jakupec MA, Drescher A, Jaehde U, Gneist M, Keyserlingk NG, Keppler BK, Hochhaus A (2005) Early results from a phase I study on orally administered tris (8-quinolinolato)gallium(III) (FFC11, KP46) in patients with solid tumors--a CESAR study (central European Society for Anticancer Drug Research--EWIV). Int J Clin Pharmacol Ther 43(12):590–591 89. Jakupec MA, Keppler BK (2004) Gallium in cancer treatment. Curr Top Med Chem 4:1575–1583 90. Bernstein PL (2004) The power of gold: the history of an obsession. Wiley, p 1 91. Macbeth MD (1831) Outlines of the ancient history of medicine. William Blackwood, p 225 92. Kean WF, Kean IRL (2008) Clinical pharmacology of gold. Inflammopharmacology 16 (3):112–125 93. Messori L, Marcon G (2004) Gold complexes in the treatment of rheumatoid arthritis. In: Sigel A (ed) Metal ions and their complexes in medication. CRC Press, pp 280–301 94. Daniel MC, Astruc D (2004) Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 104(1):293–346 95. Gibson JD, Khanal BP, Zubarev ER (2007) Paclitaxel-functionalized gold nanoparticles. J Am Chem Soc 129(37):11653–11661 96. Paciotti GF, Kingston DG, Tamarkin L (2006) Colloidal gold nanoparticles: a novel nanoparticle platform for developing multifunctional tumor-targeted drug delivery vectors. Drug Dev Res 67(1):47–54 97. Connor EE, Mwamuka J, Gole A, Murphy CJ, Wyatt MD (2005) Gold nanoparticles are taken up by human cells but do not cause acute cytotoxicity. Small 1(3):325–327 98. Ghosh P, Han G, De M, Kim CK, Rotello VM (2008) Gold nanoparticles in delivery applications. Adv Drug Del Rev 60(11):1307–1315 99. Giljohann DA, Seferos DS, Daniel WL, Massich MD, Patel PC, Mirkin CA (2010) Gold nanoparticles for biology and medicine. Angew Chem Int Ed 49(19):3280–3294 100. Pissuwan D, Niidome T, Cortie MB (2011) The forthcoming applications of gold nanoparticles in drug and gene delivery systems. J Contr Release 149(1):65–71 101. Sapsford KE, Berti L, Medintz IL, Begley TP (2007) Fluorescence spectroscopy: applications in chemical biology. In: Wiley encyclopedia of chemical biology. Wiley 102. Ahmad A, Khan F, Mishra RK, Khan R (2019) Precision cancer nanotherapy: evolving role of multifunctional nanoparticles for cancer active targeting. J Med Chem 62(23):10475–10496

68

6 Non-platinum Anticancer Agents

103. Chou LY, Ming K, Chan WC (2011) Strategies for the intracellular delivery of nanoparticles. Chem Soc Rev 40:233–245 104. Shen Z, Mu-Ping N, Li Y (2016) Decorating nanoparticle surface for targeted drug delivery: opportunities and challenges. Polymers 8(3):83 105. Huo S, Jin S, Ma X, Xue X, Yang K, Kumar A, Wang PC, Zhang J, Hu Z, Xing-Jie L (2014) Ultrasmall gold nanoparticles as carriers for nucleus-based gene therapy due to size-dependent nuclear entry. ACS Nano 8(6):5852–5862 106. Huang K, Ma H, Liu J, Huo S, Kumar A, Wei T, Zhang X, Jin S, Gan Y, Wang PC, He S, Zhang X, Xing-Jie L (2012) Size-dependent localization and penetration of ultrasmall gold nanoparticles in cancer cells, multicellular spheroids, and tumors in vivo. ACS Nano 6 (5):4483–4493 107. Feldherr CM, Akin D (1990) EM visualization of nucleocytoplasmic transport processes. Elect Microscopy Rev 3(1):73–86 108. Dworetzky SI, Lanford RE, Feldherr CM (1988) The effects of variations in the number and sequence of targeting signals on nuclear uptake. J Cell Bio 107(4):1279–1287 109. Zhang X, Shastry S, Bradforth SE, Nadeau JL (2015) Nuclear uptake of ultrasmall golddoxorubicin conjugates imaged by fluorescence lifetime imaging microscopy (FLIM) and electron microscopy. Nanoscale 7(1):240–251 110. Ghorbani M, Hamishehkar H (2017) Redox and pH-responsive gold nanoparticles as a new platform for simultaneous triple anti-cancer drugs targeting, Int. J Pharm 520(1–2):126–138 111. Sanderson B, Lam R, Alharthi J, Shapter J (2014) The potential of gold nanoparticle conjugates to kill cancer cells in culture. Process Eng 92:26–29 112. Stolarczyk EU, Leś A, Łaszcz M, Kubiszewski M, Strzempek W, Menaszek E et al (2020) The ligand exchange of citrates to thioabiraterone on gold nanoparticles for prostate cancer therapy. Int J Pharm:119319 113. Joshi P, Chakraborti S, Ramirez-Vick JE, Ansari ZA, Shanker V, Chakrabarti P, Singh SP (2012) The anticancer activity of chloroquine-gold nanoparticles against MCF-7 breast cancer cells. Coll Surf B Biointerf 95:195–200 114. Manivasagan P, Bharathiraja S, Bui NQ, Lim IG, Oh J (2016) Paclitaxel-loaded chitosan oligosaccharide-stabilized gold nanoparticles as novel agents for drug delivery and photoacoustic imaging of cancer cells. Int J Pharm 511(1):367–379 115. Oladipo AO, Iku SI, Ntwasa M, Nkambule TT, Mamba BB, Msagati TA (2020) Doxorubicin conjugated hydrophilic AuPt bimetallic nanoparticles fabricated from Phragmites australis: characterization and cytotoxic activity against human cancer cells. J Drug Del Sci Tech:101749 116. Paul P, Chatterjee S, Pramanik A, Karmakar P, Bhattacharyya SC, Kumar GS (2018) Thionine conjugated gold nanoparticles trigger apoptotic activity toward HepG2 cancer cell line. ACS Biomater Sci Eng 4(2):635–646 117. Safwat MA, Soliman GM, Sayed D, Attia MA (2018) Fluorouracil-loaded gold nanoparticles for the treatment of skin cancer: development, in vitro characterization, and in vivo evaluation in a mouse skin cancer xenograft model. Mol Pharm 15(6):2194–2205

7

Chemistry of Gold

Gold is a chemical element with the symbol Au (from Latin: aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In a pure form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal and a group 11 element (Fig. 7.1). Gold is one of the noblest—that is, least chemically reactive of the transition elements [1]. It is not attacked by oxygen or sulfur although it will react readily with halogens or with solutions containing or generating chlorine, such as aqua regia. It also will dissolve in cyanide solutions in the presence of air or hydrogen peroxide. Dissolution in cyanide solutions is attributable to the formation of the very stable dicyanoaurate ion, [Au(CN)2]
. The noble character of gold is a consequence of its low oxidation potential. The value of its electrochemical potential Eo (reduction-oxidation or redox potential) is the lowest of any metal [2]. This means that gold in any cationic form, the common ionic state for a metallic element, will accept electrons from virtually any source (reducing agents) to form neutral gold atoms which can aggregate to form gold metal in bulk. The metal can appear as a dark powder from mixtures of solids or as a dark violet precipitate from solution, which has to be collected and melted to give bright gold nuggets, but it can also be deposited as a gold mirror on the walls of the reaction vessel, either from the vapor or from solution. Aggregation of the metal atoms occurs through small cluster intermediates, which can, under suitable conditions, be stabilized as colloids in a matrix of glass or plastic. Another measure of the redox properties of gold, though less well defined by physical data, is its electronegativity. In these terms, gold is the most electronegative of all the metals. The electrochemical potential and electronegativity are used as parameters to describe the reduction of Au+ or Au3+, to mention just the most common gold cations, to gold metal (Au), whereas the electron affinity refers to the process whereby Au is converted into its anionic form Au-.

# The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_7

69

70

7

Chemistry of Gold

Fig. 7.1 King of metals, Gold, in the periodic table

Like copper, gold has a single s electron outside a completed d shell, but, in spite of the similarity in electronic structures and ionization energies, there are few close resemblances between gold on the one hand and copper on the other. The characteristic oxidation states of gold are +1 (aurous compounds) and + 3 (auric compounds). The state +1 is generally quite unstable, and most of the chemistry of gold involves the state +3. Both gold(I) and gold(III) are unstable with respect to gold(0) and are readily reduced by mild reducing agents. Gold(I) is thermodynamically more stable than gold(III). Many gold(III) complexes are strong oxidizing agents, being reduced to Au(I), and this means that they are generally toxic. This reduction can be driven by biologically occurring reductants such as thiols. Gold(I) has a very high preference for “soft” ligands such as sulfur (thiolares) and phosphorus (phosphines), with little affinity for oxygen ligands and only weakly for nitrogen. The electronic configuration of gold(0) is 5d106s1; for gold(I) it is 5d106s0 and for the gold(–I) anion it is 5d106s2. These configurations would justify the relative stability of gold(I) compounds, with 10 electrons in a closed set of 5d orbitals, or even, to some extent, the formation of the aurate anion, but they do not allow us to understand the predominance of the metallic form. Gold(I), with a d10 closed-shell configuration, generally gives rise to two-coordinate, linear compounds, which are by far the most commonly observed coordination geometry, as well as threecoordinate, trigonal and four-coordinate, tetrahedral coordination geometries [3]. Being a “soft” metal center, gold(I) has a pronounced tendency to form stable complexes with easily polarizable soft donor atoms, such as sulfur and phosphorus.

7.1 Gold and Its Complexes in Medicine

71

Gold(III), on the other hand, usually forms a tetracoordinate, square-planar geometry with a preference for hard donor atoms, including oxygen and nitrogen, due to the “hard” nature of gold(III) compared with gold(I) [4]. Among the relatively few gold compounds of practical importance are gold (I) chloride, AuCl; gold(III) chloride, AuCl3; and chloroauric acid, HAuCl4. In the first compound, gold is in the +1 oxidation state, and in the latter two, the +3 state. All three compounds are involved in the electrolytic refining of gold. Potassium cyanoaurate, K[Au(CN)2], is the basis for most gold-plating baths (the solution employed when gold is plated). Several compounds of gold have industrial applications. For example, gold mercaptides, which are obtained from sulfurized terpenes, are dissolved in certain organic solutions and used for decorating china and glass articles. The presence of the same electronic configuration and structural characteristics as cisplatin prompted the investigation of gold(III) complexes as anti-tumor agents but was initially restrained because such complexes are susceptible to reduction in the biological environment [5]. However, many significant advances with gold(III) complexes have been made in recent years with careful selection of coordination geometries.

7.1

Gold and Its Complexes in Medicine

There are two main classes of gold drugs: (1) Injectable gold(I) thiolates such as aurothiomalate, aurothioglucose, and aurothiopropanol sulfonate which are major ingredients of Myocrisin™, Solganol™, and Allocrysin™, respectively and (2) The oral complex (2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranosato-S) (triethylphosphine) gold(I) (auranofin). The oral drug has a well-defined structure with linear two-coordinate Au(I), but the injectable drugs are amorphous non-crystalline solids thought to contain thiolate sulfur-bridged oligomers and ring structures as shown in Fig. 7.2. The gold thiolate drugs undergo ligand exchange with cysteine-rich peptides and proteins such as glutathione, metallothionein, and albumin, particularly where the pK ~ of the SH group is low, e.g. the cys-34 of albumin. Gold(I) can be stabilized by cyanide or thiolate ligands, including naturally occurring thiols such as the amino acid cysteine, and gold is primarily transported in the bloodstream as an albumin adduct. The therapeutic value of gold compounds has been known since ancient times. In the modern era, the use of gold compounds in medicine was initiated with the detection of anti-tubercular activity of gold(I) dicyanide by Robert Koch [6]. This important discovery formed the basis for investigating the pharmacological activities of gold compounds. “Chrysotherapy” is the term that refers to the use of gold formulations in medicine, in particular for the treatment of joint pain and inflammatory diseases, such as rheumatoid arthritis [7]. Controlled clinical trials had proved the efficacy of gold compounds in the treatment of rheumatoid arthritis in 1960 [8]. Despite the toxicity of this compound the “gold decade” (1925–1935) followed, in which intravenously administered gold(I) thiolate salts were used for the treatment

72

7

Au

+ −

OH S

NaO O

HO OH

O

R S Au

Au S R

R S Au

Au S R

S

OH

Au

Au RS

n

n

Sodium aurothiomalate

RS

R S

O

HO

Chemistry of Gold

Aurothioglucose

Au

Au R S

S R Au

SR

Au Au S R

Hexameric structure

OAc AcO

Au SR

Oligomer

Na3 O3S S Au S SO3 Trisodium aurothiosulfate

AcO

O OAc

S

Au

P

−

Aurocyanide

Auranofin Au S

NC Au CN

OH SO3Na

Aurothiopropanol sulfonate

Fig. 7.2 Structures of some widely used gold-based drugs

of tuberculosis, despite a lack of experimental evidence for any anti-tubercular benefits [9]. Gold therapy was found to significantly reduce joint pain in a group of non-tubercular patients, which led the French physician Jacques Forestier to investigate the use of gold compounds for treatment of rheumatoid arthritis. In 1960 the results of a very large well-controlled, double-bind trial concluded that gold drugs have a beneficial effect. The early medicinal benefits of gold compounds were limited to inflammatory diseases and hence, they were initially classified as Disease Modifying Anti-Rheumatic Drugs (DMARDs) [7, 10]. As time progressed, it was noticed that the recipients of chrysotherapy for arthritis exhibited lower evidence of malignancy rates and thus it was hypothesized that gold compounds might possess anti-cancer effects [11–15]. In general, gold compounds tend to target cancer cells, and the onset of cell death mechanisms occurs as early as 2 h after receiving treatment [16, 17]. During the 1970s and early 1980s Sutton and colleagues developed an orally active Au(I) phosphine compound auranofin (see Fig. 7.2), for rheumatoid arthritis treatment, which was approved for clinical use in 1985 [18–20]. Auranofin, a goldcontaining triethylphosphine, serves as reserve therapy for rheumatoid arthritis. Although it was originally approved for the treatment of rheumatoid arthritis in 1985, its therapeutic application has been expanded to a number of other diseases, such as parasitic infections, neurodegenerative disorders, AIDS, and bacterial infections. Recently, auranofin was approved by US Food and Drug Administration for phase II clinical trials for cancer therapy [21]. The mechanism of therapeutic action of auranofin involves the inhibition of TrxR, which is a key component of the cellular antioxidant system, and induction of endoplasmic reticulum (ER) stress and subsequent activation of the unfolded protein response (UPR) [22–24].

7.2 Auranofin and Its Analogs

73

Gold salts, usually sodium aurothiomalate, are used almost exclusively in the treatment of rheumatoid and psoriatic arthritis. Apart from its antiarthritic properties, aurothiomalate has been shown to have antileishmanial activity in hamsters [25]. Auranofin has also been proposed for the treatment of psoriasis [26]. Bis (thioglucose) gold(I) has been found to inhibit the replication of one strain of the HIV virus in vitro [27]. The inhibition of the virus was suggested to be due to the interaction of gold(I) with a cysteine residue in a surface protein of the viral envelope. It was shown separately that bis(thioglucose)gold(I) inhibits solubilized reverse transcriptase in vitro. A metabolite of all gold drugs is dicyanogold(I) [28], which has been proposed for the treatment of AIDS due to its ability to penetrate cells rapidly and its suggested low toxicity [29].

7.2

Auranofin and Its Analogs

In the mid-1980s auranofin was shown to inhibit the growth of cultured tumor cells in vitro, as well as having limited in vivo antitumor activity in one mouse tumor model [30, 31]. Since that time, a large variety of other linear two-coordinate Au (I) phosphine complexes have been shown to inhibit the growth of cultured tumor cells in vitro. [15, 32–37] These complexes usually incorporate S-ligands such as thiosugars, thionucleobases and dithiocarbamates, sulfanylpropenoates and bioactive vitamin K3, azacoumarin, and naphthalimide derivatives. Extensive early mechanistic studies showed that both auranofin and Et3PAuCl affect mitochondrial function, and later these results were reinterpreted as the induction of mitochondrial apoptotic pathways [17]. Early studies on Au (I) compounds suggested that the phosphine ligands were a necessary requirement for antitumor activity, but in the last few years antitumor activity has been reported for linear Au(I) complexes with N-heterocyclic carbene (NHC) [38, 39], cyclodiphosphazene [40], phosphole [41], and N,N0-disubstituted cyclic thiourea [42] ligands. The nature of the ligand is likely to affect cellular uptake as polymeric Au(I) thiolates do not readily enter cells and were found to exhibit very low cytotoxicity to B16 melanoma cells in vitro and were inactive against ip P388 leukemia in mice. A recent study, however, has shown that both aurothiomalate and aurothioglucose exhibit potent antitumor effects in vitro and in vivo preclinical models of non-small cell lung cancer [43], as a result of selective binding of Cys-69 within the PB1 domain of protein kinase Ci [44]. The early studies on auranofin showed that its high reactivity towards protein thiols limits its antitumor activity in vivo. While a range of other Au(I) phosphine complexes has shown promising cytotoxic activity in vitro, similar reactions (a characteristic feature of linear two-coordinate Au(I) complexes that undergo facile ligand exchange reactions) are likely to limit their application as anticancer agents. The development of Au(I) complexes with chelated diphosphines such as [Au (dppe)2]Cl was with this aim of reducing the high thiol reactivity of auranofin and analogs [45]. The behavior of [Au(dppe)2]+, and related compounds, is consistent with that of the class of antitumor agents known as delocalized lipophilic cations

74

7

+ R2P

PR2 Cl

Au R2P

PR2

[Au(dppe)2]Cl R= Ph [Au(dnpype)2]Cl R= 2-,3- or 4-pyridyl

+ −

R2P R2P

Au

PR2 Cl

−

PR2

[Au(d2pypp)2]Cl R= 2-pyridyl

Chemistry of Gold

R N

R N

+

Au N R

N R

+ [(i-pr2Im)2Au] R= i-Pr

Fig. 7.3 Examples of lipophilic cationic gold(I) antitumor compounds

(DLCs) [46], that accumulate in the mitochondria of tumor cells driven by the elevated mitochondrial membrane potential that is a characteristic feature of some cancer cells [47]. However, the high lipophilicity of these compounds results in severe toxicity, a consequence of the non-selective concentration of compounds into mitochondria of both tumorigenic and non-tumorigenic cells, causing general membrane permeabilization [48, 49]. Despite the significant antitumor activity of [Au (dppe)2]Cl against a range of tumor models in mice, its clinical development was halted following preclinical toxicological studies in dogs and rabbits that identified severe toxicities to heart, liver, and lung, as a result of mitochondrial dysfunction [50, 51]. Replacing the phenyl substituents with pyridyl groups (with the N atom in either the 2, 3, or 4 position in the ring) provided a series of compounds of type [Au (dnpype)2]Cl, that are structurally similar to [Au(dppe)2]Cl with hydrophilic– lipophilic character spanning a very large range. Studies carried out in isolated rat hepatocytes, and a panel of cisplatin-resistant human ovarian carcinoma cell lines showed a general increase in cytotoxic potency (and decrease in selectivity) with increasing lipophilicity [52]. Evaluation of the in vivo antitumor activity in colon [53] tumors in mice showed that the 2-pyridyl complex, with intermediate lipophilicity, had significant antitumor activity which correlated with highest drug concentrations in plasma and tumor tissue. To further fine-tune the hydrophilic– lipophilic balance in the optimal range, the related compound [Au(d2pypp)2]Cl (Fig. 7.3) with the propyl-bridged 2-pyridyl phosphine ligand (d2pypp), was designed with the idea of combining the features of the two distinct classes of Au (I) phosphines, i.e. retaining the lipophilic cationic properties of the tetrahedral bis-chelated complexes that allow accumulation into mitochondria, but enhancing the reactivity towards protein thiols/selenols that underlies the inhibition of the selenoprotein thioredoxin reductase (TrxR) by auranofin (see below and Fig. 7.3) [54, 55]. Some findings have shown that [Au(d2pypp)2]Cl is selectively toxic to breast cancer cells but not normal breast cells, validating its rational design. Within this class of lipophilic cationic Au(I) phosphine complexes are the mixed gold phosphine compound [Au(dppp)(PPh3)Cl], which is toxic in a range of cancer cell lines in vitro [56, 57]. While the complex is neutral, it decomposes in solution to give

7.2 Auranofin and Its Analogs

75

several products including [Au(dppp)2]+ [58], so the pharmacologically active species may be positively charged complexes. Also of interest in this context is the hydrophilic four-coordinate complex [Au(P(CH2OH)3)4]Cl, which has been shown to be toxic against several human tumor cell lines and a mouse tumor model [59]. A family of linear, cationic Au(I)NHC complexes [(R2Im)2Au] + (Fig. 7.3) were prepared from simple imidazolium salt precursors where the lipophilicity was fine-tuned (from log P ¼
1.09 (R ¼ Me) to 1.73 (R ¼ cyclohexyl)) by including different functional groups [60]. It was shown that certain compounds from this series were selectively toxic to two highly tumorigenic breast cancer cell lines and not to normal breast cells, and the degree of selectivity and potency were optimized by modification of the substituents. Pharmacological research on auranofin and closely related derivatives is still the focus of a huge number of ongoing studies shedding light on the complicated pharmacodynamic and pharmacokinetic profile of these drugs. Thus, it was observed that apoptosis induction by auranofin and TEPAuCl in Jurkat T cells appeared to be mediated by the inhibition of the cytosolic and mitochondrial forms of TrxR and that it was accompanied by an increase in cellular hydrogen peroxide levels. In contrast aurothiomalate was only little effective concerning TrxR inhibition and apoptosis induction [61]. Moreover, drug resistance as one of the major problems of current cancer chemotherapy might be addressed by the use of gold(I) complexes as auranofin was also effective in cisplatin-resistant human ovarian cancer cells, which exhibited elevated levels of TrxR activity [62]. Au-Naphth-1 is a gold (I) species, which contains the triethylphosphine moiety of auranofin and a naphthalimide ligand replacing the carbohydrate ligand of auranofin [63]. The drug design strategy for this novel gold complex was motivated by the aim to replace the auranofin carbohydrate ligand, which is supposedly more relevant for the biodistribution of the compound than for its pharmacodynamic effects, by another bioactive ligand. As bioactive ligand the naphthalimide moiety was chosen based on the promising preclinical results of the naphthalimide class of antitumor drugs [64]. Au-Naphth-1 displayed promising cell growth inhibiting effects, induced apoptosis and inhibited TrxR. Mass spectrometric investigations on a cysteine containing model peptide showed that Au-Naphth-1 bound covalently to the cysteine residue under loss of its thionaphthalimide ligand, which indicates that the interaction with TrxR might also be based on a covalent binding mechanism. The interest in the investigation of gold compounds as anticancer agents is usually related to the square-planar geometry found for platinum in cisplatin. Gold in the +III oxidation state is isoelectronic with platinum(II) and forms similar squareplanar complexes. Given that the mammalian environment is generally reducing, compounds containing gold(III) may be expected to be reduced in vivo to gold (I) and metallic gold [65]. However, appropriate selection of the ligand donor set can serve to stabilize gold(III) and consequently, there is a rich literature of anti-tumor/ cytotoxicity investigations for these species. The third reason for ongoing studies of anti-tumor activity of gold compounds is one that it has a general applicability for metal-based compounds.

76

7

Chemistry of Gold

References 1. Holleman AF, Wiberg E (2001) Inorganic chemistry. Academic Press, San Diego 2. Martin J (2008) The chemistry of gold as an anion. Chem Soc Rev 37(9):1826–1835 3. Gimeno MC (2008) The chemistry of gold. In: Laguna A (ed) Modern supramolecular gold chemistry: gold-metal interactions and applications. Wiley, Weinheim 4. Vittal JJ, Puddephatt RJ (2006) Gold: inorganic & coordination chemistry in encyclopedia of inorganic chemistry. Wiley 5. Nardon C, Boscutti G, Fregona D (2014) Beyond platinums: gold complexes as anticancer agents. Anticancer Res 34:487–492 6. Kean WF, Forestier F, Kassam Y, Buchanan WW, Rooney PJ (1985) The history of gold therapy in rheumatoid disease. Semin Arthritis Rheum 14(3):180–186 7. Berners-Price SJ, Filipovska A (2011) Gold compounds as therapeutic agents for human diseases. Metallomics 3:863–873 8. The Empire Rheumatism Council (1960) Gold therapy in rheumatoid arthritis. Report of a multicentre control trial. Ann Rheum Dis 19:95–119 9. Benedek TG (2004) The history of gold therapy for tuberculosis. J Hist Med All Sci 59 (1):50–89 10. Kean WF, Kean IRL Clinical pharmacology of gold. Inflammopharmacology 3:112–125 11. Tiekink ERT (2002) Gold derivatives for the treatment of cancer. Crit Rev Oncol Hematol 42:225–248 12. Lorenzen I (1975) Treatment of rheumatoid arthritis with cytostatic drugs. Ann Clin Res 7:195–201 13. Fries JF, Bloch D, Spitz P, Mitchell DM (1985) Cancer in rheumatoid arthritis: a prospective long-term study of mortality. Am J Med 78:56–59 14. Ward JR (1988) Role of disease-modifying antirheumatic drugs versus cytotoxic agents in the therapy of rheumatoid arthritis. Am J Med 85:39–44 15. Maiore L, Cinellu MA, Michelucci E, Moneti G, Nobili S, Landini I, Mini E, Guerri A, Gabbiani C, Messori L (2011) Structural and solution chemistry, protein binding and antiproliferative profiles of gold(I)/(III) complexes bearing the saccharinato ligand. J Inorg Biochem 105:348–355 16. Ott I (2009) On the medicinal chemistry of gold complexes as anticancer drugs. Coord Chem Rev 253:1670–1681 17. McKeage MJ, Maharaj L, Berners-Price SJ (2002) Mechanisms of cytotoxicity and antitumor activity of gold(I) phosphine complexes: the possible role of mitochondria. Coord Chem Rev 232:127–135 18. Kean WF, Hart L, Buchanan WW (1997) Auranofin. Br J Rheumatol 36:560–572 19. Sutton BM, McGusty E, Walz DT, DiMartino MJ (1972) Oral gold. Antiarthritic properties of alkylphosphine gold coordination complexes. J Med Chem 15:1095–1098 20. Sutton M (1986) Gold compounds for rheumatoid arthritis. Gold Bull 19:15–16 21. Roder C, Thomson MJ (2015) Auranofin: repurposing an old drug for a golden new age. Drugs R&D 15:13–20 22. Fan C, Zheng W, Fu X, Li X, Wong Y-S, Chen T (2014) Enhancement of auranofin-induced lung cancer cell apoptosis by selenocystine, a natural inhibitor of TrxR1 in vitro and in vivo. Cell Death Dis 5:e1191 23. Fiskus W, Saba N, Shen M, Ghias M, Liu J, Gupta SD et al (2014) Auranofin induces lethal oxidative and endoplasmic reticulum stress and exerts potent preclinical activity against chronic lymphocytic leukemia. Cancer Res 74:2520–2532 24. Liu N, Li X, Huang H, Zhao C, Liao S, Yang C et al (2014) Clinically used antirheumatic agent auranofin is a proteasomal deubiquitinase inhibitor and inhibits tumor growth. Oncotarget 5:5453–5471 25. Abruzzo JL, DeHoratius RJ (1969) Gold sodium thiomalate in visceral leishmaniasis. Antileishmanial activity in hamsters. Am J Trop Med Hyg 18(4):506–510

References

77

26. Brückle W, Dexel T, Grasedyck K, Schattenkirchner M (1994) Treatment of psoriatic arthritis with auranofin and gold sodium thiomalate. Clin Rheumatol 13(2):209–216 27. Okada T, Patterson BK, Ye SQ, Gurney ME (1993) Aurothiolates inhibit HIV-1 infectivity by gold(I) ligand exchange with a component of the virion surface. Virology 192(2):631–642 28. Elder RC, Zhao Z, Zhang Y, Dorsey JG, Hess EV, Tepperman KJ (1993) Dicyanogold (I) is a common human metabolite of different gold drugs. J Rheumatol 20:268 29. Y.F. Zhang, E.V Hess, KG. Pryhuber, J.G. Dorsey, K Tepperman, R.C. Elder, (1995) Gold binding sites in red blood cells, Inorg Chim Acta, 229(1-2): 271-280 30. Simon TM, Kunishima DH, Vibert GJ, Lorber A (1979) Inhibitory effects of a new Oral gold compound on HeLa cells. Cancer 44(6):1965–1975 31. Mirabelli CK, Johnson RK, Sung CM, Faucette L, Muirhead K, Crooke ST (1985) Evaluation of the in vivo antitumor activity and in vitro cytotoxic properties of Auranofin, a coordinated gold compound, in murine tumor models. Cancer Res 45(1):32–39 32. Kim JH, Reeder E, Parkin S, Awuah SG (2019) Gold(I/III)-phosphine complexes as potent Antiproliferative agents. Sci Rep 9:12335 33. Reddy TS, Privér SH, Rao VV, Mirzadeh N, Bhargava SK (2018) Gold(i) and gold(iii) phosphine complexes: synthesis, anticancer activities towards 2D and 3D cancer models, and apoptosis inducing properties. Dalton Trans 47:15312–15323 34. Bagowski CP, You Y, Scheffler H, Vlecken DH, Schmitz DJ, Ott I (2009) Naphthalimide gold (I) phosphine complexes as anticancer metallodrugs. Dalton Trans 28(48):10799–10805 35. Milacic V, Fregona D, Dou QP (2008) Gold complexes as prospective metal-based anticancer drugs. Histol Histopathol 23:101–108 36. Fernández-Moreira V, Herrera RP, Gimeno MC (2019) Anticancer properties of gold complexes with biologically relevant ligands. Pure Appl Chem 91(2):247–269 37. Gandin V, Fernandes AP, Rigobello MP, Dani B, Sorrentino F, Tisato F, Bjornstedt M, Bindoli A, Sturaro A, Rella R, Marzano C (2010) Cancer cell death induced by phosphine gold(I) compounds targeting thioredoxin reductase. Biochem Pharmacol 79(2):90–101 38. Hickey JL, Ruhayel RA, Barnard PJ, Baker MV, Berners-Price SJ, Filipovska A (2008) Mitochondria-targeted chemotherapeutics: the rational design of gold(I) N-heterocyclic carbene complexes that are selectively toxic to cancer cells and target protein selenols in preference to thiols. J Am Chem Soc 130(38):12570–12571 39. Rubbiani R, Kitanovic I, Alborzinia H, Can S, Kitanovic A, Onambele LA, Stefanopoulou M, Geldmacher Y, Sheldrick WS, Wolber G, Prokop A, Wolfl S, Ott I (2010) Benzimidazol-2ylidene gold(I) complexes are thioredoxin Reductase inhibitors with multiple antitumor properties. J Med Chem 53(24):8608–8618 40. Suresh D, Balakrishna MS, Rathinasamy K, Panda D, Mobin SM (2008) Water-soluble cyclodiphosphazanes: synthesis, gold(i) metal complexes and their in vitro antitumor studies. Dalton Trans:2812–2814 41. Viry E, Battaglia E, Deborde V, Mueller T, Réau R, Davioud-Charvet E, Bagrel D (2008) A sugar-modified phosphole gold complex with antiproliferative properties acting as a thioredoxin reductase inhibitor in MCF-7 cells. ChemMedChem 3:1667–1670 42. K. Yan, C.-N. Lok, K. Bierla and C.-M. Che, (2010) Gold(i) complex of N,N0 -disubstituted cyclic thiourea with in vitro and in vivo anticancer properties—potent tight-binding inhibition of thioredoxin reductase, Chem Commun, 46(41): 7691–7693 43. Stallings-Mann M, Jamieson L, Regala RP, Weems C, Murray NR, Fields AP (2006) A novel small-molecule inhibitor of protein kinase Cι blocks transformed growth of non–small-cell lung cancer cells. Cancer Res 66:1767–1774 44. Edrogan E, Lamark T, Stallings-Mann M, Jamieson L, Pellechia M, Thompson EA, Johansen T, Fields AP (2006) Aurothiomalate inhibits transformed growth by targeting the PB1 domain of protein kinase ci. J Biol Chem 281(38):28450–28459 45. Berners-Price SJ, Mirabelli CK, Johnson RK, Mattern MR, McCabe FL, Faucette LF, Sung CM, Mong SM, Sadler PJ, Crooke ST (1986) In vivo antitumor activity and in vitro cytotoxic

78

7

Chemistry of Gold

properties of bis[1,2-bis(diphenylphosphino)ethane]gold(I) chloride. Cancer Res 46 (11):5486–5493 46. Modica-Napolitano JS, Aprille JR (2001) Delocalized lipophilic Cations selectively target the mitochondria of carcinoma cells. Adv Drug Delivery Rev 49(1-2):63–70 47. Chen LB (1988) Mitochondrial membrane potential in living cells. Annu Rev Cell Biol 4:155–181 48. Hoke GD, Rush GF, Bossard GE, McArdle JV, Jensen BD, Mirabelli CK (1988) Mechanism of alterations in isolated rat liver mitochondrial function induced by gold complexes of bidentate phosphines. J Biol Chem 263(23):11203–11210 49. Rush GF, Smith PF, Hoke GD, Alberts DW, Snyder RM, Mirabelli CK (1987) The mechanism of acute cytotoxicity of triethylphosphine gold(I) complexes: II. Triethylphosphine gold chloride-induced alterations in mitochondrial function. Toxicol App Pharmacol 90(3):391–400 50. Smith PF, Hoke GD, Alberts DW, Bugelski PJ, Lupo S, Mirabelli CK (1989) Mechanism of toxicity of an experimental bidentate phosphine gold complexed antineoplastic agent in isolated rat hepatocytes. J Pharmacol Exp Therap 249:944–950 51. Hoke GD, Macia RA, Meunier PC, Bugelski PJ, Mirabelli CK, Rush GF, Matthews WD (1989) In vivo and in vitro Cardiotoxicity of a gold-containing antineoplastic drug candidate in the rabbit. Toxicol Appl Pharmacol 100(2):293–306 52. Liu JJ, Galettis P, Farr A, Maharaj L, Samarasinha H, McGechan AC, Baguley BC, Bowen RJ, Berners-Price SJ, McKeage MJ (2008) In vitro antitumour and hepatotoxicity profiles of Au (I) and Ag(I) bidentate pyridyl phosphine complexes and relationships to cellular uptake. J Inorg Biochem 102(2):303–310 53. McKeage MJ, Berners-Price SJ, Galettis P, Bowen RJ, Brouwer W, Ding L, Zhuang L, Baguley BC (2000) Role of lipophilicity in determining cellular uptake and antitumour activity of gold phosphine complexes. Cancer Chemother Pharmacol 46(5):343–350 54. Humphreys AS, Filipovska A, Berners-Price SJ, Koutsantonis GA, Skelton BW, White AH (2007) Gold(i) chloride adducts of 1,3-bis(di-2-pyridylphosphino)propane: synthesis, structural studies and antitumour activity. Dalton Trans 43:4943–4950 55. Rackham O, Nichols SJ, Leedman PJ, Berners-Price SJ, Filipovska A (2007) A gold (I) phosphine complex selectively induces apoptosis in breast cancer cells: implications for anticancer therapeutics targeted to mitochondria. Biochem Pharmacol 74(7):992–1002 56. Caruso F, Rossi M, Tanski J, Pettinari C, Marchetti F (2003) Antitumor activity of the mixed phosphine gold species chlorotriphenylphosphine-1,3-bis(diphenylphosphino)propanegold(I). J Med Chem 46(9):1737–1742 57. Caruso F, Villa R, Rossi M, Pettinari C, Paduano F, Pennati M, Daidone M, Zaffaroni N (2007) Mitochondria are primary targets in apoptosis induced by the mixed phosphine gold species chlorotriphenylphosphine-1,3-bis(diphenylphosphino)propanegold(I) in melanoma cell lines. Biochem Pharmacol 73(6):773–781 58. Caruso F, Pettinari C, Paduano F, Villa R, Marchetti F, Monti E, Rossi M (2008) Chemical behavior and in vitro activity of mixed phosphine gold(I) compounds on melanoma cell lines. J Med Chem 51(6):1584–1591 59. Pillarsetty N, Katti KK, Hoffman TJ, Volkert WA, Katti KV, Kamei H, Koide T (2003) In vitro and in vivo antitumor properties of tetrakis((trishydroxy-methyl)phosphine)gold(I) chloride. J Med Chem 46(7):1130–1132 60. Baker MV, Barnard PJ, Berners-Price SJ, Brayshaw SK, Hickey JL, Skelton BW, White AH (2006) Cationic, linear Au(i) N-heterocyclic carbene complexes: synthesis, structure and antimitochondrial activity. Dalton Trans:3708–3715 61. Rigobello MP, Folda A, Dani B, Menabo R, Scutari G, Bindoli A (2008) Gold(I) complexes determine apoptosis with limited oxidative stress in Jurkat T cells. Eur J Pharmacol 582 (1-3):26–34 62. Marzano C, Gandin V, Folda A, Scutari G, Bindoli A, Rigobello MP (2007) Inhibition of thioredoxin reductase by auranofin induces apoptosis in Cisplatin-resistant human ovarian cancer cells. Free Radic Biol Med 42(6):872–881

References

79

63. Ott I, Qian X, Xu Y, Vlecken DH, Marques IJ, Kubat D, Will J, Sheldrick WS, Jesse P, Prokop A, Bagowski CP (2009) A gold(I) phosphine complex containing a naphthalimide ligand functions as a TrxR inhibiting antiproliferative agent and angiogenesis inhibitor. J Med Chem 52(3):763–770 64. Brana MF, Ramos A (2001) Naphthalimides as anti-cancer agents: synthesis and biological activity. Curr Med Chem Anticancer Agents 1(3):237–255 65. Bertrand B, Williams MRM, Bochmann M (2018) Gold(III) complexes for antitumor applications: an overview. Chem Eur J 24:11840–11851

8

Classes of Gold Complexes

8.1

Complexes with Nitrogen Donor Ligands

Among the pool of organometallic compounds, gold(III) or gold(I) complexes have attracted enormous attention in the field of medicine for their propitious antiproliferative and antitumor properties. The medicinal use of gold compounds is noticeable from several thousand years and Au(I) compounds have been used clinically to treat rheumatoid arthritis since the last century [1]. Recurrent reports revealed that a number of interesting gold complexes with different organic molecules including gold(III) porphyrins, gold(III) dithiocarbamates [2], and dioxo dinuclear gold(III) complexes containing polypyridyl ligands display exciting pharmacological profiles [3, 4]. Meanwhile gold(III) complexes being isoelectronic to platinum(II) complexes are thought to possess significant anticancer potential and by incorporating polydentate ligands their stability can be enhanced in biological environments. Besides this gold(III) complexes exert their influence not through the commonly known disruption of DNA but by inhibiting the function of intracellular proteins and/or disrupting normal mitochondrial function [5, 6]. A. N. Wein et al. explained the cytotoxicity of 5,6-dimethyl-1,10-phenanthroline and its corresponding distorted square-planar gold(III) complex 1.18 (Fig. 8.1). The complex on biological screening revealed its efficient ct-DNA binding efficiency and cytotoxicity towards a panel of human cancer cell lines than the commonly used chemotherapy agent cisplatin [7]. M. Serratrice and coworkers synthesized a variety of mononuclear (1.19–1.24) and binuclear (1.25–1.28) gold(III) and gold(I) complexes derived from 2-(20 -pyridyl)benzimidazole (Fig. 8.2) and screened them for their antiproliferative properties on a A2780 ovarian cancer cell line. The biological properties of the studied compounds were promising both in terms of significant stability under physiological like conditions and of antiproliferative potency effects. Remarkably, all tested compounds turned out to cause relevant growth inhibition of two representative ovarian cancer cell lines with IC50 values falling in the low micromolar and # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_8

81

82

8

Classes of Gold Complexes

+ BF4−

N

N Au Cl

Cl

(1.18) Fig. 8.1 Structure of Au(III) complex 1.18 with interesting anticancer activity +

N

N X

Au

H N

N

X

(1.21)

(1.22)

Cl

Cl

PPh3

Au N

N Au

(1.25)

X

Au

L = PPh3 (1.23), TPA (1.24)

+ − PF6

Au N

N

N

Cl

Au L

Ph3P

Cl

X= Cl (1.19), OAc (1.20)

N

Au

Au

PPh3 Au

X = Cl (1.26), OAc (1.27)

+ − PF6

N

N N

N

X

N

N

N

N

N

H N

N

− PF6

Au Ph3P (1.28)

Fig. 8.2 Structures of binuclear gold(III) and gold(I) complexes derived from 2-(20 -pyridyl) benzimidazole (1.19–1.28), screened for their antiproliferative properties on a A2780 ovarian cancer cell line

even nanomolar range. In addition, most compounds turned out to overcome resistance to cisplatin to a great extent [8]. Che and coworkers thoroughly studied the cytotoxic mechanisms of gold(III) porphyrins which revealed that the HONE1 cells on treatment with gold(III) porphyrin presented clear indications of apoptosis after 24 h incubation. Furthermore, functional proteomic studies presented characteristic changes in the expression of some cytosolic proteins on treatment with gold(III) porphyrin. The affected proteins primarily involved enzymes participating in energy production processes and proteins involved in the cellular redox balance. Interestingly, on treatment with gold(III) porphyrin, a rapid decrease of the mitochondrial membrane potential was detected with clear changes in Bcl-2 family proteins, and release of both cytochrome c and apoptosis-inducing factor (AIF) where cytochrome c activates both caspase-9 and caspase-3. Further studies exposed that the reactive oxygen species (ROS) play a significant role in the induction of apoptosis by regulating the mitochondrial

8.1 Complexes with Nitrogen Donor Ligands

N

83

N

N

N

Au (III)

Au (III) n(H2C)H3C N

N

N

N

N

N

N

N (CH2)nCH3

n(H2C)H3C

N

N (CH2)nCH3

(1.30)

(1.29)

N

N

N N

N Au (III) N

N

(1.31) Fig. 8.3 Structures of Au(III) complexes (1.29–1.31) used as fluorescent thiol probes as well as anticancer agents

membrane potential. These results clearly documented that gold(III) porphyrin is able to induce apoptosis through both caspase-dependent and caspase-independent mitochondrial pathways; intracellular oxidation also contributes to gold(III) porphyrin induced apoptosis. Thus, gold(III) porphyrin clearly emerged from these studies as a promising anticancer drug lead and a novel therapeutic agent directed towards mitochondria [9]. In another study, Che and coworkers came up with a new class of Au(III) complexes having NHC ligands that can be used as fluorescent thiol probes as well as anticancer agents (Fig. 8.3). These Au(III) complexes (1.29–1.31), through the formation of Au(I)–NHC upon reduction, can inhibit cellular TrxR activity and are cytotoxic to several cancer cell lines. Furthermore, the in vivo studies exposed that the complex 1.31 significantly suppressed the tumor growth in mice bearing HeLa xenografts (76% inhibition) with no mouse death or body weight loss [10]. C. Abbehausen et al. synthesized a new gold(I) complex 1.32 (Fig. 8.4) from 2-mercaptothiazoline (MTZ) having coordination formula [AuCN(C3H5NS2)] which was thoroughly characterized by several chemical and spectroscopic methods, DFT studies, and biological assays. Spectral techniques like infrared (IR) and 15N nuclear magnetic resonance (NMR) spectroscopic measurements confirmed the coordination of the ligand molecule to gold(I) via the nitrogen atom which was

84

8

Classes of Gold Complexes

S SH

N Au

N (1.32) Fig. 8.4 Molecular structure of Au–MTZ complex (1.32) screened for its antimicrobial and anticancer potential +

R

x

R'

1.33 H -

1.34 CO2 Na+

N N

Au

Cl-1

N

1.35 1.36

N

R

R

H SO3 Na+

1.37 H

R'

x

CH3

N+ I

-

CO2 Na+ -

C

+

C

SO3 Na+ NH3+Cl

C

CO2 Na

-

C

R

Fig. 8.5 Structures of gold(III) substituted porphyrins (1.33–1.37) evaluated for their in vitro cytotoxic activity

also backed by DFT studies confirming nitrogen coordination to gold(I) as a minimum of the potential energy surface with calculations of the Hessians showing no imaginary frequencies. The gold(I) complex with 2-mercaptothiazoline [Au– MTZ] is soluble in dimethyl sulfoxide (DMSO) and is insoluble in water, methanol, ethanol, acetonitrile, and hexane. This complex besides showing interesting antibacterial activity was found to be cytotoxic to HeLa cells obtained from human cervical adenocarcinoma, inducing 85% of cell death at a concentration of 2.0 μmolL1 [11]. In a study by L. Sun and coworkers, a series of new substituted gold(III) tetraarylporphyrins 1.33–1.37 (Fig. 8.5) with aqueous solubility were synthesized and evaluated for their in vitro cytotoxic activity against sarcoma 180 mouse tumor and SGC-7901 human gastric cancer cell line panel. Compound 1.34 showed extensive growth inhibitory properties against sarcoma 180 mouse tumor and SGC-7901 human gastric cancer cell examined and displayed IC50 values 200 μmol L-1 Table 8.2) in the three tumor cell lines, and neither in the normal cell lines. The IC50 values of the gold(III) complexes displayed lower values suggesting that the complexes are more efficient antitumor agents than the free ligands. Complex 1.46 was selected for DNA fragmentation and cell cycle analysis. Spectroscopic titration with calf-thymus DNA (CT DNA) showed that the complexes can bind weakly to CT DNA, probably by an external contact

8.1 Complexes with Nitrogen Donor Ligands

87

Cl

Cl HN

Cl

Au

N

Cl

O

Cl

Au

N

O

CH3 N

O

F

Cl OH

N

OH

N F

H3C

CH3

O

O

(1.46)

(1.45)

Cl

Cl

NH2 O

F

OH

Au HN

O

N

Cl

F

H3C H3C (1.47)

Fig. 8.8 Structures of biologically active gold(III) complexes (1.45–1.47) Table 8.2 Complex concentration (μmolL1) that produces 50% reduction in cellular viability (IC50) for gold(III) complexes (1.45–1.47)

Complex 1.45 1.46 1.47

IC50(μM) Normal Cells MRC-5 65  13 53  13 126  13

L919 45  4 106  21 65  4

Tumor Cells A20 25  8 49  8 48.3  0.3

B16-F10 27  15 29  9 45  7

K562 55  10 50  10 61  4

(electrostatic or groove binding). The complexes exhibited good binding propensity to bovine serum albumin (BSA) having relatively high binding constant values [15]. In a study carried by Jen-Fu Chiu, a gold(III) porphyrin complex 1.48 (Fig. 8.9) was prepared and was explored for its cellular pharmacological potential. The cytotoxicity study of gold(III) porphyrin complex 1.48 by naphthol blue black (NBB) staining assay confirmed that the higher cytotoxicity of gold(III) porphyrin complex 1.48 was not related to its photosensitizing activity. Serum dependent test revealed that serum proteins displayed lesser effects on the activity of gold(III) porphyrin complex 1.48. In addition, in vivo and in vitro binding assays displayed that gold(III) porphyrin complex 1.48 acted on DNA noncovalently, which was different from cisplatin. Flow cytometric study showed that this complex inhibited cell growth partly through abrogating cell cycle at G0–G1, and induced apoptosis in human nasopharyngeal carcinoma (NPC) cell line (SUNE1) cells. The improved

88

8

Classes of Gold Complexes

Fig. 8.9 Structure of gold (III) porphyrin complex (1.48)

N

N Au N

N

Cl

(1.48) Fig. 8.10 Molecular structure of the chloro glycylhistidinate gold(III) complex (1.49)

Table 8.3 IC50 values (μM) of chloro glycylhistidinate gold(III) complex (1.49) against the human ovarian carcinoma A2780 cell lines sensitive or resistant to cisplatin expressed as mean  S.E. of at least three determinations or mean of two determinations Compound 1.49 Cisplatin

Cell line A2780:S 5.2  1.63 1.6  0.58

A2780:R 8.5  2.3 16.1  3.85

Resistance ratio 1.63 10.0

expression of p53, a cell cycle-controlling and apoptosis-related protein, further confirmed that the cell cycle arrest and apoptosis induced by this gold porphyrin complex were p53 dependent. The results showed the potential of gold(III) porphyrin complex 1.48 as a promising anticancer drug [16]. Pierluigi Orioli and coworkers carried out the cytotoxic and DNA binding studies of a chloro glycylhistidinate gold(III) complex 1.49 (Fig. 8.10). The cytotoxicity of the complex 1.49 was assessed against the human ovarian carcinoma A2780 cell line either sensitive (A2780:S) or resistant (A2780:R) to cisplatin. The cytotoxicity of cisplatin on the same cell lines was measured only for comparison purposes. It was clearly established that the selected complex displays a relevant cytotoxic activity on both cell lines: IC50 values fall in the micromolar range and are comparable with

8.1 Complexes with Nitrogen Donor Ligands

89

O

+

H N N

N

Cl

−

N

Au

N

NH

N

+

Br

Cl

NH

−

Cl

−

N

N Au

Au Cl

−

(1.51) +

Cl

N

Cl

Cl

Cl

(1.50)

N

N Au

Cl

Cl

+

Cl

Cl

(1.52)

Cl (1.53)

Fig. 8.11 Molecular structures of gold(III) complexes (1.50–1.53)

those of cisplatin as given in Table 8.3. Remarkably, the complex 1.49 holds a high cytotoxicity towards the cisplatin-resistant cell line: the resistance ratio (i.e., IC50 of cisplatin on resistant cells: IC50 of cisplatin on sensitive parental cells) is in fact equal to 1.63 and 10.0 for the selected complex and cisplatin, respectively [17]. A series of square-planar gold(III) complexes of 2,20 -dipyridylamine (1.50), di (2-pyridyl)ketone (1.51), 2-(4-chlorophenyl)-1H-imidazo[4,5-f] [1, 10] phenanthroline (1.52), and 2-(4-bromophenyl)-1H-imidazo[4,5-f] [1, 10] phenanthroline (1.53) of type [Au(L)Cl2].Cl (Fig. 8.11) were synthesized and thoroughly characterized by using various spectral techniques. All the obtained complexes were monoionic in nature and formed by coordination through N of different neutral bidentate ligands. The UV–Visible spectra of complexes in the DMSO and buffer revealed that the complexes convert into their dihydroxy form, i.e. [Au(L)(OH)2]+ in the physiological condition. Also, the dihydroxy chromophore was stable for 24 h under the physiological conditions. The obtained complexes displayed significant cytotoxic properties in vitro for brine shrimp lethality bioassay.

90

8

HN

Cl

Cl

N O

AcO

N Au Cl

Classes of Gold Complexes

AcO

H N

N

N Au OAc

OAc

(1.54)

(1.55)

Fig. 8.12 Structures of gold(I)–chloroquine complexes (1.54 and 1.55) Table 8.4 GI50 values of new gold(I)–chloroquine complexes (1.54 and 1.56) against six tumor cell lines Compounds 1.54 1.55 1.56 Chloroquine Cisplatin

Cell Lines PC-3 15 20 9.4 20 >30

MCF-7 >30 >30 – >30 26

SKBR-3 30 >30 3.6 >30 6

HT-29 20 21 0.03 23 >30

LoVo 5 6 – 10 >30

B16/BL6 24 24 0.04 28 25

The obtained metal complexes were examined for series of DNA binding activity using UV–Visible absorption titration, hydrodynamic measurement, and thermal DNA denaturation study. The nucleolytic activity was done on plasmid pUC19 DNA and the Michaelis–Menten kinetic studies were performed to appraise the rate of enhancement in metal complexes mediated DNA cleavage over the noncatalyzed DNA cleavage [18]. Maribel Navarro and coworkers synthesized two new gold(I)–chloroquine complexes (1.54 and 1.55; Fig. 8.12) and their most feasible structures were established by various spectroscopic and analytical techniques. Their interaction with two important targets of action, DNA and thioredoxin reductase (TrxR), were investigated. These studies showed that complexes 1.54 and 1.55 displayed two types of interaction with DNA, covalent binding through the metal center, and additionally a non-covalent interaction that is electrostatic in the case of complex 1.54, but intercalative for complex 1.55, which is similar to that displayed by free chloroquine. The experimental data indicated that these gold–chloroquine complexes also possess the ability to inhibit TrxR. These results led to test their cytotoxicity against 6 tumor cell lines. The complexes displayed cytotoxic activity against the six selected human tumor cell lines including PC-3, MCF-7, SKBR-3, HT-29, LoVo, and B16/BL6 lines. Both complexes 1.54 and 1.55 exerted some degree of growth inhibition on four of the tumor cell lines and total growth inhibition on the HT-29 and LoVo lines at concentrations below 30 μM. Their general activity

8.1 Complexes with Nitrogen Donor Ligands

91

Fig. 8.13 The molecular structure of complex 1.57 along with the atomic numbering scheme. The displacement ellipsoids are drawn at the 50% probability level. Adapted with permission from the publisher

was greater than that shown by CQ and cisplatin control compounds, but not as marked as the already reported lead compound ([Au(CQ)(PPh3)]PF6; CQ is Chloroquine) 1.56, which was included here for comparison. The obtained results are shown in Table 8.4. These finding suggest that gold(I)–CQ compounds, particularly [Au(chloroquine)(PPh3)]PF6, are promising chemotherapeutic alternatives in the search of promising anticancer agents [19]. Anvarhusein A. Isab and coworkers reported the synthesis of three gold(III) complexes, [Au(npen)Cl2]Cl-2H2O (1.39), [Au(npen)2]Cl3 (1.40), and [Au(TPP)] Cl (1.41) (npen ¼ meso-1,2-di(1-naphthyl)-1,2-diaminoethane, TPP ¼ mesotetraphenylporphyrin) and characterized them using several techniques like elemental analysis, IR and NMR spectroscopy, and one of them complex 1.57 was characterized by X-ray crystallography (Fig. 8.13). The structure of complex 1.57 consisted of a [Au(npen)Cl2] complex ion, a chloride counter ion, and two water molecules. The gold atom in the complex ion adopted a distorted square-planar geometry. The interactions of 1.57 and 1.58 with L-tyrosine, glutathione, and lysozyme were studied electrochemically. The electrochemical measurements indicated that gold(III) remained stable and did not undergo reduction upon interaction with proteins. The in vitro cytotoxic potential of all the obtained complexes as well as of cisplatin was assessed on three human cancer cell lines, A549 (lung cancer cells), MCF7 (breast cancer cells), and HCT15 (colon cancer cells) using MTT assay. The results indicated that the prepared gold(III) complexes were more potent than cisplatin in inhibiting the growth of the selected cancer cells. The IC50 results

92 Table 8.5 IC50 values (in μM) of gold(III) complexes (1.57–1.59) for different cell lines

8

Compound 1.57 1.58 1.59 Cisplatin

IC50(μM) A549 28.76  4.02 10.34  2.82 5.13  1.17 42.88  1.99

Classes of Gold Complexes

MCF7 11.85  2.60 7.77  1.82 3.36  0.72 23.12  3.78

HCT15 9.40  1.93 4.23  1.31 3.50  0.73 23.12  3.78

Fig. 8.14 Molecular structure of gold(III) bis (thiosemicarbazonate) complexes (1.60–1.62)

Cl

N N R

N H

S

Au

N N S

N H

R

R = H (1.60), Me (1.61), Cyclohexane (1.62)

confirmed that complex 1.59 was the most effective antiproliferative agent as given in Table 8.5 [20]. Ana María Pizarro and coworkers in 2018 reported the synthesis and characterization of three gold(III) complexes (Fig. 8.14) of the general formula [Au(III)L]Cl, where L is benzil bis(thiosemicarbazonate) (1.60), benzil bis(4-methyl-3thiosemicarbazonate) (1.61), or benzil bis(4-cyclohexyl-3-thiosemicarbazonate) (1.62). X-ray crystal structure of complex 1.62 (Fig. 8.15) revealed a square-planar geometry around the gold(III) center. All the three synthesized gold(III) complexes (1.60–1.62) were evaluated for their cytotoxicity in comparison to cisplatin (CDDP) against three cell lines of breast cancer: the estrogen receptor positive, progesterone receptor positive, and HER2 negative MCF7 cell line, the triple-negative MDA-MB231 cell line, and normal breast epithelial immortalized MCF10A cells. Sulforhodamine B (SRB) assay was performed to determine the IC50 values from the cell viability dose response curves obtained after 24 h drug treatment (Fig. 8.16). In general, gold(III) complexes were more active in MCF10A cells. These complexes displayed similar cytotoxicity against the three selected cell lines following the order 1.62 > 1.60 > 1.61. Among all the three tested complexes, complex 1.62 was the most potent having similar activity profile to CDDP in all the three cell lines (IC50 of CDDP are 8.9 μM (MCF7), 7.1 μM (MDA-MB-231), and 4.8 μM (MCF10A)). The cytotoxicity of the three gold(III) compounds against MCF7 cells displays significant deviations from one another. Based on the obtained data the MCF7 cell line was selected to carry out further studies in order to elucidate the root cause behind the observed differences in cytotoxicity and how it is related to the gold (III) compounds’ mechanism to trigger cell death. Despite the promising results of the obtained complexes, the cytotoxic results in MCF cells were compared to the free ligand activity. The results obtained indicated that cell viability inhibition is not

8.1 Complexes with Nitrogen Donor Ligands

93

Fig. 8.15 ORTEP representation (thermal ellipsoids drawn at 50% probability level), including atom numbering scheme, of one of the cations and one chloride present in the asymmetric unit of complex 1.62. Hydrogen atoms, except those of the amine groups, have been omitted for clarity. Adapted with permission from the publisher NA (>> 50) 60

52.7

IC50 (µ µM)

50 40

37.9

34.6

30 17.4

20

10.9

8.5

10

3.5

2.5

MCF–7

MDA-MB–231

1.62

1.61

1.60

1.62

1.61

1.60

1.62

1.61

1.60

0

MCF10A

Fig. 8.16 Antiproliferative activity of gold(III) complexes (1.60–1.62). Cell viability determined in MCF7, MDA-MB-23, and MCF10A cells after 24 h of exposure to complexes 1.60, 1.61, and 1.62 and allowed to recover for 72 h. Cisplatin was used as positive control (IC50 ¼ 8.9 μM for MCF7, 7.1 μM for MDA-MB-231, and 4.8 mM for MCF10A cells). NA; not applicable

related to that of their free ligands as their cytotoxic activity does not correlate and is indeed the opposite of that of the corresponding metal complexes. Complex 1.60 displayed moderate cytotoxicity and accumulation in MCF7 breast cancer cells but did not inhibit thioredoxin reductase (TrxR) activity and did not induce reactive

94

8

+

R2

R1

N

N O

Classes of Gold Complexes

H2N

Au Cl

Cl

R1 = H, R2 = H (1.63) R1 = H, R2 = Cl (1.64)

Cl

− N Au Cl Cl Cl

Au Cl

Cl

(1.65)

(1.66)

Fig. 8.17 Structures of gold(III) complexes (1.63–1.66) synthesized and studied in the present work Table 8.6 Antiproliferative IC50 (μM) values of the complexes (1.63–1.66) after a 96 h treatment of four human cancer cell lines (averages and standard deviations of at least 3 independent determinations except where noted) Complex 1.63 1.64 1.65 1.66 Cisplatin

IC50 (μM)  SD A427 0.37  0.17 0.19  0.09 0.23 Nd 1.96  1.54

LcLC-103 h 0.26  0.16 0.22 0.29 >10 0.90  0.19

SISO 0.31  0.08 0.40  0.28 0.73  0.43 >10 0.24  0.06

5637 0.23  0.22 0.06  0.02 0.26  0.22 >10 0.35  0.10

oxygen species (ROS) production. Complex 1.61, the least cytotoxic, was found to be capable of modestly inhibiting TrxR activity and produced low levels of ROS in the MCF7 cell line. The most cytotoxic complex 1.62 had the highest cellular accumulation and its distribution pattern showed a clear preference for the cytosol and mitochondria of MCF7 cells. It readily hampered intracellular TrxR activity leading to a dramatic alteration of the cellular redox state and to the induction of cell death [21]. A series of gold(III) complexes (1.63–1.66) (Fig. 8.17) with hydroxyquinoline, aminoquinoline, and quinoline ligand were synthesized and the chemical and biological characterization of these novel gold(II) complexes was reported. All the complexes were evaluated as prospective anticancer agents. The antiproliferative activity of these gold(III) complexes was assessed on a panel of selected human tumor cell lines including A427 (lung cancer cell line), LCLC-103H (large cell lung cancer), SISO (uterine adenocarcinoma), and 5637 (human bladder carcinoma) by using the crystal violet staining assay. The obtained results were indicative of the fact that three out of the four complexes were more active than cisplatin. In particular, the 8-hydroxyquinoline substituted ligand complexes (1.63, 1.64) and complex 1.65 showed the highest antiproliferative activity in all studied cell lines (Table 8.6). DNA interaction assays with the supercoiled pBR322 plasmid DNA have

8.1 Complexes with Nitrogen Donor Ligands

95

Fig. 8.18 Structures of gold (III) complexes (1.67–1.71)

O

PF6

RO N Au N RO

Cl Cl

O R = n-Bu (1.67), n-Pe (1.68), i-Bu (1.69) i-Am (1.70), cPe (1.71) Table 8.7 IC50 (μM) concentrations of gold(III) complexes (1.67–1.71) against HeLa, Fem-x, K562, and MRC-5 cells Complex 1.67 1.68 1.69 1.70 1.71 Cisplatin

IC50 (μM) HeLa 2.07  0.15 1.61  0.11 1.99  0.81 2.14  0.76 1.72  0.67 2.10  0.20

Fem-X 2.01  0.31 1.39  0.08 1.78  0.35 1.35  0.23 1.57  0.24 5.51  0.31

K562 2.97  0.35 1.45  0.83 4.41  0.42 5.01  0.15 2.67  0.33 5.54  1.03

MRC-5 23.15  0.85 84.46  0.24 56.77  1.26 97.88  1.75 61.55  2.35 14.21  1.54

demonstrated that the interactions of these gold(III) complexes with DNA are generally weak. On the other hand, a remarkable reactivity was detected between these complexes and the model protein cyt c with formation of various kinds of adducts. These results suggest that the cellular effects produced by these gold compounds may be predominantly mediated by interaction with relevant protein targets [22]. nd not determined A series of five novel gold(III) complexes (1.67–1.71) (Fig. 8.18) of general formulas [AuCl2{(S,S)-R2eddip}]PF6, ((S,S)-eddip ¼ (S,S)-ethylenediamine-N, N´-di-2-propanoate, R ¼ n-Bu, n-Pe, i-Bu, i-Am, cPe; (1.67–1.71), respectively) were synthesized and characterized by UV/Vis, IR and NMR spectroscopy, and mass spectrometry. In vitro anticancer activity of the gold(III) complexes against human cervix adenocarcinoma HeLa, human myelogenous leukemia K562, human melanoma Fem-x tumor cell lines, as well as against non-cancerous human embryonic lung fibroblast cell line MRC-5 was determined using MTT assay. The results of in vitro cytotoxic activity are expressed as IC50 (the concentration of compound (in μM) that inhibits a proliferation rate of the tumor cells by 50% as compared to control untreated cells) and are given in Table 8.7. Among all the synthesized complexes, complex 1.70 displayed highest activity and selectivity (IC50(Fem-x) ¼ 1.3  0.2; IC50(MRC-5)/IC50(Fem-x) ¼ 72.5  12.4), 4 times

96

8

Classes of Gold Complexes +

+

−

PF6

+

N

N

N

Cl

Cl (1.72)

Cl

N

N

Au

N

Cl (1.73)

−

PF6

N N

N

N

+

−

PF6

N

Au

−

PF6

N

Au Cl

N

Cl (1.74)

Au Cl

N

Cl (1.75)

Fig. 8.19 Structures of new gold(III) complexes (1.72–1.75)

more active and 28 times more selective than cisplatin. Complexes induced apoptotic mode of cell death in a time-dependent manner in HeLa cells [23]. Gold(III) complexes have been identified as anticancer agents because of their structural and electronic resemblances with currently used platinum(II) compounds. An additional benefit of gold(III) derived complexes is the ability to overcome the cisplatin resistance. Allyn C. Ontko and coworkers reported the synthesis of four new gold(III) complexes (1.72–1.75) (Fig. 8.19), [Au(Phen)Cl2]PF6 (1.72), [Au (DPQ)Cl2]PF6 (1.73), [Au(DPPZ)Cl2]PF6 (1.74), and [Au(DPQC)Cl2]PF6 (1.75) (Phen ¼ 1,10-phenanthroline, DPQ ¼ dipyrido[3,2-d:20 ,30 -f]quinoxaline, DPPZ ¼ dipyrido[3,2-a:20 ,30 -c] phenazine, DPQC ¼ dipyrido[3,2-d:20 ,30 -f] cyclohexylquinoxaline) that displayed anticancer activity in both cisplatin-sensitive and cisplatin-resistant ovarian cancer cells. Among the synthesized complexes, two complexes 1.73 and 1.74, exhibited excellent anticancer activity and were the focus of more intensive mechanistic study. At the molecular level, complexes 1.73 and 1.74 formed DNA adducts, generated free radicals, and upregulated proapoptotic signaling molecules (p53, caspases, PARP, death effectors). Taken together, these two novel gold(III) polypyridyl complexes (1.73, 1.74) displayed potent antitumor activity in cisplatin-resistant cancer cells. These activities may be mediated, in part, by the activation of apoptotic signaling [24]. Luigi Messori and coworkers reported the synthesis of a series of new gold(I) and gold(III) complexes based on the saccharinate (sac) ligand, namely M[Au(sac)2] (1.76–1.78) (with M being Na+, K+, or NH4+), [(PTA)Au(sac)] (1.79), K[Au (sac)3Cl] (1.80), and Na[Au(sac)4] (1.81), (Fig. 8.20) and investigated for some aspects of their biological profiles. Spectrophotometric analysis revealed that these gold compounds, upon dissolution in aqueous media, at physiological pH, manifest a rather favorable balance between stability and reactivity. Their reactions with the model proteins cytochrome c and lysozyme were monitored by mass spectrometry to predict their interactions with protein targets. In the case of disaccharinato gold (I) complexes, cytochrome c adducts bearing four coordinated gold(I) ions were preferentially formed in high yield. In contrast, complex 1.79 turned out to be poorly effective, only producing a mono-metalated adduct in very low amount. In turn, the gold(III) saccharinate derivatives were less reactive than their gold(I) analogs, 1.76

8.1 Complexes with Nitrogen Donor Ligands O

O

97

Na

N Au N S S O O O O

(1.77) O

O

N

NH4

N Au N S S O O O O

S O O

O

N S O O

N Au

P

N

N

(1.79)

(1.78)

O S O O N

K

N Au N S S O O O O

(1.76) O

O

O

O O

Au Cl

N O

K

S O

(1.80)

O S O N

N S O O O

Au

O O N

Na

S O N O O S O

(1.81)

Fig. 8.20 Structures of gold(I) and gold(III) complexes Table 8.8 Drug sensitivity profiles of cisplatinsensitive and cisplatinresistant human ovarian carcinoma cell lines (A2780S and A2780R) towards the study compounds

Complex 1.76 1.77 1.78 1.79 1.80 1.81 Cisplatin

IC50 (μM)  SD A2780/S 48.2  0.1 52.7  2.4 52.4  2.2 8.5  2.4 23.6  1.5 14.9  1.4 2.1  0.3

A2780/R 54.0  2.4 44.2  3.2 40.3  3.7 15.8  0.2 40.8  1.8 14.3  0.9 16.1  0.5

and 1.77 caused moderate protein metalation, again with evidence of formation of tetragold adducts. The antiproliferative effects of the synthesized complexes, expressed as IC50 values, were measured against the human ovarian carcinoma A2780S cell line sensitive to cisplatin and its cisplatin-resistant counterpart A2780R after a continuous exposure of 72 h (Table 8.8). Cell growth inhibition by the obtained complexes was checked through the SRB assay. In some cases (i.e.,

98

8 F

+

F

F

PF6 N O

Au

F

+

-

F

PF6

N

N

O

O

(1.82)

Au

Classes of Gold Complexes

-

F

N

O

O

N O

Au

F

F

-

O

+

PF6

N

N

O

O

(1.85)

Au

-

N

(1.84)

+

PF6

+

PF6

N

(1.83) F

F

Au

-

N O

(1.86)

Fig. 8.21 Structure of Schiff base ligand-based gold(III) complexes (1.82–1.86)

for the gold(III) compounds) the cytotoxic activity was slightly affected by the intrinsic cytotoxicity of DMSO but as the final DMSO concentration was always kept below 0.5%, its effect was generally modest and could be neglected. Among the evaluated complexes, the gold(I) complex 1.79 displayed the highest cytotoxic activity, although lesser than that of cisplatin, with an IC50 value of 8.5 μM against the cisplatin-sensitive cell line and a cytotoxic activity against the resistant cell line comparable to that of cisplatin. The gold(I) disaccharinate complex 1.76 displayed a very low cytotoxic activity with an IC50 value of about 50 μM against both cell lines. Finally, the tetrasaccharinato gold(III) derivative 1.81 showed a cytotoxic effect comparable to that of cisplatin in the A2780R, with an IC50 value of 14.3 μM, while compound 1.80 was by far least active. Since gold complexes, either in +3 or +1 oxidation states, are not known to be very reactive toward DNA and nucleobases, therefore it may be concluded that the observed antiproliferative effects mainly arise from DNA-independent mechanisms and precisely from the metalation of protein targets. In recent times, a number of mechanisms have been highlighted to account for the cytotoxic properties of gold(III, I) complexes involving, for instance, potent inhibition of the selenoenzyme thioredoxin reductase, proteasome inhibition, or direct antimitochondrial effects. Gold compounds seem to act as suitable prodrugs; in fact, they first undergo chemical activation, then they produce/deliver metal containing species capable of metalating the final biomolecular targets [25]. Wukun Liu and coworkers reported the synthesis of four Schiff base ligand-based gold(III) complexes (1.82–1.86; Fig. 8.21). The obtained complexes were characterized and evaluated for antitumor activity against hepatocellular carcinoma cells (HCC) (HepG2, SMMC-7721, and Hep3B) by MTT assay after 72 h incubation, with cisplatin and auranofin being the positive controls [26]. All complexes triggered noteworthy antiproliferative effects against HCC cells, particularly the most active complex 1.82 induced HepG2 cells apoptosis by initiating the

8.1 Complexes with Nitrogen Donor Ligands

99

endoplasmic reticulum stress (ERS). Complex 1.82 could clearly inhibit the TrxR activity to elevate reactive oxygen species (ROS), mediate ERS, and lead to mitochondrial dysfunction. Notably, treatment with 1.82 improved the CCl4-induced liver damage in vivo by downregulation of TrxR expression and inflammation level.

8.1.1

Complexes with Sulfur Donar Ligands

New metal-based complexes are synthesized with the aim of enhancing the cytotoxicity potential of anticancer drugs to get more success in cancer treatment. The lipophilicity and stability of metal complexes strongly hinges on the nature of the ligand systems. In this regard several suitable ligand systems have been developed and among them dithiocarbamate ligands have appeared as one of the ligand systems of choice for various applications in medicine. Positive outcomes were obtained as metal complexes of dithiocarbamate ligands have the ability of modulating key proteins engaged in biological processes such as apoptosis, transcription, oxidative stress, and degradation. Numerous gold dithiocarbamate coordinated complexes have been designed to display potential chemoprotective function and facilitate the transport of metal ions to their active sites [27]. In this regard, huge interest in a new class of gold(III) complexes that contain dithiocarbamate ligands has developed. Fregona and coworkers reported the synthesis of some remarkably active gold (I) and gold(III) dithiocarbamate complexes containing N,Ndimethyldithiocarbamate (DMDT) and ethylsarcosinedithiocarbamate (ESDT) demonstrating promising chemical and biological profile. In particular gold(III) derivatives of DMDT and ESDT have been proved to be much more cytotoxic in vitro than cisplatin, with IC50 values about onefold to fourfold lower than that of the reference drug, even toward human tumor cell lines naturally resistant to cisplatin itself [28]. Similarly, significant inhibitory effect of dibromo(N,Ndimethyldithiocarbamato) gold(III) in vivo was reported against growth of MDA-MB-231 breast tumor cells (BTC). Cell-free proteasome activity assay using a purified 20S proteasome and compounds offers direct indication for proteasome inhibition by the gold complexes [29]. Gold(I)-dithiocarbamato complexes have showed promising potential to inhibit the A549 (lung cancer), MCF7 (breast cancer), HeLa (cervical cancer), and chymotrypsin-like activity of purified 20S proteasome and 26S proteasome in human breast cancer MDA-MB-231 cells [30]. A series of six dialkyl/diaryldithiocarbamato gold(III) complexes 1.87–1.92 (Fig. 8.22) were synthesized and characterized by various spectral techniques like elemental analysis, IR and NMR spectroscopy, and single crystal X-ray analysis. The complexes were evaluated for their anticancer potential against MCF7, A2780, and A2780R human cancer cell lines as well as against a healthy MRC5 cell line. Among the complexes, the most promising results were obtained for complex 1.90, a cytotoxicity similar to cisplatin against A2780 cells (EC50  9.5 μM vs. EC50  10 μM), and significantly higher cytotoxicity as compared to cisplatin on A2780R and MCF7 cell lines, with EC50  2.8 μM vs. EC50  21 μM, and

100

8

S N

+

S N

Au S

Classes of Gold Complexes

Cl

S

−

N S

S (1.87)

C2H5 N C2H5

S

Cl

(1.88) C2H5 N C2H5

S Au

S

S

+

Cl

−

C2H5 N C2H5

(1.89) C2H7 N C2H7

Cl Au

S

Cl Au

S

Cl

(1.90) C2H5 N C2H5

S Au

S

S

S

+

Cl

−

C2H7 N C2H7

(1.91)

S

Cl Au

S

Cl

(1.92)

Fig. 8.22 Molecular structures of gold(I) complexes (1.87–1.92) displaying significant activity than the other tested compounds +

N

R

N N Cl

Au

S

Cl

−

NH

R = 2-Cl (1.93), 3-Cl (1.94), 4-Cl (1.95), 3-NO2 (1.96) , 4-OCH3 (1.97), 3,4-OCH2O (1.98) 4-CH3 (1.99), H (2.0) Fig. 8.23 Structure of gold(III) complexes (1.93–2.0)

EC50  2.2 μM vs. EC50 > 50 μM, respectively. Furthermore, the interactions of the representative complexes 1.89 and 1.90 with a mixture of physiological levels of L-cysteine (Cys) and reduced L-glutathione (GSH) were also studied [31]. A series of eight new gold(III) complexes (1.93–2.0; Fig. 8.23) from 5-aryl-3(pyridin-2-yl)-4,5-dihydropyrazole-1-carbothioamide were synthesized and characterized by various spectral techniques. All the synthesized complexes were evaluated for their cytotoxic potential against HeLa and A549 cell lines tested by MTT assay. The cytotoxic effects of the complexes against the tested carcinoma cell lines with a lower IC50 value are given in Table 8.9. The results indicate that the complexes exert cytotoxic effects against HeLa and A549 cell lines. Moreover, the

8.1 Complexes with Nitrogen Donor Ligands Table 8.9 The cytotoxicity of the complexes (1.93–2.0) against A549 and HeLa cell lines

IC50(μM) A549 4.66  0.07 36.23  1.04 7.18  0.22 34.21  0.38 10.04  0.39 12.69  0.30 12.73  0.13 29.22  0.26 51.18  3.21

Complex Cisplatin 1.93 1.94 1.95 1.96 1.97 1.98 1.99 2.0

OAc AcO AcO

101

OAc

OAc OAc

O OAc S

Au

O

O

N

AcO

N

OAc

O AcO

OAc

AcO OAc

(2.3)

N N

OAc

OAc OAc S Au

S Au

(2.2)

(2.1) OAc OAc O

HeLa 6.86  0.18 3.86  0.13 8.60  0.15 8.70  0.49 3.91  0.10 2.55  0.08 13.82  0.42 5.14  0.18 4.41  0.37

N N

O AcO OAc

O O AcO

OAc

O

S Au

N N

(2.4)

Fig. 8.24 Structures of gold(I) complexes (2.1–2.4) displaying significant activity than the other tested compounds

complexes 1.97 showed comparatively higher cytotoxicity than cisplatin and other complexes against HeLa cell line. It suggests that the substituent groups on benzene have significant effect on cytotoxicity [32]. N-heterocyclic carbenes (NHCs) and their derived metal complexes are extensively studied as a novel source of metallopharmaceuticals due to their high stability and ease of modification. Inspired by the imperative potential of these magnificent compounds, O. Dada and coworkers synthesized a series of gold(I) complexes (2.1–2.4; Fig. 8.24) from glucose, lactose, and galactose bearing phosphine and NHCs as stabilizing agents and screened them for their potential as apoptosisinducing anticancer drug candidates. All the four synthesized complexes were investigated at National Cancer Institute (NCI) against a range of cancer cell lines, and the results revealed that the complex 2.1 displayed better activity than the other tested compounds. The complex 2.1 contains thioglucose that has a significant impact on the cellular uptake and activity profiles of metallodrugs. Besides D-glucose can be substrates for GLUT transporters and can thus be considered as candidates for GLUT-targeting approach for the delivery of the drug candidates [33].

102

Br Br

8

CH3 O N

S Au

S

N H

O O

Br Br

(2.5)

Classes of Gold Complexes

CH3 O N

S

Au

S

N H

O O

(2.6)

Fig. 8.25 Structures of gold(III) complexes (2.5 and 2.6)

In order to enhance the chemotherapeutic index and widen the therapeutic spectrum of current anticancer drugs, two gold(III) complexes (2.5 and 2.6; Fig. 8.25) were designed as carrier-mediated delivery systems exploiting peptide transporters, which are upregulated in some cancers. The two complexes were selected and verified against human MDA-MB-231 (resistant to cisplatin) breast cancer cell cultures and xenografts, discovering the proteasome as a major target both in vitro and in vivo. 3% inhibition of breast tumor growth in mice was observed after 27 days of treatment at 1.0 mg kg1d1, compared to control. Remarkably, if only the most responsive mice are taken into account, 85% growth inhibition, with some animals showing tumor shrinkage, was observed after 13 days. These results led to file an international patent, recognizing this class of gold(III) peptidomimetics as suitable candidates for entering phase I clinical trials [34]. Similarly, Dolores Fregona reported gold(III)-dithiocarbamato (dtc) derivatives of oligopeptides as promising anticancer agents. A series of complexes [AuIIIX2(dtc-Sar-L-Ser(t-Bu)-O(t-Bu))] (X ¼ Br (2.7); Cl (2.8)), [AuIIIX2(dtc-AA-Aib2-O(t-Bu))] (AA ¼ Sar (sarcosine, N-methylglycine), X ¼ Br (2.9); Cl (2.10); AA ¼ D,L-Pro, X ¼ Br (2.11); Cl (2.12)), [AuIIIX2(dtc-Sar-Aib3-O(t-Bu))] (X ¼ Br (2.13); Cl (2.14)), and [AuIIIX2(-dtc-Sar-Aib3-Gly-OEt)] (X ¼ Br (2.15); Cl (2.16)) (Aib ¼ “alpha”-aminoisobutyric acid, 2-methylalanine) (Fig. 8.26) were designed to obtain metalbased peptidomimetics that may specifically target two peptide transporters (namely, PEPT1 and PEPT2) upregulated in several human tumor cells. All the complexes were characterized by various spectral techniques like FT-IR and mono- and multidimensional NMR spectroscopy. According to in vitro cytotoxicity investigations against Hodgkin’s lymphoma (L540), human androgen-resistant prostate cancer (PC3) cell lines, human ovarian adenocarcinoma (2008) cell line, and the parent cisplatin-resistant subclone (C13), complex (1.83b) turned out to be the most active complex toward these four selected human tumor cell lines, for which the IC50 values were lower than cisplatin. Exposure of L540 cells to increasing concentrations of complexes 2.7–2.16 produced significant inhibition of cell growth, with IC50 values (1.4–5.4 μM) similar to that recorded for cisplatin (2.5 μM). The gold(III) complexes 2.7–2.12 displayed cytotoxicity levels equal or even lower than cisplatin on both prostate cancer and ovarian adenocarcinoma cells, whereas the tetra- and penta-peptide derivatives 2.13–2.16 proved to be less effective. When tested against the cisplatin-resistant C13 subclone, the gold(III) complexes were proved to be much more active than the reference drug. Overall, the complex (2.12)

8.1 Complexes with Nitrogen Donor Ligands

S X Au S X

O O

N

S X Au S X

OC(CH3)3

N H

103

OC(CH3)3

X = Br (2.7), Cl (2.8)

O X X Au S S N

H N

OC(CH3)3 O

S X Au S X

N

N H

OC(CH3)3

O

O

N

N H

X = Br (2.11), Cl (2.12)

S X Au S X

O

H N

X = Br (2.9), Cl (2.10)

O N H

O

N

H N O

O N H

OC(CH3)3 O

X = Br (2.13), Cl (2.14)

O N H

H N O

O N H

H N O

O OC(CH3)3

X = Br (2.15), Cl (2.16)

Fig. 8.26 Molecular structures of gold(III)-dithiocarbamato complexes

turned out to be the most active on all the studied tumor cell lines, displaying IC50 values similar to those previously reported for the model complex [AuIIIBr2(dtc-SarAib-O(t-Bu))] [35]. Remarkably, it showed no cross-resistance with cisplatin itself and was proved to inhibit tumor cell proliferation by inducing almost exclusively late apoptosis/necrosis [36]. Six gold(III) bis(dithiolene) complexes (2.17–2.22; Fig. 8.27) were prepared and evaluated for their anticancer properties against A2780/A2780cisR ovarian cancer cells [37]. The IC50 values were calculated from dose–response curves using the colorimetric MTT assay. The results showed that at 48 h incubation the complexes exhibited high cytotoxic activity for both cancer cell lines with IC50 values ranging from 0.4 to 1.3 μM (A2780) and 0.08 to 2.0 μM (A2780cisR). Furthermore, the complexes were assessed against normal fibroblasts to govern their selectivity for cancer cells, the ovarian cancer cells used in this study. The results obtained displayed that the gold complexes were more active against the ovarian cancer cells than the normal fibroblasts, presenting selectivity indexes (SI) of 4–7 for 2.17–2.20, 2 for 2.21 and 2.22, and 2 for cisplatin and auranofin. Complex 2.17 also showed substantial antimicrobial activity against Staphylococcus aureus with MIC values of 12.1–3.9 μg mL1 and against Candida glabrata and Candida albicans with MICs of 9.7–2.7 and 19.9–2.4 μg mL1, respectively. In addition, all complexes showed antiplasmodial activity against the Plasmodium berghei parasite liver stages, even exhibiting better results than the ones obtained using primaquine, an anti-malarial drug. Mechanistic studies support the idea that thioredoxin reductase, but not DNA, is a possible target of these complexes. Complex 2.17 was found to be stable under biological conditions, which would be

104

S

8

N

S

S

S

Au

S

S

S

N

NEt4

S

S

Classes of Gold Complexes

N

S

S

S

Au

(2.17)

S

N S

S S

N S

S S

Au

S

S

N

PPh4

S

(2.20)

S

S N

S

S

PPh4

S

S

S

Au

S

(2.18)

HO

S

Au

S

S

PPh4

S

(2.21)

S

S

S

N

S

NEt4

S

S S

OH

(2.19)

S

Au

S

PPh4

S

(2.22)

Fig. 8.27 Structures of gold(III) bis(dithiolene) complexes (2.17–2.22) R S

O

Au Cl

1. N

R

−

N

S R

S

MeOH, 16 h

a

+ PF

6

-

S Au

N

2. NH4PF6, H2O

Cl

O (2.23a-e) (2.24a-e)

1. benzoylpyridine 2. benzylpyridine

N

N

R

N b

N c

N

N

Br d

N

N

O

e

Fig. 8.28 Structures of highly potent organometallic gold(III) complexes (2.23a–e and 2.24a–e)

important if this compound is ever to be considered as a drug. Overall, the results obtained demonstrated the promising biological activity of this complex, which might have potential as a novel anticancer, antimicrobial, and antiplasmodial agent to be used as an alternative to current therapeutics. A series of ten highly potent organometallic gold(III) complexes (2.23a–e and 2.24a–e; Fig. 8.28) from dithiocarbamate ligands were synthesized and characterized [38]. The structure of dithiocarbamates employed dictates the

8.2 Complexes with Phosphorus Donar Ligands

105

biological stability and cellular cytotoxicity. Most of the compounds exhibit 50% inhibitory concentration (IC50) in the low micromolar (0.50–2.9 μM) range when tested against a panel of aggressive cancer types (breast, ovarian, lung, and leukemia cancers) with considerable selectivity for cancer cells over normal cells. Consequently, there is great interest in the mechanism of action of gold chemotherapeutics, particularly, considering that DNA is not the major target of most gold complexes. The mechanism of action of representative complexes 2.23a and 2.24a in the recalcitrant triple-negative breast cancer (TNBC) cell line, MDA-MB-231 was investigated. Whole-cell transcriptomics sequencing revealed genes related to three major pathways, namely: cell cycle, organelle fission, and oxidative phosphorylation. 2.24a irreversibly and rapidly inhibits maximal respiration in TNBC with no effect on normal epithelial cells, implicating mitochondrial OXPHOS as a potential target. Furthermore, the modulation of cyclin-dependent kinases and G1 cell cycle arrest induced by these compounds is promising for the treatment of cancer. This work contributes to the need for mitochondrial respiration modulators in biomedical research and outlines a systematic approach to study the mechanism of action of metal-based agents.

8.2

Complexes with Phosphorus Donar Ligands

The increasing success of auranofin triggered the development of gold (phosphane) complexes as potential metallodrugs. Phosphanes are good donating agents and are readily attached to the gold ions. A highly effective gold phosphole complex 2.25 (Fig. 8.29) was found to be an efficient inhibitor of glutathione reductase and thioredoxin reductase. X-ray crystallography revealed that the ligands of the gold phosphole complex were replaced by the cystine residues of the enzyme [39]. Oliver Rackham and coworkers extensively studied the mechanism of anticancer activity of new bis-chelated Au(I) bidentate phosphine complexes 2.26–2.29 (Fig. 8.29). Bis-chelated Au(I) complexes are having tremendous anticancer potential but their use is limited due to their high toxicity and non-selectivity to cancer cells. The complex 2.26 displayed more efficiency at submicromolar concentrations and also selectively caused apoptosis in breast cancer cells but not in normal breast cells R=

R2 P N

Au N Cl (2.25)

R2 P

Au

PR2 PR2

N Cl (2.26) N (2.28)

(2.27) N (2.29)

Fig. 8.29 Structures of gold(I) complexes (2.25–2.29) with tremendous anticancer potential

106 Fig. 8.30 Molecular structures of the prepared gold (I) complexes 2.30–2.38 showing considerably higher in vitro anticancer activity

8 R1

O N

N R2

N

N Au P

Classes of Gold Complexes

2.30 = R1 = Ethyl R2 = H 2.31 = R1 = n-butyl R2 = H 2.32 = R1 = allyl R2 = H 2.33 = R1 = benzyl R2 = H 2.34 = R1 = phenethyl R2 = H 2.35 = R1 = Ethyl R2 = Cl 2.36 = R1 = n-butyl R2 = Cl 2.37 = R1 = allyl R2 = Cl 2.38 = R1 = benzyl R2 = Cl

through depolarization of mitochondrial potential and depletion of the glutathione pool and caspase-3 and caspase-9 activation. Some of the complexes effectively inhibited the activity of both thioredoxin and thioredoxin reductase in cancer cells, providing new insights into the mechanism of action of bis-chelated gold(I)– diphosphine complexes in the development of mitochondria-targeted chemotherapeutics [40]. Many triphenylphosphine gold complexes are outstanding therapeutic agents with high in vitro cytotoxicity and the property of inhibiting DNA and protein synthesis. Initial experiments have revealed that triethylphosphine gold chloride (TEPAu) may stimulate the peroxidative decomposition of cellular membrane lipids. Furthermore, preliminary studies have indicated that TEPAu is a strong cytotoxic compound in vitro against a range of cultured cell types and isolated hepatocytes. TEPAu caused a rapid loss of cell viability at concentrations above 25 μm which was significantly different from that of control by 60 min and complete by 180 min of incubation. TEPAu also depleted cells of reduced glutathione (GSH) and increased the formation of malondialdehyde (MDA) by 60 min. Incubation of cells with either of the antioxidants, N,N0 -diphenyl-p-phenylenediamine (DPPD) or promethazine blocked the formation of MDA but did not alter the time course of cell death or GSH depletion induced by TEPAu. TEPAu also caused a decrease in cellular NADPH and NADH by 10 min. TEPAu also caused a concentration-dependent decrease in cellular ATP and oxygen consumption in isolated rat hepatocytes. These data suggest that lipid peroxidation, as indicated by the formation of MDA, is probably not a major mechanism by which triethylphosphine gold complexes lethally injure cells. These data, therefore, suggest that mitochondria may be target organelles in TEPAu-induced toxicity to isolated rat hepatocytes. The changes that could be noticed in hepatocytes comprised of bleb formation in the plasma membrane, alterations in the mitochondrial morphology, and fast decreases in cellular ATP and oxygen utilization [41]. A series of gold(I) complexes (2.30–2.38; Fig. 8.30) of the general formula [Au (Ln)(PPh3)], where Ln represents mono- and di-substituted derivatives of hypoxanthine, were synthesized and fully characterized by several spectral techniques like high-resolution NMR spectroscopy and single crystal X-ray diffraction (Fig. 8.31) and were assessed for their cytotoxicity against a panel of nine human cancer cell lines (MCF7, HOS, THP-1, A549, G361, HeLa, A2780, A2780R (cisplatin resistant), and 22Rv1 (Table 8.10). Complexes 2.33, 2.34, and 2.35 showed a

8.2 Complexes with Phosphorus Donar Ligands

107

Fig. 8.31 X-ray crystal structure of complex 2.30 and 2.32 showing the atom numbering scheme. Non-hydrogen atoms are displayed as ellipsoids at the 50% probability level. Adapted with permission from the publisher

considerably higher in vitro anticancer activity against the used cancer cells, except for G-361, as compared with the normally used anticancer drug cisplatin, with IC50 < 1–30 μM [42]. Cationic tri- and tetra-nuclear gold(I) phosphine complexes (2.39–2.42; Fig. 8.32) [Au3(μ-dppen)3]X3 and [Au4(μ-dppa)4]X4 (X ¼ OTf, PF6) [OTf ¼ trifluoromethanesulfonate, dppen ¼ trans-1,2-bis(diphenylphosphino) ethene, dppa ¼ bis(diphenylphosphino)acetylene] have been reported to show interesting cytotoxicity against four different cancer cells [prostate (DU145), cervical (HeLa), breast (MDAMB-231), and fibro sarcoma (HT1080). The cytotoxicity test results indicated that these complexes own remarkable tumor cell growth inhibitory effects and high selectivity towards cancer cells than the standard drug cisplatin. The obtained IC50 values for all synthesized complexes were in the range of 0.04–4.65 μM with strong cytotoxicity towards fibro sarcoma cells. Furthermore, the possible antitumor mechanism of these complexes in fibro sarcoma cells (HT1080) was studied using wound healing assay, actin staining, flow-cytometry, and Hoechst nuclear staining. The treatment of cells with these complexes resulted in condensation of actin polymers and inhibition of migration potential. Flow-

Comp 2.30 2.31 2.32 2.33 2.34 2.35 2.36 2.37 2.38 Cisplatin

MCF7 15.2  0.7 27.1  0.2 12.4  1.0 3.66  0.4 6.30  0.7 5.23  0.6 >50 >25 >25 17.9  1.1

Cell line HOS 19.2  0.7 30.0  2.1 15.6  1.1 11.6  0.3 6.56  0.9 3.96  0.3 >50 18.8  0.7 16.8  0.3 20.5  0.1 THP-1 1.17  0.04 1.03  0.04 1.87  0.09 2.15  0.19 1.89  0.11 1.94  0.12 1.97  0.14 5.28  0.21 >10 –

A549 – – – 16.8  0.7 19.3  0.8 21.7  0.4 – – – >50

G-361 – – – 3.9  0.5 3.4  0.1 3.0  0.1 – – – 5.3  0.2

HeLa – – – 14.2  0.5 19.8  0.9 21.6  0.3 – – – >50

Table 8.10 In vitro cytotoxicity of complexes 2.30–2.38 and cisplatin against different cancer cell lines A2780 – – – 3.6  0.1 4.2  0.2 4.2  0.3 – – – 12.0  0.3

A2780R – – – 4.3  0.2 4.8  0.6 5.1  0.4 – – – 27.0  1.5

22Rv1 – – – 3.8  0.1 4.4  0.3 3.6  0.1 – – – 26.9  1.2

108 8 Classes of Gold Complexes

8.2 Complexes with Phosphorus Donar Ligands

Ph2HP

Au

Ph2 P

Ph2 P Au PPh2

Ph2P

Au

Au

P Ph2

Ph2P

109

X3

Ph2P Au H2PP Ph2P

X = OTf (2.39) X = PF6 (2.40)

Ph2 P PPh2

Au

P Ph2

X4

Au PPh2

X = OTf (2.41) X = PF6 (2.42)

Fig. 8.32 Structure of [Au3(μ-dppen)3]X3 (2.39, 2.40) and [Au4(μ-dppa)4]X4 (2.41, 2.42) [X ¼ OTf, PF6] complex

HO HO O

O OH O

O P

OH OH

Au P O O

OH

Fig. 8.33 Structure of the gold(I) complex 2.43 and its ORTEP diagram. Adapted with permission from the publisher

cytometry results indicated that the treatment resulted in cell cycle arrest at the G0/G1 phase. Moreover, the complexes could induce apoptosis in HT1080 cells through mitochondrial dysfunction and increased ROS production [43]. A new linear and hydrosoluble gold(I) complex 2.43 with tris(2-carboxyethylphosphine) was synthesized and its structure was solved by single crystal X-ray diffraction study (Fig. 8.33), which showed that the gold(I) atom coordinates linearly through the two phosphorus atoms of the phosphines. In vitro growth inhibition (GI50) in a panel of nine tumorigenic and one non-tumorigenic cell lines demonstrated the complex is highly selective to ovarium adenocarcinoma (OVCAR-3) with GI50 of 3.04 nmolmL1. Biophysical evaluation with the sulfurrich amino acids and proteins showed the compound does not interact with two types of zinc fingers, bovine serum albumin, N-acetyl-L-cysteine and also L-histidine, revealing to be inert to ligand substitution reactions with these molecules. The unique inert behavior of complex is interesting to the field and represents the

110

8

R2 P

Classes of Gold Complexes

PR 2

Au

Au

Cl

Cl

(2.44) Fig. 8.34 Gold(I) complexes containing bis(diphenylphosphino)alkanes

F Br (2.45)

F

F

PPh2

F Br

F F

F

Cl

(2.46)

F

Ph2 P

F Br

(2.48)

PPh2

Br

F

Br

F

Au

F Ph2 P Au

F

Au

+

F

F

F

F F

F

F

Br

PPh2 Au I

(2.47)

−

PF6

+

PPh2 Au Cl (2.49)

Ph2 P Au

−

PF6

Ph2 P

(2.50)

Fig. 8.35 Structures of mononuclear gold complexes (2.45–2.50)

modulation of the reactivity through coordination chemistry to decrease the toxicity associated with Au(I) complexes and its lack of specificity, generating very selective compounds [44]. A number of gold complexes [(AuCl)2(PR2-CH2CH2-PR`2)] (2.44; Fig. 8.34, R, R` ¼ substituted phenyl) were prepared and the antitumor activity of complexes evaluated against P388 leukemia cells was found to be higher than that for free ligands when R ¼ phenyl, cyclohexyl, p-fluorophenyl, m-fluorophenyl, p-(methylthio)phenyl, and o-(methylthio)phenyl. In two instances with 2-pyridyl and o-methoxyphenyl, free ligand showed no activity, whereas coordination to gold enhanced the antitumor activity of the resulting metal complexes. In contrast, for R ¼ 2-furyl and 2-thienyl, the free ligand was more active and coordination to gold diminished the antitumor activity of gold complexes against P388 leukemia cells [45]. S. K. Bhargava and coworkers in 2018 reported the synthesis of mononuclear gold complexes 2.45–2.50 (Fig. 8.35) containing the 2-BrC6F4PPh2 ligand, which were characterized by various spectral techniques and then evaluated for their

8.2 Complexes with Phosphorus Donar Ligands

111

anticancer activity against five human tumor [prostate (PC3), glioblastoma (U87MG), cervical (HeLa), fibro sarcoma (HT1080), ovarian (SKOV-3)] and normal human embryonic kidney (Hek-293 T) cell lines. Among the synthesized gold complexes, the cationic complexes 2.48 and 2.50 demonstrated higher cytotoxicity in comparison with cisplatin towards PC-3, HeLa, and U87MG cells and inhibited the thioredoxin reductase (TrxR) enzyme that is believed to be a potential target for newly synthesized compounds in cancer treatment. These cationic complexes get accumulated in higher amounts in the cancer cells than the neutral complexes, leading to higher cytotoxicity. The more physiologically appropriate tumor spheroid assay confirmed the superior potency of these gold phosphine complexes in inhibiting the growth of cervical carcinoma cell line HeLa (3D) spheroidal models. The mechanism of cell death was shown to be apoptotic cell death through cell cycle arrest, mitochondrial membrane depolarization, and increased ROS production as is evident from different imaging techniques including flow cytometric imaging as shown in Fig. 8.36 [46]. Basal like breast cancer (BLBC) is a very disparaging subtype of breast cancer with only a few chances of survival, against which cisplatin-based therapy is a compromise among the anticancer activity, the resistance development, and the severe side effects. With the aim of finding new anticancer agents alternative to cisplatin, a series of seven 2.51–2.57 gold(I) azolate/phosphane complexes (Fig. 8.37) containing N–Au–P or P–Au–Cl backbones were synthesized and evaluated in vitro by MTT assay at 24 h treatment against human MDA-MB-231, human mammary epithelial HMLE cells overexpressing FoxQ1, and murine A17 cells as models of BLBC. Among the synthesized complexes, only two compounds 2.51 and 2.52 were found very active and chosen for an in vivo study in A17 tumors transplanted in syngeneic mice, being able to decrease cell viability in a dosedependent manner with IC50 values at low μM concentrations, whereas compounds 2.53–2.57 failed in reducing the cell viability significantly even at the highest tested dose. Interestingly, the antitumor efficacy was confirmed also on both human MDA-MB-468 and HMLE/FoxQ1 BLBC cells, which resulted to be the most sensible cells to treatment with both gold compounds. The compounds proved to be more active than cisplatin, less nephrotoxic and generally more tolerated by the mice. This study also provides evidence that both gold(I) complexes inhibited the 19S proteasome-associated deubiquitinase USP14 and induced apoptosis, while the mechanism of action of the compounds depends also on their ability to downregulate key molecules governing cancer growth and progression, such as STAT3 and Cox-2 [47]. Neutral mononuclear or dinuclear gold(I) diphosphines (2.58–2.76; Fig. 8.38) from N-heterocyclic sources have been screened in vitro and in vivo for anticancer activity against HeLa, CoLo, Jurkat cancer cell lines, normal resting, and PHA (phytohemagglutinin) stimulated human lymphocytes. The antitumor activity of the acyclic gold(I) compounds is highly dependent on the substituents on the phosphorus atoms being highest for phenyl groups and lower for methyl groups. The activity of these compounds against selected cell lines is linked to the length of the carbon bridge between the two phosphorus atoms being highest with a bridge

8

Fig. 8.36 Flow cytometric analysis of the apoptotic and necrotic cells induced by complexes 2.45–2.50 and cisplatin. HeLa cells were stained with Annexin V-FITC and PI after 48 h of incubation with IC50 concentrations of the metal complexes. (LL: live; LR: early apoptotic; UR: late apoptotic; UL: necrotic). Adapted with permission from the publisher

112 Classes of Gold Complexes

8.2 Complexes with Phosphorus Donar Ligands N C

Cl Cl

113

N C

N N

N C N C

N

N

N

Au

N

Au

P

Au

P

P HO O

(2.52)

(2.51)

HO

O

P

O

HO

HO

Cl Au HO

(2.53)

N

O

Au

P

N

Cl P

Au OH O

P

Au Cl 3

(2.54)

(2.55)

(2.56)

(2.57)

Fig. 8.37 Molecular structures of gold(I) complexes (2.51–2.57)

(CH2)n

R R

R

P

P

Au

Au

N X Y Z

N X Y Z

R

2.58 2.59 2.60 2.61 2.62 2.63 2.64 2.65 2.66 2.67

R

n

X

Y

Z

Me Ph Ph Ph Me Ph Ph Ph Ph Me

2 3 5 6 2 2 3 5 6 2

CH CH CH CH N N N N N N

N N N N CH CH CH CH CH N

CH CH CH CH CH CH CH CH CH CH

2.68 2.69 2.70 2.71 2.72 2.73 2.74 2.75 2.76

R

n

X

Y

Z

Ph Ph Ph Ph Ph Ph Ph Ph Me

2 2 2 2 2 3 5 2 2

N N N N N N N N CH

N N N N CH CH CH CH CH

CH CH CH CH N N N N CH

Fig. 8.38 Structures of neutral mononuclear or dinuclear gold(I) diphosphines (2.58–2.76)

consisting of 5 or 6 carbons. Two compounds with the highest tumor specificities that contain dpppe and pyrazolate (a lipophilic compound) or 1,2,4-triazolate (a hydrophilic compound) induce an apoptotic cell death pathway and a maximum dose to Balb/C mice is tolerated [48]. In a study by Niu Huang and coworkers, the synthesis of a novel soluble bis-chelated gold(I)  diphosphine compound 2.77 with strong anticancer activity and low toxicity was reported (Fig. 8.39). The complex inhibited 50% of cell proliferation of both cell lines [non-small cell lung cancer (NSCLC) cells, NCI-H460, and gastric cancer cells, BGC-823] at a concentration of 0.5 μM and inhibited 80.38% of tumor growth of sarcoma S180 xenografts in vivo at a dose of 8 mg/kg without loss of body weight at the same time. Mechanistic studies revealed

114

8

N

P

N

P

Au

P

N

P

N

Cl

Classes of Gold Complexes

−

Fig. 8.39 Structure of a novel soluble bis-chelated gold(I)  diphosphine complex (2.77)

P Au Cl

P Au Br

(2.78)

(2.79)

P Au S C N

P Au C N (2.80)

P Au S C O

S (2.82)

S (2.83)

P Au S C N (2.81) NH2 P Au S C H NH2 (2.84)

Fig. 8.40 Structure of linear, “auranofin-like” gold(I) complexes (2.78–2.84)

that complex 2.77 specifically inhibits the enzymatic activity of thioredoxin reductase by binding to selenocysteine residue, without targeting other well-known selenol and thiol groups contained in biomolecules. Remarkably, in animal studies complex 2.77 was shown to be well tolerated even at the high dose of 8 mg/kg. The obtained results strongly suggest that complex 2.77 represents a promising candidate for the development of novel anticancer drugs [49]. In order to obtain effective and selective TrxR inhibitors, Cristina Marzano and coworkers in 2010 synthesized a series of linear, “auranofin-like” gold(I) complexes 2.78–2.84 (Fig. 8.40) all containing the [Au(PEt3)]+ synthon and the ligands: Cl, Br, cyanate, thiocyanate, ethylxanthate, diethyldithiocarbamate, and thiourea. Stability of the synthesized complexes was checked in reconstituting solvent DMSO and it was found to be stable for the time necessary before addition to the growth medium. All the compounds were studied for their cytotoxic properties against a panel of human tumor cell lines including lung (A549), colon (HCT-15), breast (MCF-7), and cervical (HeLa) cancers along with leukemia (HL60) and melanoma (A375). IC50 values, calculated from the dose–survival curves obtained after 72 h of drug treatment from the MTT test, are shown in Table 8.11. The synthesized phosphine gold(I) complexes proficiently inhibited cytosolic and mitochondrial TrxR at concentrations that did not affect the two related oxidoreductases glutathione reductase (GR) and glutathione peroxidase (GPx). The inhibitory effect of the redox proteins was also observed intracellularly in cancer cells pretreated with gold (I) complexes. Gold(I) compounds were found to induce antiproliferative effects towards several human cancer cells some of which were endowed with cisplatin or

8.2 Complexes with Phosphorus Donar Ligands

115

Table 8.11 Cytotoxicity of synthesized gold(I) complexes (2.78–2.84) Compound 2.78 2.79 2.80 2.81 2.82 2.83 2.84 Auronafin Cisplatin

IC50 (μM)  SD HL60 A549 0.52 0.85  0.13  0.01 0.62 1.21  0.06  0.54 0.19 0.41  0.07  0.06 0.21 0.71  0.12  0.02 0.19 1.22  1.21  0.96 0.52 0.33  0.06  0.47 2.84 3.01  1.29  1.01 0.23 0.75  0.11  0.05 4.56 29.21  1.13  1.92

Cl Au P

(2.85)

Cl Au P

(2.86)

MCF-7 0.84  0.01 1.44  0.41 0.45  0.12 0.65  0.11 1.63  0.58 0.41  0.21 3.28  1.12 0.98  0.32 19.04  1.51

Cl Au P

(2.87)

A375 0.78  0.31 1.02  1.00 0.21  0.23 0.37  0.29 1.47  0.77 0.13  0.27 2.23  1.03 0.34  0.21 2.37  1.23

HCT-15 0.97  0.17 1.08  0.82 0.08  0.01 0.32  0.02 0.92  0.01 0.61  0.13 2.75  1.43 0.11  0.02 20.34  1.31

HeLa 0.90  0.21 0.63  0.32 0.09  0.02 0.18  0.01 0.54  0.07 0.13  0.61 1.84  0.25 0.15  0.03 8.50  1.51

Cl Au P

(2.88)

Fig. 8.41 Structure of gold(I) phosphine complexes (Cl–Au–P(R)3, R ¼ Me, Et, tert-But, Ph) (2.85–2.88)

multidrug resistance. In addition, they were able to activate caspase-3 and induce apoptosis observed as nucleosome formation and sub-G1 cell accumulation. The complexes with thiocyanate and xanthate ligands were particularly effective in inhibiting thioredoxin reductase and inducing apoptosis. Pharmacodynamic studies in human ovarian cancer cells allowed for the correlation of intracellular drug accumulation with TrxR inhibition that leads to the induction of apoptosis via the mitochondrial pathway [50]. Gold(I) phosphine complexes have revealed promising results as an important class of anticancer drugs in various biochemical and pharmacological researches. Investigations on pharmacokinetic properties of such types are rare. Ingo Ott and coworkers reported the results of a comparative study on the cytotoxic potential, cellular and nuclear uptake of a series of selected chloro gold(I) phosphine complexes (Cl–Au–P(R)3, R ¼ Me, Et, tert-But, Ph) (2.85–2.88; Fig. 8.41) holding

116

8

Table 8.12 Antiproliferative effects of selected chloro gold(I) phosphine complexes (2.46–2.49)

N N O

Cell line HT-29 (μM) 5.2  0.6 5.3  1.9 5.2  2.1 4.2  0.9

Compound 2.85 2.86 2.87 2.88

N N S

Au

Classes of Gold Complexes

P

O

MCF-7 (μM) 3.9  0.9 3.2  1.3 3.1  0.4 2.6  0.1

S

Au

P

X

X X = H (2.89), Cl (2.90), F (2.91) NO2(2.92), OCH3 (2.93)

X = H (2.94), Cl (2.95), F (2.96) NO2 (2.97), OCH3 (2.98)

Fig. 8.42 Structures of gold(III) complexes (2.89–2.98) from 5-phenyl-1,3,4-oxadiazole-2-thione derivatives evaluated for their in vitro cytotoxic activity

different ligands on the phosphor. Cellular and nuclear gold levels in HT-29 colon carcinoma and MCF-7 breast cancer cells were measured by electrothermal atomic absorption spectrometry. The antiproliferative effects of the studied gold phosphine complexes in two human tumor cell lines are given in Table 8.12. All studied complexes showed significant antiproliferative properties in both studied cell lines. Cellular and nuclear gold levels were enhanced especially for Cl–Au–P(Ph)3 indicating a positive influence of larger and more lipophilic substituents [51].

8.3

Complexes with Sulfur-Phosphorus Donor Ligands

Novel gold(III) complexes (2.89–2.98; Fig. 8.42) from 5-phenyl-1,3,4-oxadiazole-2thione derivatives have shown promising anticancer and antileishmanial activities. On biological screening it was observed that all gold(I) complexes were active at low micromolar or nanomolar range with IC50 values ranging from 2.30 and 1.00, respectively [52]. Further studies were carried by H. Silva and coworkers on some novel antitumor adamantane–azole gold(I) complexes 2.99–3.2 (Fig. 8.43) for their thioredoxin reductase inhibition potential. Spectroscopic studies revealed that gold is coordinated to the exocyclic sulfur atom in all cases, as was confirmed by X-ray crystallographic data obtained for complex 2.99 and backed by quantum–mechanical

8.3 Complexes with Sulfur-Phosphorus Donor Ligands

P P

Au S O

P N N

Au S O

(2.99)

117

S

N N

P

Au S

Au S

N

(3.0)

(3.1)

S N

(3.2)

Fig. 8.43 Structure of novel antitumor adamantane–azole gold(I) complexes (2.99–3.2) O N

S Au P

O (3.3)

Fig. 8.44 Structure of luminescent gold(I) complex (3.3)

calculations.The cytotoxicity of the synthesized complexes has been evaluated in comparison to cisplatin and auranofin in three different tumor cell lines, colon cancer (CT26WT), metastatic skin melanoma (B16F10), mammary adenocarcinoma (4 T1) and kidney normal cell (BHK-21). The obtained gold complexes were more active than their respective free ligands and showed their ability to inhibit the thioredoxin reductase (TrxR) enzyme which is considered as a potential target for newly designed compounds in the treatment of cancer. Among all the complexes 3.2 with triethylphosphine and thiazolidine ring appeared to be more selective than auranofin and was the most cytotoxic and active against TrxR among the tested analogs [53]. A highly effective luminescent gold(I) complex [N-(N0 ,N0 -dimethylaminoethyl)1,8-naphthalimide-4-sulfide](triethylphosphine)gold(I) 3.3 (Fig. 8.44) was reported and evaluated for its biological activity. Antiproliferative studies of the complex were carried against HT-29 colon carcinoma and MCF-7 breast cancer cells with Et3PAuCl as the reference. The gold(I) phosphine complex 3.3 exhibited noteworthy antiproliferative effects in cultured tumor cells. The IC50 values obtained with the complex were slightly lower than those obtained with the reference compound (Et3PAuCl) in both cell lines (HT-29 and MCF-7), 1.9 μM for Et3PAuCl and 0.4–0.7 μM for 3.3, indicating that the replacement of the chlorine of Et3PAuCl by the naphthalimide ligand offers a suitable strategy to obtain bioactive compounds. Biodistribution experiments by fluorescence microscopy showed an efficient uptake of the complex into specific cell compartments, and AAS experiments confirmed an elevated uptake of this complex into the nuclei. As the non-naphthalimide Et3PAuCl displayed a much lower nuclear uptake, it may be speculated that the naphthalimide

118

8

+

tBu

HO

Classes of Gold Complexes

tBu

S Au PPh3

tBu

HO

Au PPh3 S Au PPh3

tBu

(3.4)

−

BF4

(3.5)

Fig. 8.45 Molecular structure of biologically active gold(I) complexes (3.4 and 3.5)

ligand of the complex might be a useful vector to enable the transport of metals into the nucleus. Studies on the mode of action showed that the inhibition of TrxR and induction of apoptosis via the mitochondrial pathway also play major roles in the pharmacodynamics of this novel gold antitumor agent. For a cysteine containing model peptide the covalent binding of complex and Et3PAuCl to the cysteine side residue after loss of the respective non-phosphine ligand could be confirmed. Interestingly, the obtained complex displayed significant antiangiogenic effects in developing zebrafish embryos in comparison to Et3PAuCl, which was almost inactive in this assay. Consequently, the antiangiogenic properties can be attributed to the naphthalimide ligand of the complex, which is not present in Et3PAuCl. The above-described results reveal a rather complex pharmacological profile of the complex and indicates the presence of multiple biological targets, of which nuclear (DNA) and mitochondrial (TrxR) macromolecules may have high relevance. The mechanism underlying the observed antiangiogenic effects, however, remains to be clarified [54]. Two new gold(I) complexes 3.4 and 3.5 (Fig. 8.45) were synthesized and the biological impact of both these complexes on lipid peroxidation, mitochondrial functions, the tubulin polymerization, activity of glutathione reductase, and cell viability was investigated. The complexes displayed high antioxidant activity and were found to induce mitochondrial lipid peroxidation. Furthermore, the complexes were evaluated for their in vitro cytotoxicity against a primary culture of rat cerebellar granule cells. The high in vitro toxicity against a primary culture of rat cerebellar granule cells was determined for AuPPh3Cl only, while the introduction of antioxidant hindered phenol groups into complex molecules decreases their cytotoxicity. The IC50 values were found to increase in the order: AuPPh3Cl (3  1.7 μM) < complex 3.5 (16.1  4.3 μM) < complex 3.4 (27.6  7.4 μM). The results of this study suggest that polytopic compounds combining in one molecule 2,6-di-tert-butylphenol pendants, a gold center, and a thiol ligand are membrane active compounds and may be studied with the aim to find novel goldcontaining agents that possess lower undesirable toxicity [55]. M. Concepcion Gimeno and coworkers in 2013 reported the synthesis of ferrocenyl amide phosphines such as PPh2CH2CH2-NHCOFc (3.6), (PPh2CH2CH2)2NCOFc (3.7), and (PPh2CH2-CH2NHCO)2Fc (3.8), and used them to prepare different gold(I) and silver(I) complexes as anticancer agents. Complexes with the well-known dppf (3.9) ligand were also prepared for comparative studies. The gold chloride derivatives [AuCl(PPh2CH2CH2NHCOFc)], [Au2Cl2{(PPh2CH2-CH2)2NCOFc}], and [Au2Cl2{(PPh2CH2CH2NHCO)2Fc}]

8.3 Complexes with Sulfur-Phosphorus Donor Ligands

119

have been synthesized in which the chloro ligand can be easily substituted by several biologically relevant thiolates such as 2-mercaptonicotinic acid, 2-thiocytosine, 2-thiouracil, 2-mercaptopurine, and 2,3,4,6-tetra-6-acetyl-1-thiol-β-Dglucopyranosato, affording the gold phosphine thiolate species (3.10–3.23) (Fig. 8.46). The antiproliferative activity of these compounds has been tested by the MTT viability assay in two murine cell lines, NIH-3 T3 (mouse embryonic fibroblasts) and PC-12 (pheochromocytoma of the rat adrenal medulla), and two human cell lines, A-549 (adenocarcinomic human alveolar basal epithelial cells) and Hep-G2 (hepatocellular carcinoma). The ferrocenyl amide phosphine ligands are not active, whereas the metal complexes are. The silver complexes are less active than the gold species. Among the ferrocenyl amide phosphines good to excellent values are found for the gold complexes with the monosubstituted ferrocenyl phosphines. With the monophosphine ligand 3.6 excellent values are found in the murine cells for all the complexes, but only for the nicotinic acid thiolate species in the human cell lines. However, the complexes with the monosubstituted ferrocenyl diphosphine ligand 3.7 have good values in all the cell lines. The complexes with the disubstituted ferrocenyl amide diphosphine (3.8) present the poorest antiproliferative values. The IC50 values for the compounds with dppf (3.9) are good but are of the same order as those with ligand 3.7 [56]. Ali Alhoshani and his coworkers described the cytotoxic potential of a gold complex bis(diethyldithiocarbamato-gold(I)) bis(diphenylphosphino) methane 3.24 (Fig. 8.47) against different cancer cell lines and compared it with the cytotoxic potential of the most commonly used cisplatin. Besides this the molecular mechanism underlying the toxic effects of the synthesized complex 3.24 against the A549 cell line and the identification of cancer-related miRNAs possible to be involved in killing the lung cancer cells was investigated. Using microarray, global miRNA expression profiling in A549 cells treated with complex 3.24 revealed 64 upregulated and 86 downregulated miRNAs, which targeted 4689 and 2498 genes, respectively. Biological network connectivity of the miRNAs was significantly higher for the upregulated miRNAs than for the downregulated miRNAs. Two of the 10 most upregulated miRNAs (hsa-mir-20a-5p and hsa-mir-15b-5p) were associated with lung cancer. AmiGo2 server and Panther pathway analyses indicated significant enrichment in transcription regulation of miRNA target genes that promoted intrinsic kinase mediated signaling, TGF-b, and GnRH signaling pathways, as well as oxidative stress responses. Crystal structure X-ray diffraction studies revealed gold–gold intramolecular interaction [Au. . .Au ¼ 3.1198 (3) Å] for a single independent molecule, reported to be responsible for its activity against cancer. This study sheds light on the development of novel gold complex 3.24 with favorable anticancer therapeutic functionality, showing that the treatment with complex 3.24 affects the micro-RNA network, and is a strong disruptor of basic cellular mechanisms in lung cancer cell lines and provides insight into the importance of the development of new gold(I) complexes and the elucidation of their potential as novel anticancer therapeutic agents [57]. Metal complexes derived from thiosemicarbazone ligands are of considerable interest because of their fascinating chemical and biological properties. Several gold (I) complexes 3.25–3.36 (Fig. 8.48) as potential anticancer agents are currently

120

8

O

O

O PPh2

N H

Fe

Classes of Gold Complexes

PPh2

N PPh2

(3.6) O

Fe

Fe

PPh2

(3.8)

N H

Fe

(3.10)

(3.9)

O Ph2 P Au OTf PPh3

N H

Fe

Ph2 P Au S

N H

Fe

Au

P Ph2

O

O N

NH2

Fe Ph2P

Fe

N H

N

O N

Ph2 P S Au

Ph2P

N H

(3.16)

N

NH2

N

Fe

N

N

O

(3.15)

Cl Au

(3.17)

N

HN

Ph2 P Au Cl

N

Ph2 P S Au

Ph2 P S Au

N H

Fe O

(3.14)

O

COOH

(3.13)

N S

Fe

N

(3.12) H N

O N H

(3.11)

O

Fe

Ph Ph2 H 2 P P Au OTf

O

Ph2 P Cl Au

N H

PPh2

O

(3.7)

PPh2

PPh2

N H H N

Fe

Fe

(3.18)

Au S

N

N NH2

AcO O

Ph2 O P S Au AcO

N Fe Ph2P

OAc OAc

OAc Au O S AcO OAc AcO (3.19)

O N H H N

Fe O

Ph2 P Au SR

Ph2 P Au SR Fe Au P Ph2

SR

SR = TG (3.20), 2-thiocytosine (3.21)

P Au Ph2

SR

SR = TG (3.22), thiouracil (3.23)

Fig. 8.46 Molecular structure of biologically active ferrocene amide phosphine derived compounds

8.3 Complexes with Sulfur-Phosphorus Donor Ligands

121

Fig. 8.47 Crystal structure of gold(I) complex 3.24, showing the atom labeling scheme and displacement ellipsoids at the 50% probability level. Adapted with permission from the publisher

S R

KAuCl4

R

N

H N

NH2 Au S

N

N H

N H

R = Phenyl, NH2 p-methoxyphenyl thiophene, furan, pyridinyl, p-hydroxyphenyl

Au(PPh3)Cl

+ R

N

+

H N

NH2 S Au P

S

R

N

NH2

(3.25-3.30)

(3.31-3.36)

R= phenyl (3.25, 3.31), p-methoxyphenyl (3.26, 3.32), thiophene (3.27, 3.33) furan (3.28, 3.34), pyridinyl (3.29, 3.35) or p-hydroxyphenyl (3.30, 3.36)

Fig. 8.48 Structures of gold(I) complexes derived from thiosemicarbazone ligands (3.25–3.36)

122

8

Classes of Gold Complexes

undergoing intense preclinical investigations. Ana Paula Soares Fontes and coworkers reported the synthesis and characterization of novel gold(I) complexes through the reaction of aryl-thiosemicarbazone ligands with KAuCl4 and Au(PPh3) Cl. The cytotoxicity of the synthesized complexes was evaluated against tumor cell lines like mouse metastatic skin melanoma (B16-F10) and colon cancer cells (CT26. WT) and non-tumor cell line, Baby Hamster Kidney (BHK-21). Most of the tested complexes displayed reasonable cytotoxic activity, some of them being even more cytotoxic in tumor cells than cisplatin with a high selectivity index in the tested cells. The gold complexes with phosphine derivatives showed a better biological response and higher selectivity indices than their respective non-phosphine gold complexes. All the phosphine derived complexes displayed activity at concentrations below 2 μM. The addition of phenylphosphine group enhanced the lipophilicity of the complexes and thus may be responsible for the increased cytotoxicity since this property could facilitate cell membrane penetration. A preliminary drug-receptor docking study advocates the interaction of thiosemicarbazone ligands with Y116 and E30, which are part of the catalytic N-terminal motif of thioredoxin reductase (TrxR). All compounds were assessed for their ability to inhibit the activity of the TrxR enzyme. The non-substituted phosphine gold complexes turned out to be effective inhibitors of TrxR when compared to the phosphine analogs. Both complexes, in most of the cases, were more efficient TrxR inhibitors than the free ligands. Among all tested compounds, the ligand having R-group as p-hydroxyphenyl and complex 3.33 were the most potent TrxR inhibitors, with complex 3.36 displaying good activity with the series containing -PPh3 as ligand [58]. Elena Cerrada and coworkers report the synthesis of new gold(I) thiolate derivatives (3.37–3.49) (Fig. 8.49) with water-soluble phosphanes derived from PTA via [{Au-(thiolate}n] formation in good yields. These complexes were studied for their antiproliferative effects on human colon cancer cell lines and found to be potent cytotoxic agents (Table 8.13). All of the synthesized complexes have confirmed more chemical stability in buffered solutions and better anticancer activity than the previous derivatives with the same phosphanes but with halogen or pseudohalogen ligands. This fact supports the relevance of the appropriate choice of ligands in the coordination sphere of the metallic center. Thus, thiolate ligands confer higher stability under physiological conditions, which leads to an improvement of their effectiveness as cytotoxic derivatives. Besides the relevance of the thiolate incorporation, the presence of electron-withdrawing substituents (NO2) in the para position of the benzene moiety in the alkylated PTA phosphane improves the stability and lipophilicity, although without improvement of the anticancer activity. Moderate binding constant values for interaction of the compounds with BSA have been calculated, which makes them suitable for transportation by the protein through blood and easily released to the target. Some of these new compounds have demonstrated a selective cytotoxic behavior because high viabilities (values around 100%) have been found in normal enterocytes (Caco2 cells under confluence) and synergism with 5-FU when administered in combination with gold derivatives to cancerous cells, leading to a reduction of up to 40 times

8.3 Complexes with Sulfur-Phosphorus Donor Ligands N

N

R'S Au P

123

Br

N R

N

R'S

S-

R 3.37

COOH

COOH

NO2

N N

S-

3.38

3.41

3.42

3.45

3.46

N N

S-

N N

S-

3.39

3.40

3.43

3.44

3.47

3.48

O 3.49

HN O

OMe

Fig. 8.49 Molecular structures of new gold(I) thiolate complexes (3.37–3.49) Table 8.13 IC50 values of the thiolate complexes (3.37–3.49) against Caco-2/ PD7 and Caco-2/TC7 colon cancer cell lines compared with auranofin and cisplatin

Complex 3.37 3.38 3.39 3.40 3.41 3.42 3.43 3.44 3.45 3.46 3.47 3.48 3.49 Cisplatin Auranofin

IC50 (μM) PD7 2.00  0.58 2.30  1.02 2.73  2.08 6.83  0.50 6.99  0.53 13.37  3.93 8.90  1.79 5.36  1.95 2.98  0.83 6.43  1.70 4.01  1.29 6.18  0.01 8.36  1.00 37.24  5.15 1.8  0.1

TC7 2.68  0.30 6.47  2.15 1.92  1.64 7.66  0.09 4.61  0.14 10.66  2.56 5.06  0.78 6.29  1.07 3.08  0.79 2.98  1.28 6.08  0.38 3.96  1.33 6.13  0.92 45.6  8.08 2.1  0.4

124

8 S PH Au S

N

(3.50)

Classes of Gold Complexes S

PH Au S

N

(3.51)

Fig. 8.50 Molecular structures of gold(I) complexes (3.50 and 3.51) that contain tri-tertbutylphosphine and dialkyldithiocarbamate ligands Table 8.14 IC50 Values (μM) of gold(I) complexes (3.50 and 3.51) against A549, MCF7, and HeLa cancer cell lines Compound 3.50 3.51 Cisplatin

A549 29.90  1.04 19.4  0.64 41.60  3.00

MCF7 21.07  0.95 16.00  0.83 22.38  0.62

HeLa 2.11  0.43 3.18  0.54 19.40  1.85

the dose required of 5-FU for the same effect. Preliminary studies of the possible mechanism has shown that the complexes induce apoptosis and cause an increase in the intracellular ROS levels, probably due to interaction with the enzyme TrxR, altering, therefore, the redox balance [59]. Two new gold(I) complexes containing tri-ter-butylphosphine and dialkyl dithiocarbamate ligands (3.50, 3.51) were reported (Fig. 8.50). The in vitro anticancer activity of both these complexes was studied towards A549 (lung cancer), MCF7 (breast cancer), and HeLa (cervical cancer) human cancer cell lines. Both complexes exhibited very strong in vitro cytotoxic effects against A549, MCF7, and HeLa cell lines. The screening of the cytotoxic activity based on IC50 data against the A549, MCF7, and HeLa lines showed that the synthesized gold(I) complexes were highly effective, particularly against HeLa cancer cell line. Based on IC50 data, the cytotoxic activity of both complexes was found to be better than the well-known commercial anticancer drug cisplatin against all the three cancer lines tested (Table 8.14). The in vitro cytotoxicity results against A549 cell line were found as 41.60  3.00, 26.90  1.04, and 19.40  0.64 μM for cisplatin and gold (I) complexes (3.50) and (3.51), respectively. These complexes (3.50) and (3.51) have revealed cytotoxicity almost two-three folds higher than cisplatin. In MCF7 cancer cell line, in vitro anticancer activity in terms of IC50 values was 22.38  0.62, 21.07  0.95, and 16.00  0.83 mM for cisplatin and gold(I) complexes (3.50) and (3.51), respectively. For HeLa cancer cell line, in vitro cytotoxicity in terms of IC50 values was 19.40  1.85, 2.11  0.43, and 3.18  0.54 mM for cisplatin and gold (I) complexes (3.50) and (3.51), respectively. The cytotoxicity of gold(I) complexes (3.50 and 3.51) against HeLa cell line is six to eight times higher than cisplatin. The better inhibition of growth of cancer cells by these complexes (3.50 and 3.51) can be credited to dithiocarbamate as labile co-ligands bonded with central gold(I) ions. Overall, these complexes (3.50 and 3.51) showed higher anticancer activity against A549, MCF7, and HeLa human cancer cell lines than previously reported anticancer gold compounds [60].

8.3 Complexes with Sulfur-Phosphorus Donor Ligands Fig. 8.51 Structure of gold (III) complexes (3.52–3.55)

125 +

Cl

Ph2 P SH

Au

S

Cl

Au

-

PF6

S

Et2N

(3.52)

Ph2 P S

(3.53) +

Ph2P

Au

S P Ph2 Cl

2+

Cl-

Ph2P

Au

PPh2

(3.55)

P

P

Au Cl NH2

(3.58)

Au S

N

(3.57)

S

Au S NH2

S

NH2

(3.56)

P

-

(PF6) 2

S

PPh2

(3.54)

Fig. 8.52 Molecular structure of the gold (I) complexes (3.56–3.59)

PPh2

P

N

Au S NH2 (3.59)

S N

Ph Ph

Anticancer gold(III) complexes (3.52–3.55) (Fig. 8.51) bearing cyclometalated triphenylphosphine sulfide ligands have been prepared and investigated toward five human cancer cell lines, HeLa, A549, PC3, HT1080, and MDA-MB-231, respectively [61]. In vitro cytotoxicity studies showed that compounds 3.53–3.55 exhibited potent cell growth inhibition with IC50 of 0.17–2.50 μM, comparable to, or better than, clinically used cisplatin (0.63–6.35 μM). Primary mechanistic studies using HeLa cells specified that the cytotoxic effects of the compounds involved induction of apoptosis by the accumulation of reactive oxygen species (ROS). Compound 3.53 also confirmed significant inhibition of endothelial cell migration and tube formation in the angiogenesis process. Assessment of the in vivo antitumor activity of compound 3.53 in nude mice bearing cervical cancer cell (HeLa) xenografts showed substantial tumor growth inhibition (55%) with 1 mg/kg dose (every 3 days) compared with the same dose of cisplatin (28%). These results demonstrate the potential of gold(III) complexes containing cyclometalated triphenylphosphine sulfide ligands as novel metal-based anticancer agents. Gold(I) complexes from 2-(diphenylphosphanyl)-1-aminocyclohexane and dithiocarbamates (3.56–3.59; Fig. 8.52) were synthesized and evaluated for their

126 Fig. 8.53 Structure of gold (I)–phosphane dithiocarbamate complexes (3.60–3.63)

8

tBu

NH3 Cl Pt NH3 Cl

tBu

S P Au

cDDP

tBu

tBu

S

Classes of Gold Complexes

N

tBu

tBu

S

(3.60) S

P Au

(3.62)

S

N

S

P Au

N

(3.61)

tBu

tBu

S P Au

S

N

(3.63)

anticancer activity against A549, HeLa, and HepG2 human cancer cell lines. The dose-dependent inhibition of cell proliferation was observed by specific increase of the concentration of the complexes against three cell lines (A549, HeLa, and HepG2). The IC50 values of the complexes were in the range 2.2– ˃100 μM, compared to that of the cisplatin IC50 values of 13.75, 21.39, and 9.6 μM for A549, HeLa, and HepG2 cell lines, respectively. The data clearly showed that the complexes 3.56, 3.57, and 3.59 cause excellent inhibition of cell proliferation; complex 3.57 being the best. Their IC50 values were several folds higher than that of cisplatin. Surely, the effectiveness of the complex 3.56 was better for HeLa cells, while 3.57 and 3.59 were more active towards HepG2 cell lines. Complex 3.58 exhibited less activity with respect to cisplatin. The higher potency of the inhibition of cell proliferation of the examined compounds could be related to the presence of the labile phosphane and dithiocarbamate ligands around the gold(I) ion, which augments the activity and selectivity of the complexes [62]. Highly cytotoxic gold(I)–phosphane dithiocarbamate complexes (3.60–3.63; Fig. 8.53) were synthesized and unambiguously characterized by 1H and 31P NMR spectroscopy, IR spectroscopy, and mass spectrometry [63]. Furthermore, their purity was confirmed by elemental analysis and high-resolution mass spectrometry. Complexes 3.60–3.63 were poorly soluble in H2O, MeOH, and DMSO and wellsoluble in CH2Cl2, CHCl3, and pyridine. The molecular structures of complexes 3.60, 3.62, and 3.63 were determined by solid-state X-ray diffraction analyses. The in vitro anticancer activity of all the complexes 3.60–3.63 and unexpectedly formed gold chain polymer (3.61) were determined in human ovarian carcinoma cell line A2780 and its cDDP-resistant variant form A2780cis, as well as healthy embryonic kidney cells HEK293. All tested Au(I) complexes demonstrated low micromolar to nanomolar toxicity, surpassing cDDP. The maximum cytotoxicity of the lead complex 3.61 was associated to its effective cellular accumulation (Table 8.15). The mechanism of action of 3.61 was based on the induction of integrative stress, including ROS insult, cell cycle interference, and endoplasmic reticulum stress, leading to apoptosis. The activation of endoplasmic reticulum stress by 3.61 was

8.3 Complexes with Sulfur-Phosphorus Donor Ligands Table 8.15 IC50 Values (nM) of gold(I)–phosphane complexes (3.60 and 3.63)

Compound 3.60 3.61 3.61´ 3.62 3.63 cDDP

127

IC50 (nM) A2780 51  5 23  2 243  66 133  29 106  7 560  140

Fig. 8.54 Structure of gold (I) complexes with HMPT (3.64–3.71)

A2780cis 82  31 38  13 nd 145  38 190  76 5352  1100

N N P Au Cl N

N N P Au SN N

(3.64)

SN = N

(3.65-3.71) N

S

N

(3.65)

N

S

N

(3.66) N S

(3.69)

HEK293 74  6 21  7 312  89 176  20 303  39 3966  1151

S

S

N

(3.67)

S

(3.68) O

S

N N (3.70)

NH

S N

S

(3.71)

followed by the CRT translocation to the cell membrane, which is associated with the activation of an immune system response. New thiolate gold(I) complexes (3.64–3.71; Fig. 8.54) with P(NMe2)3 (HMPT) as a phosphane group were prepared and characterized by several sophisticated techniques, including X-ray studies of complexes with SMe2pyrim, S-benzothiazole, and 2-thiouracil moieties [64]. In addition, their potential application as anticancer drugs was investigated by determining their pharmacokinetic activity (water solubility, cell permeability, and BSA transport protein affinity). Furthermore, cell viability studies against different cell lines (A2780, A2780R, and Caco-2/TC7) cells with the synthesized compounds were performed, displaying more cytotoxic activity than cisplatin in all cases. Besides, two of the synthesized complexes exhibited specific selectivity for cancerous Caco-2 cells, arising complexes with S-benzimidazole and 2-thiouracil groups, as potential candidates for anticancer drugs. These complexes were able to induce a strong inhibition of the thioredoxin reductase (TrxR) protein and oxidative damage in membrane lipids. Additional studies in primary cultures from mouse colon tumors showed that these two complexes were proapoptotic by the exposure of phosphatidylserine. Based on these results, it was concluded that two of the synthesized thiolate gold(I) complexes were good and effective candidates to be used in chemotherapy. Six dithiocarbamate phosphanegold(I) complexes (3.72–3.77; Fig. 8.55) were reported and evaluated for their anticancer activity towards the A549, HeLa, and

128

8 S

S

NH2

NH2 P

Au

Au

P

N

S

(3.72)

(3.73) NH2 S

NH2 Au

Ph

N

S

P

Au

N

(3.75)

NH2 S Au

S

Ph

(3.74)

P

N

S

S P

Classes of Gold Complexes

NH2 S N

S

P

Au

S

Ph

N Ph

(3.76)

(3.77)

Fig. 8.55 Structures of dithiocarbamate phosphanegold(I) complexes (3.72–3.77) Table 8.16 In vitro cytotoxicity (IC50, inμM) of dithiocarbamate phosphanegold (I) complexes (3.712–3.77) and cisplatin in A549, HeLa, and HepG2 cancer cell lines

Complexes 3.72 3.73 3.74 3.75 3.76 3.77 Cisplatin

A549 27.51 (0.45) >100 >100 >100 3.45 (0.19) 30.30 (0.57) 13.75 (0.40)

HeLa 95.5 (0.31) 83.57 (0.32) 29.20 (0.35) >100 >100 53.27 (0.34) 21.30 (0.72)

HepG2 18.55 (0.03) 49.30 (0.20) 5.29 (0.11) >100 33.15 (0.45) >100 9.60 (0.90)

HepG2 human cancer cell lines. The dose-dependent response of cell proliferation was found by specific increase of concentrations of the complexes against selected human cancer cell lines. The IC50 values (Table 8.16) were obtained from the plot of concentrations of the complexes against cell viability percentage. The obtained results revealed that complexes 3.74 and 3.76 showed best inhibition of cell proliferation in HepG2 and A549 cells, respectively. Complex 3.74 showed IC50 value 5.29 μM approximately two-fold to cisplatin (IC50 ¼ 9.60 μM) for HepG2 cell line, while that of complex 3.76 (IC50 3.45 μM) is about four-fold compared to cisplatin (IC50 ¼ 13.74 μM) against A549 cell line. In addition, complexes 3.72 and 3.77 exhibited moderate cytotoxicity against A549 or (and) HepG2 cell, while 3.74 is moderately active for HeLa cells. Complex 3.75 is less effective towards all three cell lines. Previously, the anticancer potential of similar dithiocarbamate gold

8.4 Organometallic Gold Complexes

129

(I) complexes was investigated using another phosphane, 2-(diphenylphosphanyl)1-aminocyclohexane [62]. In that case, three of the four complexes displayed excellent antiproliferative activity indicating the positive influence of diphenyl groups of the phosphane. This data reflects that the nature of the phosphane as well as the subsidiary ligand bound to the gold(I) center is important in determining the anticancer activity and tumor selectivity of these complexes [65].

8.4

Organometallic Gold Complexes

Metal complexes with N-heterocyclic carbene (NHC) ligands were primarily investigated for their antimicrobial activities and then as potential antiproliferative agents against cancer cells [66]. Undeniably, several cytotoxic Ag–NHC complexes have displayed promising antibacterial effects and excellent in vivo anticancer activity against an ovarian cancer xenograft model [67]. Several Au(I)–NHC metal complexes have been reported to exhibit thioredoxin reductase (TrxR) enzyme inhibition favoring cancer cell death [68]. In view of the excellent potential of gold complexes and the facile reduction of gold(III) to gold(I), researchers have developed gold complexes bearing N-heterocyclic carbene (NHC) ligands as promising anticancer agents. A series of new gold(I) complexes 3.78–3.81 with benzimidazole derived Nheterocyclic carbene (NHC) ligands (Fig. 8.56) were synthesized and investigated for their proficient biological profiles. Biological studies on the inhibition of disulfide reductases demonstrated efficient and selective inhibition of the selenocysteine containing TrxR and verified this enzyme as a major target for the synthesized gold (I) complexes. Regarding the enzymatic inhibition studies, this is the first report offering EC50 values for the direct and selective TrxR inhibition by Au(I)–NHC complexes while as TrxR inhibition in cellular lysates has been reported previously. Antiproliferative effects for 3.78–3.81 were noted in the low micromolar range, and more detailed studies were carried on selected complex (3.79) which displayed a distinct pharmacodynamic profile with enhanced reactive oxygen species formation, apoptosis induction, strong effects on cellular metabolism, inhibition of mitochondrial respiration, and activity against resistant cell lines [69]. Similarly, a series of organometallic gold(I) complexes 3.82–3.85 (Fig. 8.57) containing different water-soluble phosphanes and chromophoric units were reported. Biological evaluation of the complexes revealed their low potency Fig. 8.56 Molecular structures of new gold (I) complexes (3.78–3.81)

R N N R

Au Cl

3.78: R = methyl 3.79: R = ethyl 3.80: R = benzyl 3.81: R = diphenylmethyl

130

8 S-O3Na+

S-O3Na+

+NaO -S 3

+

P

P

O

O

Au

+NaO -S 3

(3.82)

N N P

NaO3-S

N

Au

+NaO -S 3

Classes of Gold Complexes

O (3.83)

N N N P Au O

N

O

Au

O N N

NP

N

O

Au

O

O

O

N NP

O

Au

O

O

O

O (3.85)

(3.84)

Fig. 8.57 Molecular structures of Au–NHC complexes (3.82–3.85) with promising anticancer properties O

O

Br

AuPPh3

N H O

AuPPh3

3.86

O O

O

FcCH=CH

O N H

O

3.88

AuPPh3

3.87

Fig. 8.58 Molecular structures of gold(I) complexes (3.86–3.88) evaluated for their anticancer potential

toward tumor cell growth but an inhibitory potency against thioredoxin reductase (TrxR). The lower cytotoxic results have been attributed to low bioavailability which was exposed by measuring the cellular gold levels using atomic absorption spectroscopy. Further studies are needed to improve the bioavailability of the synthesized complexes while maintaining their strong TrxR inhibition [70]. In another report by K. Kowalski et al. three more gold(I) complexes 3.86–3.88 (Fig. 8.58) of alkynyl chromones were synthesized and evaluated as anticancer agents against four human cancer cell lines with auranofin as a reference drug. All complexes showed antiproliferative activity at lower micromolar concentrations with complex 3.86 showing broad activity profile, being more active than the reference drug auranofin. In order to investigate the mechanism for the cytotoxicity

8.4 Organometallic Gold Complexes

N

N

N

N

N +

NO3−

N

N

Au N

131

N

(3.89) N

N

Au N

N (3.91)

NO3− N

N (3.90)

O

N +

N

Au +

N

N

NO − 3

N

N

N

Au+

NO3−

N

N

O

(3.92)

Fig. 8.59 Structure of cationic gold(I) complexes (3.89–3.92) evaluated for their in vitro anticancer activities in PC-3 (prostate cancer) and T24 (bladder cancer) cell lines and in the non-cancerous MC3T3 (osteoblast) cell line

of the synthesized complexes, cellular uptake, thioredoxin reductase (TrxR) inhibition, genotoxic effect, caspase activation, and cell cycle analysis were performed. The compounds were taken up by cancer cells, as shown by the atomic absorption spectroscopy experiments, and displayed cytotoxic activity at low micromolar concentrations. In general, the obtained results showed pro-apoptic activity of the investigated gold complexes and suggested that the mitochondria were their major cellular target [71]. Catherine Hammert and coworkers in 2018 reported a series of four new mononuclear cationic gold(I) complexes (3.89–3.92; Fig. 8.59) containing nitrogen functionalized N-heterocyclic carbenes (NHCs), fully characterized by various spectroscopic methods. X-ray type crystals for complexes 3.89, 3.90, and 3.91 were obtained and structures of these three complexes were elucidated. In all the three complexes a common feature was observed, that was the trans position of the substituents relative to the NHC–Au–NHC unit in the solid state. These lipophilic gold(I) complexes originated from a pharmacomodulation of previously described gold(I)–NHC complexes, by replacing an aliphatic spacer with an aromatic one. The LogP values of the resulting complexes increased by 0.7–1.5, depending on the substituents in comparison to the aliphatic-linker systems. The newly synthesized series of complexes 3.89–3.92 (Fig. 8.59) were examined in vitro for their anticancer potencies in PC-3 (prostate cancer) and T24 (bladder cancer) cell lines and in the non-cancerous MC3T3 (osteoblast) cell line. All screened complexes displayed high activities towards the cancer cell lines with GI50 values lower than 500 nM. Among the obtained complexes, complex 3.91 has been chosen for further investigations and tested in vitro against six cancer cell lines from different origins (prostate, bladder, lung, bone, liver, and breast) and two non-cancerous cell lines (osteoblasts, fibroblasts). The GI50 values of the complexes are given in Table 8.17 which clearly indicate that the complexes 3.89 and 3.90 display strongest effects with GI50 values of 50 and 30 nM on PC-3 cells, respectively, and 60 nM for both complexes on T24

132

8

Table 8.17 The GI50 values of the complexes (3.89–3.92)

GI50 (nM) Compound 3.89 3.90 3.91 3.92 Auronofin

N Cl Cl

N N

Au S N

(3.93)

N

Classes of Gold Complexes

PC-3 0.05 0.03 0.25 0.16 1.05

T24 0.06 0.06 0.25 0.14 0.84

N

Au S

N

N

N N N

Au S N

(3.94)

MC3T3 0.12 0.11 0.90 0.35 1.05

(3.95)

N

Au Cl

N

(3.96)

Fig. 8.60 Molecular structures of the cytotoxic Au(I)–NHC complexes (3.93–3.96)

cells. Moreover, cellular uptake measurements were indicative of a good bioavailability. Various biochemical assays indicated that the complex 3.91 efficiently inhibited the thioredoxin reductase (TrxR) and its cytotoxicity towards prostate PC-3, bladder T24, and liver HepG2 cells was found to be ROS-dependent [72]. In another finding, gold(I) complexes (Fig. 8.60) of the type NHC–Au–L (NHC ¼ N-heterocyclic carbene L ¼ Cl or 2-mercapto-pyrimidine) with 1,3-substituted imidazole-2-ylidene and benzimidazole-2-ylidene ligands have been evaluated against human ovarian cancer cells (A2780S/R), as well in the non-tumorigenic human embryonic kidney cell line (HEK-293 T), for any potent cytotoxic effects. The cell viability IC50 values of the potent complexes against human ovarian carcinoma cell lines sensitive (A2780S) or resistant to cisplatin (A2780R) and against human embryonic kidney cell line (HEK-293 T) after 24 h of incubation were in the range of 3.2  0.7–11.5  2.6 μM, 4.9  1.5–12.7  3.1 μM, and 5.3  1.2–16.2  1.9 μM, respectively. The most effective compounds (3.93–3.96) were the 2-pyrimidinethiolato derivatives as compared to the chloro-substituted NHC–Au(I) complexes. This could be due to the more labile nature of the chloride ligand, making the chloro derivatives more reactive and prone to deactivation by different cellular components. The more potent cytotoxic complexes were also screened for their TrxR inhibition properties both on the purified enzyme and on cell extracts in vitro. The compounds inhibited cytosolic TrxR1 better than mitochondrial TrxR2. The intended mechanism of TrxR inhibition involves direct coordination of the gold center to the Sec in the enzyme active site, as indicated by the reduced inhibition of the enzyme glutathione reductase lacking the Sec-containing domain, as well as by the BIAM (biotin-conjugate iodoacetamide)

8.4 Organometallic Gold Complexes

133 O

O O

N

N

N

O N

O

O

O

Au O

S

(3.97)

S

O

O

Cl

O

N

N

Au O

Au

O

O

HO

O

OH OH (3.99)

(3.98)

O

OH O

Fig. 8.61 Structure of N-heterocyclic carbene gold(I) complexes (3.97–3.99) bearing a fluorescent coumarin-type carbene ligand as antiproliferative agents

Fe

N

N Au

N

N

Fe

(4.0)

Fig. 8.62 Structure of Au(I) complex (4.0) exhibiting prominent anticancer activity

assay results. The BIAM assay also showed that the compounds were not markedly oxidative agents as other cytotoxic gold(III) complexes [73]. A new series of NHC–gold(I) complexes (3.97–3.99; Fig. 8.61) bearing a fluorescent coumarin type carbene ligand were synthesized and characterized by various spectral techniques. The complexes were investigated for their antiproliferative effects in normal (i.e., HEK-293 T) and tumor cells (A2780, MCF-7, and A549) in vitro, showing moderate activity and some degree of selectivity. Among the complexes, complex 3.99 displayed better activity against A2780 and MCF-7 cells with an IC50 ¼ 11.6  0.8 μM and 12.9  3.8 μM than the reference drug cisplatin (1.9  0.6–20.0  3.1 μM). The complexes effectively inhibited selenoenzyme thioredoxin reductase (TrxR) but were weakly effective for the glutathione reductase (GR) and glutathione peroxidase enzymes [74]. A series of homoleptic gold(I) complexes having ferrocenyl groups as Nsubstituents were synthesized and evaluated for their cytotoxic effects. Among the synthesized complexes, complex (4.0; Fig. 8.62) was assumed to show antimitochondrial activity, by analogy with other gold NHC complexes. The ferrocenyl groups were introduced for their electrophoric properties, because they are supposed to act as cytotoxic radical generators while cycling between +2 and +3 redox states. The cytotoxicity results were reported on three cancerous cell lines (HeLa, CoLo 320 DM: colon adenocarcinoma, Jurkat: leukemia) and the values are

134

8

Table 8.18 IC50 values for ferrocenyl Au(I)–NHC complex (4.0) (μM)

Cl

Au

N Cl

(4.1)

Complex 4.0 Cisplatin

HeLa 0.57 0.638

P

N

Au

N

CoLo 320 DM 1.01 0.41

PF6

Cl

OAc AcO AcO

N

N

(4.2)

OAc AcO AcO

O

Classes of Gold Complexes

Au

Au

Jurkat 0.25 0.78

N Cl

O AcO

(4.3)

N AcO

S S

O

AcO (4.4)

OAc

OAc OAc

Fig. 8.63 Structures of novel (C^N) gold(III) cyclometallated complexes (4.1–4.4) evaluated for their anticancer potential R1 NH2 Au

R2 Cl

Cl

R1= R2 = OMe (4.5) R1= R2 = OCH2O (4.6)

Fig. 8.64 Structure of two novel organometallic gold(III) complexes (4.5 and 4.6)

given in Table 8.18. Interestingly, a sevenfold selectivity for cancerous cells versus human lymphocytes was demonstrated [75]. In order to demonstrate the tremendous anticancer potential of organometallic compounds a new series of (C^N) cyclometallated gold(III) complexes (4.1–4.4; Fig. 8.63) clubbed with different ancillary ligands to confer different reactivity and efficient biological properties to the resulting compounds, like 1,3,5triazaphosphaadamantane for increased water solubility, and thiosugar moieties to influence uptake and reduce exchange with biological nucleophiles. All the compounds were tested in vitro against five human cancer cell lines including the lung, breast, colon, and ovarian cancer cells and against a model of healthy human cells from the embryonic kidney for comparison purposes. Classical MTT assay was employed to determine the IC50 value after 72 hours of incubation with different concentrations of the compounds. The starting compound 4.1 was poorly cytotoxic on all cell lines, while the phosphane-containing complex 4.2 displayed the most promising results against the HCT116 cancer cell line overexpressing p53 [76]. In an another report, two novel organometallic gold(III) complexes (4.5 and 4.6; Fig. 8.64) resembling the structure of tetrahydroisoquinolines were evaluated for

8.4 Organometallic Gold Complexes

135

Fig. 8.65 Structure of gold (III) biotin-linked N-heterocyclic carbene complex (4.7)

+

OTf

-

N Au N

C

N

S

O

NH

O

HN

(4.7)

Fig. 8.66 Structure of gold (III)–NHC complex (4.8)

O

Cl Cl

Au N

C

Cl

N C

N

(4.8)

their stability, lipophilicity, uptake ability by cells, and the anticancer activity of these gold complexes was studied in detail. On screening the complexes for their in vitro activity, it was found that both the gold complexes demonstrated lower toxicity, reduce resistance factors, and increased anticancer activity than that of cisplatin. Complex 4.6 exhibited higher antiproliferative activity against the tested cancer cells, including cisplatin-resistant cells. Importantly, this complex also showed low toxicity against the nontumoral HL-7702 and WI-38 cell lines. It was confirmed that 4.6 mainly affected mitochondria, where it initiated a cascade of events indicative of mitochondrial dysfunction, including ATP depletion, mitochondrial membrane depolarization, increased ROS levels, and ER stress. Additional experiments with 4.6 implicated both prodeath autophagy and the intrinsic apoptotic pathway in the cytotoxicity of the complexes [77]. A biotin-linked cyclometalated gold(III) complex (4.7; Fig. 8.65) has been described and coupled with avidin resulting in a conjugate species capable of binding to proteins and DNA [78]. Both the complex and its avidin conjugate presented antiproliferative activity towards a panel of cancer cell lines (HeLa, HepG2, MDA-MB-231) with IC50 values ranging from 1.1 to 6.9 μM. The cyclometalated Au(III)–avidin conjugate is luminescent (520 nm emission with a lifetime of 1.8 ms in open air in phosphate-buffered saline (PBS) solution), so it was possible to assess its localization in the cytoplasm with negligible nuclear uptake. Similarly another gold(III)–NHC complex 4.8 (Fig. 8.66) was recently prepared by Haque and coworkers [79]. This complex showed a remarkable in vitro cytotoxic activity against MCF-7, PC3, and U937 with IC50 values in the nanomolar range (0.00031  0.00002 μM, 0.00034  0.00002 μM, 0.19  0.002 μM, respectively). Dinuclear gold(I) complexes (4.9–4.11) containing both a bridging bis N-heterocyclic carbene (Fig. 8.67) and a diphosphine ligand were reported by Zou [80]. These complexes showed good stability in the presence of thiols at blood concentrations, favorable thiol reactivity and were able to inhibit TrxR through a

136 Fig. 8.67 Structure of dinuclear gold(I)–carbene complexes (4.9–4.11)

8

nBu N

C

Classes of Gold Complexes

N

N

C

Au

Au

P

P

2+

N nBu

2X

−

4.9; X = PF6 4.10; X = Cl 4.11; X = OTf

Fig. 8.68 Structure of dinuclear gold(I)–carbene complexes (4.12)

AuCl N C

N

AuCl

N

C N

(4.12)

tight-binding mode [81]. In particular, complex 4.9 showed good antiproliferative activity in vitro towards MCF-7, nasopharyngeal carcinoma (SUNE1), lung adenocarcinoma (H1975), and mouse melanoma (B16-F10) cell lines with IC50 values ranging from 1.3  0.2 to 3.2  1.0 μM. It is worth noting that complex 4.9 was also used for in vivo studies giving a significant tumor inhibition in two independent animal models: HeLa xenografts (62%, p < 0.05 after i.p. administration with 15 mgkg1 of complex 4.9 once every 2–3 days for 8 days) and highly aggressive mouse B16-F10 melanoma with no detectable side effects. Moreover, complex 4.9 was the first example of gold(I) complex able to inhibit cancer stem cell activity and in vivo angiogenesis, as assessed by immune histochemical detection of the CD31 (cluster of differentiation 31) of the blood microvessels in the tumor tissue of mice bearing HeLa xenografts. A reduction of the average number of microvessels per microscopic field of treated tumor with respect to the control was also observed (4.34, p < 0.05). Another dinuclear complex 4.12 (Fig. 8.68) was obtained by reaction of the ligand 1,10 -(1,2-ethylene)-3,30-dimethyldiimidazolium dibromide with Ag2O and the gold salt Au(Cl)(tetrahydrothiophene). The resulting complex 4.12 is a non-electrolytic, neutral compound with two coordinated chloride ligands and coordinated two different gold atoms in a bridging mode. The synthesized complex 4.12 and the free ligand were tested against six tumor cell lines (MCF-7, PC3, A459, HeLa, HT-29, and 4 T1, murine breast tumor line) and cytotoxicity was evaluated by sulphorhodamine-B (SRB) assay in order to differentiate between the cytostatic effect (reduction in cell proliferation) and the cytotoxic effect (decrease in the number of viable cells). Complex 4.12 displayed a good cytostatic activity against the MCF7, PC3, and HeLa cell lines (GI50 0.6, 1.3, and 6.4 μM, respectively), although it did not appear to be cytotoxic at the concentrations used in these

8.4 Organometallic Gold Complexes X Au R' N C N

R

R

137

4.13; R = 2-MeO, R' = Et, X = Br 4.14; R = 3-MeO, R' = Et, X = Br 4.15; R = 4-MeO, R' = Et, X = Br 4.16; R = 2-F R' = Et, X = Br 4.17; R = 3-F R' = Et, X = Br 4.18; R = 4-F R' = Et, X = Br 4.19; R = H R' = Et, X = Br 4.20; R = 4-OH R' = Et, X = Br 4.21; R = 4-MeO R' = PhCH2, X = Cl 4.22; R = 4-F R' = PhCH2, X = Cl

Br Au N

C

N

4.23

Fig. 8.69 Structure of complexes 4.13–4.23 Table 8.19 IC50 values (μM) of complexes 4.13–4.23, cisplatin and auranofin against different human cancer cells, and TrxR inhibition (IC50 (nM)) Compound Ligand 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 Auranofin Et3PAuCl Cisplatin

Cell Lines MCF 7 >50 1.2  0.1 1.6  0.6 1.4  0.1 0.80  0.06 3.1  0.1 1.1  0.3 0.87  0.07 4.5  0.3 2.6  0.3 1.8  0.1 3.0  0.4 1.1 3.2 1.6  0.5

MDA-MB-231 >50 2.4  0.5 2.9  0.3 3.7  0.9 1.7  0.9 6.4  0.1 3.9  0.1 3.1  0.5 >20 3.3  0.7 2.4  0.1 6.9  0.8

7.8  0.8

HT-29 >50 3.1  0.4 4.2  1.0 2.9  0.1 3.3  1.0 4.2  0.3 2.3  0.1 3.3  0.7 17.0  2.8 4.3  0.1 3.2  0.3 7.7  0.8 2.6 5.3 4.1  0.3

Inhibition of TrxR TrxR (IC50 (nM)  S.D.) >50,000 597.5  89.6 668.3  38.1 1505.5  27.3 703.9  61.5 1202.0  110.3 815.4  74.1 1036.1  40.4 4371.3  322.2 802.7  68.1 374.4  9.0 379.8  105.1 18.6  7.2 25.8  13.0

experiments (LC50 > 30 μM). In addition, the activity of the free ligand was very low as it inhibited cell growth (GI50/TGI) at concentrations below 20 μM only on PC3 prostate cell line. In 2011 Gust et al. reported a series of neutral gold(I)–NHC halide complexes derived from 4,5-diarylimidazoles 4.13–4.23 (Fig. 8.69) [82]. All complexes were screened against MCF-7, MDA-MB-231, and HT29 cancer cells. The comparison with the known drugs auranofin, cisplatin, and with Et3PAuCl show that, generally, the exchange of the triethylphosphine ligand with NHC, as well as the exchange of chloride with bromide did not significantly affect the activity of gold derivatives, whereas the introduction of phenyl rings at different positions of the heterocycle increased the growth inhibitory effects. Complexes 4.16 and 4.19 were the most active within the series, even more active than cisplatin. As outlined in Table 8.19, methoxy substituents at the aromatic rings did not change the growth inhibitory properties of complex 4.19, while F substituents in the ortho-position of complex

138 Fig. 8.70 Structures of gold (I) complexes (4.24–4.32)

8

R N N R`

Au

R N

+

X

Classes of Gold Complexes

−

N R`

4.24: R = R` = Me, X = Br 4.25: R = Me, R` = Et, X = PF6 4.26: R = R` = iPr, X = Cl 4.27: R = R` = tBu, X = Cl n 4.28: R = R` = Bu, X = Cl 4.29: R = R` = Cy, X = Cl 4.30: R = R` = nPr, X = Br 4.31: R = R` = Et, X = Br

Fc +

N N

Au

N

BF4

−

N

Fc 4.32: Fc = Ferrocenyl

4.16 increased the activity against MDA-MB-231 cells and reduced the activity at all cell lines in meta-position of complex 4.17. The results of complex 4.18 bearing a 4-F substituent were comparable to those of complex 4.15 (unsubstituted) and complex 4.15 (4-OCH3). Exchange of the 4-OCH3 groups by more hydrophilic 4-OH groups strongly decreased the cytotoxicity of complex 4.20. The substituents at the nitrogen atoms play a less significant role because complexes with either a N1-ethyl chain or a bulky N1-benzyl moiety yielded nearly identical results. With the exception of the complex 4.20, all gold complexes displayed activity against TrxR. In 2008, Berners-Price et al. developed a new approach in the design of antitumor organometallic complexes capable of influencing the mitochondria and inhibit the enzyme TrxR [83]. Lipophilic cationic Au(I)–NHC complexes 4.24–4.32 were synthesized displaying two-step ligand exchange reactions with cysteine and selenocysteine with the release of NHC ligands (Fig. 8.70). The rate constants for the reactions with selenocysteine were 20- to 80-fold higher than those with cysteine at physiological pH. The logP of these complexes were 0.02 and 0.84, respectively. Antiproliferative assessments were performed on breast cancer cell lines (MDAMB231 and MDA-MB-468) and normal human mammary epithelial cells (HMEC) displaying selectivity toward infected cells. Among the synthesizes complexes 4.26–4.31, complex 4.26 exhibited the highest cytotoxicity (Fig. 8.62) which can be attributed to its accumulation in mitochondria of cancer cells, instigating cell death by a mitochondrial apoptotic pathway and inhibited TrxR activity, but not the closely related and Se-free enzyme GR. Caspase-3 and -9 proteases that have been linked to the mitochondrial death pathway were detected in complex 4.26 treated cells confirming the apoptosis. A 50% inhibition of the TrxR activity was evident with 5 μM of complex 4.26. In a study by Heinz Gornitzka and coworkers, three new heterobimetallic gold(I)– ruthenium(II) complexes 4.33–4.35 (Fig. 8.71) containing heteroditopic bipyridineNHC ligands were synthesized and their in vitro cytotoxic activities were investigated [84]. The complexes involved the tethering of a gold NHC unit with a Ru(bipy)3 unit which was having luminescence properties. The synthesis procedure

8.4 Organometallic Gold Complexes

139

Fig. 8.71 Structure of heterobimetallic gold(I)– ruthenium(II) complexes (4.33–4.35)

2+

R Au

Ru N

−

N

N N

2X

N N

Cl

4.33: R = Me; X = PF6 4.34: R = nBu; X = Cl, Br 4.35: R = Bn; X = PF6

involved the complexation of Ru(II), starting from Ru(bipy)2Cl2, by the substituted imidazolium salts giving the imidazolium Ru-derivatives, followed by transformation of the imidazolium into the NHC gold(I) unit to give 4.33–4.35 complexes. Whereas the imidazolium Ru-complexes were inactive against the cell line Hep3B (IC50 > 100 μM), heterobimetallic 4.33–4.35 derivatives showed a moderate activity (IC50 ¼ 30.4, 17.1 and 18.9 μM, respectively), anyway lower in comparison to previously synthesized analogous homonuclear gold complexes. N-heterocyclic carbenes (NHC) are ubiquitous ligands which are largely exploited for their potential transition metal complexes for medicinal applications. Keeping this fact in mind a series of mononuclear and dinuclear compounds 4.35–4.40 (Fig. 8.72) were synthesized and screened for their antiproliferative properties in human ovarian cancer cell lines (A2780) and in human embryonic kidney HEK293T cells. Among the complexes, three complexes were having a pentafluorophenolic ester group as a possible “activable” moiety for further functionalization. Biological evaluation studies revealed a dose-dependent inhibition of cell growth in all cell lines with IC50 values ranging from ca. 2–122 μM (Table 8.20). Differently from auranofin, which showed high cytotoxicity in both cell lines, compounds 4.33–4.40 displayed selectivity towards cancer cells, being at least twofold less cytotoxic in the non-tumorigenic HEK293T cells [85]. A series of gold(I) pyrrolidinedithiocarbamato complexes 4.41–4.46 (Fig. 8.73) comprising a mono or a bidentate carbene ligand have been reported [86]. The in vitro anticancer activities were determined toward a panel of human cancer cell lines (A2780 and its cisplatin-resistant variant A2780cis, HepG2, U-87 MG) and a normal MadineDarby canine kidney epithelial cells (MDCK) cell line. Due to its rigid scaffold, the very active dinuclear gold(I) pyrrolidinedithiocarbamato complex 4.41 enabled a zinc(II)-based metal-organic framework (Zn-MOF) to be used as a carrier in facilitating its uptake and release in solutions. The use of a Zn-MOF as a carrier, in this case a biodegradable blue-fluorescent zinc-based MOF composed of biocompatible substances including zinc(II), adenine, and a benzene-1,3,5-tricarboxylic acid linker, was a new approach to enhance the in vivo bioavailability, modulate the release, and confine the biological activities of metal-based anticancer drugs. The biocompatible Zn-MOF utilized did not exert cytotoxicity towards

140

8

Classes of Gold Complexes O

F

Me N

F

O

N

F F

F O

F

F O

N

F F

O N H

Me N

Au Cl

N Me (4.38)

O

(4.36) X = Cl (4.37) X = PPh3

Me N

F

N Au

N Au X

F

F

F

O Me N

O

F

F

N Au Cl

(4.39)

P

AuC

l

(4.40)

Fig. 8.72 Structures of complexes 4.36–4.40 Table 8.20 IC50 (μM) values of compounds 4.36–4.40 in two different cell lines

Complex 4.36 4.37 4.38 4.39 4.40 Auranofin

Cell lines A2780 53.0  2.4 5.2  0.7 19.7  2.0 – 2.2  0.4 1.2  0.5

HEK293T 122.0  6.2 11.2  0.5 41.5  1.4 – 6.2  0.7 1.7  0.3

A2780cis cell line, whereas 4.41 Zn-MOF for short co-incubation times (2–12 h) showed a reduced toxicity with respect to complex 4.41 (cellular survival percentages of around 95% versus 28%) but for longer co-incubation periods (>24 h) a decrease of cellular survival percentage to around 50% was observed. Three new linear gold(I) complexes 4.47–4.49 (Fig. 8.74) were prepared by the reaction of equimolar amounts of 1,3-bis(2,6-di-isopropylphenyl)imidazol-2ylidenegold(I)chloride, [(Ipr)Au(Cl)] with sodium dimethyldithiocarbamate monohydrate, sodium diethyl dithiocarbamatetrihydrate, and dibenzyl dithiocarbamate, respectively. The structures of the complexes have been determined by single X-ray crystallography. All gold(I) complexes (4.47–4.49) are iso-structural,

8.4 Organometallic Gold Complexes

R

N

N

N

Au S

N

141

R N R

N Au S

Au S

R

S Au S

N

N

4.41: R = Bu

4.43: R = nBu 4.44: R = Me

4.42: R = Me

4.45: R = iPr

n

N

N

S

N

(4.46)

Fig. 8.73 Structures of gold(I) pyrrolidinedithiocarbamato complexes (4.41–4.46)

N N

Au

S

S N

N N

(4.47)

Au

S

S

N

N

N

(4.48)

Au

S

S N

(4.49)

Fig. 8.74 Structures of novel gold(I)–carbene complexes (4.47–4.49) Table 8.21 IC50 values (μM) of gold(I) complexes (4.47–4.49) against A549, HeLa, and HCT15 cancer cell lines

Complex 4.47 4.48 4.49 Cisplatin

A549 133.9 91.7 139.9 41.6

HeLa 108.3 79.6 124.6 19.4

HCT15 41.1 24.5 27.4 29.5

i.e. linear geometry. The complex (4.47) crystallized in monoclinic space group “P21/a,” while the complexes (4.48) and (4.49) crystallized in orthorhombic space group “Pna21.” All the complexes were evaluated for their cytotoxicity profiles against a panel of cell lines like A549 (human lung carcinoma), HCT15 (human colon cancer), and HeLa (human cervical cancer) cell lines and compared to the standard anticancer drug cisplatin using MTT assay (Table 8.21). These gold (I) complexes showed moderate anticancer activities against all the selected cell lines. The high IC50 values against all three cancer cell lines were probably due to strong Au–C bond and bulky carbene ligand. These factors slowed down the dissociation of gold complex and its interaction with cancer cells, that could be the major factor for slow inhibition of growth of cancer cell lines for all (4.47, 4.48, and 4.49) complexes. The in vitro cytotoxicity of gold complexes (4.48 and 4.49) was quite promising and better than cisplatin. All the three complexes showed good selectivity and growth inhibition of HCT15 cancer line [87].

142

8 +

OTf

−

+

2 2OTf

N

N

Au N

Classes of Gold Complexes

N Au

N

Au

N

N

(4.50)

N

N

(4.51) + +

2 2OTf N

N

−

S

N

Au

N

OTf

−

N Au

N

−

Au N

(4.52)

N

N

(4.53)

Fig. 8.75 Structure of complexes 4.50–4.53

In another study by Che and coworkers, the cytotoxic properties of some newly synthesized complexes 4.50–4.53 (Fig. 8.75) were evaluated against a panel of human cancer cell lines including HepG2, KB, and its camptothecin-resistant cell line KB-CPT-100, NCI-H460, SUNE1, and non-cancerous originated cells (lung fibroblast cells, CCD-19Lu) (Table 8.22). These complexes exhibited promising cytotoxicity with IC50 values in the range of 0.17–28 μM, equivalent to those shown by cisplatin and camptothecin. Among the synthesized complexes, 4.50 exhibited the highest cytotoxic activity (~18- to 28-fold higher than that of cisplatin). It was reported that all the synthesized gold(III) complexes were selective to tumor cell lines. The in vivo anticancer activity of complex 4.50 was examined by treating nude mice models bearing primary liver cancer (PLC). The treatment significantly blocked tumor growth in comparison to the control vehicles, without apparent induced toxic side-effects from complex 3.81 [88]. Maftei et al. reported the synthesis of a series of nine gold(I) complexes 4.54–4.62 (Fig. 8.76) with 1,2,4-oxadiazole containing N-heterocyclic carbene ligands [89]. The 1,2,4-oxadiazole unit was chosen as it is present in many biologically active natural products and was incorporated in the NHC framework, along with a variety of other biologically active moieties, in order to tune the lipophilicity of the resulting gold(I) complexes. In vitro evaluation of antitumor activity against a panel of twelve human tumor cell lines (GXF 251, LXFA629, LXFL529, MAXF 401, MEXF 462, 22RV1, UXF1138, OVXF 899, PAXF1657, PRXF 22Rv1, PXF 1752, RXF 486) exposed very high mean potency (IC50 < 0.1 μM) and tumor sensitivity against OVXF 899 and RXF 486 cell lines for six complexes (4.54–4.62). Additionally, good influence was spotted for complex 4.56 (with an IC50 ¼ 0.208 μM) followed by complex 4.54 (IC50 in the range from 2 to 20 μM). Furthermore, two

Compound 4.50 4.51 4.52 4.53 Cisplatin Camptothecin

Cell lines HepG2 0.37  0.03 7.9  0.6 1.1  0.1 18  1.5 10.5  0.5 0.57  0.08 KB 0.56  0.10 10  0.6 2.3  0.3 20  1.0 9.8  2.8 0.52  0.12

KB-CPT-100 1.2  0.2 28  2 12  1.3 9.4  0.5 10.1  2.5 33  7.8

SUNE1 0.25  0.02 3.3  0.6 3.0  0.4 9.4  0.8 4.9  0.8 0.37  0.06

Table 8.22 IC50 (μM) of complexes 4.50–4.53, cisplatin, and camptothecin against selected human cell lines NCI-H460 0.17  0.05 3.0  0.3 1.2  0.3 11  1.0 3.5  1.0 0.09  0.03

CCD-19Lu 25  3.8 >100 16  2.5 48  14 51  7 99  10

8.4 Organometallic Gold Complexes 143

144

8 Cl Au

N O N

N R

N

N O R=

Classes of Gold Complexes

R = Me (4.55)

R=

N (4.54)

(4.56) R=

R=

N

NH

R=

(4.58)

(4.59)

(4.57) AcO

N

N R=

R=

R=

OAc O

H

OAc OAc

(4.62)

(4.61)

(4.60)

Fig. 8.76 Structure of complexes 4.54–4.62

N

N

N

N

Au Cl

N O

N Au Cl

N O N

(4.63)

N

N

N O

N

N

Au

Au

N O N

N

N

N

N

N

N O N O N N

(4.64)

Fig. 8.77 Structure of complexes 4.63 and 4.64

new binuclear gold 1,2,4-oxadiazole derived complexes (4.63 and 4.64) (Fig. 8.77) were also synthesized, displaying lower activity with respect to analogous mononuclear species. The NHC ligand 2-pyridin-2-yl-2H-imidazo [1,5-a]pyridin-4-ylium and its derivative 1-methyl-2-pyridin-2-yl-2H-imidazo [1,5-a]pyridin-4-ylium were used for the synthesis of monocarbene [(NHC)AuCl] (4.67) or cationic [(NHC)2Au]PF6 bis-carbene complexes (4.65 and 4.69, respectively) that, in turn, in the presence of [Au(SMe2)Cl], gave the Au(III) derivatives [(NHC)AuCl3] (4.66 and 4.68) or

8.4 Organometallic Gold Complexes +

145

−

PF6

N

N

Au

N

N N

N

N

N

N

(4.66) +

N

N

(4.67) +

−

PF6

N

N

N

(4.69)

+

N

Au

N N

N

N

(4.70)

−

PF6

N

N

N N

−

Cl Au Cl

Au

(4.68)

PF6

N

N

N

N

N

N (4.65)

N

Cl Cl Au Cl

Cl Au

Cl Cl Au Cl

N

N

(4.71)

Cl Cl Au Cl N

N

(4.72)

Fig. 8.78 Molecular structure of gold complexes (4.65–4.72)

[(NHC)2AuCl2]PF6 (4.70) (Fig. 8.78) [90–92]. Generally linear Au(I) complexes exhibited higher cytotoxicities than tetracoordinated Au(III) complexes, and in most cases both displayed higher cytotoxicity than cisplatin (Table 8.23). The lower potency of Au(III) complexes could be due to the reduction of Au(III) complexes via cellular compounds containing thiols. Mechanism studies on cellular death suggested in many cases an apoptotic pathway. The strongest difference in IC50 was seen for 4.65, 4.66 pair, taking into account that Au(I) complex is positively charged, while Au(III) derivative is neutral. In addition, the biological activity of Au (I) and Au(III) derivatives containing 1-naphthyl-2-pyridin-2-yl-2H-imidazo [1,5-a] pyridin-4-yliumwas reported [93]. The cytotoxicity of the complexes 4.71 and 4.72 (Fig. 8.78) was tested against A549, HCT-116, and MCF-7 tumor cell lines. Again, the Au(I) complex 4.71 was found more potent than the Au(III) one (Table 8.23). Biological profiles revealed that free ligands did not show any activity. A new N-heterocyclic carbene gold(I) complex 4.73 (Fig. 8.79) was synthesized and examined for its cytotoxicity and its in vitro and in vivo anti-melanoma activity. Viability of cancer cells was determined by MTT assay upon treatment with various concentrations of the complex 4.73 in a dose- and time-dependent manner. The complex 4.73 markedly inhibited the growth of HCT 116, HepG2, and A549 and induced apoptosis in B16F10 cells with nuclear condensation, DNA fragmentation, externalization of phosphatidylserine, activation of caspase-3 and caspase-9, PARP cleavage, downregulation of Bcl-2, upregulation of Bax, cytosolic cytochrome c elevation, ROS generation, and mitochondrial membrane potential loss, indicating the involvement of an intrinsic mitochondrial death pathway. Further, upregulation

Complex 4.65 4.66 4.67 4.68 4.69 4.70 4.71 4.72

Cell Lines B16-F10 2.13  0.08 4.87  0.04 HCT-116 2.25  0.14 4.73  0.34 5.08  3.8 5.98  2.17 3.6  4.1 5.9  3.6 5.2  1.26 6.78  2.01

Table 8.23 IC50 values (μM) of complexes 4.65–4.72 at 24 h HepG2 2.44  0.35 5.12  0.26 4.91  3.6 7.01  1.65 3.7  2.3 5.1  3.8

MCF-7 2.15  0.37 5.34  0.32 5.18  1.35 4.96  1.43 4.7  0.8 6.2  1.4 6.78  1.82 7.72  2.25

PBMCs

>10 >10

A549

5.23  2.96 6.56  1.42 5.2  1.5 5.2  3.0 3.2  0.78 5.2  1.25

146 8 Classes of Gold Complexes

8.4 Organometallic Gold Complexes

147 PF6 N

O HN

NH

Au

O

N (4.73)

Fig. 8.79 Structure of N-heterocyclic carbene gold(I) complex (4.73)

O N

N O

O

N

N (4.74)

Au

N

N

I O

R

N

N

−

O

BF4 +

Au

N N

N N

O

R R=H (4.75) (4.76) Vinyl (4.77) E-propenyl Ph (4.78) (4.79) p-C6H4NO2 p-C6H4CO2Me (4.80) (4.81) p-C6H4CF3

Fig. 8.80 Structures of gold(I)–NHC complexes (4.74–4.81)

of p53, p-p53 (ser 15), and p21 indicated the role of p53 in complex 4.73 mediated apoptosis. This complex caused reduction of tumor size and caused upregulation of p53 and p21 along with downregulation of NF-κB (p65 and p50), VEGF, and MMP-9. These results suggest that it induced anti-melanoma effect in vitro and in vivo by modulating p53 and other apoptotic factors [94]. A new series of gold(I) NHC complexes 4.74–4.81 (Fig. 8.72) from xanthinebased ligands were synthesized, characterized, and evaluated for their in vitro antiproliferative properties in human cancer cells and non-tumorigenic cells and ex vivo toxicity studies in healthy tissues. Among the synthesized complexes, complex 4.77 appeared to be selective against human ovarian cancer cell lines and less toxic in healthy organs. The IC50 values of gold(I)–NHC complexes against various cancer cell lines and non-cancerous cells HEK-293 T are given in Table 8.24. To gain preliminary insights into their actual mechanism of action, two biologically relevant in cellulo targets were studied, namely, G-quadruplex DNA and a pivotal enzyme of the DNA damage response (DDR) machinery (poly-(adenosine diphosphate (ADP)-ribose) polymerase 1 (PARP-1), strongly involved in the cancer resistance mechanism). The results specified that the complex 4.77 acted as an effective and selective G-quadruplex ligand while being a modest PARP-1 inhibitor (i.e., poor DDR impairing agent) and thus provided preliminary insights into the molecular mechanism that underlies its antiproliferative behavior [95] (Fig. 8.80, Table 8.24).

148

8

Classes of Gold Complexes

Table 8.24 The IC50 values of gold(I)–NHC complexes (4.74–4.81) against various cancer cell lines and non-cancerous cells HEK-293 T compared to cisplatin Compound 4.74 4.75 4.76 4.77 4.78 4.79 4.80 4.81 Cisplatin

IC50 (μM) A2780 37  15 16.2  2.1 26.0  2.2 28.4  4.0 12.4  0.2 23.4  4.0 21.9  2.4 13.1  2.4 5.2  1.9

A2780/R 49  15 15.6  2.7 17.2  1.7 25.8  1.7 17.1  0.4 20.7  2.8 22.1  3.2 17.8  1.7 35  7

O

SKOV3 37.3  9.8 62.7  7.8 60  14 25.6  4.5 21.8  2.3 53.8  4.6 37.6  7.2 30.3  3.4 13.2  3.5

A549 25.4  2.2 >100 52.8  5.2 46.7  5.6 47.7  0.6 90.0  4.8 56.0  7.9 26.1  2.1 8.0  0.5

O

Au P O

O

HEK-293 T 22.9  6.9 >100 42.0  4.0 38.7  8.3 32.5  4.4 82  13 84  11 37.9  2.1 11.0  2.9

Au P

HO

HO (4.82)

(4.83) Au

P

O

O O

Au P

HO HO

(4.84)

(4.85)

Fig. 8.81 Structures of gold(I) alkyne complexes (4.82–4.85)

A series of gold(I)alkyne complexes (4.82–4.85) (Fig. 8.81) were evaluated for their antiproliferative activities against breast cancer (MCF-7), colon cancer (HT-29), liver cancer (HepG2), and ovarian cancer (A2780) cells using the MTT assay [96]. All the synthesized complexes 4.82–4.85 revealed lower IC50 values in A2780 cancer cells than in other three cancer cells. However, gold(I) complex containing an oleanolic acid derivative (4.85) exhibited the highest cytotoxicity in A2780 cancer cells with an IC50 value of 10.24 μM, compared to that of its parent compound oleanolic acid (IC50 ¼ 78.42 μM) and other compounds. The IC50 value of complex 4.85 in A2780 cancer cells was like that of cisplatin (IC50 ¼ 6.49 μM) and auranofin derivative Et3PAuCl (IC50 ¼ 5.74 μM). This complex was two to sevenfold more active against A2780 cells than against HepG2 cells (IC50 ¼ 27.11 μM), MCF-7 cells (IC50 ¼ 33.26 μM), and HT-29 cells

8.4 Organometallic Gold Complexes

149

Fig. 8.82 Structure of gold complexes (4.86, 4.87) from NHC ligands

O Au

O

N N

(4.86)

O Au

O

P

(4.87)

Fig. 8.83 Structure of organometallic Au(I) complex (4.88)

O N

N O

N

+

O

N

Au

N N

N N

BF4

-

O

(4.88)

(IC50 ¼ 81.75 μM). In addition, the lower anticancer activity of oleanolic acid showed that the cytotoxicity of complex 4.85 is primarily because of the alkynyl gold(I) PPh3 scaffold. Furthermore, it was revealed that the complex 4.85 can inhibit thioredoxin reductase (TrxR) enzyme activity to elevate ROS, mediate endoplasmic reticulum stress (ERS) and mitochondrial dysfunction, finally leading to the A2780 cells cycle arrest and apoptosis. It is worth noticing that complex 4.85 inhibited the growth of A2780 xenograft tumor, accompanied with no clear weight loss in mouse. A series of gold(I) complexes from NHC ligands (4.86, 4.87) (Fig. 8.82) were prepared, characterized, and investigated for their anticancer activity against MCF-7, MDAMB 231 breast cancer cells, and HT-29 colon cancer cells with cisplatin as the reference drug [97]. Complex 4.87 displayed more cytotoxic activity than complex 4.86. Complex 4.87 was 4.2-, 3.7-, and 1.7-fold more active than cisplatin towards HT-29, MDA-MB-231, MCF-7 cancer cell line, respectively. The activity of cytosolic TrxR (TrxR1) was inhibited by the complexes 4.86 and 4.87 with IC50 values in the nanomolar range and more active than auranofin. Also, the complexes 4.86 and 4.87 were 10 times more active than auranofin against mitochondrial thioredoxin reductase (TrxR2). For both TrxR1 and TrxR2, complex 4.87 was more active than complex 4.86. However, 4.86 and 4.87 were less active against glutathione reductase (GR) than auranofin that confirmed the inhibiting ability of complexes 4.86 and 4.87 for purified TrxR. Both complexes 4.86 and 4.87 increased the ROS levels to 5.1 and 8.3 folds as compared with the control and more than antimycin as positive control. Therefore, the mechanism of anticancer activity of the complexes 4.86 and 4.87 could be due to strong inhibition of thioredoxin reductase with the increased ROS generation. A cationic organometallic Au(I) complex 4.88 (Fig. 8.83) was reported for its indepth investigation of the mechanisms of reactivity and anticancer action [98]. The results showed a multimodal mode of action, involving metallodrug binding to

150

8

Fig. 8.84 Structure of ligand (4.89) and gold(I) alkenyl complexes (4.90, 4.91)

Classes of Gold Complexes

Au O

O

PR3

O O Au

(4.89)

PR3

R = Ph (4.90), Cy (4.91)

Table 8.25 IC50 value of ligand (4.89) and gold(I) alkenyl complexes (4.90, 4.91) against MCF-7, HEPG-2, PC-3, and MOLT-4 cancer cell lines Compound 4.89 4.90 4.91 Cisplatin

IC50  SD (μM) MCF-7 283.48  0.13 22.58  0.03 18.63  0.03 16.00  0.06

HEPG-2 245.78  0.15 26.01  0.03 27.94  0.03 –

PC-3 227.63  0.06 27.46  0.01 27.31  0.01 39.99  0.05

MOLT-4 269.02  0.10 25.91  0.03 20.28  0.03 –

nuclear and cytoplasmic components in human ovarian cancer cells A2780. The antiproliferative effect of complex 4.88 against A2780 cancer cells displayed a half maximal effective concentration (EC50) of 13.3  1.1 μM after 24 h treatment. Proteomic studies exposed that treatment with complex 4.88 at sub-cytotoxic concentrations brought the regulation of proteins involved in stress-induced transcriptional activation, as well as in telomere function. In parallel, in the nuclear fraction, downregulation of proteins related to actin stress-fibers was observed which were further supported by pharmacological assays, fluorescence microscopy, and cellular accumulation experiments. 3,6-Diethynyl-9,10-diethoxyphenanthrene (4.89) and two binuclear gold (I) alkynyl complexes (4.90) and (4.91) (Fig. 8.84) were synthesized and characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. UV–Vis spectroscopy studies showed strong π-π* transitions in the near UV region which shifted by about 50 nm upon coordination at the gold centers [99]. The emission spectrum of 4.89 displayed an intense fluorescence band at 420 nm which red shifted, slightly upon coordination of 4.89 to gold. Binding studies of 4.89, 4.90, and 4.91 with calf-thymus DNA revealed that 4.89, 4.90, and 4.91 have 40% stronger binding affinities than the commonly used intercalating agent ethidium bromide. The molecular docking scores of 4.89, 4.90, and 4.91 with B-DNA advocated a similar trend in behavior to that detected in the DNA binding study. Unlike the ligand 4.89, promising anticancer activity for 4.90 and 4.91 were observed against four cancer cell lines (MCF-7, HEPG-2, PC-3, and MOLT-4, Table 8.25); the DNA binding capability of the precursor alkyne was maintained, and its anticancer efficiency improved by the gold centers. Such phenanthrenyl complexes could be promising candidates in certain biological applications because

8.4 Organometallic Gold Complexes R

151

R N N

Au

R N

Cl

N

R

Au

R R= F (4.92) MeO (4.93)

N

Au

I

R (4.94) (4.95)

(4.96) (4.97) R

R N N R

N

Br

N Au

(4.98) (4.99)

NCO

N R

Au

OAc

(5.0) (5.1)

Fig. 8.85 Structure of Halo and pseudohalo gold(I)  NHC complexes (4.92–5.1) Table 8.26 IC50 values of the test compounds (4.92–5.1) against HepG2, SMMC-7721, and Hep3B Cells after 72 h of incubation

Compound 4.92 4.93 4.94 4.95 4.96 4.97 4.98 4.99 5.0 5.1 Cisplatin Auranofin

HepG2 >20 15.50  1.21 3.72  0.15 4.44  0.32 1.08  0.12 0.50  0.02 12.10  1.82 3.12  0.16 1.22  0.11 0.71  0.05 1.35  0.12 1.74  0.32

SMMC-7721 16.14  2.79 7.70  0.41 9.41  0.71 10.15  1.29 1.88  0.15 0.92  0.08 8.76  0.82 3.83  0.18 2.47  0.24 1.07  0.09 3.54  0.28 2.24  0.24

Hep3B >20 7.90  0.89 4.95  0.11 4.71  0.22 1.05  0.10 0.52  0.04 7.86  1.22 2.41  0.31 1.21  0.11 0.77  0.08 1.15  0.11 1.93  0.21

the two components (phenanthrenyl bridge and metal centers) can be altered independently to improve the targeting of the complex, as well as the biological and physicochemical properties. Halo and pseudohalo gold(I)  NHC complexes (NHC  Au  X) (X ¼ Cl, Br, I, NCO, and OAc) (4.92–5.1; Fig. 8.85) have been synthesized, structurally characterized, and investigated for their biological profiles [100]. The antiproliferative activity of all synthesized gold complexes on HepG2, SMMC7721, and Hep3B cancer cells with auranofin and cisplatin as the positive controls has been performed (Table 8.26). The most active complex, the iodo-complex (4.97) was at least twofold more cytotoxic than cisplatin and auranofin against hepatocellular carcinoma cells. In vivo studies showed that complex 4.97 showed a significantly higher anticancer efficiency (IRT ¼ 75.7%) than cisplatin (IRT ¼ 44.4%) in a

152

8

Classes of Gold Complexes

HepG2 xenograft mouse model and ameliorated liver injury caused by CCl4 in chronic hepatocellular carcinoma cells. Further studies exposed that the complex 4.97 could inhibit the expression of the thioredoxin reductase (TrxR) both in vitro and in vivo, block the HepG2 cells in the G2/M phase, induce reactive oxygen species (ROS) production, damage mitochondrial membrane potential (MMP), and promote HepG2 cell apoptosis.

References 1. Berners-Price SJ, Filipovska A (2011) Gold compounds as therapeutic agents for human diseases. Metallomics 3(9):863–873 2. Shaik N, Martinez A, Augustin I, Giovinazzo H, Varela-Ramírez A, Sanaú M, Contel M (2009) Synthesis of apoptosis-inducing iminophosphorane organogold (III) complexes and study of their interactions with biomolecular targets. Inorg Chem 48(4):1577–1587 3. Casini A, Cinellu MA, Minghetti G, Gabbiani C, Coronnello M, Mini E, Messori L (2006) Structural and solution chemistry, antiproliferative effects, and DNA and protein binding properties of a series of dinuclear gold (III) compounds with bipyridyl ligands. J Med Chem 49(18):5524–5531 4. Hartinger CG, Metzler-Nolte N, Dyson PJ (2012) Challenges and opportunities in the development of organometallic anticancer drugs. Organometallics 31(16):5677–5685 5. Milacic V, Dou QP (2009) The tumor proteasome as a novel target for gold (III) complexes: implications for breast cancer therapy, Coord. Chem Rev 253(11–12):1649–1660 6. Sun RWY, Che CM (2009) The anti-cancer properties of gold (III) compounds with dianionic porphyrin and tetradentate ligands. Coord Chem Rev 253(11–12):1682–1691 7. Wein AN, Stockhausen AT, Hardcastle KI, Saadein MR, Peng SB, Wang D et al (2011) Tumor cytotoxicity of 5, 6-dimethyl-1, 10-phenanthroline and its corresponding gold (III) complex. J Inorg Biochem 105(5):663–668 8. Serratrice M, Cinellu MA, Maiore L, Pilo M, Zucca A, Gabbiani C et al (2012) Synthesis, structural characterization, solution behavior, and in vitro antiproliferative properties of a series of gold complexes with 2-(20 -pyridyl) benzimidazole as ligand: comparisons of gold (III) versus gold (I) and mononuclear versus binuclear derivatives. Inorg Chem 51 (5):3161–3171 9. Bindoli A, Rigobello MP, Scutari G, Gabbiani C, Casini A, Messori L (2009) Thioredoxin reductase: a target for gold compounds acting as potential anticancer drugs. Coord Chem Rev 253(11–12):1692–1707 10. Zou T, Lum CT, Chui SSY, Che CM (2013) Gold (III) complexes containing N-heterocyclic carbene ligands: Thiol “switch-on” fluorescent probes and anti-cancer agents. Angew Chem Int Ed 52(10):2930–2933 11. Abbehausen C, Castro JF, Spera MB, Heinrich TA, Costa-Neto CM, Lustri WR et al (2011) Synthesis, spectroscopic characterization, DFT studies and biological assays of a novel gold (I) complex with 2-mercaptothiazoline. Polyhedron 30(13):2354–2359 12. Sun L, Chen H, Zhang Z, Yang Q, Tong H, Xu A, Wang C (2012) Synthesis and cancer cell cytotoxicity of water-soluble gold (III) substituted tetraarylporphyrin. J Inorg Biochem 108:47–52 13. Maiore L, Cinellu MA, Nobili S, Landini I, Mini E, Gabbiani C, Messori L (2012) Gold (III) complexes with 2-substituted pyridines as experimental anticancer agents: solution behavior, reactions with model proteins, antiproliferative properties. J Inorg Biochem 108:123–127 14. Gabbiani C, Cinellu MA, Maiore L, Massai L, Scaletti F, Messori L (2012) Chemistry and biology of three representative gold (III) compounds as prospective anticancer agents. Inorg Chim Acta 393:115–124

References

153

15. Gouvea LR, Garcia LS, Lachter DR, Nunes PR, de Castro Pereira F, Silveira-Lacerda EP et al (2012) Atypical fluoroquinolone gold (III) chelates as potential anticancer agents: relevance of DNA and protein interactions for their mechanism of action. Eur J Med Chem 55:67–73 16. Wang Y, He QY, Sun RWY, Che CM, Chiu JF (2007) Cellular pharmacological properties of gold (III) porphyrin 1a, a potential anticancer drug lead. Eur J Pharmacol 554(2–3):113–122 17. Carotti S, Marcon G, Marussich M, Mazzei T, Messori L, Mini E, Orioli P (2000) Cytotoxicity and DNA binding properties of a chloro glycylhistidinate gold (III) complex (GHAu). Chem Biol Int 125(1):29–38 18. Patel MN, Bhatt BS, Dosi PA (2013) DNA binding, cytotoxicity and DNA cleavage promoted by gold (III) complexes. Inorg Chem Commun 29:190–193 19. Navarro M, Castro W, González S, Abad MJ, Taylor P (2013) Synthesis and anticancer activity of gold (I)-chloroquine complexes. J Mex Chem Soc 57(3):220–229 20. Altaf M, Ahmad S, Kawde AN, Baig N, Alawad A, Altuwaijri S et al (2016) Synthesis, structural characterization, electrochemical behavior and anticancer activity of gold (iii) complexes of meso-1, 2-di (1-naphthyl)-1, 2-diaminoethane and tetraphenylporphyrin. New J Chem 40(10):8288–8295 21. Rodriguez-Fanjul V, Lopez-Torres E, Mendiola MA, Pizarro AM (2018) Gold (III) bis (thiosemicarbazonate) compounds in breast cancer cells: cytotoxicity and thioredoxin reductase targeting. Eur J Med Chem 148:372–383 22. Martín-Santos C, Michelucci E, Marzo T, Messori L, Szumlas P, Bednarski PJ et al (2015) Gold (III) complexes with hydroxyquinoline, aminoquinoline and quinoline ligands: synthesis, cytotoxicity, DNA and protein binding studies. J Inorg Biochem 153:339–345 23. Pantelić N, Stanojković TP, Zmejkovski BB, Sabo TJ, Kaluđerović GN (2015) In vitro anticancer activity of gold (III) complexes with some esters of (S, S)-ethylenediamine-N, N0 -di-2-propanoic acid. Eur J Med Chem 90:766–774 24. Palanichamy K, Sreejayan N, Ontko AC (2012) Overcoming cisplatin resistance using gold (III) mimics: anticancer activity of novel gold (III) polypyridyl complexes. J Inorg Biochem 106(1):32–42 25. Maiore L, Cinellu MA, Michelucci E, Moneti G, Nobili S, Landini I et al (2010) Structural and solution chemistry, protein binding and antiproliferative properties of novel gold (I, III) compounds bearing the saccharinato ligand. J Inorg Biochem 105(3):348–355 26. Bian M, Wang X, Sun Y, Liu W (2020) Synthesis and biological evaluation of gold (III) Schiff base complexes for the treatment of hepatocellular carcinoma through attenuating TrxR activity. Eur J Med Chem 193:112234 27. Adokoh CK (2020) Therapeutic potential of dithiocarbamate supported gold compounds. RSC Adv 10(5):2975–2988 28. Ronconi L, Giovagnini L, Marzano C, Bettf o F, Graziani R, Pilloni G, Fregona D (2005) Gold dithiocarbamate derivatives as potential antineoplastic agents: design, spectroscopic properties, and in vitro antitumor activity. Inorg Chem 44(6):1867–1881 29. Milacic V, Chen D, Ronconi L, Landis-Piwowar KR, Fregona D, Dou QP (2006) A novel anticancer gold (III) dithiocarbamate compound inhibits the activity of a purified 20S proteasome and 26S proteasome in human breast cancer cell cultures and xenografts. Cancer Res 66(21):10478–10486 30. Zhang X, Frezza M, Milacic V, Ronconi L, Fan Y, Bi C et al (2010) Inhibition of tumor proteasome activity by gold–dithiocarbamato complexes via both redox-dependent and-independent processes. J Cell Biochem 109(1):162–172 31. Altaf M, Isab AA, Vančo J, Dvořák Z, Trávníček Z, Stoeckli-Evans H (2015) Synthesis, characterization, and in vitro cytotoxicity of gold (III) dialkyl/diaryldithiocarbamato complexes. RSC Adv 5(99):81599–81607 32. Wang S, Shao W, Li H, Liu C, Wang K, Zhang J (2011) Synthesis, characterization and cytotoxicity of the gold (III) complexes of 4, 5-dihydropyrazole-1-carbothioamide derivatives. Eur J Med Chem 46(5):1914–1918

154

8

Classes of Gold Complexes

33. Dada O, Sánchez-Sanz G, Tacke M, Zhu X (2018) Synthesis and anticancer activity of novel NHC-gold (I)-sugar complexes. Tetrahedron Lett 59(30):2904–2908 34. Nardon C, Schmitt SM, Yang H, Zuo J, Fregona D, Dou QP (2014) Gold (III)-dithiocarbamato peptidomimetics in the forefront of the targeted anticancer therapy: Preclinical studies against human breast neoplasia. PloS one 9(1):e84248 35. Negom Kouodom M, Ronconi L, Celegato M, Nardon C, Marchiò L, Dou QP et al (2012) Toward the selective delivery of chemotherapeutics into tumor cells by targeting peptide transporters: tailored gold-based anticancer peptidomimetics. J Med Chem 55(5):2212–2226 36. Kouodom MN, Boscutti G, Celegato M, Crisma M, Sitran S, Aldinucci D et al (2012) Rational design of gold (III)-dithiocarbamato peptidomimetics for the targeted anticancer chemotherapy. J Inorg Biochem 117:248–260 37. Fontinha D, Sousa SA, Morais TS, Prudêncio M, Leitão JH, Le Gal Y, Lorcy D, Silva RAL, Velho MFG, Belo D, Almeida M, Guerreiro JF, Pinheirog T, Marques F (2020) Gold (III) bis (dithiolene) complexes: from molecular conductors to prospective anticancer, antimicrobial and antiplasmodial agents. Metallomics 12:974–987 38. Mertens RT, Parkin S, Awuah SG (2020) Cancer cell-selective modulation of mitochondrial respiration and metabolism by potent organogold (iii) dithiocarbamates. Chem Sci 11:10465–10482 39. Urig S, Fritz-Wolf K, Réau R, Herold-Mende C, Tóth K, Davioud-Charvet E, Becker K (2006) Undressing of phosphine gold (I) complexes as irreversible inhibitors of human disulfide reductases. Angew Chem Int Ed 45(12):1881–1886 40. Rackham O, Nichols SJ, Leedman PJ, Berners-Price SJ, Filipovska A (2007) A gold (I) phosphine complex selectively induces apoptosis in breast cancer cells: implications for anticancer therapeutics targeted to mitochondria. Biochem Pharmacol 74(7):992–1002 41. Rush GF, Smith PF, Alberts DW, Mirabelli CK, Snyder RM, Crooke ST et al (1987) The mechanism of acute cytotoxicity of triethylphosphine gold (I) complexes: I. Characterization of triethylphosphine gold chloride-induced biochemical and morphological changes in isolated hepatocytes. Toxicol Appl Pharmacol 90(3):377–390 42. Křikavová R, Hošek J, Vančo J, Hutyra J, Dvořák Z, Trávníček Z (2014) Gold (I)triphenylphosphine complexes with hypoxanthine-derived ligands: in vitro evaluations of anticancer and anti-inflammatory activities. PLoS One 9(9):e107373 43. Reddy TS, Privér SH, Mirzadeh N, Bhargava SK (2017) Anti-cancer gold (I) phosphine complexes: cyclic trimers and tetramers containing the P-Au-P moiety. J Inorg Biochem 175:1–8 44. Gomes SQ, Vitoriano L, de Arruda EGR, Ruiz ALTG, Candido T, de Carvalho JE et al (2018) Linear gold (I) complex with tris-(2-carboxyethyl) phosphine (TCEP): Selective antitumor activity and inertness toward sulfur proteins. J Inorg Biochem 186:104–115 45. Mirabelli CK, Hill DT, Faucette LF, McCabe FL, Girard GR, Bryan DB et al (1987) Antitumor activity of bis (diphenylphosphino) alkanes, their gold (I) coordination complexes, and related compounds. J Med Chem 30(12):2181–2190 46. Reddy TS, Privér SH, Mirzadeh N, Bhargava SK (2018) Synthesis of gold (I) phosphine complexes containing the 2-BrC6F4PPh2 ligand: evaluation of anticancer activity in 2D and 3D spheroidal models of HeLa cancer cells. Eur J Med Chem 145:291–301 47. Gambini V, Tilio M, Maina EW, Andreani C, Bartolacci C, Wang J et al (2018) In vitro and in vivo studies of gold (I) azolate/phosphane complexes for the treatment of basal like breast cancer. Eur J Med Chem 155:418–427 48. Horvath UE, Dobrzańska L, Strasser CE, Bouwer W, Joone G, van Rensburg CEJ et al (2012) Amides of gold (I) diphosphines prepared from N-heterocyclic sources and their in vitro and in vivo screening for anticancer activity. J Inorg Biochem 111:80–90 49. Wang Y, Liu M, Cao R, Zhang W, Yin M, Xiao X et al (2013) A soluble bis-chelated gold (I) diphosphine compound with strong anticancer activity and low toxicity. J Med Chem 56 (4):1455–1466

References

155

50. Gandin V, Fernandes AP, Rigobello MP, Dani B, Sorrentino F, Tisato F et al (2010) Cancer cell death induced by phosphine gold (I) compounds targeting thioredoxin reductase. Biochem Pharmacol 79(2):90–101 51. Scheffler H, You Y, Ott I (2010) Comparative studies on the cytotoxicity, cellular and nuclear uptake of a series of chloro gold (I) phosphine complexes. Polyhedron 29(1):66–69 52. Chaves JDS, Tunes LG, Franco CHDJ, Francisco TM, Correa CC, Murta SM et al (2017) Novel gold (I) complexes with 5-phenyl-1, 3, 4-oxadiazole-2-thione and phosphine as potential anticancer and antileishmanial agents. Eur J Med Chem 127:727–739 53. Garcia A, Machado RC, Grazul RM, Lopes MTP, Corrêa CC, Santos HFD et al (2016) Novel antitumor adamantane–azole gold (I) complexes as potential inhibitors of thioredoxin reductase. J Biol Inorg Chem 21(2):275–292 54. Ott I, Qian X, Xu Y, Vlecken DH, Marques IJ, Kubutat D et al (2009) A gold (I) phosphine complex containing a naphthalimide ligand functions as a TrxR inhibiting antiproliferative agent and angiogenesis inhibitor. J Med Chem 52(3):763–770 55. Milaeva ER, Shpakovsky DB, Dyadchenko VP, Gryzlov AI, Gracheva YA, Antonenko TA et al (2017) Synthesis and biological activity of novel Au(I) complexes with a protective antioxidant 2, 6-di-tert-butylphenol group. Polyhedron 127:512–519 56. Goitia H, Nieto Y, Villacampa MD, Kasper C, Laguna A, Gimeno MC (2013) Antitumoral gold and silver complexes with ferrocenyl-amide phosphines. Organometallics 32 (20):6069–6078 57. Alhoshani A, Alrashdi A, Alhosaini K, Alanazi FE, Alajez NM, Altaf M et al (2018) Goldcontaining compound BDG-I inhibits the growth of A549 lung cancer cells through the deregulation of miRNA expression. Saudi Pharm J 26(7):1035–1043 58. Tavares TT, Azevedo GC, Garcia A, Carpanez AG, Lewer PM, Paschoal D et al (2017) Gold (I) complexes with aryl-thiosemicarbazones: molecular modeling, synthesis, cytotoxicity and TrxR inhibition. Polyhedron 132:95–104 59. Atrian-Blasco E, Gascon S, Rodrıguez-Yoldi MJ, Laguna M, Cerrada E (2017) Novel gold (I) thiolate derivatives synergistic with 5-fluorouracil as potential selective anticancer agents in colon cancer. Inorg Chem 56(14):8562–8579 60. Altaf M, Monim-ul-Mehboob M, Seliman AA, Sohail M, Wazeer MI, Isab AA et al (2015) Synthesis, characterization and anticancer activity of gold (I) complexes that contain tri-tertbutylphosphine and dialkyl dithiocarbamate ligands. Eur J Med Chem 95:464–472 61. Reddy TS, Privér SH, Mirzadeh N, Luwor RB, Reddy VG, Ramesan S, Bhargava SK (2020) Antitumor and antiangiogenic properties of gold (III) complexes containing cycloaurated triphenylphosphine sulfide ligands. Inorg Chem 59(8):5662–5673 62. Sulaiman AAA, Alhoshani A, As Sobeai HM, Alghanem M, Abogosh AK, Ahmad S, Altaf M, Monim-ul-Mehboob M, Stoeckli-Evans H, Isab A (2020) Anticancer activity and X-ray structure determination of gold (I) complexes of 2-(diphenylphosphanyl)-1aminocyclohexane. Polyhedron 183:114532 63. Le HV, Babak MV, Ehsan MA, Altaf M, Reichert L, Gushchin AL, Ang WH, Isab AA (2020) Highly cytotoxic gold (i)-phosphane dithiocarbamate complexes trigger an ER stressdependent immune response in ovarian cancer cells. Dalton Trans 49(22):7355–7363 64. Abas E, Pena-Martinez R, Aguirre-Ramírez D, Rodriguez-Dieguez A, Laguna M, Grasa L (2020) New selective thiolate gold (i) complexes inhibit the proliferation of different human cancer cells and induce apoptosis in primary cultures of mouse colon tumors. Dalton Trans 49 (6):1915–1927 65. Sulaiman AA, Zierkiewicz W, Michalczyk M, Malik-Gajewska M, Ahmad S, Alhoshani A, Sobeai HMA, Bieńko D, Isaba AA (2020) Synthesis, characterization, DFT optimization and anticancer evaluation of phosphanegold (I) dithiocarbamates. J Mol Str 1218:128486 66. Zhang P, Sadler PJ (2017) Advances in the design of organometallic anticancer complexes. J Organomet Chem 839:5–14

156

8

Classes of Gold Complexes

67. Patil S, Deally A, Gleeson B, Müller-Bunz H, Paradisi F, Tacke M (2011) Novel benzylsubstituted N-heterocyclic carbene–silver acetate complexes: synthesis, cytotoxicity and antibacterial studies. Metallomics 3(1):74–88 68. Medvetz DA, Hindi KM, Panzner MJ, Ditto AJ, Yun YH, Youngs WJ (2008) Anticancer activity of Ag (I) N-heterocyclic carbene complexes derived from 4, 5-dichloro-1H-imidazole. Metal-Based Drugs:2008 69. Rubbiani R, Kitanovic I, Alborzinia H, Can S, Kitanovic A, Onambele LA et al (2010) Benzimidazol-2-ylidene gold (I) complexes are thioredoxin reductase inhibitors with multiple antitumor properties. J Med Chem 53(24):8608–8618 70. Gavara R, Aguiló E, Schur J, Llorca J, Ott I, Rodríguez L (2016) Study of the effect of the chromophore and nuclearity on the aggregation and potential biological activity of gold (I) alkynyl complexes. Inorg Chim Acta 446:189–197 71. Hikisz P, Szczupak Ł, Koceva-Chyła A, Guśpiel A, Oehninger L, Ott I et al (2015) Anticancer and antibacterial activity studies of gold (I)-alkynyl chromones. Molecules 20 (11):19699–19718 72. Zhang C, Maddelein ML, Sun RWY, Gornitzka H, Cuvillier O, Hemmert C (2018) Pharmacomodulation on gold-NHC complexes for anticancer applications–is lipophilicity the key point? Eur J Med Chem 157:320–332 73. Schuh E, Pflüger C, Citta A, Folda A, Rigobello MP, Bindoli A et al (2012) Gold (I) carbene complexes causing thioredoxin 1 and thioredoxin 2 oxidation as potential anticancer agents. J Med Chem 55(11):5518–5528 74. Bertrand B, de Almeida A, van der Burgt EP, Picquet M, Citta A, Folda A et al (2014) New gold (I) organometallic compounds with biological activity in cancer cells. Eur J Inorg Chem 2014(27):4532–4536 75. Horvath UE, Bentivoglio G, Hummel M, Schottenberger H, Wurst K, Nell MJ et al (2008) A cytotoxic bis (carbene) gold (I) complex of ferrocenyl complexes: synthesis and structural characterisation. New J Chem 32(3):533–539 76. Bertrand B, Spreckelmeyer S, Bodio E, Cocco F, Picquet M, Richard P et al (2015) Exploring the potential of gold (III) cyclometallated compounds as cytotoxic agents: variations on the C^ N theme. Dalton Trans 44(26):11911–11918 77. Huang KB, Wang FY, Tang XM, Feng HW, Chen ZF, Liu YC et al (2018) nOrganometallic gold (III) complexes similar to tetrahydroisoquinoline induce ER-stress-mediated apoptosis and pro-death autophagy in A549 cancer cells. J Med Chem 61(8):3478–3490 78. Tsai JLL, Chan AOY, Che CM (2015) A luminescent cyclometalated gold (iii)–avidin conjugate with a long-lived emissive excited state that binds to proteins and DNA and possesses anti-proliferation capacity. Chem Commun 51(40):8547–8550 79. Haque RA, Ghdhayeb MZ, Budagumpi S, Ahamed MBK, Majid AMA (2016) Synthesis, crystal structures, and in vitro anticancer properties of new N-heterocyclic carbene (NHC) silver (I)-and gold (I)/(III)-complexes: a rare example of silver (I)–NHC complex involved in redox transmetallation. RSC Adv 6(65):60407–60421 80. Zou T, Lum CT, Lok C-N, W.-P. To, Low K-H, Che CM (2014) A binuclear gold(I) complex with mixed bridging diphosphine and Bis(N-heterocyclic Carbene) ligands shows favorable Thiol reactivity and inhibits tumor growth and angiogenesis in vivo. Angew Chem Int Ed 53 (23):5810–5814 81. Zou T (2016) Springer theses. Springer, Singapore, p 175 82. Liu W, Bensdorf K, Proetto M, Abram U, Hagenbach A, Gust R (2011) NHC gold halide complexes derived from 4, 5-diarylimidazoles: synthesis, structural analysis, and pharmacological investigations as potential antitumor agents. Eur J Med Chem 54(24):8605–8615 83. Hickey JL, Ruhayel RA, Barnard PJ, Baker MV, Berners-Price SJ, Filipovska A (2008) Mitochondria-targeted chemotherapeutics: the rational design of gold (I) N-heterocyclic carbene complexes that are selectively toxic to cancer cells and target protein selenols in preference to thiols. J Am Chem Soc 130(38):12570–12571

References

157

84. Boselli L, Carraz M, Mazères S, Paloque L, González G, Benoit-Vical F et al (2015) Synthesis, structures, and biological studies of heterobimetallic Au (I)–Ru (II) complexes involving N-heterocyclic carbene-based multidentate ligands. Organometallics 34(6):1046–1055 85. Bertrand B, Bodio E, Richard P, Picquet M, Le Gendre P, Casini A (2015) Gold (I) N-heterocyclic carbene complexes with an “activable” ester moiety: possible biological applications. J Organomet Chem 775:124–129 86. Sun RWY, Zhang M, Li D, Zhang ZF, Cai H, Li M et al (2015) Dinuclear gold (I) pyrrolidinedithiocarbamato complex: cytotoxic and antimigratory activities on cancer cells and the use of metal–organic framework. Chem A Eur J 21(51):18534–18538 87. Altaf M, Monim-ul-Mehboob M, Seliman AA, Isab AA, Dhuna V, Bhatia G, Dhuna K (2014) Synthesis, x–ray structures, spectroscopic analysis and anticancer activity of novel gold (I) carbene complexes. J Organomet Chem 765:68–79 88. Yan JJ, Chow ALF, Leung CH, Sun RWY, Ma DL, Che CM (2010) Cyclometalated gold (III) complexes with N-heterocyclic carbene ligands as topoisomerase I poisons. Chem Commun 46(22):3893–3895 89. Maftei CV, Fodor E, Jones PG, Freytag M, Franz MH, Kelter G et al (2015) N-heterocyclic carbenes (NHC) with 1, 2, 4-oxadiazole-substituents related to natural products: synthesis, structure and potential antitumor activity of some corresponding gold (I) and silver (I) complexes. Eur J Med Chem 101:431–441 90. Dinda J, Samanta T, Nandy A, Saha KD, Seth SK, Chattopadhyay SK, Bielawski CW (2014) N-heterocyclic carbene supported Au (I) and Au (III) complexes: a comparison of cytotoxicities. New J Chem 38(3):1218–1224 91. Dinda J, Nandy A, Rana BK, Bertolasi V, Saha KD, Bielawski CW (2014) Cytotoxicity of silver (I), gold (I) and gold (III) complexes of a pyridine wingtip substituted annelated N-heterocyclic carbene. RSC Adv 4(105):60776–60784 92. Rana BK, Nandy A, Bertolasi V, Bielawski CW, Das Saha K, Dinda J (2014) Novel gold (I)– and gold (III)–N-heterocyclic carbene complexes: synthesis and evaluation of their anticancer properties. Organometallics 33(10):2544–2548 93. Samanta T, Munda RN, Roymahapatra G, Nandy A, Saha KD, Al-Deyab SS, Dinda J (2015) Silver (I), gold (I) and gold (III)-N-heterocyclic carbene complexes of naphthyl substituted annelated ligand: synthesis, structure and cytotoxicity. J Organomet Chem 791:183–191 94. Nandy A, Dey SK, Das S, Munda RN, Dinda J, Saha KD (2014) Gold (I) N-heterocyclic carbene complex inhibits mouse melanoma growth by p53 upregulation. Mol Cancer 13 (1):1–14 95. Bertrand B, Stefan L, Pirrotta M, Monchaud D, Bodio E, Richard P et al (2014) Caffeine-based gold (I) N-heterocyclic carbenes as possible anticancer agents: synthesis and biological properties, Inorg. Chem 53(4):2296–2303 96. Bian M, Sun Y, Liu Y, Xu Z, Fan R, Liu Z, Liu W (2020) A gold (I) complex containing an Oleanolic acid derivative as a potential anti-ovarian-Cancer agent by inhibiting TrxR and activating ROS-mediated ERS. Chem A Eur J 26:7092–7108 97. Tabrizi L, Romanova J (2020) Antiproliferative activity of gold (I) N-heterocyclic carbene and triphenylphosphine complexes with ibuprofen derivatives as effective enzyme inhibitors. App Organomet Chem 34(5):e5618 98. Meier-Menches S, Neuditschko B, Zappe K, Schaier M, Gerner M, Schmetterer K, Favero GD, Bonsignore R, Cichna-Makl M, Koellensperger G, Casini A, Gerner C (2020) An organometallic gold (I) bis-N-heterocyclic carbene complex with multimodal activity in ovarian cancer cells. Chem A Eur J. https://doi.org/10.1002/chem.202003495 99. Alsaeedi MS, Babgi BA, Hussien MA, Abdellattif MH, Humphrey MG (2020) DNA-binding and anticancer activity of binuclear gold (I) Alkynyl complexes with a Phenanthrenyl bridging ligand. Molecules 25(5):1033 100. Bian M, Fan R, Jiang G, Wang Y, Lu Y, Liu W (2020) Halo and pseudohalo gold (I)–NHC complexes derived from 4, 5-diarylimidazoles with excellent in vitro and in vivo anticancer activities against HCC. J Med Chem 63(17):9197–9211

9

Mechanisms of Action of Anticancer Gold Complexes

So far a broad variety of various gold (I) and gold(III) (as well as one gold(II)) agents have been investigated for their antitumor effects in vitro and in vivo mostly showing promising preclinical results. Under physiological conditions, the naked Au+ ion is unstable and tends to undergo disproportionation to yield Au3+ and Au0. Auþ ! Au3þ þ Au0 Thus, the Au+ ion needs to be stabilized by ligands. The ligands commonly used include thiolate (RS ), phosphine (PR3), NHC, and acetylide (RCC ). Gold (I) complexes can be two-, three-, or four-coordinated but two-coordinated gold (I) complexes are mostly encountered [1]. In biological systems, gold(I) complexes can undergo a two-step ligand exchange reaction via a three-coordinated gold (I) intermediate [2] (Fig. 9.1). Upon administering a gold(I) complex to cells, a ligand exchange reaction with GSH can rapidly occur [3]. In animal studies, the ligand exchange reaction of gold(I) complexes with Cys34 of serum albumin (SA) to form gold(I)–SA adducts has been reported [4]. It has been suggested that SA may serve as a drug carrier or, more likely, function as a drug “scavenger” to abrogate the activity of gold(I) complexes [5, 6]. Sadler and coworkers reported that the thermodynamic stability of gold complexes is dependent on the auxiliary ligands in the following order: ~ 3 >> SMet N~His > Cl >> COO CN S~cys PR Based on the different ligands and structures a unique mode of action most probably does not exist. However, some trends can be considered as general biological effects of antiproliferative gold species. The most relevant biochemical property is the inhibition of the enzyme TrxR demonstrated for many compounds, which is supposedly based on the covalent binding of the gold center to a # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_9

159

160

9

Mechanisms of Action of Anticancer Gold Complexes

−

RS R1

I

Au

R2

R1

I

Au

−

SR + R2

+ RS −

RS

I

Au

SR

Fig. 9.1 Ligand exchange reaction of gold(I) complexes with thiolate

selenocysteine residue in the active site of the enzyme. Other properties common for gold drugs are the triggering of antimitochondrial effects and the induction of apoptotic events. The interaction with other biological targets (e.g. protein tyrosine phosphatase or DNA) has also been confirmed for different gold compounds which contributes to the pharmacological profile of the complexes (Fig. 9.2). Unlike cisplatin and its derivatives that target DNA, gold complexes exhibit a high propensity to target enzymes, especially those that contain thiols. This targeting feature is attributed to the strong binding affinity of gold ions with thiols. Most of the thiol-containing enzymes, such as TrxR, glutathione reductase (GR), and cysteine protease, are overexpressed in cancer cells, thus providing potential anticancer targets for gold complex based therapy [7, 8]. Some of the important anticancer targets for gold complexes are discussed as under:

9.1

Thioredoxin System

Mitochondria play a key role in the regulation of apoptosis and the regulation of the intracellular redox state, and almost all mechanistic studies indicate that mitochondria are the biological targets for gold antitumor compounds [9, 10]. Mitochondria contain a specific thioredoxin reductase (TrxR2) (which is different from the cytosolic form, TrxR1). Thioredoxin reductases are a class of homodimeric selenoenzymes that catalyze the NADPH-dependent reduction of thioredoxins, a family of ubiquitous disulfide reductases responsible for maintaining proteins in their reduced state. Thioredoxin reductases belong to the flavoprotein family of pyridine nucleotide-disulfide reductases that include glutathione reductase. The thioredoxin system plays a key role in regulating the overall intracellular redox balance [11]. It includes the small redox protein thioredoxin (Trx), nicotinamide adenine dinucleotide phosphate, in its reduced form (NADPH), and thioredoxin reductase (TrxR), a large homodimeric selenoenzyme governing the redox state of thioredoxin. A crystal structure of human thioredoxin reductase 1 is given in Fig. 9.3 [12]. The work of Bindoli and coworkers first demonstrated that inhibition of TrxR2 was linked to mitochondrial permeability transition and the initiation of the apoptotic process [13]. The mechanism of cytotoxicity of auranofin [14–17] and a diverse range of other Au(I) compounds with phosphine (both monodentate [18] and bidentate [19], NHC [20], and phosphole [21] ligands have now been linked to inhibition of TrxR. It is assumed that inhibition is due to interaction of Au(I) with the active site selenocysteine residue. Indirect evidence for this is that the inhibition of native TrxR by Au(I) phosphole compounds is orders of magnitude stronger than

9.1 Thioredoxin System

161

Fig. 9.2 Gold compounds can induce cell death in cancer cells by targeting proteins in different cellular compartments. The inhibition of thiol or selenol protein targets leads to programmed cell death that often may involve mitochondrial pathways. Inhibition of the Trx system (Trx and TrxR) by gold compounds in mitochondria, or in the cytoplasm, can lead to increased reactive oxygen species (ROS), dysfunctional gene expression, via the inability to reduce transcription factors (TF), and that can cause cell death. The reduced form of Trx interacts with the apoptosis stimulating kinase 1 (ASK1) and the inhibition of Trx reduction could result in increased free ASK1 that would lead to programmed cell death. For simplicity the Au(I) compounds are represented here as two-coordinate L–Au–L, but they may be neutral or charged, and can be four-coordinate (where ligands (L) are bidentate phosphines). Note that the ligands may be substituted completely on binding to protein target sites. For Au(III) compounds the active metabolites could be also Au (I) species produced by Au(III) reduction in vivo (Adapted with permission from the publisher)

that of a mutant where the active site selenocysteine is replaced by a cysteine residue [22]. The binding of the [Au(PMe3)]+ fragment to Cys, Sec. and the tetrapeptides H2NGlyCysA-GlyCOOH (A ¼ Cys, Sec) has been investigated by DFT methods and these theoretical calculations support the binding of auranofin to the Sec residue of TrxR [23]. Similarly, an ESI-MS study on binding of auranofin to the tetrapeptide Ac-GlyCysSecGlyNH2 (a model for the TrxR C-terminus) shows preferential

162

9

Mechanisms of Action of Anticancer Gold Complexes

Fig. 9.3 Crystal structure of human thioredoxin reductase 1 (PDB: 2CFY)

Fig. 9.4 (a) Crystal structure of glutathione reductase with Au(I) phosphine complex (PDB: 2AAQ) and (b) crystal structure of thioredoxin glutathione reductase in complex with auranofin (PDB: 3H4K)

binding of Au(I) to the Sec residue with replacement of the thiosugar [24]. However, mass spectrometry studies of TrxR1 treated with auranofin are consistent with binding of four AuPEt3+ fragments, showing that other binding sites, in addition to the selenenylsulfide/selenolthiol active center, are bound by Au(I) [25]. A crystal structure of glutathione reductase (GR, an enzyme related to TrxR but lacking the Sec residue) modified with a linear, two coordinate Au(I) phosphine complex shows Au bound to the active center thiols with S–Au–S coordination in the inactive GR product and in another crystal structure thioredoxin glutathione reductase in complex with auranofin shows similar interactions as can be seen in

9.2 Cysteine Proteases

163

Fig. 9.4a, b, respectively [26, 27]. Various Au(III) compounds have also been shown to be potent inhibitors of either mitochondrial or cytosolic TrxR [28–31] and the mechanism of cytotoxicity of three organogold(III) compounds and also Au(III) dithiocarbamate compounds has been attributed to mitochondrial apoptotic pathways stemming from TrxR inhibition [29]. Some studies on the inhibition of isolated TrxR by gold(III) compounds suggest that oxidative damage to the enzyme through indiscriminate oxidation of thiol/selenol groups may be important, rather than metal coordination. However, it is important to bear in mind that for almost all of the known active Au(III) compounds the active metabolites could be Au (I) species produced by Au(III) reduction in vivo. Howsoever, inhibition of TrxR finally leads to apoptosis of cancer cells [16, 32], consequently many emerging anticancer therapies use TrxR as a target for drug development [22, 31].

9.2

Cysteine Proteases

The cysteine protease cathepsin contains a nucleophilic cysteinyl thiol in its active site [33, 34]. The molecular mechanism of proteolysis by cysteine proteases includes: (1) deprotonation of thiol by an adjacent histidine; (2) nucleophilic attack of carbonyl group carbon by the S atom of the deprotonated thiol; (3) hydrolysis of the thioester bond as shown in Fig. 9.5. In the literature, cathepsins K and S have been reported to be efficiently inhibited by auranofin and are at least ten times more efficiently inhibited by gold thiomalate. The crystal structure of the gold thiomalate–cathepsin K adduct (Fig. 9.6) shows a linear Cys-S–Au(I)–S-thiomalate coordination, with the thiomalate group

Fig. 9.5 Catalytic cycle of cysteine proteases. The covalent (coordination) binding of Au(I) with cysteinyl thiol can block the catalytic cycle (Adopted with permission from the publisher)

164

9

Mechanisms of Action of Anticancer Gold Complexes

Fig. 9.6 Crystal structure of Human Cathepsin K in complex with myocrisin (2ATO)

surrounded by several amino acid residues that form hydrogen bonds with thiomalate, leading to an increased stability of the covalent adduct [35]. It is noted that deprotonation of thiol by the adjacent base (e.g., His) plays a crucial role in the enzyme inhibition by gold complexes.

9.3

Protein Tyrosine Phosphatases (PTP)

The protein tyrosine phosphatase (PTP) superfamily of enzymes functions in a coordinated manner with the protein tyrosine kinases to control signaling pathways that underlie a number of fundamental physiological processes. PTPs catalyze the removal of phosphate groups from phosphorylated tyrosine residues on proteins and play important roles in various physiological processes including regulation of signaling pathways such as T-cell signaling, cell growth, cell differentiation, immune response, and survival. PTPs contain a highly activated cysteine residue in their active site with an unusually low pKa (~4.7) [36]; therefore, the thiol is mainly in the deprotonated form at physiological pH and can be a molecular target of anticancer gold(I) complexes. Barrios and coworkers have identified a library of [Au (PR3)Cl] complexes that show micromolar IC50 values (1.5–33 mM) for the inhibition of the lymphoid tyrosine phosphatase (LYP) [37].

9.4 Thiol/Selenol Containing Enzymes

9.4

165

Thiol/Selenol Containing Enzymes

Besides the redox regulation of disulfide reductases, cysteine proteases, and protein tyrosine phosphatases, other thiol/selenol containing enzymes may also be involved in the mechanisms of action of anticancer gold complexes. For example, glutathione peroxidase (GPx) and iodothyronine deiodinase (ID) have been reported to be effectively inhibited by gold(I) complexes. The activity of IkB kinase (IKK) can be blocked by thiol-reactive auranofin, aurothiomalate, aurothioglucose, and AuCl3, leading to blocked activation of NF-kB, a transcription factor involved in the expression of many inflammatory genes and cancer development. This inhibition is probably caused by the binding of gold with the cysteinyl thiol in the catalytic subunits of IKK. Though there are no direct structural data of an Au(III)–protein adduct yet, the intracellular reduction of Au(III) to Au(I) is well documented. This lends support to redox reactive gold(III) complexes as potential inhibitors of the aforementioned enzymes. By using high-resolution mass spectrometry, the binding of gold(III) with thiol proteins has also been observed. The membrane water/glycerol channel protein (aquaporin 3) and deubiquitinase have also been reported to be potential targets of gold(III) complexes due to the binding of gold(III) with cysteinyl thiols. Unlike the redox and/or substitution reactive gold(III)–gold(I) complexes that can form tight Au–S/Se bonds, gold(III) porphyrins, which are not reactive towards thiols, have been reported to show anticancer activity. Although cysteine is widely considered to be the primary binding site of gold ions, in some cases, coordination of N-donor ligands, such as histidine, to gold ions has also been observed. Sadler and coworkers [38] reported the crystal structure of an adduct of [Au(PEt3)Cl] with cyclophilin 3 (Cyp3) that contains four cysteine residues, two of which (Cys163 and Cys168) are accessible for Au–S bonding (Fig. 9.7). However, the crystal structure of the Au(I)–Cyp3 adduct does not show any Scys–Au(I) coordination. Instead, there is NHis–AuI–PEt coordination. A chymotrypsin-coupled assay revealed that the binding of His133 of Cyp-3 to a gold ion can significantly inhibit the PPIase activity with gold in the nanomolar range. Messori and coworkers have reported the crystal structures of a ribonuclease A–gold adduct and a hen egg white lysozyme– gold adduct, where the gold ion is coordinated with His groups, forming His–Au(I)– His or His–Au(I)–Cl [39, 40]. A possible reason for binding with His, but not Cys, based on DFT calculations, is that the activation enthalpy for Cys binding is higher than His binding despite the reaction free energy for the latter being much higher than the former. That is, the reaction of gold(I) complex with His is kinetically favorable while the reaction with Cys is thermodynamically favorable [41]. 20.5. UBIQUITIN-PROTEASOME SYSTEM. Proteasomes are very large protein complexes located in the nucleus and in the cytoplasm of eukaryotic cells [42]. The main function of the proteasome is to degrade -by proteolysis- proteins that are either in excess or damaged; proteins are tagged for degradation with a small protein called ubiquitin. Thus, proteasomes are

166

9

Mechanisms of Action of Anticancer Gold Complexes

Fig. 9.7 Crystal structure of gold(I)–cyclophilin 3 adduct (PDB code:1E3B, gold source: [Au (PEt3)Cl])

part of a major mechanism by which cells regulate the concentration of particular proteins and degrade misfolded proteins. The degradation process typically produces small peptides (about 7–8 amino acids long), which can be further degraded into amino acids and used for the synthesis of new proteins. The ubiquitin-proteasome pathway (Fig. 9.8) therefore plays a major role in the degradation of oxidatively damaged and mutated proteins as well as proteins involved in cell cycle progression, proliferation, and apoptosis [43]. The ubiquitin-proteasome system has been identified as a major in vitro and in vivo target for gold(III)-dithiocarbamato derivatives [44, 45] (Fig. 9.9) potently reducing proliferation in different breast cancer cell lines, including premalignant MCF10K.cl2, malignant MCF10dcis.com, estrogen receptor a-positive MCF7, and estrogen receptor a-negative MDA-MB-231 cells. The tested gold(III) complexes inhibit the proteasomal chymotrypsin-like activity in highly metastatic and invasive MDA-MB-231 whole cell extract in a concentration-dependent way. This evidence is particularly important because it has been reported that inhibition of proteasomal chymotrypsin-like but not trypsin-like activity is associated with growth arrest and/or apoptosis induction in cancer cells [46, 47]. Moreover, inhibition of proteasome activity and accumulation of p27 were also detected on xenograft tumors: treatment of MDA-MB-231 tumor-bearing nude mice resulted in a significant inhibition (about 50%) of tumor growth, as a consequence of the proteasomal inhibition and the massive induction of apoptosis. During the daily treatment at 1.0 mg kg1 for 29 days, no toxicity was observed, and mice did not display any sign of weight loss, decreased activity, or anorexia [44].

Fig. 9.8 The ubiquitin-proteasome pathway as a target for gold complexes (Adopted with permission from the publisher)

9.4 Thiol/Selenol Containing Enzymes 167

168

9

H N

Br Br Au S S

O

N O

Mechanisms of Action of Anticancer Gold Complexes

O

O O Au O N N

N

N H

N

S

N O

Au

Au

P

P

Fig. 9.9 Gold complexes as proteasome inhibitors

Deubiquitinases (DUBs) are a class of enzymes that deconjugate ubiquitin from ubiquitinated proteins and 19S proteasome-associated DUBs considered as potential drug targets for cancer treatment [48, 49]. A gold(I) complex-pyrithione, i.e. Au (PPh3)PT has been found to be a potent antitumor agent, able to inhibit ubiquitinproteasome system (UPS) proteolytic function [50]. Au(PPh3)PT complex was selective in inhibiting the 19S proteasome-associated DUBs and other non-proteasomal DUBs with minimal effects on the function of 20S proteasome, which makes it a potential anticancer agent.

References 1. Gimeno MC, Laguna A (1997) Three- and four-coordinate gold(I) complexes. Chem Rev 97 (3):511–522 2. Sadler PJ, Sue RE (1994) The chemistry of gold drugs. Metal Based Drugs 1(2–3):107–144 3. Shaw CF (1999) Gold-based therapeutic agents. Chem Rev 99(9):2589–2600 4. Cottrill SM, Sharma HL, Dyson DB, Parish RV, McAuliffe CA (1989) The role of the ligand in chrysotherapy, a kinetic study of gold-199 and sulfur-35-labeled myocrisin and auranofin. J Chem Soc Perkin Trans 1:53–58 5. Meyer A, Bagowski CP, Kokoschka M, Stefanopoulou M, Alborzinia H, Can S, Vlecken DH, Sheldrick WS, Wolfl S, Ott I (2012) On the biological properties of alkynyl phosphine gold (I) complexes. Angew Chem Int Ed 51(35):8895–8899 6. Berners-Price SJ, Filipovska A (2011) Gold compounds as therapeutic agents for human diseases. Metallomics 3(9):863–873 7. Mohamed MM, Sloane BF (2006) Cysteine cathepsins: multifunctional enzymes in cancer. Nat Rev Cancer 6(10):764–775 8. Urig S, Fritz-Wolf K, Réau R, Herold-Mende C, Toth K, Davioud-Charvet E, Becker K (2006) Undressing of phosphine gold(I) complexes as irreversible inhibitors of human disulfide reductases. Angew Chem Int Ed 45(12):1881–1886 9. Battogtokh G, Cho Y-Y, Lee JY, Lee HS, Kang HC (2018) Mitochondrial-targeting anticancer agent conjugates and nanocarrier systems for cancer treatment. Front Pharmacol 9:922 10. Barnard PJ, Berners-Price SJ (2007) Targeting the mitochondrial cell death pathway with gold compounds. Coord Chem Rev 251:1889–1902 11. Gromer S, Urig S, Becker K (2004) The thioredoxin system—from science to clinic. Med Res Rev 24:40–89 12. Arnér ES (2009) Focus on mammalian thioredoxin reductases—important selenoproteins with versatile functions. Biochim Biophys Acta 1790:495–526

References

169

13. Rigobello MP, Callegaro MT, Barzon E, Benetti M, Bindoli A (1998) Purification of mitochondrial thioredoxin reductase and its involvement in the redox regulation of membrane permeability. Free Radic Biol Med 24(2):370–376 14. Rigobello MP, Scutari G, Boscolo R, Bindoli A (2002) Induction of mitochondrial permeability transition by Auranofin, a gold(I)-phosphine derivative. Br J Pharmacol 136(8):1162–1168 15. Rigobello MP, Scutari G, Folda A, Bindoli A (2004) Mitochondrial thioredoxin reductase inhibition by gold(I) compounds and concurrent stimulation of permeability transition and release of cytochrome c. Biochem Pharmacol 67(4):689–696 16. Marzano C, Gandin V, Folda A, Scutari G, Bindoli A, Rigobello MP (2007) Inhibition of thioredoxin reductase by auranofin induces apoptosis in cisplatin-resistant human ovarian cancer cells. Free Radic Biol Med 42:872–881 17. M.P. Rigobello, A. Folda, B. Dani, R. Menabo`, G. Scutari, and A. Bindoli, (2008) Gold (I) complexes determine apoptosis with limited oxidative stress in Jurkat T cells, Eur J Pharmacol 582(1–3): 26–34 18. Ott I, Qian X, Xu Y, Vlecken DHW, Marques IJ, Kubutat D, Will J, Sheldrick WS, Jesse P, Prokop A, Bagowski CP (2009) A gold(I) phosphine complex containing a naphthalimide ligand functions as a TrxR inhibiting Antiproliferative agent and angiogenesis inhibitor. J Med Chem 52:763–770 19. Rackham O, Nichols SJ, Leedman PJ, Berners-Price SJ, Filipovska A (2007) A gold (I) phosphine complex selectively induces apoptosis in breast cancer cells: implications for anticancer therapeutics targeted to mitochondria. Biochem Pharmacol 74:992–1002 20. Hickey JL, Ruhayel RA, Barnard PJ, Baker MV, Berners-Price SJ, Filipovska A (2008) Mitochondria-targeted chemotherapeutics: the rational design of gold(I) N-heterocyclic carbene complexes that are selectively toxic to cancer cells and target protein selenols in preference to thiols. J Am Chem Soc 130(38):12570–12571 21. Viry E, Battaglia E, Deborde V, Mueller T, Reau R, Davioud-Charvet E, Bagrel D (2008) A sugar-modified phosphole gold complex with antiproliferative properties acting as a thioredoxin reductase inhibitor in MCF-7 cells. Chem Med Chem 3(11):1667–1670 22. Urig S, Fritz-Wolf K, Reau R, Herold-Mende C, Toth K, Davioud-Charvet E, Becker K (2006) Undressing of phosphine gold(I) complexes as irreversible inhibitors of human disulfide reductases. Angew Chem Int Ed 45(12):1881–1886 23. Howell JAS (2009) DFT investigation of the interaction between gold(I) complexes and the active site of thioredoxin reductase. J Organomet Chem 694(6):868–873 24. Pratesi A, Gabbiani C, Ginanneschi M, Messori L (2010) Reactions of medicinally relevant gold compounds with the C-terminal motif of thioredoxin reductase elucidated by MS analysis. Chem Commun 46(37):7001–7003 25. Bindoli A, Rigobello MP, Scutari G, Gabbiani C, Casini A, Messori L (2009) Thioredoxin reductase: a target for gold compounds acting as potential anticancer drugs. Coord Chem Rev 253(11–12):1692–1707 26. Angelucci F, Sayed AA, Williams DL, Boumis G, Brunori M, Dimastrogiovanni D, Miele AE, Pauly F, Bellelli A (2009) Inhibition of Schistosoma mansoni thioredoxin-glutathione reductase by auranofin: structural and kinetic aspects. J Biol Chem 284:28977–28985 27. Urig S, Fritz-Wolf K, Reau R, Herold-Mende C, Toth K, Davioud-Charvet E, Becker K (2006) Undressing of phosphine gold(I) complexes as irreversible inhibitors of human disulfide Reductases. Angew Chem Int Ed Engl 45:1881–1886 28. Rigobello MP, Messori L, Marcon G, Agostina CM, Bragadin M, Folda A, Scutari G, Bindoli A (2004) Gold complexes inhibit mitochondrial thioredoxin reductase: consequences on mitochondrial functions. J Inorg Biochem 98(10):1634–1641 29. Saggioro D, Rigobello MP, Paloschi L, Folda A, Moggach SA, Parsons S, Ronconi L, Fregona D, Bindoli A (2007) Gold(III)-dithiocarbamato complexes induce Cancer cell death triggered by thioredoxin redox system inhibition and activation of ERK pathway. Chem Biol 14 (10):1128–1139 30. Omata Y, Folan M, Shaw M, Messer RL, Lockwood PE, Hobbs D, Bouilaguet S, Sano H, Lewis JB, Wataha JC (2006) Sublethal concentrations of diverse gold compounds inhibit mammalian cytosolic thioredoxin reductase (TrxR1). Toxicol In Vitro 20(6):882–890

170

9

Mechanisms of Action of Anticancer Gold Complexes

31. Engman L, McNaughton M, Gajewska M, Kumar S, Birmingham A, Powis G (2006) Thioredoxin reductase and cancer cell growth inhibition by organogold(III) compounds. AntiCancer Drugs 17(5):539–544 32. Anestal K, Arner ESJ (2003) Rapid induction of cell death by selenium-compromised thioredoxin reductase 1 but not by the fully active enzyme containing selenocysteine. J Biol Chem 278:15966–15972 33. Palermo C, Joyce JA (2008) Cysteine cathepsin proteases as pharmacological targets in cancer. Trends Pharmacol Sci 29(1):22–28 34. Olson OC, Joyce JA (2015) Cysteine cathepsin proteases: regulators of cancer progression and therapeutic response. Nat Rev Cancer 15(12):712–729 35. Weidauer E, Yasuda Y, Biswal BK, Kerr LD, Cherney MM, Gordon RE, James MNG, Bromme D (2011) Crystal structure of human cathepsin K in complex with myocrisin. https://doi.org/10. 2210/pdb2ato/pdb 36. Zhang ZY, Dixon JE (1993) Active site labeling of the Yersinia protein tyrosine phosphatase: the determination of the pKa of the active site cysteine and the function of the conserved histidine 402. Biochemistry 32:9340–9345 37. Karver MR, Krishnamurthy D, Kulkarni RA, Bottini N, Barrios AM (2009) Gold(I) phosphine mediated selective inhibition of lymphoid tyrosine phosphatase. J Med Chem 52:6912–6918 38. Zou J, Taylor P, Dornan J, Robinson SP, Walkinshaw MD, Sadler PJ (2000) Cyclophilin 3 from C. elegans complexed with AuP(Et)3. Angew Chem Int Ed 39:2931–2934 39. Messori L, Scaletti F, Massai L, Cinellu MA, Gabbiani C, Vergara A, Merlino A (2013) The mode of action of anticancer gold-based drugs: a structural perspective. Chem Commun 49:10100–10102 40. Messori L, Scaletti F, Massai L, Cinellu MA, Russo Krauss I, di Martino G, Vergara A, Paduano L, Merlino A (2014) Interactions of gold-based drugs with proteins: crystal structure of the adduct formed between ribonuclease A and a cytotoxic gold(iii) compound. Metallomics 6:233–236 41. Dos Santos HF (2014) Reactivity of auranofin with S-, Se-and N-containing amino acids. Comput Theor Chem 1048:95–101 42. Gallastegui N, Groll M (2010) The 26S proteasome: assembly and function of a destructive machine. Trends Biochem Sci 35:634–642 43. Nalepa G, Rolfe M, Harper JW (2006) Drug discovery in the ubiquitin-proteasome system. Nat Rev Drug Discov 5:596–613 44. Milacic V et al (2006) A novel anticancer gold(III) dithiocarbamate compound inhibits the activity of a purified 20S proteasome and 26S proteasome in human breast cancer cell cultures and xenografts. Cancer Res 66:10478–11048 45. Milacic V et al (2008) Gold complexes as prospective metal-based anticancer drugs. Histol Histopathol 23:101–108 46. An B et al (1998) Novel dipeptidyl proteasome inhibitors overcome Bcl-2 protective function and selectively accumulate the cyclin dependent kinase inhibitor p27 and induce apoptosis in transformed, but not normal, human fibroblasts. Cell Death Differ 5:1062–1107 47. Saggioro D et al (2007) Gold(III)-dithiocarbamato complexes induce cancer cell death triggered by thioredoxin redox system inhibition and activation of ERK pathway. Chem Biol 14:1128–1139 48. Tian Z, D'Arcy P, Wang X, Ray A, Tai YT, Hu Y et al (2014) A novel small molecule inhibitor of deubiquitylating enzyme USP14 and UCHL5 induces apoptosis in multiple myeloma and overcomes bortezomib resistance. Blood 123(5):706–716 49. Liu N, Huang H, Dou QP, Liu J (2015) Inhibition of 19S proteasome-associated deubiquitinases by metal-containing compounds. Onco Targets Ther 2(5):457–466 50. Li X, Huang Q, Long H, Zhang P, Su H, Liu J (2019) A new gold(I) complex-Au(PPh3)PT is a deubiquitinase inhibitor and inhibits tumor growth. Biomedicine 39:159–172

Perspectives

10

In its elemental form, gold(0) appears as a bright-yellow metal with high resistance to harsh conditions, but is susceptible to reactions with aqua regia, during which oxidation to gold(III) occurs. In general, gold exists in several oxidation states, ranging from I to +V, with gold(I) and gold(III) being the most common forms employed for biological studies. Relativistic effects, leading to the stabilization of the 6s valence electrons and the destabilization of the 5d electrons, give rise to many of the physical and chemical properties of gold, including its distinctive color. Although metal compounds in particular have been used in therapy for many centuries, there has been little molecular understanding of their mechanisms of action. This is now changing. The success of cisplatin, currently the leading anticancer drug, has established that structure–activity relationships can be constructed for metal compounds, and that they can be designed to achieve selectivity in biological activity. In general, the activity of a metal complex will depend not only on the metal itself, but also on its oxidation state, on the number and types of bound ligands, and the coordination geometry of the complex. Metal drugs will often be “pro-drugs” which undergo ligand substitution and redox reactions before they reach the target site. It is important to learn how to control such processes and to devise novel drug delivery procedures for metal complexes. Control can involve both thermodynamics and kinetics. Improved understanding of the biological chemistry of elements will eventually require new techniques and new methods of study. Gold(I), with a d10 closed-shell configuration, generally gives rise to two-coordinate, linear coordination compounds or complexes, which are by far the most commonly observed coordination geometry, as well as three-coordinate, trigonal and four-coordinate, tetrahedral coordination geometries. Being a “soft” metal center, gold(I) has a pronounced tendency to form stable complexes with easily polarizable soft donor atoms, such as sulfur and phosphorus. Gold(III), on the other hand, usually forms a tetracoordinate, square-planar geometry with a preference for hard donor atoms, including oxygen and nitrogen, due to the “hard” nature of gold (III) compared with gold(I). The presence of the same electronic configuration and # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 M. Y. Wani, M. A. Malik, Gold and its Complexes in Anticancer Chemotherapy, https://doi.org/10.1007/978-981-33-6314-4_10

171

172

10

Perspectives

structural characteristics as cisplatin prompted the investigation of gold(III) complexes as antitumor agents but was initially restrained because such complexes are susceptible to reduction in the biological environment. However, many significant advances with gold(III) complexes have been made in recent years with careful selection of coordination geometries. To date, a wide range of gold compounds with promising anticancer properties have been designed by taking into account the following: (1) having chemical structures/properties resembling gold(I) antiarthritic agents; (2) gold(III) complexes analogous to platinum(II) complexes, sharing a common d8 closed-shell configuration; and (3) incorporating molecules with known anticancer properties to prepare gold(I) and gold(III) complexes with enhanced activities—the “Trojan-horse” strategy. Given the strict similarity to cisplatin, gold(III) complexes are of interest for two main reasons: on the one side, they may constitute a new class of anticancer compounds with a novel profile of antitumor activity; on the other side, they represent a further attempt to elucidate the mechanism of action of antitumor d8 square-planar metal complexes, which still remains largely unknown. In this context, it is of fundamental importance to determine to what extent gold(III) compounds are cytotoxic and to define their structure–activity relationships. Previous investigations have already revealed favorable cytotoxic properties for some gold(III) complexes; however, the data present in the literature are still very scarce, probably as a consequence of the high reactivity of gold(III) complexes. The stability of gold-based drugs under physiological condition remains a challenge for the development of effective therapeutic agents. The nature of the ligand coordinated to the gold center greatly determines the pharmacokinetic profiles of both gold(I) and gold(III) compounds. Compared with platinum-based complexes, gold compounds with oxidation states of +I and + III possess better selectivity and potency towards cancer cells than normal cells due to their weaker DNA binding activity and greater affinity towards the sulfhydryl, thiol, and selenocysteine groups of several protein targets. In short, the biological benefits of gold compounds are prompted from a series of complex interactions between metal, cellular components, and genes. However, like many other drugs, gold compounds induce deleterious toxic side effects and future research, based on experimental trials, augmented by in silico studies, e.g., molecular docking studies, needs to be directed towards selective targeting of cancer cells to enhance the effectiveness of these compounds and to minimize unwanted biological responses. So far a broad variety of ligands have been investigated as ligands for bioactive gold(I) species. Of those mainly phosphine ligands play a major role. However, recent reports demonstrate that the use of different ligands can also lead to promising preclinical results. For example, N-heterocyclic carbene complexes have shown interesting biological properties. Although gold in the oxidation state +III in a physiological medium is often easily reduced to gold(I) or gold nanoparticles, the development of chelating ligands with nitrogen donors, cyclometalated structures, and dithiocarbamates has opened the way to enable Au(III) complexes to be used for chemotherapeutic purposes. The mechanisms by which these compounds are able to exert antiproliferative effects are

10

Perspectives

173

slowly beginning to emerge. Different trends have been highlighted although details remain very sketchy. Improving the selectivity of these compounds for cancer cells over healthy tissues in order to limit the side effects remains a major challenge although some promising results have been obtained. More sophisticated synthetic approaches are required to fully utilize the potential of Au(III) complexes for the development of new generations of anticancer drugs. The bioconjugation of Au(III) complexes has emerged as a promising strategy for improving the selectivity of this class of compounds for cancer cells over healthy tissues, and recent developments have shown that combining gold complexes with molecular structures that are specifically recognized by the cell can exploit the cell’s own transport mechanisms to improve selective metal uptake. There has been increasing realization that the unique chemistry of gold, particularly the high affinity for protein thiols and selenols, can be exploited to develop new gold-based therapeutics, targeting a particular disease. To do so, however, it requires a sound understanding of the biological chemistry of gold, together with knowledge of the biotransformation reactions that gold drugs are known to undergo in vivo. New techniques which allow following the reaction of new Au complexes and nucleic acids and proteins will allow the detection of otherwise invisible intermediate products. For example, the reduction possibility of L-glutathione is well-known. Its concentration in the cells is 10 mM, so the chosen complexes must avoid reduction before reaching to the target. In general, square-planar complexes undergo two pathways for substitution. The first is solvolytic, which includes formation of solvent-complex before substitution with nucleophile. The second is direct attack by the nucleophile. It is generally appreciated that enormous progress has been made in the understanding of the mode of action of Au complexes. Application of this knowledge in drug design is close, and it is generally expected that in the next decade improved antitumor drugs will be developed based on the knowledge of the interactions (and their repair) and on the kinetics of binding of Au compounds to proteins and DNA. Kinetic understanding is of great importance for controlling the toxic side effects of such compounds. The next stage in drug design is likely to be the development of dedicated drugs that comprise the transport (through the membranes), survival in the cell, binding to the DNA, and eventually excretion from the body with minimum side effects. In the perception of this process, both (kinetically controlled) metal coordination and hydrogen bonding will be key factors at the molecular level.