Following the serendipitous discovery of the anticancer activity of cisplatin over 50 years ago, a deep understanding of the chemical and biochemical transformations giving rise to its medicinal properties has developed allowing for improved treatment regimens and rational design of second and third generation drugs. This chapter begins with a brief historical review detailing initial results that led to the worldwide clinical approval of cisplatin and development of the field of metal anticancer agents. Later sections summarize our understanding of key mechanistic features including drug uptake, formation of covalent adducts with DNA, recognition and repair of Pt-DNA adducts, and the DNA damage response, with respect to cisplatin and oxaliplatin. The final section highlights known shortcomings of classical platinum anticancer agents, including problems with toxicity and mutagenicity, and the development of resistance and enrichment of cancer stem cells brought about through treatment. Instances where specific differences in the response or mechanism of action of cisplatin versus oxaliplatin have been demonstrated are discussed in the text. In this manner the chapter provides a broad overview of our current understanding of the mechanism of action of platinum anticancer agents, providing a framework for improving the rational design of better Pt-based anticancer agents.
Clinical trials have shown gallium nitrate, a group 13 (formerly IIIa) metal salt, to have antineoplastic activity against non-Hodgkin’s lymphoma and urothelial cancers. Interest in gallium as a metal with anticancer properties emerged when it was discovered that 67Ga(III) citrate injected in tumor-bearing animals localized to sites of tumor. Animal studies showed non-radioactive gallium nitrate to inhibit the growth of implanted solid tumors. Following further evaluation of its efficacy and toxicity in animals, gallium nitrate, Ga(NO3)3, was designated an investigational drug by the National Cancer Institute (USA) and advanced to Phase 1 and 2 clinical trials. Gallium(III) shares certain chemical characteristics with iron(III) which enable it to interact with iron-binding proteins and disrupt iron-dependent tumor cell growth. Gallium’s mechanisms of action include the inhibition of cellular iron uptake and disruption of intracellular iron homeostasis, these effects result in inhibition of ribonucleotide reductase and mitochondrial function, and changes in the expression in proteins of iron transport and storage. Whereas the growth-inhibitory effects of gallium become apparent after 24 to 48 hours of incubation of cells, an increase in intracellular reactive oxygen species (ROS) is seen with 1 to 4 hours of incubation. Gallium-induced ROS consequently triggers the upregulation of metallothionein and hemoxygenase-1 genes. Beyond the first generation of gallium salts such as gallium nitrate and gallium chloride, a new generation of gallium-ligand complexes such as tris(8-quinolinolato)gallium(III) (KP46) and gallium maltolate has emerged. These agents are being evaluated in the clinic while other ligands for gallium are in preclinical development. These newer agents appear to possess greater antitumor efficacy and a broader spectrum of antineoplastic activity than the earlier generation of gallium compounds.
The most effective class of anticancer drugs in clinical use are the platins which act by binding to duplex B-DNA. Yet duplex DNA is not DNA in its active form, and many other structures are formed in cells; for example, Y-shaped fork structures are involved in DNA replication and transcription and 4-way junctions with DNA repair. In this chapter we explore how large, cationic metallo-supramolecular structures can be used to bind to these less common, yet active, nucleic acid structures.
Guanine-rich sequences of DNA can readily fold into tetra-stranded helical assemblies known as G-quadruplexes (G4s). It has been proposed that these structures play important biological roles in transcription, translation, replication, and telomere maintenance. Therefore, over the past 20 years they have been investigated as potential drug targets for small molecules including metal complexes. This chapter provides an overview of the different classes of metal complexes as G4-binders and discusses the application of these species as optical probes for G-quadruplexes as well as metallo-drugs.
Anticancer platinum-based drugs are widely used in the treatment of a variety of tumorigenic diseases. They have been identified to target DNA and thereby induce apoptosis in cancer cells. Their reactivity to biomolecules other than DNA has often been associated with side effects that many cancer patients experience during chemotherapy. The development of metal compounds that target proteins rather than DNA has the potential to overcome or at least reduce the disadvantages of commonly used chemotherapeutics. Many exciting new metal complexes with novel modes of action have been reported and their anticancer activity was linked to selective protein interaction that may lead to improved accumulation in the tumor, higher selectivity and/or enhanced antiproliferative efficacy. The development of new lead structures requires bioanalytical methods to confirm the hypothesized modes of action or identify new, previously unexplored biological targets and pathways. We have selected original developments for review in this chapter and highlighted compounds on track toward clinical application.
As the carrier of the inheritable information in cells, DNA has been the target of metal complexes for over 40 years. In this chapter, the focus will be on non-covalent recognition of the highly structured DNA surface by substitutionally inert metal complexes capable of either sliding in between the normal base pairs as metallointercalators or flipping out thermodynamically destabilized mispaired nucleobases as metalloinsertors. While most of the compounds discussed are based on ruthenium(II) and rhodium(III) due to their stable octahedral coordination environment and low-spin 4d6 electronic configuration, most recent developments of alternative metal complexes, based on both transition metals and main group elements, will also be highlighted. A particular focus of the coverage is on structural data from X-ray structure analysis, which now provides details of the interaction at unprecedented details and will enable development of novel DNA binding probes for fundamental studies as well as new anticancer drug candidates.
Iron (Fe) is an essential metal, vital for biological functions, including electron transport, DNA synthesis, detoxification, and erythropoiesis that all contribute to metabolism, cell growth, and proliferation. Interactions between Fe and O2 can result in the generation of reactive oxygen species (ROS), which is based on the ability of Fe to redox cycle. Excess Fe may cause oxidative damage with ensuing cell death, but DNA damage may also lead to permanent mutations. Hence Fe is carcinogenic and may initiate tumor formation and growth, and also nurture the tumor microenvironment and metastasis. However, Fe can also contribute to cancer defense. Fe may induce toxic ROS and/or initiate specific forms of cell death, including ferroptosis that will benefit cancer treatment. Furthermore, Fe-binding and Fe-regulatory proteins, such as hepcidin, lipocalin-2/NGAL, heme oxygenase-1, ferritin, and iron-sulfur clusters can display antitumor properties under specific conditions and in particular cancer types. In addition, the milk protein lactoferrin may synergize with other established anticancer agents in the prevention and therapy of cancer. Consequently, drugs that target Fe metabolism in tumors are promising candidates for the prevention and therapy of cancer, but consideration of context specificity (e.g., tumor type; systemic versus tumor microenvironment Fe homeostasis) is mandatory.
Copper homeostasis is tightly regulated in both prokaryotic and eukaryotic cells to ensure sufficient amounts for cuproprotein biosynthesis, while limiting oxidative stress production and toxicity. Over the last century, copper complexes have been developed as antimicrobials and for treating diseases involving copper dyshomeostasis (e.g., Wilson’s disease). There now exists a repertoire of copper complexes that can regulate bodily copper through a myriad of mechanisms. Furthermore, many copper complexes are now being appraised for a variety of therapeutic indications (e.g., Alzheimer’s disease and amyotrophic lateral sclerosis) that require a range of copper-related pharmacological affects. Cancer therapy is also drawing considerable attention since copper has been recognized as a limiting factor for multiple aspects of cancer progression including growth, angiogenesis, and metastasis. Consequently, ‘old copper complexes’ (e.g., tetrathiomolybdate and clioquinol) have been repurposed for cancer therapy and have demonstrated anticancer activity in vitro and in preclinical models. Likewise, new tailor-made copper complexes have been designed based on structural and biological features ideal for their anticancer activity. Human clinical trials continue to evaluate the therapeutic efficacy of copper complexes as anticancer agents and considerable progress has been made in understanding their pharmacological requirements. In this chapter, we present a historical perspective on the main copper complexes that are currently being repurposed for cancer therapy and detail several of the more recently developed compounds that have emerged as promising anticancer agents. We further provide an overview of the known mechanisms of action, including molecular targets and we discuss associated clinical trials.
Zinc is an important element that is gaining momentum as a potential target for cancer therapy. In recent years zinc has been accepted as a second messenger that is now recognized to be able to activate many signalling pathways within a few minutes of an extracellular stimulus by release of zinc(II) from intracellular stores. One of the major effects of this store release of zinc is to inhibit a multitude of tyrosine phosphatases which will prevent the inactivation of tyrosine kinases and hence, encourage further activation of tyrosine kinasedependent signalling pathways. Most of these signalling pathways are not only known to be involved in driving aberrant cancer growth, they are usually the main driving force. All this data together now positions zinc and zinc signalling as potentially important new targets to prevent aggressive cancer growth.
Polynuclear platinum complexes (PPCs) represent a discrete structural class of DNA-binding agents with excellent antitumor properties. The use of at least two platinum coordinating units automatically means that multifunctional DNA binding modes are possible. The structural variability inherent in a polynuclear platinum structure can be harnessed to produce discrete modes of DNA binding, with conformational changes distinct from and indeed inaccessible to, the mononuclear agents such as cisplatin. Since our original contributions in this field a wide variety of dinuclear complexes especially have been prepared, their DNA binding studied, and potential relevance to cytotoxicity examined. This chapter focuses on how DNA structure and reactivity is modulated through interactions with PPCs with emphasis on novel aspects of such structure and reactivity. How these major changes are further reflected in damaged DNA-protein binding and cellular effects are reviewed. We further review, for the first time, the great structural diversity achieved in PPC complex design and summarize their major DNA binding effects.