Current Medicinal Chemistry - Volume 24, Issue 15, 2017

Amongst all currently used drugs in the field of cancer therapy, the most prominent group of agents which induce DNA, damage both directly or indirectly. Intuitively DNA should not be a perfect target for relatively unspecific small molecular weight drugs. However, the current understanding is that not damage per se but cellular response to DNA damage induced by antitumor agents is responsible for their specific targeted effect towards cancer cells in comparison to the normal cells. DNA damaging chemotherapeutics include compounds with diferent activities namely: directly or indirectly induce DNA strand breaks, covalently modify DNA bases, change the chromatin structure and topology by inhibiting chromatin-modifying enzymes. In this special issue of Current Medicinal Chemistry entitled “DNA Damage as a Strategy for Anticancer Chemotherapy”, Blasiak [1], D'Arcy, N and Gabrielli [2], Stojanovska et al. [3], McQuade et al. [4], Muller [5], Basourakos et al. [6] and Mordente et al. [7] comprehensively reviewed the most recent literatures concerning the various strategies for cancer chemotherapy specifically using DNA damage drugs. The articles present in this special edition cover a wide range of topics that include both clinical practice and translational research. The authors of the presented papers will critically discuss the advantages and disadvantages of the current therapies and the factors that explain decreasing eradication rates. We believe that a more comprehensive understanding of the complexity of the DNA damage response could help in the rational designing of new DNA damaging agents. Such agents can lead to the successful discovery of drugs or innovative combination strategies with improved activity, selectivity toward tumor cells. We hope that the readers find this issue useful and as well as can implement these strategies in their daily course of work.

DNA-damaging drugs in cancer present two main problems: therapeutic resistance and side effects and both can associate with DNA repair, which can be targeted in cancer therapy. Bleomycin (BLM) induces complex DNA damages, including strand breaks, base loss and 3’-phosphoglycolate (3’PG) residues repaired by several pathways, but 3’PGs must be processed to the 3’-OH ends, usually by tyrosyl-DNA phosphodiesterase 1 (Tdp1). Therefore, targeting Tdp1 can improve anticancer therapy with BLM. Mitomycin C (MMC) produces a variety of adducts with DNA, including inter-strand cross-links (ICLs) and Xeroderma pigmentosum (XP) proteins, including XPG, XPE and XPF can be crucial for the initial stage of ICL repair, so they can be targeted by inhibitors to increase toxicity of MMC in cancer cells. Although these proteins are essential for nucleotide excision repair (NER), their decreased activity may not be fatal in normal cells as almost all NER substrates can be repaired by other pathways. Four-stranded DNA, resulted mainly from guanine quadruplexes (G-4s), are highly overexpressed at the end of telomeres, where they can inhibit telomerase, hence stabilization G-4s at the telomeres ends can hamper proliferation of cancer cells. Quadruplexes are also found in the promoters of genes important for cancer and are resolved by DNA helicases, which can be targeted in cancer along with stabilization of quadruplexes. As cancer cells often have defects in DNA repair pathway(s), they can be subjected by synthetic lethality, with the most promising results with poly(ADP-ribose) polymerase 1 (PARP-1) and DNA-dependent protein kinase, catalytic subunit (DNA-PKCS).

Interactions between the decatenation checkpoint and Topoisomerase II (TopoII) are vital for maintaining integrity of the genome. Agents that target this enzyme have been in clinical use in cancer therapy for over 30 years with great success. The types of compounds that have been developed to target TopoII are broadly divided into poisons and catalytic inhibitors. The TopoII poisons are in clinical use as anti-cancer therapies, although in common to most chemotherapeutic agents, they display considerable normal tissue toxicity. Inhibition of the TopoIIb isoform has been implicated in this cytotoxicity. Response to TopoII active agents is determined by several factors, but cell cycle checkpoints play a large role in sensitivity and resistance. The G2/M phase checkpoints are of particular importance in considering the effectiveness of these drugs and are reviewed in this article. Functionality of the ATM dependent decatenation checkpoint may represent a new avenue for selective cancer therapy. Here we review the function of TopoII, the anti-cancer mechanisms and limitations of current catalytic inhibitors and poisons, and their influence on cell cycle checkpoints. We will also assess potential new mechanisms for targeting this enzyme to limit normal tissue toxicity, and how the cell cycle checkpoint triggered by these drugs may provide an alternative and possibly better target for novel therapies.

Platinum-based anti-cancer agents, which include cisplatin, carboplatin and oxaliplatin, are an important class of drugs used in clinical setting to treat a variety of cancers. The cytotoxic efficacy of these drugs is mediated by the formation of inter-strand and intrastrand crosslinks, or platinum adducts on nuclear DNA. There is also evidence demonstrating that mitochondrial DNA is susceptible to platinum-adduct damage in dorsal root ganglia neurons. Although all platinum-based agents form similar DNA adducts, they are quite different in terms of activation, systemic toxicity and tolerance. Platinum-based agents are well known for their neurotoxicity and gastrointestinal side-effects which are major causes for dose limitation and treatment discontinuation compromising the efficacy of anti-cancer treatment. Accumulating evidence in non-neuronal cells shows that the copper transport system is associated with platinum drug sensitivity and resistance. There is minimal research concerning the role of copper transporters within the central and peripheral nervous systems. It is unclear whether neurons are more sensitive to platinum-based drugs, are insufficient in drug clearance, or whether platinum accumulation affects intracellular copper status and coppermediated functions. Understanding these mechanisms is important as neurotoxicity is the predominant side-effect of platinum-based chemotherapy. This review highlights the role of copper transporters in drug influx, differences in drug activation and side-effects caused by platinum-based agents, as well as their association with central and peripheral neuropathies and gastrointestinal toxicities.

Colorectal cancer (CRC) is one the greatest contributors to cancer related mortality. Although 5 year survival rate for patients at the early stage of CRC (stages I and II) is above 60%, more than 50% of patients are diagnosed at or beyond stage III when distant metastasis has already occurred, in which case 5 year survival rate drops to 10%. Chemotherapeutic intervention coupled with surgery is the backbone of metastatic CRC treatment and the only means of enhanced survival. For decades following its discovery, an antimetabolite 5-fluorouracil (5-FU) was the only chemotherapeutic agent available to successfully improve 12 month survival in CRC patients. Treatment of metastatic CRC has been considered palliative for many years; aiming to increase the duration and quality of the patient's remaining life, with little hope of cure, highlighting the need for novel DNA and RNA targeted therapies in the treatment of CRC. Over the last several decades, combinations of several chemotherapeutic agents have been incorporated into routine clinical practice. Combination regimes incorporating irinotecan, a semisynthetic inhibitor of topoisomerase, oxaliplatin, a third-generation platinum compound that causes mitotic arrest via the formation of DNA adducts, and capecitabine, a 5-FU prodrug, are now all established options for use as first-line, second-line and sequential treatment of CRC. This review provides a brief overview of the evolution of CRC chemotherapy as well as new and emerging treatment options.

Compounds causing DNA damage have been used widely in molecular biology and some are used as therapeutic agents in cancer therapy. In most cases, their cellular response is pleiotropic, making it challenging to develop these agents efficiently for potential therapeutic use. Furthermore, this means that such compounds can also affect healthy tissues, which is a major drawback for the use in therapy. Thus, dissecting and understanding not only their molecular mode of action, but also identifying all their cellular targets is critical. With the advent of high throughput DNA sequencing technologies our understanding of the genomic targets of such compounds has increased significantly over recent years. This review gives an overview of some well-studied DNA-damaging agents and dissects what is known about their molecular mode of action, their cellular response and use in clinical settings. It then describes how high throughput-sequencing approaches can be used (a) to study DNAdamaging compounds and (b) to gain insight into their biological activity in vivo.

Maintenance of genomic stability is a critical determinant of cell survival and is necessary for growth and progression of malignant cells. Interstrand crosslinking (ICL) agents, including platinum-based agents, are first-line chemotherapy treatment for many solid human cancers. In malignant cells, ICL triggers the DNA damage response (DDR). When the damage burden is high and lesions cannot be repaired, malignant cells are unable to divide and ultimately undergo cell death either through mitotic catastrophe or apoptosis. The activities of ICL agents, in particular platinum-based therapies, establish a “molecular landscape,” i.e., a pattern of DNA damage that can potentially be further exploited therapeutically with DDR-targeting agents. If the molecular landscape created by platinum-based agents could be better defined at the molecular level, a systematic, mechanistic rationale(s) could be developed for the use of DDR-targeting therapies in combination/maintenance protocols for specific, clinically advanced malignancies. New therapeutic drugs such as poly(ADP-ribose) polymerase (PARP) inhibitors are examples of DDR-targeting therapies that could potentially increase the DNA damage and replication stress imposed by platinum-based agents in tumor cells and provide therapeutic benefit for patients with advanced malignancies. Recent studies have shown that the use of PARP inhibitors together with platinum-based agents is a promising therapy strategy for ovarian cancer patients with “ BRCAness”, i.e., a phenotypic characteristic of tumors that not only can involve loss-of-function mutations in either BRCA1 or BRCA2, but also encompasses the molecular features of BRCA-mutant tumors. On the basis of these promising results, additional mechanism-based studies focused on the use of various DDR-targeting therapies in combination with platinum-based agents should be considered. This review discusses, in general, (1) ICL agents, primarily platinum-based agents, that establish a molecular landscape that can be further exploited therapeutically; (2) multiple points of potential intervention after ICL agent–induced crosslinking that further predispose to cell death and can be incorporated into a systematic, therapeutic rationale for combination/ maintenance therapy using DDR-targeting agents; and (3) available agents that can be considered for use in combination/maintenance clinical protocols with platinum-based agents for patients with advanced malignancies.

Topoisomerases are ubiquitous enzymes involved in maintaining genomic stability of the cell by regulating the over- or underwinding of DNA strands. Besides their customary functions, topoisomerases are important cellular targets of widely used anticancer drugs. In particular, topoisomerase IIα (Top2α) has been postulated as the primary molecular target of anthracycline’s anticancer activity, whereas topoisomerase IIβ (Top2β), the only Top2 present in heart tissue, seems to be involved in the development of anthracycline-induced cardiotoxicity. Noteworthy, cardiotoxicity is the most frequent adverse effect of both conventional and modern anticancer targeted therapy, representing the leading noncancer-related cause of morbidity and mortality in long-term survivors. The molecular mechanisms of anthracyclineinduced cardiotoxicity have been investigated for decades and, despite the numerous mechanistic hypotheses put forward, its aetiology and pathogenesis still remain controversial. This review is aimed at focusing on the double edge sword of topoisomerase-anthracycline interaction, and, in particular, on the potential role of topoisomerases in anthracyclines anticancer activity as well as in the pathogenesis of anthracycline-induced cardiotoxicity.