DNA-Damaging Anticancer Drugs - A Perspective for DNA Repair- Oriented Therapy.

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).