Base excision repair of reactive oxygen species–initiated 7,8-dihydro-8-oxo-2′-deoxyguanosine inhibits the cytotoxicity of platinum anticancer drugs

Anticancer therapy with cisplatin and oxaliplatin is limited by toxicity and onset of tumor resistance. Both drugs form platinum-DNA cross-linked adducts, and cisplatin causes oxidative DNA damage including the 7,8-dihydro-8-oxo-2′-deoxyguanosine (8-oxodG) lesion. To assess oxidative DNA damage as a mechanism of cisplatin and oxaliplatin cytotoxicity, 8-oxodG–directed base excision repair was stably enhanced in human embryonic kidney cells by FLAG-tagged expression of human oxoguanine glycosylase 1 (α-OGG1) or its functional homologue, Escherichia coli formamidopyrimidine glycosylase (fpg). Both drugs increased reactive oxygen species and 8-oxodG levels, and cytotoxicity was decreased by antioxidant pretreatment. Ectopic expression of α-OGG1 or fpg in cell clones increased nuclear and mitochondrial 8-oxodG repair, and reduced death by reactive oxygen species initiators (H2O2, menadione) and both platinum drugs. Exposure to oxaliplatin caused a more marked and sustained block of cell proliferation than exposure to cisplatin. We conclude that the 8-oxodG lesion is cytotoxic, and base excision repair a likely determinant of risk. The greater antitumor efficacy of oxaliplatin seems unrelated to oxidative DNA damage, suggesting a novel strategy for improving the therapeutic index in cancer therapy. [Mol Cancer Ther 2009;8(7):2015–26]