Oxidative Stress and 1-Methyl-2-nitroimidazole Cytotoxicity

Abstract

The 2-nitroimidazoles have been used clinically to radiosensitize resistant hypoxic cells, but a dose-limiting peripheral neuropathy has restricted their therapeutic effectiveness. A model compound, 1-methyl-2-nitroimidazole (INO 2), was used to investigate the possible role of oxidative stress in this normal tissue toxicity. Chinese hamster ovary (CHO) cells were 10–15 times more resistant to 20 mM INO 2 under aerobic than hypoxic conditions. In comparison, a pair of transformed rat embryo fibroblasts (ER17-1 wtp53 and ER12L5 mtp53), differing in their p53 genotype, were approximately 3- to 4-fold more sensitive than Chinese hamster ovary cells to INO 2 under aerobic conditions, but had the same sensitivity as Chinese hamster ovary cells under hypoxic conditions. These results are consistent with an earlier hypothesis that the mechanism of aerobic toxicity is different from that of hypoxic toxicity (nitroreduction) and show that neither toxicity is dependent on cellular p53 status. There was an increase in the production of reactive oxygen intermediates and a decrease in the antioxidant glutathione following aerobic exposure to INO 2, which correlated with cell survival in all three cell lines. No evidence of reductive adducts of the 2-nitroimidazole 2-(2-nitro-1 H-imidazol-1-yl)- N-(2,2,3,3,3-pentafluoropropyl)acetamide (EF5) was found by immunofluorescent techniques in aerobic cells. Differing activities of the antioxidant enzymes superoxide dismutase and catalase could be correlated with INO 2 aerobic cytotoxicity. DNA strand breaks, as measured by the comet assay, paralleled the appearance of INO 2 aerobic cytotoxicity in all three cell lines. Taken together, these results strongly support the conclusion that the aerobic toxicity of INO 2 is due to active oxygen species created by futile redox cycling of the parent compound.