Mechanism of the selective hypoxic cytotoxicity of 1-methyl-2-nitroimidazole

2-Nitroimidazoles were introduced into radiation therapy to test their ability to radiosensitize hypoxic cells in solid human tumours. In addition, they are selectively reduced in hypoxic cells to form reactive metabolites that may be effective cytotoxins. 1-Methyl-2-nitroimidazole (INO2) was investigated as a model compound to study the mechanism of selective bioreduction in hypoxic cells. Results demonstrated that INO2 was toxic under hypoxic conditions (tested via colony-forming assay) at concentrations where no toxicity was observed for aerobic cells. This selective hypoxic toxicity was a function of both concentration and time. The depletion of both glutathione and protein thiols occurred under hypoxic conditions and preceded a rise in intracellular calcium levels. Previous work with INO, the nitroso intermediate of INO2 reduction, also showed concentration-dependent cytotoxicity, and glutathione and protein thiol depletion, which was followed by an increase in intracellular calcium levels. The kinetics of cytotoxicity and cellular reactions were slower for the parent compound, INO2, as compared with the 2e- reductive metabolite, INO, reflecting the limited enzymatic production of the reactive intermediate in the INO2 experiments. Zeiosis (membrane blebbing) and chromatin condensation occurred shortly after treatment of cells with equitoxic concentrations of both INO2 (under hypoxic conditions) and INO (under aerobic conditions), suggesting that an apoptotic-like death mechanism may be involved. However, analysis of DNA isolated from both INO2- and INO-treated cells, up to 2 hr after treatment, did not reveal any nucleosomal fragmentation, another characteristic feature of cells undergoing apoptosis. The toxicity of high INO2 concentrations toward CHO cells is consistent with the production of an INO intermediate and has several features characteristic of an apoptotic mechanism of cell death.