Role of Glutathione in the Hypoxie Cell Cytotoxicity of Misonidazole1

Misonidazole (MIS) is a hypoxic cell radiosensitizer currently undergoing Phase III clinical trials in the treatment of cancer by radiation. It is also a cytotoxic agent with specificity toward hypoxic cells, and consequently has a tumoricidal effect in laboratory animals. This tumoricidal effect has not been clini cally applicable, in part because the initial resistance to the cytotoxic action of MIS (the nonexponential portion, or shoul der, of the semilogarithmic plot of the surviving fraction of the cell population versus the time of exposure to MIS, referred to hereafter as the shoulder of the survival curve) for cells treated with MIS under hypoxic conditions is too large to be overcome at clinically tolerable doses of MIS. We report here that pre treatment of Chinese hamster ovary cells in vitro with diethylmaleate to deplete ¡ntracellular glutathione results in a sub stantial decrease in the shoulder of the survival curve for MIStreated hypoxic cells. Restoration of glutathione results in restoration of the shoulder of the survival curve and a slight extension beyond that seen with control cells. These results demonstrate that glutathione protects against the cytotoxic effect of MIS. However, glutathione depletion does not signif icantly affect the rate of binding of MIS metabolites to cellular macromolecules, indicating that the cytotoxicity of MIS is not simply a reflection of massive binding of MIS metabolites to cellular constituents. We propose that the cytotoxicity of MIS toward hypoxic cells is a result of hydrogen abstraction from target molecules by free radicals formed in the reduction of the nitro group.