Studies on the Toxicity and Radiosensitizing Ability of Misonidazole under Conditions of Prolonged Incubation

In vitro studies with CHO cells indicate that the 2-nitroimidazole, misonidazole, is markedly more toxic under hypoxic conditions than under aerobic conditions. Also, survival curves determined using prolonged exposure to the drug under hypoxic conditions before irradiation have shown a reduction in the extrapolation number (ñ) as well as the mean lethal dose (D0). Studies with radioactive misonidazole have indicated that at 37°C there is a gradual accumulation of radioactive material within hypoxic as compared to aerobic cells and that over an incubation period of 5 hr, a 2- to 3-fold increase in uptake by hypoxic cells is observed. Chromatographic analysis has shown that this increase in uptake is due to the accumulation of metabolites of misonidazole. It is suggested that both the increase in toxicity and reduction in ñ as a result of prolonged incubation with misonidazole under hypoxic conditions are related to the formation of these metabolites.