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Selective Toxicity of 5-(3,3-Dimethyl-1-triazeno)imidazole-4-carboxamide toward Hypoxic Mammalian Cells¹

Physics Division, Ontario Cancer Institute, and Department of Medical Biophysics, University of Toronto, Toronto, Ontario, Canada M4X 1K9

The chemotherapeutic agent 5-(3,3-dimethyl-1-triazeno)-imidazole-4-carboxamide (DTIC) is used in the treatment of malignant melanoma where response rates of 15 to 30% have been reported. Some current interest exists in combining DTIC chemotherapy with localized high-dose (800 rads)-per-fraction radiotherapy in the treatment of unresectable metastatic melanoma. The present work investigates the radiosensitizing and chemotherapeutic properties of DTIC in an in vitro system using Chinese hamster ovary or HeLa cells and in vivo using the KHT transplantable murine tumor. No evidence of a radiosensitizing effect of DTIC was found towards hypoxic or aerobic cells either in vitro or in vivo. In vitro, high drug concentrations (1 mg/ml) were approximately 5 times more effective in killing hypoxic Chinese hamster ovary or HeLa cells than in killing aerobic cells over exposure times of 0 to 12 hr. The degree of toxicity was drug dose and temperature dependent but was not highly dependent on cell number or cell type. In vivo plasma levels of DTIC were measured with high-pressure liquid chromatography after i.p. injection of drug into C3H mice. At the highest drug doses tested, near the 50% lethal dose in mice for DTIC (0.5 mg/g), the drug was toxic to both aerobic and hypoxic tumor cells with some evidence of increased toxicity towards hypoxic cells. The present work suggests that DTIC may be more efficiently activated under hypoxic conditions as compared to aerobic conditions. The increased toxicity of DTIC under hypoxic versus aerobic conditions may prove to be a feature of this drug that can be exploited in its clinical use and in the design of new analogs of DTIC.

¹ This work was supported by grants from the National Cancer Institute, the Medical Research Council of Canada, and the Ontario Cancer Treatment and Research Foundation.

² To whom requests for reprints should be addressed.

Received 5/12/81. Accepted 8/24/81.