REDUCTION OF 3-AMINO-1,2,4-BENZOTRIAZINE-1,4-DI-N-OXIDE (TIRAPAZAMINE, WIN-59075, SR-4233) TO A DNA-DAMAGING SPECIES - A DIRECT ROLE FOR NADPH-CYTOCHROME P450 OXIDOREDUCTASES

3-amino-1,2,4-benzotriazine-1,4-di-N-oxide (tirapazamine, WIN 59075, S R 4233, NSC 130181) has entered phase 1 clinical trials as a bioreduct ive hypoxic cell cytotoxin because of its novel structure and impressi ve selective cytotoxicity towards hypoxic cells. Understanding the enz ymology and underlying mechanism of oxidative and reductive DNA damage may allow more optimal development and use of this agent and contribu te to the rational design of new bioreductive drugs. Here we provide u nambiguous evidence that WIN 59075 undergoes one-electron reduction by purified rat liver NADPH:cytochrome P450 oxidoreductase to generate s ingle- and double-strand breaks in plasmid DNA. The DNA damage caused may be important for the therapeutic toxicity of the drug, Enzyme kine tic parameters for this oxidoreductase reaction are in the range 1.01- 1.61 mM for K-m and 4416 - 5099 nmol/min/mg for V-max. The relative le vels of expression and cellular localization of target tumour NADPH:cy tochrome P450 oxidoreductase may contribute to the therapeutic selecti vity of WIN 59075.