Reduction of 3-amino-l,2,4-benzotriazine-l,4-di-N-oxide (tirapazamine, WIN 59075, SR 4233) to a DNA-damaging species: a direct role for NADPH: cytochrome P450 oxidoreductase

3-amino-l,2,4-benzotriazine-l,4-di-N-oxide (tirapazamine, WIN 59075, SR 4233, NSC 130181) has entered phase 1 clinical trials as a bioreductive hypoxic cell cytotoxin because of its novel structure and impressive selective cytotoxicity towards hypoxic cells. Understanding the enzymology and underlying mechanism of oxidative and reductive DNA damage may allow more optimal development and use of this agent and contribute to the rational design of new bioreductive drugs. Here we provide unambiguous evidence that WIN 59075 undergoes one-electron reduction by purified rat liver NADPH:cytochrome P450 oxidoreductase to generate single- and double-strand breaks in plasm id DNA. The DNA damage caused may be important for the therapeutic toxicity of the drug. Enzyme kinetic parameters for this oxidoreductase reaction are in the range 1.01-1.61 mM for Km and 4416–5099 nmol/min/ mg for Vmax. The relative levels of expression and cellular localization of target tumour NADPH:cytochrome P450 oxidoreductase may contribute to the therapeutic selectivity of WIN 59075.