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Direct evidence for bimodal DNA damage induced by tirapazamine.

The ability of tirapazamine (1, 3-amino-1,2,4-benzotriazine 1, 4-dioxide, SR4233) to fix DNA radical lesions is demonstrated by studying the reaction between the antitumor drug and an oligonucleotide radical that is independently produced at a defined site within a biopolymer. Using beta-mercaptoethanol as a competitor, it was determined that tirapazamine traps a C1'-nucleotide radical with a rate constant of approximately 2 x 10(8) M-1 s-1. Product and isotopic labeling studies suggest that tirapazamine reacts with the radical via covalent adduct formation, resulting primarily from reaction at the N-oxide oxygen. Intermediate covalent adducts could not be observed, but are postulated to decompose to the alkaline labile 2'-deoxyribonolactone lesion. These experiments affirm recent proposals suggesting that tirapazamine can serve as a surrogate for O2 in converting DNA radicals into toxic strand damage events.