Nitrobenzyl derivatives as bioreductive alkylating agents: evidence for the reductive formation of a reactive intermediate.

o- and p-nitrobenzyl chlorides and carbamates were chemically and electrochemically reduced in the presence and absence of the nucleophile morpholine; activation of these compounds by reduction was required to produce an intermediate capable of alkylation. The reduction products formed by the catalytic hydrogenation of each compound were examined by gas chromatography-mass spectrometry. In addition, the products generated by controlled-potential electrolysis were examined by ESR and NMR spectrometry. After a one-electron reduction, o- and p-nitrobenzyl chlorides were activated to the nitrobenzyl radicals, which subsequently dimerized to the dinitrobibenzyl derivatives or reacted with morpholine when present in the reaction medium to form the (nitrobenzyl)morpholine adducts. The nitrobenzyl carbamates were not activated after a one-electron reduction; however, the morpholine and the ether adducts of these agents were observed after catalytic hydrogenation. It was assumed that an intermediate or intermediates formed after the one-electron reduction product, or the full reduction product of the carbamates, were capable of alkylating various nucleophiles. Chemical reduction of the potential bioreductive alkylating agent (o-nitrobenzyl)-6-thioguanine produced (o-aminobenzyl)-6-thioguanine, indicating a lack of formation of a reactive electrophile by reduction. (o-, (m-, and (p-nitrobenzyl)-6-thioguanine analogues were also examined for cytotoxic activity toward EMT6 tumor cells under aerobic and hypoxic conditions. In agreement with the inability of (o-nitrobenzyl)-6-thioguanine to form a reactive species after chemical reduction, no decrease in the survival of neoplastic cells exposed to 10(-4) M drug occurred under either aerobic or hypoxic conditions.