Hypoxia-selective antitumor agents .12. Nitrobenzyl quaternary salts as bioreductive prodrugs of the alkylating agent mechlorethamine

A series of benzene-substituted analogues of the novel hypoxia-selective cytotoxin N,N-bis(2-chloroethyl)-N-methyl-N-(2-nitrobenzyl)ammonium chloride (3a), together with three corresponding tetrahydroisoquinolinium ''cyclic'' analogues 21a-23a and two naphthalene derivatives (19a and 20a), have been prepared and evaluated for cytotoxicity in cultured mammalian tumor cells under aerobic and hypoxic conditions. The parent compound 3a has a one-electron reduction potential of -358 mV and undergoes reductively-induced fragmentation to release the nitrogen mustard mechlorethamine. The compounds were prepared by halogenation (SOCl2) of the corresponding quaternary diols, which in turn were synthesized from N-methyldiethanolamine and substituted nitrobenzyl chlorides. The reduction potentials of the benzene-substituted compounds were generally well-predicted by Hammett substituent relationships. All of the compounds were much more toxic toward repair-deficient UV4 cells than the corresponding wild-type AA8 cells, as expected if the active cytotoxic species is a DNA alkylating agent. They were also more toxic toward the human cell lines EMT6 and FME compared to AA8, but the reasons for this are not known. Analogues of 3a substituted in the phenyl ring with electron-donating substituents provided compounds with widely differing selectivities for hypoxic AA8 cells, ranging from no selectivity for the 3-Me compound 9a to 3000-fold (at least as great as that of the parent 3a) for the 4-OMe compound 14a. The naphthalene derivatives 19a and 20a and the tetrahydroisoquinolinium compounds 21a-23a showed no hypoxic selectivity. Selective chemical reduction of 22a and 23a with nickel boride resulted in isolation of the corresponding stable amino derivatives, indicating that reduction of these compounds does not result in fragmentation. The reason(s) for the marked differences in hypoxic selectivity of the nitrobenzyl quaternary mustards is unknown, but may reflect differences in radical chemistry, cell uptake, or sensitivity to enzymatic reduction.