Metabolic N-hydroxylation. Use of substituent variation to modulate the in vitro bioactivation of 4-acetamidostilbenes.

N-Hydroxylation is an obligate step in the bioactivation of carcinogenic aryl amides. Previous reports from this laboratory demonstrated that variation of the 4' substituent of trans-4-acetamidostilbene (1) has a marked effect on the rate of its in vitro microsomal N-hydroxylation. In order to further investigate the effects of electronegative and aliphatic substituents, the 4'-CN, 4'-CH3, 4'-C(CH3)3, and 4'-CF3 analogues of 1 were synthesized and subjected to metabolic transformation by hamster hepatic microsomes. Each compound was synthesized in radiolabeled form, and the metabolites were identified and quantified by TLC, mass spectrometry, and liquid scintillation counting. The Vmax for N-hydroxylation of the 4'-CN analogue was 24% and the Km was 11% of that of 1. The glycolamide was a minor metabolite of the 4'-CN compound. The principal metabolite of the 4'-CH3 compound was the 4'-CH2OH derivative, the N-hydroxylated product being formed in small quantities. Similarly, the 4'-C(CH3)3 analogue was metabolized to yield trans-4'-[2-(hydroxymethyl)-2-propyl]-4-acetamidostilbene (26) along with trace quantities of the hydroxamic acid. The 4'-CF3 substrate yielded small amounts of the N-hydroxylated material as the only detectable metabolite. Thus, introduction of a 4' substituent into 1 resulted in a decreased rate of N-hydroxylation for all compounds studied. The reduction in N-hydroxylation depends on both the physicochemical properties of the 4' substituent and upon the susceptibility of the substituent to metabolic oxidation.