New heterocyclic modifiers of oxidative drug metabolism—II: Steric factors in the interaction of isomeric 2-(naphthyl)methylbenzimidazoles with rat hepatic microsomal cytochrome P-450 and monooxygenase activities

Abstract

The inhibitory potency of the two isomeric 2-(naphthyl)methylbenzitnidazoles towards three monooxygenase activities (aminopyrine N-demethylase, 7-ethoxycoumarin O-deethylase and aniline p-hydroxylase) was assessed in hepatic microsomal fractions from untreated, phenobarbitone-induced and β-naphthoflavone-induced rats. The isomers were essentially equipotent with each other as inhibitors of the phenobarbitone-induced monooxygenases (the ratio of the I 50 s of the isomers was about 1.0 in each case) but differences between the isomers were noted in the inhibition potencies against three monooxygenase activities from β-naphthoflavone-induced liver. The isomer 2-(1'-naphthyl)methyl-benzimidazole was approximately twice as potent as the 2'-naphthyl isomer against 7-ethoxyresorufin O-deethylase activity, whereas the opposite was observed with respect to 7-ethoxycoumarin O-deethylase inhibition; aniline p-hydroxylase was poorly inhibited by both isomers. The binding affinity and extent of binding, assessed from double-reciprocal plots of spectral binding studies, of the 1'-isomer was much greater than that of the 2'-isomer in β-naphthoflavone-induced microsomes. Inhibition data in untreated hepatic microsomes were more complex and the finding of principal interest was that the 1'-isomer was poorly inhibitory towards aniline p-hydroxylase activity whereas the 2'-isomer enhanced this activity. These studies suggest (1) that the steric conformations of the isomeric naphthylmethylbenzimidazoles at the cytochrome P-450 active centre determines the extent to which the inhibitors modulate a specific monooxygenase activity, and (2) that multiple binding sites with the capacity to interact to different extents with benzimidazole derivatives are present in P-450 in β-naphthoflavone-induced hepatic microsomes. The apparent importance of steric conformation as a determinant of inhibition and enhancement of aniline p-hydroxylase in untreated microsomal fractions may well reflect specific interactions with multiple binding sites.