Vol. 30, Issue 10, 1070-1076, October 2002

Quaternary Ammonium-Linked Glucuronidation of 1-Substituted Imidazoles by Liver Microsomes: Interspecies Differences and Structure-Metabolism Relationships

Sarvesh C. Vashishtha,¹ Edward M. Hawes, Denis J. McCann,² Omar Ghosheh, and Lawrence Hogg

Drug Metabolism and Drug Disposition Group (S.C.V., E.M.H., O.G., L.H.), University of Saskatchewan, and PharmaLytics (L.H.), Saskatoon, Saskatchewan, Canada; and Drug Disposition and Metabolism Department, AstraZeneca, Wilmington, DE (D.J.M.)

N-Glucuronidation at an aromatic tertiary amine of 5-membered polyaza ring systems was investigated for a model series of eight 1-substituted imidazoles in liver microsomes from five species. The major objectives were to investigate substrate specificities of the series in human microsomes and interspecies variation for the prototype molecule, 1-phenylimidazole. The formed quaternary ammonium-linked metabolites were characterized by positive ion electrospray mass spectrometry. The incubation conditions for the N-glucuronidation of 1-substituted imidazoles were optimized; where for membrane disrupting agents, alamethicin was more effective than the detergents examined. The need to optimize alamethicin concentration was indicated by 4-fold interspecies variation in optimal concentration and by a change in effect from removal of glucuronidation latency to inhibition on increasing concentration. For the four species with quantifiable N-glucuronidation of 1-phenylimidazole, there were 8- and 18-fold variations in the determined apparent K_m (range, 0.63 to 4.8 mM) and V_max (range, 0.08 to 1.4 nmol/min/mg of protein) values, respectively. The apparent clearance values (V_max/K_m) were in the following order: human  guinea pig  rabbit > rat  dog (no metabolite detected). Monophasic kinetics were observed for the N-glucuronidation of seven substrates by human liver microsomes, which suggests that one enzyme is involved in each metabolic catalysis. No N-glucuronidation was observed for the substrate containing the para-phenyl substituent with the largest electron withdrawing effect, 1-(4-nitrophenyl)imidazole. Linear correlation analyses between apparent microsomal kinetics and substrate physicochemical parameters revealed significant correlations between K_m and lipophilicity (_para or log P values) and between V_max/K_m and both electronic properties (_para value) and pKa.