Troglitazone quinone formation catalyzed by human and rat CYP3A: an atypical CYP oxidation reaction

Oxidative ring opening of troglitazone (TGZ), a thiazolidine 2,4-dione deri vative used for the treatment of type II diabetes mellitus, leads to the fo rmation of a quinone metabolite. The formation of TGZ quinone was shown to be NADPH dependent and to require active microsomal enzymes. Quinone format ion was not affected by co-incubation with catalase or sodium azide and was partially inhibited (25%) by superoxide dismutase (SOD). Kinetic analysis of TGZ quinone formation in human liver microsomes implied single enzyme in volvement. CYP3A isoforms were characterized as the primary enzymes involve d in quinone formation by several lines of evidence including: (a) troleand omycin and ketoconazole almost completely inhibited microsomal quinone form ation when SOD was present, whereas other CYP inhibitors had minimal effect s (<20%); (b) TGZ quinone formation was highly correlated with regard to bo th contents (r(2): 0.9374) and activities (r(2): 0.7951) of CYP3A4 in human liver microsomes (HLM); (c) baculovirus insect cell-expressed human CYP3A4 was able to catalyze TGZ quinone formation at a higher capacity (V-max/K-m ) than other human CYPs with the relative contribution of CYP3A4 in HLM est imated to be 20-fold higher than that of other CYPs; (d) TGZ quinone format ion was increased by 350% in liver microsomes from rats pretreated with dex amethasone (DEX); and (e) plasma concentrations of TGZ quinone were increas ed by 260-680% in rats pretreated with DEX. The chemical nature of the quin one metabolite suggests an atypical CYP reaction consistent with a one-elec tron oxidation mechanism where an intermediate phenoxy radical combines wit h ferryl oxygen to subsequently form the quinone metabolite. (C) 2001 Elsev ier Science Inc. All rights reserved.