Significantly Different Covalent Binding of Oxidative Metabolites, Acyl Glucuronides, and S-Acyl CoA Conjugates Formed from Xenobiotic Carboxylic Acids in Human Liver Microsomes.

Xenobiotic carboxylic acids may be metabolized to oxidative metabolites, acyl glucuronides, and/or S-acyl-CoA thioesters (CoA conjugates) in vitro, e.g., in hepatocytes, and in vivo. These metabolites can potentially be reactive species and bind covalently to tissue proteins and are generally considered to mediate adverse drug reactions in humans. Acyl glucuronide metabolites have been the focus of reactive metabolite research for decades, whereas drug-CoA conjugates, which have been shown to be up to 40-70 times more reactive, have been given much less attention. In an attempt to dissect the contribution of different pathways to covalent binding, we utilized human liver microsomes supplemented with NADPH, uridine 5'-diphosphoglucuronic acid (UDPGA), or CoA to evaluate the reactivity of each metabolite separately. Seven carboxylic acid drugs were included in this study. While ibuprofen and tolmetin are still on the market, ibufenac, fenclozic acid, tienilic acid, suprofen, and zomepirac were stopped before their launch or withdrawn. The reactivities of the CoA conjugates of ibuprofen, ibufenac, fenclozic acid, and tolmetin were higher compared to those of their corresponding oxidative metabolites and acyl glucuronides, as measured by the level of covalent binding to human liver microsomal proteins. The highest covalent binding was observed for ibuprofenyl-CoA and ibufenacyl-CoA, to levels of 1000 and 8600 pmol drug eq/mg protein, respectively. In contrast and in agreement with the proposed P450-mediated toxicity for these drug molecules, the reactivities of oxidative metabolites of suprofen and tienilic acid were higher compared to the reactivities of their conjugated metabolites, with NADPH-dependent covalent binding of 250 pmol drug eq/mg protein for both drugs. The seven drugs all formed UDPGA-dependent acyl glucuronides, but none of these resulted in covalent binding. This study shows that, unlike studies with hepatocytes or in vivo, human liver microsomes provide an opportunity to investigate the reactivity of individual metabolites.