Cytochrome P450 3A4-Mediated Bioactivation and Its Role in Nomilin-Induced Hepatotoxicity

Nomilin is a furan-containing triterpenoid isolated from the medicinal plants of citrus. The aim of this study was to investigate the in vitro and in vivo bioactivation of nomilin and the role in nomilin-induced hepatotoxicity. Microsomal incubations of nomilin supplemented with NADPH and GSH or NAL resulted in the detection of six conjugates (M1-M6). The structures of the metabolites were characterized based on LC-HRMS and NMR. Nomilin was bioactivated to a reactive cis-butene-dial (BDA) intermediate dependent on NADPH, and this intermediate suffered from the reaction with the nucleophiles (GSH and NAL) to form stable adducts. M1-M4 were identified as pyrrole derivatives, and M5 and M6 were pyrrolinone derivatives. M1 was further chemically synthesized and characterized by ¹³C NMR spectroscopy. M1 was the major metabolite detected in mice bile. Pretreatment with ketoconazole significantly reduced the formation of M1 in mice bile, while pretreatment with rifampicin significantly increased the formation of M1. Chemical inhibition together with recombinant human CYP450 phenotyping demonstrated that CYP3A4 was the major enzyme contributing to the bioactivation of nomilin. Toxicity study suggested that nomilin displayed dose-dependent liver injury in mice, while tetrahydro-nomilin was found to be nonhepatotoxic. Pretreatment with ketoconazole prevented mice from nomilin-induced liver injury. The liver injury induced by nomilin was deteriorated when the mice were pretreated with rifampicin. These findings provide evidence that CYP3A4-mediated bioactivation was indispensable in nomilin-induced hepatotoxicity.