Mechanism-Based Inactivation of Cytochrome P450 3A4 by Benzbromarone

Benzbromarone (BBR), a potent uricosuric agent for the management of gout, is known to cause fatal fulminant hepatitis. Although the mechanism of BBR-induced idiosyncratic hepatotoxicity remains unelucidated, cytochrome P450 enzyme-mediated bioactivation of BBR to electrophilic reactive metabolites is commonly regarded as a key molecular initiating event. However, apart from causing aberrant toxicities, reactive metabolites may result in mechanism-based inactivation (MBI) of cytochrome P450. Here, we investigated and confirmed that BBR inactivated CYP3A4 in a time-, concentration-, and NADPH-dependent manner with K _I, k _inact, and partition ratio of 11.61 µM, 0.10 minutes^-1, and 110, respectively. Coincubation with ketoconazole, a competitive inhibitor of CYP3A4, attenuated the MBI of CYP3A4 by BBR, whereas the presence of glutathione and catalase did not confer such protection. The lack of substantial recovery of enzyme activity postdialysis and after oxidation with potassium ferricyanide, combined with the absence of a Soret peak in spectral difference scans, implied that MBI of CYP3A4 by BBR did not occur through the formation of quasi-irreversible metabolite-intermediate complexes. Analysis of the reduced CO-difference spectrum revealed an ∼44% reduction in ferrous-CO binding and hinted that inactivation is mediated via irreversible covalent adduction to both the prosthetic heme moiety and the apoprotein. Finally, our in silico covalent docking analysis further suggested the modulation of substrate binding to CYP3A4 via the covalent adduction of epoxide-derived reactive intermediates of BBR to two key cysteine residues (Cys239 and Cys58) vicinal to the entrance of the orthosteric binding site. SIGNIFICANCE STATEMENT: Although the bioactivation of benzbromarone (BBR) to reactive metabolites has been well characterized, its potential to cause mechanism-based inactivation (MBI) of cytochrome P450 has not been fully investigated. This study reports the MBI of CYP3A4 by BBR via irreversible covalent adduction and develops a unique covalent docking methodology to predict the structural molecular determinants underpinning the inactivation for the first time. These findings lay the groundwork for future investigation of clinically relevant drug-drug interactions implicating BBR and mechanisms of BBR-induced idiosyncratic hepatotoxicity.