Sequential metabolism and bioactivation of the hepatotoxin benzbromarone: formation of glutathione adducts from a catechol intermediate.

Benzbromarone (BBR) is a uricosuric agent that has been used as a treatment for chronic gout. Although never approved in the United States, BBR was recently withdrawn from European markets due to several clinical cases linking the drug to an idiosyncratic hepatotoxicity that is sometimes fatal. We report here a possible mechanism of toxicity that involves the bioactivation of BBR through sequential hydroxylation of the benzofuran ring to a catechol, which can then be further oxidized to a reactive quinone intermediate capable of adducting protein. NADPH-supplemented human liver microsomes generated a single metabolite that was identified as 6-OH BBR by comparison with the synthesized chemical standard. CYP2C9 was the major recombinant enzyme capable of catalyzing the formation of 6-OH BBR, although CYP2C19 also showed a lower degree of activity. Further oxidation of either 6-OH BBR or 5-OH BBR by human liver microsomes resulted in the formation of a dihydroxy metabolite with identical chromatographic and mass spectral properties. This product of sequential metabolism of BBR was identified as the catechol, 5,6-dihydroxybenzbromarone. Incubation of the catechol with liver microsomes, in the presence of glutathione, resulted in the formation of two glutathione adducts that could derive from a single ortho-quinone intermediate. Isoform profiling with recombinant human P450s suggested that CYP2C9 is primarily responsible for the formation of this reactive quinone intermediate.