HUMAN CYP2C9 AND CYP2A6 MEDIATE FORMATION OF THE HEPATOTOXIN 4-ENE-VALPROIC ACID

Cytochrome P450-dependent desaturation of the anticonvulsant drug valp roic acid (VPA) results in formation of the hepatotoxin, 4-ene-VPA. Po lytherapy with other anticonvulsants which are known P450 inducers inc reases the flux through this bioactivation pathway. The aim of the pre sent study was to identify specific, inducible forms of human liver P4 50 which catalyze terminal desaturation of VPA. Oxidized VPA metabolit es formed in an NADPH-dependent manner by human liver microsomes were quantified by gas-chromatography/mass spectrometry. In vitro reaction conditions were established which reflected the product profile found in vivo. Production of 4-ene-VPA by microsomal P450s could be inhibite d significantly by coumarin, sulfaphenazole and diethyldithiocarbamate , but not by triacetyloleandomycin, quinidine or furafylline. Recombin ant human CYP3A4 did not form detectable levels of 4-ene-VPA and, of n ine additional isoforms expressed in either HepG2 or lymphoblastoid ce lls which were screened for VPA desaturase activity, only CYP2C9 and C YP2A6 formed detectable levels of metabolite. Consequently, CYP3A4, th e isoform usually associated with induction by anticonvulsants cannot be responsible for the enhanced 4-ene-VPA formation that occurs during polytherapy. Instead, enhanced activity in vivo likely results from i nduction of CYP2A6 and/or CYP2C9.