title: Contribution of CYP2C9, CYP2A6, and CYP2B6 to valproic acid metabolism in hepatic microsomes from individuals with the CYP2C9*1/*1 genotype.
abstract: The present study investigated the role of specific human cytochrome P450 (CYP) enzymes in the in vitro metabolism of valproic acid (VPA) by a complementary approach that used individual cDNA-expressed CYP enzymes, chemical inhibitors of specific CYP enzymes, CYP-specific inhibitory monoclonal antibodies (MAbs), individual human hepatic microsomes, and correlational analysis. cDNA-expressed CYP2C9*1, CYP2A6, and CYP2B6 were the most active catalysts of 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA formation. The extent of 4-OH-VPA and 5-OH-VPA formation by CYP1A1, CYP1A2, CYP1B1, CYP2C8, CYP2C19, CYP2D6, CYP2E1, CYP4A11, CYP4F2, CYP4F3A, and CYP4F3B was only 1-8% of the levels by CYP2C9*1. CYP2A6 was the most active in catalyzing VPA 3-hydroxylation, whereas CYP1A1, CYP2B6, CYP4F2, and CYP4F3B were less active. Correlational analyses of VPA metabolism with CYP enzyme-selective activities suggested a potential role for hepatic microsomal CYP2A6 and CYP2C9. Chemical inhibition experiments with coumarin (CYP2A6 inhibitor), triethylenethiophosphoramide (CYP2B6 inhibitor), and sulfaphenazole (CYP2C9 inhibitor) and immunoinhibition experiments (including combinatorial analysis) with MAb-2A6, MAb-2B6, and MAb-2C9 indicated that the CYP2C9 inhibitors reduced the formation of 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA by 75-80% in a panel of hepatic microsomes from donors with the CYP2C9*1/*1 genotype, whereas the CYP2A6 and CYP2B6 inhibitors had a small effect. Only the CYP2A6 inhibitors reduced VPA 3-hydroxylation (by approximately 50%). The extent of inhibition correlated with the catalytic capacity of these enzymes in each microsome sample. Overall, our novel findings indicate that in human hepatic microsomes, CYP2C9*1 is the predominant catalyst in the formation of 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA, whereas CYP2A6 contributes partially to 3-OH-VPA formation.
authors.affiliation: Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada.
authors.forename: Tony K L
authors.initials: TK
authors.lastname: Kiang
authors.forename: Ping C
authors.initials: PC
authors.lastname: Ho
authors.forename: M Reza
authors.initials: MR
authors.lastname: Anari
authors.forename: Vincent
authors.initials: V
authors.lastname: Tong
authors.forename: Frank S
authors.initials: FS
authors.lastname: Abbott
authors.forename: Thomas K H
authors.initials: TK
authors.lastname: Chang
mesh_terms: D000927:Anticonvulsants / Q000378:metabolism* / Q000493:pharmacokinetics; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics / Q000378:metabolism*; D065637:Cytochrome P-450 CYP2A6; D065702:Cytochrome P-450 CYP2B6; D065729:Cytochrome P-450 CYP2C9; D018076:DNA, Complementary / Q000378:metabolism; D015971:Gene Expression Regulation, Enzymologic; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000235:genetics / Q000378:metabolism*; D010089:Oxidoreductases, N-Demethylating / Q000235:genetics / Q000378:metabolism*; D014635:Valproic Acid / Q000378:metabolism* / Q000493:pharmacokinetics
chemical_list: D000927:Anticonvulsants; D018076:DNA, Complementary; D014635:Valproic Acid; D006899:Mixed Function Oxygenases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; C585710:CYP2A6 protein, human; C585599:CYP2B6 protein, human; D065637:Cytochrome P-450 CYP2A6; D065702:Cytochrome P-450 CYP2B6; D010089:Oxidoreductases, N-Demethylating