Contribution of CYP2C9, CYP2A6, and CYP2B6 to valproic acid metabolism in hepatic microsomes from individuals with the CYP2C9*1/*1 genotype

Toxicol Sci. 2006 Dec;94(2):261-71. doi: 10.1093/toxsci/kfl096. Epub 2006 Aug 31.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anticonvulsants / metabolism*
  • Anticonvulsants / pharmacokinetics
  • Aryl Hydrocarbon Hydroxylases / genetics
  • Aryl Hydrocarbon Hydroxylases / metabolism*
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 CYP2B6
  • Cytochrome P-450 CYP2C9
  • DNA, Complementary / metabolism
  • Gene Expression Regulation, Enzymologic
  • Humans
  • In Vitro Techniques
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology*
  • Mixed Function Oxygenases / genetics
  • Mixed Function Oxygenases / metabolism*
  • Oxidoreductases, N-Demethylating / genetics
  • Oxidoreductases, N-Demethylating / metabolism*
  • Valproic Acid / metabolism*
  • Valproic Acid / pharmacokinetics


  • Anticonvulsants
  • DNA, Complementary
  • Valproic Acid
  • Mixed Function Oxygenases
  • CYP2C9 protein, human
  • Cytochrome P-450 CYP2C9
  • Aryl Hydrocarbon Hydroxylases
  • CYP2A6 protein, human
  • CYP2B6 protein, human
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 CYP2B6
  • Oxidoreductases, N-Demethylating