In vitro evaluation of valproic acid as an inhibitor of human cytochrome P450 isoforms: preferential inhibition of cytochrome P450 2C9 (CYP2C9)

Br J Clin Pharmacol. 2001 Nov;52(5):547-53. doi: 10.1046/j.0306-5251.2001.01474.x.


Aims: To evaluate the potency and specificity of valproic acid as an inhibitor of the activity of different human CYP isoforms in liver microsomes.

Methods: Using pooled human liver microsomes, the effects of valproic acid on seven CYP isoform specific marker reactions were measured: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), tolbutamide hydroxylase (CYP2C9), S-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1) and midazolam 1'-hydroxylase (CYP3A4).

Results: Valproic acid competitively inhibited CYP2C9 activity with a Ki value of 600 microM. In addition, valproic acid slightly inhibited CYP2C19 activity (Ki = 8553 microM, mixed inhibition) and CYP3A4 activity (Ki = 7975 microM, competitive inhibition). The inhibition of CYP2A6 activity by valproic acid was time-, concentration- and NADPH-dependent (KI = 9150 microM, Kinact=0.048 min(-1)), consistent with mechanism-based inhibition of CYP2A6. However, minimal inhibition of CYP1A2, CYP2D6 and CYP2E1 activities was observed.

Conclusions: Valproic acid inhibits the activity of CYP2C9 at clinically relevant concentrations in human liver microsomes. Inhibition of CYP2C9 can explain some of the effects of valproic acid on the pharmacokinetics of other drugs, such as phenytoin. Co-administration of high doses of valproic acid with drugs that are primarily metabolized by CYP2C9 may result in significant drug interactions.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aryl Hydrocarbon Hydroxylases*
  • Cytochrome P-450 CYP1A2
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 CYP2C9
  • Cytochrome P-450 Enzyme Inhibitors
  • Cytochrome P-450 Enzyme System / metabolism*
  • Dose-Response Relationship, Drug
  • Enzyme Inhibitors / pharmacology*
  • Humans
  • Hydroxylation / drug effects
  • Isoenzymes / antagonists & inhibitors
  • Isoenzymes / metabolism
  • Microsomes, Liver / drug effects*
  • Microsomes, Liver / enzymology
  • Mixed Function Oxygenases / antagonists & inhibitors
  • Mixed Function Oxygenases / metabolism
  • Steroid 16-alpha-Hydroxylase*
  • Steroid Hydroxylases / antagonists & inhibitors
  • Steroid Hydroxylases / metabolism
  • Time Factors
  • Tolbutamide / metabolism
  • Valproic Acid / pharmacology*


  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Isoenzymes
  • Valproic Acid
  • Cytochrome P-450 Enzyme System
  • Tolbutamide
  • Mixed Function Oxygenases
  • Steroid Hydroxylases
  • CYP2C9 protein, human
  • Cytochrome P-450 CYP2C9
  • Aryl Hydrocarbon Hydroxylases
  • CYP1A2 protein, human
  • CYP2A6 protein, human
  • Cytochrome P-450 CYP1A2
  • Cytochrome P-450 CYP2A6
  • Steroid 16-alpha-Hydroxylase