Inhibition of CYP2C9 by selective serotonin reuptake inhibitors: in vitro studies with tolbutamide and (S)-warfarin using human liver microsomes

Eur J Clin Pharmacol. 1999 Feb;54(12):947-51. doi: 10.1007/s002280050580.


Objective: To investigate the in vitro potential of selective serotonin reuptake inhibitors (SSRIs) to inhibit two CYP2C9-catalysed reactions, tolbutamide 4-methylhydroxylation and (S)-warfarin 7-hydroxylation.

Methods: The formation of 4-hydroxytolbutamide from tolbutamide and that of 7-hydroxywarfarin from (S)-warfarin as a function of different concentrations of SSRIs and some of their metabolites was studied in microsomes from three human livers.

Results: Both tolbutamide 4-methylhydroxylation and (S)-warfarin 7-hydroxylation followed one enzyme Michaelis-Menten kinetics. Kinetic analysis of 4-hydroxytolbutamide formation yielded a mean apparent Michaelis-Menten constant (Km) of 133 microM and a mean apparent maximal velocity (Vmax) of 248 pmol x min(-1) x mg(-1); formation of 7-hydroxywarfarin yielded a mean Km of 3.7 microM and a mean Vmax of 10.5 pmol x min(-1) x mg(-1). Amongst the SSRIs and some of their metabolites tested, only fluvoxamine markedly inhibited both reactions. The average computed inhibition constant (Ki) values and ranges of fluvoxamine when tolbutamide and (S)-warfarin were used as substrate, were 13.3 (6.4-17.3) microM and 13.0 (8.4-18.7) microM, respectively. The average Ki value of fluoxetine for (S)-warfarin 7-hydroxylation was 87.0 (57.0-125) microM.

Conclusion: Amongst the SSRIs tested, fluvoxamine was shown to be the most potent inhibitor of both tolbutamide 4-methylhydroxylation and (S)-warfarin 7-hydroxylation. Fluoxetine, norfluoxetine, paroxetine, sertraline, desmethylsertraline, citalopram, desmethylcitalopram had little or no effect on CYP2C9 activity in vitro. This is consistent with in vivo data indicating that amongst the SSRIs, fluvoxamine has the greatest potential for inhibiting CYP2C9-mediated drug metabolism.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Aryl Hydrocarbon Hydroxylases*
  • Cytochrome P-450 CYP2C9
  • Cytochrome P-450 Enzyme Inhibitors*
  • Dose-Response Relationship, Drug
  • Fluvoxamine / pharmacology
  • Humans
  • In Vitro Techniques
  • Microsomes, Liver / metabolism*
  • Middle Aged
  • Selective Serotonin Reuptake Inhibitors / pharmacology*
  • Statistics as Topic
  • Steroid 16-alpha-Hydroxylase*
  • Steroid Hydroxylases / antagonists & inhibitors*
  • Time Factors
  • Tolbutamide / analogs & derivatives
  • Tolbutamide / analysis
  • Tolbutamide / metabolism*
  • Warfarin / analogs & derivatives
  • Warfarin / analysis
  • Warfarin / metabolism*


  • Cytochrome P-450 Enzyme Inhibitors
  • Serotonin Uptake Inhibitors
  • hydroxymethyltolbutamide
  • Warfarin
  • Tolbutamide
  • 7-hydroxywarfarin
  • Steroid Hydroxylases
  • CYP2C9 protein, human
  • Cytochrome P-450 CYP2C9
  • Aryl Hydrocarbon Hydroxylases
  • Steroid 16-alpha-Hydroxylase
  • Fluvoxamine