A patient who had taken a stable dose of phenytoin for 2 years had a coronary stent placed for unstable angina and ticlopidine was added to this therapeutic regimen. Twenty-five days later, he was hospitalized with acute symptomatic phenytoin toxicity and a serum concentration of 46.5% μg/ml. Determination of metabolic genotype revealed that the patient had a wild-type genotype for CYP2C9, CYP2C19, and CYP2D6. Using human liver microsomes, we showed that ticlopidine is a potent inhibitor of cytochrome P450 2C19, with an estimated inhibition constant (K(i)) of 3.7 ± 0.2 μmol/L. The influence of ticlopidine on CYP2C9, the other cytochrome P450 isoform that metabolizes phenytoin, is relatively weak, with a calculated K(i) of 38.8 ± 27 μmol/L. These data suggest that, in this patient, phenytoin toxicity was caused by inhibition of CYP2C19 by ticlopidine, and the data emphasize the importance of CYP2C19 in the metabolism of phenytoin.
ASJC Scopus subject areas
- Pharmacology (medical)