In vivo inhibition of CYP2C19 but not CYP2D6 by fluvoxamine

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Abstract
Studies were performed in eight healthy extensive metabolizers of mephenytoin and debrisoquine to determine the effect of fluvoxamine on the activities of S‐mephenytoin 4′‐hydroxylase (CYP2C19) and metoprolol α‐hydroxylase (CYP2D6). Therapeutic dosing with fluvoxamine (100 mg day−1) for 2 weeks caused a significant increase in the 0–8 h urinary S/R ratio of mephenytoin from 0.16 to 0.55 (95% confidence interval for difference between means: 0.28–0.50; PP0.05) or the 0–8 h urinary recovery of α‐hydroxymetoprolol (95% confidence interval for difference between means: −0.61–0.70 mg; P >0.05). These results indicated fluvoxamine has a modest inhibitory effect on the activity of CYP2C19, but no effect on that of CYP2D6 in vivo.