Background: Phenytoin, a cytochrome P450 (CYP) 2C9 substrate, has a narrow therapeutic index and nonlinear pharmacokinetics. Therefore there is the potential for significant concentration-related adverse effects when phenytoin is coadministered with other CYP2C9 substrates. Losartan, an antihypertensive agent, is also a substrate for CYP2C9.
Objective: Our objective was to assess the effects of losartan on the pharmacokinetics of phenytoin and the effects of phenytoin on the pharmacokinetics of losartan in a healthy population of volunteers.
Methods: A prospective, randomized, 3-period crossover study was conducted in 16 healthy volunteers with phenytoin alone, phenytoin in combination with losartan, and losartan alone. Each treatment was given for 10 days with a 3-week washout period between treatments. On day 10, plasma concentrations of phenytoin and plasma and urine concentrations of losartan and its active carboxylic-acid metabolite E3174 were measured to determine steady-state pharmacokinetic parameters.
Results: Coadministration of losartan had no effect on the pharmacokinetics of phenytoin. Coadministration of phenytoin increased the mean area under the concentration-time curve from time zero to 24 hours [AUC(0-24)] of losartan by 17% (355 +/- 220 ng x h/mL versus 427 +/- 177 ng x h/mL; P =.1), but this difference was not statistically significant. In the 14 CYP2C9*1/*1 subjects, the mean AUC(0-24) of losartan was increased by 29% (284 +/- 84 ng x h/mL versus 402 +/- 128 ng x h/mL; P =.008). Coadministration of phenytoin significantly reduced the AUC(0-24) of E3174 by 63% (1254 +/- 256 ng x h/mL versus 466 +/- 174 ng x h/mL; P =.0001) and the formation clearance of losartan to E3174 (1.91 +/- 0.8 mL/h per kilogram versus 0.62 +/- 0.4 mL/h per kilogram; P =.0001).
Conclusions: Losartan, a CYP2C9 substrate, had no effect on the pharmacokinetics of phenytoin. However, phenytoin inhibited the CYP2C9-mediated conversion of losartan to E3174.