Losartan, an angiotensin II receptor antagonist, is oxidized by hepatic cytochromes P450 to an active carboxylic acid metabolite, E-3174. The aim of the present investigation was to study the contribution of CYP2C9 and CYP3A4 in losartan oxidation in vitro and to evaluate the role of CYP2C9 polymorphism. Kinetic properties of different genetic CYP2C9 variants were compared both in a yeast expression system and in 25 different samples of human liver microsomes where all known genotypes of CYP2C9 were represented. Microsomes were incubated with losartan (0.05-50 microM), and the formation of E-3174 was analyzed by high-performance liquid chromatography to estimate V(max), K(m), and intrinsic clearance for all individual samples. Sulfaphenazole, a CYP2C9 inhibitor, blocked the formation of E-3174 at low losartan concentrations (<1 microM), whereas the inhibitory effect of triacetyloleandomycin, a CYP3A4 inhibitor, was significant only at high concentrations of losartan (>25 microM). In comparison to the CYP2C9.1 variant, oxidation of losartan was significantly reduced in yeast expressing the rare CYP2C9.2 or CYP2C9.3 variants. Moreover, the rate of losartan oxidation was lower in liver microsomes from individuals hetero- or homozygous for the CYP2C9*3 allele, or homozygous for the CYP2C9*2 allele. The difference between the common and rare CYP2C9 variants was mainly explained by a lower V(max), both in yeast and human liver microsomes. In summary, these in vitro results indicate that CYP2C9 is the major human P450 isoenzyme responsible for losartan oxidation and that the CYP2C9 genotype contributes to interindividual differences in losartan oxidation and activation.