Role of CYP2C9 polymorphism in losartan oxidation

Losartan, an angiotensin II receptor antagonist, is oxidized by hepatic cyt ochromes P450 to an active carboxylic acid metabolite, E-3174. The aim of t he present investigation was to study the contribution of CYP2C9 and CYP3A4 in losartan oxidation in vitro and to evaluate the role of CYP2C9 polymorp hism. Kinetic properties of different genetic CYP2C9 variants were compared both in a yeast expression system and in 25 different samples of human liv er microsomes where all known genotypes of CYP2C9 were represented. Microso mes were incubated with losartan (0.05-50 muM), 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 concentra tions (<1 <mu>M), whereas the inhibitory effect of triacetyloleandomycin, a CYP3A4 inhibitor, was significant only at high concentrations of losartan (>25 muM). In comparison to the CYP2C9.1 variant, oxidation of losartan was significantly reduced in yeast expressing the rare CYP2C9.2 or CYP2C9.3 va riants. Moreover, the rate of losartan oxidation was lower in liver microso mes from individuals hetero- or homozygous for the CYP2C9*3 allele, or homo zygous 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 hu man liver microsomes. In summary, these in vitro results indicate that CYP2 C9 is the major human P450 isoenzyme responsible for losartan oxidation and that the CYP2C9 genotype contributes to interindividual differences in los artan oxidation and activation.