Abstract

Losartan is a 4-chloro-5-hydroxymethylimidazole derivative that is a potent and highly selective angiotensin II receptor antagonist. Losartan is metabolized in vivo in rats, monkeys, and humans to a carboxylic acid derivative E3174 that is pharmacologically more active than the parent compound. We have investigated the mechanism of this biotransformation in human liver preparations. The oxidation of both losartan and the putative aldehyde intermediate E3179 was catalyzed by the microsomal fraction, required both NADPH and molecular oxygen, and was inhibited by SKF 525-A, implicating cytochrome P450 (CYP). When incubations with each substrate were performed under an atmosphere of 18O2, the extent of 18O incorporation into the carboxylic acid product was consistent with a mechanism for losartan oxidation involving an aldehyde intermediate. To substantiate the involvement of CYP in these reactions, incubations with losartan and the aldehyde E3179 were performed in the presence of isoform-selective inhibitors. Inhibitors of CYP3A4/5 (gestodene and ketoconazole) and CYP2C9/10 (sulfaphenazole) attenuated the oxidation of both substrates. It was then demonstrated that microsomes containing either recombinant human liver CYP2C9 or CYP3A4 were capable of oxidizing both losartan and the aldehyde E3179 to the carboxylic acid E3174. Subsequently, it was shown that rabbit anti-CYP2C9 and anti-CYP3A3/4 inhibited the oxidation of losartan to E3174 in incubations with human liver microsomes. These studies support the hypothesis that the aldehyde E3179 is an intermediate in the oxidation of losartan and that this two-step reaction is catalyzed in human liver microsomes by members of the CYP3A and CYP2C subfamilies.