Valsartan is known to be excreted largely as unchanged compound and is minimally metabolized in man. Although the only notable metabolite is 4-hydroxyvaleryl metabolite (4-OH valsartan), the responsible enzyme has not been clarified at present. The current in vitro studies were conducted to identify the cytochrome P450 (CYP) enzymes involved in the formation of 4-OH valsartan. Valsartan was metabolized to 4-OH valsartan by human liver microsomes and the Eadie-Hofstee plots were linear. The apparent Km and Vmax values for the formation of 4-OH valsartan were 41.9-55.8 microM and 27.2-216.9 pmol min(-1) mg(-1) protein, respectively. There was good correlation between the formation rates of 4-OH valsartan and diclofenac 4'-hydroxylase activities (representative CYP2C9 activity) of 11 individual microsomes (r = 0.889). No good correlation was observed between any of the other CYP enzyme marker activities (CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4 and CYP4A). Among the recombinant CYP enzymes examined (CYPs 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5 and 4A11), CYP2C9 notably catalysed 4-hydroxylation of valsartan. For the specific CYP inhibitors or substrates examined (furafylline, diclofenac, S(+)-mephenytoin, quinidine and troleandomycin), only diclofenac inhibited the formation of 4-OH valsartan. These results showed that CYP2C9 is the only form responsible for 4-hydroxylation of valsartan in human liver microsomes. Although CYP2C9 is involved in valsartan metabolism, CYP-mediated drug-drug interaction between valsartan and other co-administered drugs would be negligible.