KR-60436 ([1-(4-methoxy-2-methylphenyl)-4-[(2-hydroxyethyl)amino]-6-trifluoromethoxy-2,3-dihydropyrrolo [3,2-c]quinoline]) is a new reversible H+/K+-ATPase inhibitor. The isoforms of human liver cytochrome P450 (CYP) responsible for the hepatic transformation of KR-60436 is identified. Dihydropyrrole oxidation and O-demethylation are major pathways for the metabolism of KR-60436 in human liver microsomes, whereas N-dehydroxyethylation and hydroxylation are minor pathways. The specific CYP isozymes responsible for KR-60436 oxidation to four major metabolites, pyrrole-KR-60436, O-demethylpyrrole-KR-60436, N-dehydroxyethyl-KR-60436 and an active metabolite, O-demethyl-KR-60436 were identified using the combination of correlation analysis, immuno-inhibition, chemical inhibition in human liver microsomes and metabolism by expressed recombinant CYP enzymes. The inhibitory potency of KR-60436 on clinically major CYPs was investigated in human liver microsomes. The results show that CYP3A4 contributes to the oxidation of KR-60436 to pyrrole-KR-60436, O-demethylpyrrole-KR-60436 and N-dehydroxyethyl-KR-60436, and CYP2C9 and CYP2D6 play roles in demethylation of KR-60436 to form the active metabolite, O-demethyl-KR-60436. KR-60436 was found to inhibit potently the metabolism of CYP1A2 substrates.