Vol. 28, Issue 1, 73-78, January 2000

Identification of Cytochrome P-450 Isoform(s) Responsible for the Metabolism of Pimobendan in Human Liver Microsomes

Shin-ichiro Kuriya,¹ Shigeru Ohmori,¹ Mayuko Hino, Itsuko Ishii, Hiroyoshi Nakamura, Chiaki Senda, Takashi Igarashi, Masahiro Kiuchi, and Mitsukazu Kitada

Division of Pharmacy, University Hospital (S.K., S.O., H.N., M.K.) and Department of Legal Medicine (M.K.), Chiba University School of Medicine, Faculty of Pharmaceutical Sciences, Chiba University (M.H., I.I.); and Department of Drug Metabolism and Pharmacokinetics, Kawanishi Pharma Research Institute, Nippon Boehringer Ingelheim Co. (C.S., T.I.), Japan

Pimobendan, 4,5-dihydro-6-(2-(4-methoxyphenyl)-1H-benzimidazol-5-yl)-5-methyl-3(2-H)-pyridazinone, is a new inotropic drug that augments Ca²⁺ sensitivity and inhibits phosphodiesterase in cardiomyocytes. Pimobendan is well absorbed after oral administration and is metabolized in the liver to the O-demethyl metabolite, which is also active. This study was conducted to identify the cytochrome P-450 (CYP) isoform(s) responsible for the pimobendan O-demethylation in human liver microsomes. Pimobendan O-demethylase activity in human liver microsomes was significantly correlated with phenacetin O-deethylase activity. CYP1A2 antibody and specific inhibitors of CYP1A2 strongly inhibited the metabolism of pimobendan. CYP1A2 was the only one of 10 recombinant human CYP isoforms tested that catalyzed pimobendan O-demethylation at the substrate concentration of 1 µM. At a high substrate concentration (100 µM), recombinant CYP3A4 also catalyzed the reaction, and antibody to CYP3A4 partially inhibited the activity in human liver microsomes. The contribution of CYP1A2 to pimobendan O-demethylation in human liver microsomes varied in the range of 18 to 76%, whereas CYP3A4 accounted for less than 10%, as calculated using the relative activity factor method. We conclude that CYP1A2 is one of the major enzymes responsible for the O-demethylation of pimobendan and CYP3A may make a minor contribution at clinically relevant concentrations of the drug.