Identification of cytochrome P-450 isoform(s) responsible for the metabolism of pimobendan in human liver microsomes

Pimobendan, 4,5-dihydro-6-(2-(4-methoxyphenyl)-1H-benzimidazol-5-yl)-5-meth yl-3(2-H)-pyridazinone, is a new inotropic drug that augments Ca2+ sensitiv ity and inhibits phosphodiesterase in cardiomyocytes. Pimobendan is well ab sorbed after oral administration and is metabolized in the liver to the O-d emethyl metabolite, which is also active. This study was conducted to ident ify 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-de ethylase activity. CYP1A2 antibody and specific inhibitors of CYP1A2 strong ly inhibited the metabolism of pimobendan. CYP1A2 was the only one of 10 re combinant human CYP isoforms tested that catalyzed pimobendan O-demethylati on at the substrate concentration of 1 mu M. At a high substrate concentrat ion (100 mu M), recombinant CYP3A4 also catalyzed the reaction, and antibod y to CYP3A4 partially inhibited the activity in human liver microsomes. The contribution of CYP1A2 to pimobendan O-demethylation in human liver micros omes varied in the range of 18 to 76%, whereas CYP3A4 accounted for less th an 10%, as calculated using the relative activity factor method. We conclud e that CYP1A2 is one of the major enzymes responsible for the O-demethylati on of pimobendan and CYP3A may make a minor contribution at clinically rele vant concentrations of the drug.