The purpose of the study was to elucidate human intestinal cytochrome P450 isoform(s) involved in the metabolism of an antihistamine, ebastine, having two major pathways of hydroxylation and N-dealkylation. The ebastine dealkylase in human intestinal microsomes was CYP3A4, based on the inhibition studies with antibodies against CYP1A, CYP2A, CYP2C, CYP2D, CYP2E, and CYP3A isoforms and their selective inhibitors. However, ebastine hydroxylase could not be identified. We then examined the inhibitory effects of anti-CYP4F antibody and 17-octadecynoic acid, an inhibitor of the CYP4 family, on ebastine hydroxylation in intestinal microsomes, since CYP4F was recently found to be the predominant ebastine hydroxylase in monkey intestine; and a novel CYP4F isoform (CYP4F12), also capable of hydroxylating ebastine, was found to exist in human intestine. However, the inhibitory effects were only partial (about 20%) and thus it was thought that, although human CYP4F was involved in ebastine hydroxylation, another predominant enzyme exists. Further screening showed that the hydroxylation was inhibited by arachidonic acid. CYP2J2 was selected as a candidate expressed in the intestine and closely related to arachidonic acid metabolism. The catalytic activity of recombinant CYP2J2 was much higher than that of CYP4F12. Anti-CYP2J antibody inhibited the hydroxylation to about 70% in human intestinal microsomes. These results demonstrate that CYP2J2 is the predominant ebastine hydroxylase in human intestinal microsomes. Thus, the present paper for the first time indicates that, in human intestinal microsomes, both CYP2J and CYP4F subfamilies not only metabolize endogenous substrates but also are involved in the drug metabolism. Ebastine is a potent nonsedative H 1 -receptor antagonist (Fig. 1), and after oral administration to experimental animals and humans, the agent is almost completely metabolized to the pharmacologically active principle, the carboxylated metabolite (carebastine), and other inactive metabolites Matsuda et al., 1994;Yamaguchi et al., 1994). Carebastine alone was the major metabolite detectable in the blood. Our previous in situ studies using rats indicated that the small intestine extensively converted the orally given ebastine to carebastine via hydroxylated ebastine and the dealkylated metabolite (Fujii et al., 1997). Therefore, it seemed that small intestine plays an important role in the first-pass metabolism of this drug, and the enzymes responsible for its metabolism exist there.We reported that ebastine was primarily metabolized by human liver microsomes to two metabolites, hydroxy-and desalkyl-ebastine (Hashizume et al., 1998). N-Dealkylation to desalkyl-ebastine was mediated by CYP3A4, whereas hydroxylation to hydroxy-ebastine, the most important intermediate metabolite yielding carebastine, was mediated by unidentified P450(s) other than CYP3A4. Our recent studies revealed that two novel CYP4F isoforms (P450 MI-2 and CYP4F12) obtained from monkey and human small intestine, respectively, were ...
