N-DEALKYLATION AND HYDROXYLATION OF EBASTINE BY HUMAN LIVER CYTOCHROME-P450

Ebastine yl-4-[4-(diphenylmethoxy)piperidino]butyrophenone] is a new-g eneration, nonsedative, H(1 )antihistamine. The present study was perf ormed to characterize the cytochrome P450 (CYP) isoforms responsible f or ebastine N-dealkylation and hydroxylation. Human liver microsomes m etabolized ebastine to two major metabolites, i.e, a desbutyrophenone metabolite (des-BP) and hydroxyebastine (M-OH), and the ratio of V-max values was 3:1. N-Dealkylation yielded des-BP, whereas M-OH, an hydro xylation product, could be further oxidized to the pharmacologically a ctive carebastine. In a panel of 14 human liver microsomal preparation s, the rate of dealkylation showed a highly significant correlation wi th CYP3A-mediated testosterone 6 beta-hydroxylation but not with react ions of seven other CYP isoforms. However, there was no correlation be tween the two pathways for ebastine (dealkylation and hydroxylation). Differential chemical inhibition in liver microsomes, in which dealkyl ation was more sensitive than hydroxylation, was demonstrated with ket oconazole, troleandomycin, cyclosporin A, and midazolam, Anti-CYP3A an tibodies markedly reduced the dealkylation rate (>95%) in liver micros omes but exhibited insignificant effects on hydroxylation (<5%). Among 12 cDNA-expressed human CYP isoforms, which account for up to 70% of the total CYP enzyme content in human liver, CYP3A4 alone metabolized ebastine; the ratio of des-BP to M-OH formation was 12:1. This ratio f or metabolism by the pure enzyme was much larger than the ratio (3:1) observed for the microsomal reaction mixture. This change in ratio, wh ich is attributed to a decrease in M-OH formation, indicates that, alt hough ebastine is metabolized to two major metabolites, N-dealkylation to des-BP is mediated by CYP3A, whereas hydroxylation to M-OH appears to be mediated mainly by unidentified enzymes other than CYP3A.