CHARACTERIZATION OF THE CYTOCHROME-P450 ENZYMES INVOLVED IN THE IN-VITRO METABOLISM OF DOLASETRON - COMPARISON WITH OTHER INDOLE-CONTAINING5-HT3 ANTAGONISTS

Dolasetron mesilate [(2 alpha,6 alpha,8 alpha,9 alpha methano-2H-quino lizin-8-yl-1H-indole-3-carboxylate monomethane-sulfonate], is a 5-HT3 receptor antagonist, which is in development for the treatment of chem otherapy-induced emesis. The compound is rapidly reduced by carbonyl r eductase to form its major pharmacologically active metabolite reduced dolasetron (red-dolasetron), which is further metabolized by cytochro me P450 (CYP450). Studies were conducted, using human liver microsomes , to characterize the CYP450 enzymes responsible for the in vitro meta bolism of red-dolasetron, Red-dolasetron underwent oxidation of the In dole aromatic ring at positions 5, 6, and 7, acid also N-oxidation. En zyme-selective inhibition and correlation studies showed that hydroxyl ation of red-dolasetron was CYP2D6-dependent, and N-oxidation was cond ucted by CYP3A4, The rate of formation of 6-hydroxy red-dolasetron was significantly correlated with that of B-hydroxy red-dolasetron, which further suggested that these metabolites were formed by the same CYP4 50 enzyme(s), Inhibition studies also demonstrated that 6-hydroxylatio n was, to a lesser extent, CYP3A4-dependent, This was confirmed by cor relation experiments, wherein formation of this metabolite was signifi cantly correlated with that of N-oxide formation, in quinidine-inhibit ed microsomes. Results were compared with those obtained with two othe r indole-containing 5-HT3 receptor antagonists: tropisetron and ondans etron, Tropisetron hydroxylation was CYP2D6-dependent, whereas that of ondansetron was both CYP2D6- and CYP2E1-dependent. Results were furth er confirmed, when compounds were incubated with microsomes containing recombinant human liver CYP2D6, CYP3A4, and CYP2E1, Red-dolasetron wa s a competitive inhibitor of CYP2D6, with an IC50 value of 70 mu M, wh ich is 2 orders of magnitude above maximum plasma concentrations found in humans, The implications of these in vitro results to the in vivo metabolism of these compounds in humans and their potential pharmacoki netic consequences is discussed.