Experiments were conducted to characterize the metabolism of ezlopitant alkene (CJ-12,458), an active metabolite of ezlopitant, in human liver microsomes. In incubations with human liver microsomes and cofactors required for cytochrome P450 (CYP) activity, CJ-12,458 was converted to two metabolites: a diol (CP-611,781) and a 1 degrees alcohol (CP-616,762). In human liver microsomes, apparent K(M) values of 5.4 and 8.5 microM were determined for the formation of diol and 1 degrees alcohol metabolites, respectively. High K(M) activities were also observed for formation of these metabolites; however, the aforementioned low K(M) activities accounted for greater than 90% of the total intrinsic clearance. In pooled human liver microsomes, formation of both metabolites was partially inhibited by both quinidine and ketoconazole, suggesting that CYP2D6 and CYP3A enzymes are involved in the metabolism of CJ-12,458. This evidence was corroborated through the use of heterologously expressed CYP enzymes and correlation analysis with a panel of human liver microsomes. The data suggest that CYP2D6 is quantitatively more important than CYP3A in the metabolism of CJ-12,458 by a factor of about 2 to 1. The conversion of an alkene to a 1 degrees alcohol represents a novel biotransformation reaction. Incubations using (18)O(2), (2)H(2)O, [(2)H(5)]CJ-12,458, and [(2)H]NADPH were conducted and the 1 degrees alcohol product was characterized by ion trap mass spectrometry. From these data, a mechanism for this reaction is proposed involving epoxidation, an exocyclic hydride shift, and reduction at the benzylic position.