Vol. 29, Issue 7, 1057-1067, July 2001

SHORT COMMUNICATION
Cytochrome P450-Catalyzed Metabolism of Ezlopitant Alkene (CJ-12,458), a Pharmacologically Active Metabolite of Ezlopitant: Enzyme Kinetics and Mechanism Of an Alkene Hydration Reaction

R. Scott Obach

Drug Metabolism, Pfizer, Inc., Groton, Connecticut

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° alcohol (CP-616,762). In human liver microsomes, apparent K_M values of 5.4 and 8.5 µM were determined for the formation of diol and 1° 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° alcohol represents a novel biotransformation reaction. Incubations using ¹⁸O₂, ²H₂O, [²H₅]CJ-12,458, and [²H]NADPH were conducted and the 1° 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.