title: Involvement of CYP2B6 in n-demethylation of ketamine in human liver microsomes.
abstract: Ketamine is metabolized by cytochrome P450 (CYP) leading to production of pharmacologically active products and contributing to drug excretion. We identified the CYP enzymes involved in the N-demethylation of ketamine enantiomers using pooled human liver microsomes and microsomes from human B-lymphoblastoid cells that expressed CYP enzymes. The kinetic data in human liver microsomes for the (R)- and (S)-ketamine N-demethylase activities could be analyzed as two-enzyme systems. The K(m) values were 31 and 496 microM for (R)-ketamine, and 24 and 444 microM for (S)-ketamine. Among the 12 cDNA-expressed CYP enzymes examined, CYP2B6, CYP2C9, and CYP3A4 showed high activities for the N-demethylation of both enantiomers at the substrate concentration of 1 mM. CYP2B6 had the lowest K(m) value for the N-demethylation of (R)- and (S)-ketamine (74 and 44 microM, respectively). Also, the intrinsic clearance (CL(int): V(max)/K(m)) of CYP2B6 for the N-demethylation of both enantiomers were 7 to 13 times higher than those of CYP2C9 and CYP3A4. Orphenadrine (CYP2B6 inhibitor, 500 microM) and sulfaphenazole (CYP2C9 inhibitor, 100 microM) inhibited the N-demethylase activities for both enantiomers (5 microM) in human liver microsomes by 60 to 70%, whereas cyclosporin A (CYP3A4 inhibitor, 100 microM) failed to inhibit these activities. In addition, the anti-CYP2B6 antibody inhibited these activities in human liver microsomes by 80%, whereas anti-CYP2C antibody and anti-CYP3A4 antibody failed to inhibit these activities. These results suggest that the high affinity/low capacity enzyme in human liver microsomes is mediated by CYP2B6, and the low affinity/high capacity enzyme is mediated by CYP2C9 and CYP3A4. CYP2B6 mainly mediates the N-demethylation of (R)- and (S)-ketamine in human liver microsomes at therapeutic concentrations (5 microM).
authors.affiliation: Department of Hospital Pharmacy, Tokyo Postal Services Agency Hospital, Chiyoda-ku, Tokyo, Japan. yyanagihara@tpth.go.jp
authors.forename: Y
authors.initials: Y
authors.lastname: Yanagihara
authors.forename: S
authors.initials: S
authors.lastname: Kariya
authors.forename: M
authors.initials: M
authors.lastname: Ohtani
authors.forename: K
authors.initials: K
authors.lastname: Uchino
authors.forename: T
authors.initials: T
authors.lastname: Aoyama
authors.forename: Y
authors.initials: Y
authors.lastname: Yamamura
authors.forename: T
authors.initials: T
authors.lastname: Iga
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D002851:Chromatography, High Pressure Liquid; D065702:Cytochrome P-450 CYP2B6; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D007649:Ketamine / Q000378:metabolism*; D008745:Methylation; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism*
chemical_list: D007649:Ketamine; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; D065702:Cytochrome P-450 CYP2B6; D010089:Oxidoreductases, N-Demethylating