title: Identification of the human cytochromes p450 responsible for in vitro formation of R- and S-norfluoxetine.
abstract: The formation of R- and S-norfluoxetine was analyzed in vitro in human liver microsomes. Low apparent K(m) values for R-norfluoxetine formation of < or =8 microM and S-norfluoxetine of <0.2 microM were determined. R-Norfluoxetine formation rates in a characterized microsomal bank correlated with the catalytic activities for cytochrome P450 (CYP) 2D6, CYP2C9, and CYP2C8. Expressed CYP2C9, CYP2C19, and CYP2D6 formed R-norfluoxetine following incubation with 1 microM R-fluoxetine and exhibited apparent K(m) values of 9.7, 8.5, and 1.8 microM, respectively. Multivariate correlation analysis identified CYP2C9 and CYP2D6 as significant regressors with R-norfluoxetine formation. Antibodies to the CYP2C subfamily and CYP2D6 each exhibited moderate inhibition of R-norfluoxetine formation. Therefore, CYP2D6 and CYP2C9 contribute to this biotransformation. At pharmacological concentrations of S-fluoxetine, S-norfluoxetine formation rates in the bank of microsomes were found to correlate only with CYP2D6 catalytic activity and only expressed CYP2D6 was found to be capable of forming S-norfluoxetine. Thus, it would appear that both CYP2D6 and CYP2C9 contribute to the formation of R-norfluoxetine, whereas only CYP2D6 is responsible for the conversion to S-norfluoxetine. Since the enantiomers of fluoxetine and norfluoxetine are inhibitors of CYP2D6, upon chronic dosing, the CYP2D6-mediated metabolism of the fluoxetine enantiomers would likely be inhibited, resulting in R-norfluoxetine formation being mediated by CYP2C9 and S-norfluoxetine formation being mediated by multiple high K(m) enzymes.
authors.affiliation: Department of Drug Disposition, Lilly Research Laboratories, Eli Lilly and Co., Indianapolis, Indiana, USA. ring_barbara_j@lilly.com
authors.forename: B J
authors.initials: BJ
authors.lastname: Ring
authors.forename: J A
authors.initials: JA
authors.lastname: Eckstein
authors.forename: J S
authors.initials: JS
authors.lastname: Gillespie
authors.forename: S N
authors.initials: SN
authors.lastname: Binkley
authors.forename: M
authors.initials: M
authors.lastname: VandenBranden
authors.forename: S A
authors.initials: SA
authors.lastname: Wrighton
mesh_terms: D000911:Antibodies, Monoclonal / Q000494:pharmacology; D001189:Aryl Hydrocarbon Hydroxylases; D001711:Biotransformation; D002384:Catalysis / Q000187:drug effects; D065731:Cytochrome P-450 CYP2C19; D065727:Cytochrome P-450 CYP2C8; D065729:Cytochrome P-450 CYP2C9; D019389:Cytochrome P-450 CYP2D6 / Q000737:chemistry / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry* / Q000378:metabolism*; D005473:Fluoxetine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism* / Q000493:pharmacokinetics; D006801:Humans; D007700:Kinetics; D008745:Methylation; D008862:Microsomes, Liver / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000737:chemistry / Q000378:metabolism; D013237:Stereoisomerism; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000737:chemistry / Q000378:metabolism
chemical_list: D000911:Antibodies, Monoclonal; D005473:Fluoxetine; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; C450259:CYP2C8 protein, human; D065731:Cytochrome P-450 CYP2C19; D065727:Cytochrome P-450 CYP2C8; D019389:Cytochrome P-450 CYP2D6; D039181:Steroid 16-alpha-Hydroxylase; C036139:norfluoxetine