title: Comprehensive evaluation of tamoxifen sequential biotransformation by the human cytochrome P450 system in vitro: prominent roles for CYP3A and CYP2D6.
abstract: We performed comprehensive kinetic, inhibition, and correlation analyses in human liver microsomes and experiments in expressed human cytochromes P450 (P450s) to identify primary and secondary metabolic routes of tamoxifen (TAM) and the P450s catalyzing these reactions at therapeutically relevant concentrations. N-Desmethyl-TAM formation catalyzed by CYP3A4/5 was quantitatively the major primary metabolite of TAM; 4-hydroxy-TAM formation catalyzed by CYP2D6 (and other P450s) represents a minor route. Other minor primary metabolites include alpha -, 3-, and 4'-hydroxyTAM and one unidentified metabolite (M-I) and were primarily catalyzed by CYP3A4, CYP3A5, CYP2B6/2C19, and CYP3A4, respectively. TAM secondary metabolism was examined using N-desmethyl- and 4-hydroxy-TAM as intermediate substrates. N-Desmethyl-TAM was predominantly biotransformed to alpha-hydroxy N-desmethyl-, N-didesmethyl-, and 4-hydroxy N-desmethyl-TAM (endoxifen), whereas 4-hydroxy-TAM was converted to 3,4-dihydroxyTAM and endoxifen. Except for the biotransformation of N-desmethyl-TAM to endoxifen, which was exclusively catalyzed by CYP2D6, all other routes of N-desmethyl- and 4-hydroxy-TAM biotransformation were catalyzed predominantly by the CYP3A subfamily. TAM and its primary metabolites undergo extensive oxidation, principally by CYP3A and CYP2D6 to metabolites that exhibit a range of pharmacological effects. Variable activity of these P450s, brought about by genetic polymorphisms and drug interactions, may alter the balance of TAM effects in vivo.
authors.affiliation: Indiana University School of Medicine, Department of Medicine/Division of Clinical Pharmacology, 1001 West 10th street, WD Myers Bldg., W7123, Indianapolis, IN 46202, USA. zdesta@iupui.edu
authors.forename: Zeruesenay
authors.initials: Z
authors.lastname: Desta
authors.forename: Bryan A
authors.initials: BA
authors.lastname: Ward
authors.forename: Nadia V
authors.initials: NV
authors.lastname: Soukhova
authors.forename: David A
authors.initials: DA
authors.lastname: Flockhart
mesh_terms: D001711:Biotransformation; D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism*; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D010084:Oxidation-Reduction; D013223:Statistics as Topic; D013629:Tamoxifen / Q000378:metabolism*
chemical_list: D013629:Tamoxifen; D003577:Cytochrome P-450 Enzyme System; C104464:CYP3A protein, human; C510164:CYP3A5 protein, human; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human