title: Identification of cytochrome P450 isoforms involved in the metabolism of paroxetine and estimation of their importance for human paroxetine metabolism using a population-based simulator.
abstract: We identify here for the first time the low-affinity cytochrome P450 (P450) isoforms that metabolize paroxetine, using cDNA-expressed human P450s measuring substrate depletion and paroxetine-catechol (product) formation by liquid chromatography-tandem mass spectrometry. CYP1A2, CYP2C19, CYP2D6, CYP3A4, and CYP3A5 were identified as paroxetine-catechol-forming P450 isoforms, and CYP2C19 and CYP2D6 were identified as metabolizing P450 isoforms by substrate depletion. Michaelis-Menten constants K(m) and V(max) were determined by product formation and substrate depletion. Using selective inhibitory studies and a relative activity factor approach for pooled and single-donor human liver microsomes, we confirmed involvement of the identified P450 isoforms for paroxetine-catechol formation at 1 and 20 muM paroxetine. In addition, we used the population-based simulator Simcyp to estimate the importance of the identified paroxetine-metabolizing P450 isoforms for human metabolism, taking mechanism-based inhibition into account. The amount of active hepatic CYP2D6 and CYP3A4 (not inactivated by mechanism-based inhibition) was also estimated by Simcyp. For extensive and poor metabolizers of CYP2D6, Simcyp-estimated pharmacokinetic profiles were in good agreement with those reported in published in vivo studies. Considering the kinetic parameters, inhibition results, relative activity factor calculations, and Simcyp simulations, CYP2D6 (high affinity) and CYP3A4 (low affinity) are most likely to be the major contributors to paroxetine metabolism in humans. For some individuals CYP1A2 could be of importance for paroxetine metabolism, whereas the importance of CYP2C19 and CYP3A5 is probably limited.
authors.affiliation: Section of Forensic Chemistry, Department of Forensic Medicine, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark. jakob.jornil@forensic.ku.dk
authors.forename: Jakob
authors.initials: J
authors.lastname: Jornil
authors.forename: Klaus Gjervig
authors.initials: KG
authors.lastname: Jensen
authors.forename: Frank
authors.initials: F
authors.lastname: Larsen
authors.forename: Kristian
authors.initials: K
authors.lastname: Linnet
mesh_terms: D002396:Catechols / Q000737:chemistry / Q000378:metabolism; D003198:Computer Simulation; D019388:Cytochrome P-450 CYP1A2 / Q000235:genetics / Q000378:metabolism; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D019389:Cytochrome P-450 CYP2D6 / Q000235:genetics / Q000378:metabolism; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D051544:Cytochrome P-450 CYP3A / Q000235:genetics / Q000378:metabolism; D065692:Cytochrome P-450 CYP3A Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D050216:Metabolic Detoxication, Phase I; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D008954:Models, Biological; D017711:Nonlinear Dynamics; D017374:Paroxetine / Q000378:metabolism* / Q000493:pharmacokinetics*; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D017367:Selective Serotonin Reuptake Inhibitors / Q000378:metabolism* / Q000493:pharmacokinetics*
chemical_list: D002396:Catechols; D019388:Cytochrome P-450 CYP1A2; D065609:Cytochrome P-450 CYP1A2 Inhibitors; D019389:Cytochrome P-450 CYP2D6; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D051544:Cytochrome P-450 CYP3A; D065692:Cytochrome P-450 CYP3A Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D017374:Paroxetine; D011994:Recombinant Proteins; D017367:Selective Serotonin Reuptake Inhibitors; C504573:CYP1A2 protein, human; C510163:CYP3A4 protein, human