title: Identification of the human cytochromes P450 responsible for atomoxetine metabolism.
abstract: Studies were performed to determine the human enzymes responsible for the biotransformation of atomoxetine to its major metabolite, 4-hydroxyatomoxetine, and to a minor metabolite, N-desmethylatomoxetine. Utilizing human liver microsomes containing a full complement of cytochrome P450 (P450) enzymes, average K(m) and CL(int) values of 2.3 microM and 103 microl/min/mg, respectively, were obtained for 4-hydroxyatomoxetine formation. Microsomal samples deficient in CYP2D6 exhibited average apparent K(m) and CL(int) values of 149 microM and 0.2 microl/min/mg, respectively. In a human liver bank characterized for P450 content, formation of 4-hydroxyatomoxetine correlated only to CYP2D6 activity. Of nine expressed P450s examined, 4-hydroxyatomoxetine was formed at a rate 475-fold greater by CYP2D6 compared with the other P450s. These results demonstrate that CYP2D6 is the enzyme primarily responsible for the formation of 4-hydroxyatomoxetine. Multiple P450s were found to be capable of forming 4-hydroxyatomoxetine when CYP2D6 was not expressed. However, the efficiency at which these enzymes perform this biotransformation is reduced compared with CYP2D6. The formation of the minor metabolite N-desmethylatomoxetine exhibited average K(m) and CL(int) values of 83 microM and 0.8 microl/min/mg, respectively. Utilizing studies similar to those outlined above, CYP2C19 was identified as the primary enzyme responsible for the biotransformation of atomoxetine to N-desmethylatomoxetine. In summary, CYP2D6 was found to be the primary P450 responsible for the formation of the major oxidative metabolite of atomoxetine, 4-hydroxyatomoxetine. Furthermore, these studies indicate that in patients with compromised CYP2D6 activity, multiple low-affinity enzymes will participate in the formation of 4-hydroxyatomoxetine. Therefore, coadministration of P450 inhibitors to poor metabolizers of CYP2D6 substrates would not be predicted to decrease the clearance of atomoxetine in these individuals.
authors.affiliation: Department of Drug Disposition, Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana 46285, USA. ring_barbara_j@lilly.com
authors.forename: Barbara J
authors.initials: BJ
authors.lastname: Ring
authors.forename: Jennifer S
authors.initials: JS
authors.lastname: Gillespie
authors.forename: James A
authors.initials: JA
authors.lastname: Eckstein
authors.forename: Steven A
authors.initials: SA
authors.lastname: Wrighton
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D000069445:Atomoxetine Hydrochloride; D065731:Cytochrome P-450 CYP2C19; D019389:Cytochrome P-450 CYP2D6 / Q000235:genetics / Q000378:metabolism; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D008862:Microsomes, Liver / Q000201:enzymology; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000378:metabolism; D010636:Phenols / Q000378:metabolism; D010647:Phenyl Ethers / Q000378:metabolism; D011437:Propylamines / Q000378:metabolism* / Q000493:pharmacokinetics; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D065690:Cytochrome P-450 CYP2D6 Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D018076:DNA, Complementary; D007527:Isoenzymes; D010636:Phenols; D010647:Phenyl Ethers; D011437:Propylamines; D011994:Recombinant Proteins; C454680:N-desmethylatomoxetine; D000069445:Atomoxetine Hydrochloride; C454679:4-hydroxyatomoxetine; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; D019389:Cytochrome P-450 CYP2D6