title: The role of P-glycoprotein in the bioactivation of raloxifene.
abstract: Drug transporters have been shown to alter drug metabolism. Similarly, bioactivation of drugs may also be altered by drug transporters. The aim of this work was to examine the role of P-glycoprotein (Pgp) in the bioactivation of a Pgp substrate, raloxifene, and a non-Pgp substrate, naphthalene. To evaluate the extent of bioactivation, covalent binding was measured. In both freshly isolated and cryopreserved hepatocytes, the extent of raloxifene covalent binding increased significantly (p < 0.05) in the presence of verapamil, whereas no change was observed with the covalent binding of naphthalene. To ascertain that the change was a Pgp effect, covalent binding was examined in microsomes in which raloxifene and naphthalene covalent binding was not altered in the presence of verapamil. In addition, the measure of raloxifene-glutathione adducts in the cryopreserved hepatocytes showed that the formation of the adducts increased in the presence of verapamil, which supports the idea that blocking Pgp in the liver increases metabolism and, therefore, the bioactivation of raloxifene. Because raloxifene and naphthalene are known to undergo bioactivation mediated by CYP3A4, covalent binding in the presence of ketoconazole was examined. In both hepatocytes and microsomes, raloxifene covalent binding decreased significantly (p < 0.01). It is interesting that naphthalene covalent binding was not affected. In the presence of the CYP2E inhibitor 4-methylpyrazole, a decrease in naphthalene covalent binding was observed, suggesting that the formation of the 1,2-epoxide may be the main culprit contributing to naphthalene covalent binding. In conclusion, these data suggest that in addition to other "protective" mechanisms, Pgp may attenuate bioactivation of drugs.
authors.affiliation: Merck & Co., Inc., West Point, PA 19486-0004, USA. jae_chang@merck.com
authors.forename: Jae H
authors.initials: JH
authors.lastname: Chang
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Kochansky
authors.forename: Magang
authors.initials: M
authors.lastname: Shou
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D001711:Biotransformation; D002478:Cells, Cultured; D065692:Cytochrome P-450 CYP3A Inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D000077604:Fomepizole; D005978:Glutathione / Q000378:metabolism; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007654:Ketoconazole / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D009281:Naphthalenes / Q000378:metabolism*; D011720:Pyrazoles / Q000494:pharmacology; D020849:Raloxifene Hydrochloride / Q000378:metabolism*; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D065692:Cytochrome P-450 CYP3A Inhibitors; D004791:Enzyme Inhibitors; D009281:Naphthalenes; D011720:Pyrazoles; C031721:naphthalene; D020849:Raloxifene Hydrochloride; D000077604:Fomepizole; D014700:Verapamil; D005978:Glutathione; D007654:Ketoconazole