title: Silybin inactivates cytochromes P450 3A4 and 2C9 and inhibits major hepatic glucuronosyltransferases.
abstract: Silybin, a major constituent of the milk thistle, is used to treat several liver disorders. Silybin inactivated purified, recombinant cytochromes P450 (P450) 3A4 and 2C9 in a mechanism-based manner. The inactivations were time-, concentration-, and NADPH-dependent. The inactivation of the 7-benzyloxy-4-(trifluoromethyl-)coumarin O-debenzylation activity (P450 3A4) was characterized by a K(I) of 32 microM, a k(inact) of 0.06 min(-1), and a t(1/2) of 14 min. Testosterone metabolism to 6-beta-hydroxytestosterone (P450 3A4) was also inactivated with a K(I) of 166 microM, a k(inact) of 0.08 min(-1), and a t(1/2) of 9 min. The 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of purified human P450 2C9 was inactivated with a K(I) of 5 microM, a k(inact) of 0.14 min(-1), and a t(1/2) of 7 min. Parallel loss of heme was observed with both P450s. Activity of both P450 enzymes was not recovered after removal of silybin either by dialysis or by spin gel filtration. In addition, silybin inhibited the glucuronidation of 7-hydroxy-4-trifluoromethylcoumarin catalyzed by recombinant hepatic UDP-glucuronosyltransferases (UGTs) 1A1, 1A6, 1A9, 2B7, and 2B15, with IC(50) values of 1.4 microM, 28 microM, 20 microM, 92 microM, and 75 microM, respectively. Silybin was a potent inhibitor of UGT1A1 and was 14- and 20-fold more selective for UGT1A1 than for UGT1A9 and UGT1A6, respectively. Thus, careful administration of silybin with drugs primarily cleared by P450s 3A4 or 2C9 is advised, since drug-drug interactions cannot be excluded. The clinical significance of in vitro UGT1A1 inhibition is unknown.
authors.affiliation: Department of Pharmacology, The University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: Chitra
authors.initials: C
authors.lastname: Sridar
authors.forename: Theunis C
authors.initials: TC
authors.lastname: Goosen
authors.forename: Ute M
authors.initials: UM
authors.lastname: Kent
authors.forename: J Andrew
authors.initials: JA
authors.lastname: Williams
authors.forename: Paul F
authors.initials: PF
authors.lastname: Hollenberg
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors* / Q000302:isolation & purification / Q000378:metabolism; D065729:Cytochrome P-450 CYP2C9; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000302:isolation & purification / Q000378:metabolism; D014453:Glucuronosyltransferase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D006418:Heme / Q000378:metabolism; D007700:Kinetics; D009249:NADP; D010936:Plant Extracts / Q000494:pharmacology; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D000077385:Silybin; D012838:Silymarin / Q000494:pharmacology*; D013739:Testosterone / Q000378:metabolism
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D010936:Plant Extracts; D011994:Recombinant Proteins; D012838:Silymarin; D013739:Testosterone; D006418:Heme; D000077385:Silybin; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; D014453:Glucuronosyltransferase