Silybin Inactivates Cytochromes P450 3a4 and 2c9 and Inhibits Major Hepatic Glucuronosyltransferases

Abstract

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 NADPHdependent. The inactivation of the 7-benzyloxy-4-(trifluoromethyl-)coumarin O-debenzylation activity (P450 3A4) was characterized by a K I of 32 M, a k inact of 0.06 min ؊1 , and a t 1/2 of 14 min.Testosterone metabolism to 6-␤-hydroxytestosterone (P450 3A4) was also inactivated with a K I of 166 M, 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 M, 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 M, 28 M, 20 M, 92 M, and 75 M, 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.

Effects of Herbal Supplements on Drug Glucuronidation. Review of Clinical, Animal, andIn VitroStudies

“…C max of total flavonolignans was 24 ng/mL following intake of 600 mg milk thistle extract. Thus, plasma concentrations of flavonolignans are not expected to reach IC 50 concentrations reported by our group and others [65,102]. Therefore, milk thistle intake is not expected to affect systemic metabolism of UGT substrates.…”

“…Based on a range of intestinal volume of 0.5 to 5.0 L, the expected intestinal concentration of milk thistle extract is 40 to 1200 µg/mL following intake of 200 to 600 mg of milk thistle [73]. Therefore, further research is warranted on the effect of milk thistle on drugs cleared primarily by first pass glucuronidation -particularly substrates of UGT1A1, 1A6, and 1A9, which showed the lowest IC 50 values [65,102].…”

Improving the oral bioavailability of beneficial polyphenols through designed synergies

“…However, to the best of our knowledge, there are no studies demonstrating inhibition of UGTs or SULTs by polyphenols resulting in clinically relevant pharmacokinetic changes in humans. There are a number of reports indicating that various flavonoids, such as hexamethoxyflavone, tangeretin and silybin are potent inhibitors of UGTs in vitro, with IC 50 values \1 lM [72,83,84]. Similarly, a number of flavonoids (fisetin, galangin, quercetin, myricetin, chrysin, kaempferol, apigenin and genistein) have been identified in vitro as potent inhibitors of various SULTs [15,26,52], but as with the UGTs, it is not known whether such in vitro interactions translate into significant in vivo effects.…”

Herb–Drug Interactions in Oncology: Focus on Mechanisms of Induction

“…For detailed information about CAM-drug interactions resulting from inhibition, the reader is referred to some recent reviews [10][11][12][13][14]23 [25], Echinacea purpurea [24], milk thistle (silybin) [26][27][28], and evening primrose oil (cis-linoleic acid) [23]. Pgp activity was shown to be inhibited by curcumin, ginsenosides, piperine, some cathechins from green tea, quercetin, and silymarin [14,29,30].…”