Two Flavonolignans from Milk Thistle (Silybum marianum) Inhibit CYP2C9-Mediated Warfarin Metabolism at Clinically Achievable Concentrations

Abstract

Milk thistle (Silybum marianum) is a popular herbal product used for hepatoprotection and chemoprevention. Two commercially available formulations are the crude extract, silymarin, and the semipurified product, silibinin. Silymarin consists of at least seven flavonolignans, of which the most prevalent are the diastereoisomers silybin A and silybin B; silibinin consists only of silybin A and silybin B. Based on a recent clinical study showing an interaction between a silymarin product and the CYP2C9 substrate losartan, the CYP2C9 inhibition properties of silybin A and silybin B and corresponding regioisomers, isosilybin A and isosilybin B, were evaluated using human liver microsomes (HLMs), recombinant CYP2C9 (rCYP2C9) enzymes, and the clinically relevant probe, (S)-warfarin. Silybin B was the most potent inhibitor in HLMs, followed by silybin A, isosilybin B, and isosilybin A (IC 50 of 8.2, 18, 74, and Ͼ100 M, respectively). Next, silybin A and silybin B were selected for further characterization. As with HLMs, silybin B was more potent than silybin A toward rCYP2C9*1 (6.7 versus 12 M), rCYP2C9*2 (9.3 versus 19 M), and rCYP2C9*3 (2.4 versus 9.3 M). Using a matrix of five substrate (1-15 M) and six inhibitor (1-80 M) concentrations and HLMs, both diastereoisomers inhibited (S)-warfarin 7-hydroxylation in a manner described best by a mixed-type inhibition model (K i values of 4.8 and 10 M for silybin B and silybin A, respectively). These observations, combined with the high systemic silibinin concentrations (Ͼ5-75 M) achieved in a phase I study involving prostate cancer patients, prompt clinical evaluation of a potential warfarin-milk thistle interaction.Milk thistle [Silybum marianum (L.) Gaertn.] is a resilient and sometimes noxious plant that has been valued for its medicinal qualities for more than 2000 years Post-White et al., 2007). In modern herbal compendia, milk thistle is used to self-treat hepatic disorders, including hepatitis C and cirrhosis, and as a hepatoprotectant, particularly for mushroom poisoning. Milk thistle is available mainly as an extract prepared from the seeds of the plant. The two most common commercial preparations are termed silymarin and silibinin. Silymarin, a crude extract, is a complex mixture of at least seven flavonolignans and one flavonoid (taxifolin) (Davis-Searles et al., 2005;Kroll et al., 2007). The most abundant flavonolignans are the diastereoisomers silybin A and silybin B (Fig. 1). The diastereoisomers isosilybin A and isosilybin B (Fig. 1) also are present and are regioisomers of silybin A and silybin B. The remaining three flavonolignans are silychristin, isosilychristin, and silydianin, all of which are constitutional isomers of the aforementioned compounds (Davis-Searles et al., 2005). Silibinin is a semipurified extract, representing approximately a 1:1 mixture of silybin A and silybin B. Because silymarin and silibinin are mixtures of compounds derived from a natural source, batch-to-batch variation in bioactive ingredient composition occurs, which can c...

Self-nanoemulsifying drug delivery systems ameliorate the oral delivery of silymarin in rats with Roux-en-Y gastric bypass surgery

“…Both silymarin and silibinin undergo extensive first-pass metabolism in the liver. 37 The nanodroplets containing oils and emulsifiers can be incorporated with lipoproteins and chylomicrons in the intestine. Such mixtures are largely taken up via the lymphatics, thus minimizing the first-pass effect.…”

Pharmacokinetic Herb-Drug Interactions: Insight into Mechanisms and Consequences

“…In a study by Gurley et al (2006), [89] insignificant changes in the disposition of digoxin were observed on co-administration with silymarin, posing no clinically significant risk for P-gp-mediated herb-drug interactions. On the other hand, a study has shown that silybin A and silybin B at clinically relevant concentrations inhibit CYP 2C9-mediated metabolism of warfarin [90]. This HDI needs to be further evaluated because of the narrow therapeutic index of warfarin.…”

The Effects of Milk Thistle (Silybum marianum) on Human Cytochrome P450 Activity

“…Despite the apparent ability of milk thistle extracts to produce significant inhibition of one or more P450 enzymes, as reported in several published in vitro studies, in vivo human data have been unable to replicate in vitro predictions (Leber and Knauff, 1976;Piscitelli et al, 2002;DiCenzo et al, 2003;Gurley et al, 2004Gurley et al, , 2006aGurley et al, ,b, 2008Mills et al, 2005;van Erp et al, 2005;Fuhr et al, 2007;Rao et al, 2007;Deng et al, 2008). In a recent report, an in vitro study utilizing human liver microsomes evaluated a number of individual flavonolignans as potential P450 inhibitors and identified silybin B as the most potent inhibitor of CYP2C9 with an IC 50 value of 8.2 mM, followed by silybin A at 18 mM (Brantley et al, 2010). Isosilybin A and isosilybin B were noted to be much weaker CYP2C9 inhibitors with IC 50 values of 74 mM and .100 mM, respectively.…”

“…Isosilybin A and isosilybin B were noted to be much weaker CYP2C9 inhibitors with IC 50 values of 74 mM and .100 mM, respectively. Brantley et al (2010) noted that there is precedent for some silymarin extracts formulated to provide significantly enhanced bioavailability (i.e., silibinin-phosphatidylcholine complex) to produce systemic concentrations of silibinin (i.e., combined silybin A and silybin B) between 5 and 75 mM (Flaig et al, 2007;Brantley et al, 2010). However, a previous dose escalation study assessing the pharmacokinetics of the formulation used in the present study produced silybin B plasma concentrations over an order of magnitude less than even the lowest concentration reported in the study by Flaig et al (2007) (Zhu et al, 2013).…”