Intestine versus liver? Uncovering the hidden major metabolic organs of silybin in rats

Silybin, a milk thistle extract, is a flavonolignan compound with hepatoprotective effect. It is commonly used in dietary supplements, functional foods, and nutraceuticals. However, the metabolism of silybin has not been systematically characterized in organisms to date. Therefore, we established a novel high-performance liquid chromatography quadrupole time-of-flight mass spectrometry method to analyze and identify the prototype and metabolites of silybin in rats. In total, 29 (of 32) new metabolic pathways and 56 (of 59) unreported metabolite products were detected. Moreover, we found that the liver had a high first-pass effect of 63.30% ± 13.01% for silybin, and only 1 metabolite was detected. Moreover, the metabolites identified in gastrointestinal tract possessed 88% of all unreported metabolite products (52 of 59). At the same time, the high concentration of silybin in the liver also indicated that large amounts of silybin may be accumulated in the liver instead of being metabolized. These results indicated that the primary metabolizing organ of silybin in rats was intestine rather than liver, which offers a solid chemical foundation for exploring more pharmacological effects of silybin. SIGNIFICANCE STATEMENT: This study confirmed that the primary location of metabolism of silybin in rats after intragastric administration was the gastrointestinal tract instead of the liver and that intestinal microbes were closely involved. In total, 29 (of 32) metabolism pathways and 56 (of 59) metabolites were identified for the first time in rats, to the authors' knowledge. To further study the liver disposition of silybin, its hepatic first-pass effect was determined for the first time. This work is capable of furnishing a robust material foundation for the forthcoming pharmacological investigations regarding silybin.