Urinary excretion of silibinin diastereoisomers and their conjugated metabolites in rat and human at different dosages

Silibinin is a mixture of two flavonoid lignan silibinins A and B from the seeds of milk thistle (Silybum marianum L.). Using ultra-performance liquid chromatography/quadrupole time-of-flight-MS (UPLC/Q-TOF-MS), a total of 18 metabolites were identified in rat and human urine samples after oral administration of silibinin capsule. Furthermore, nine glucuronides and/or sulfated metabolites and two prototype compounds were simultaneously quantified in rat urine after oral administration of silibinin capsule at 50 and 100 mg/kg. Over a 72-h period, 27.6% and 23.3% of silibinin were excreted in the form of metabolites (n = 11) in urine, among which five major metabolites, including silibinin A-7-O-beta-glucuronide (SA-7G), silibinin B-7-O-beta-glucuronide (SB-7G), silibinin A-5-O-beta-glucuronide (SA-5G), silibinin B-5-O-beta-glucuronide (SB-5G) and silibinin A-20-O-glucuronide (SA-20G), accounted for 20.5% and 15.5% of the dosages, respectively, when administered at doses of 50 and 100 mg/kg. These results suggested that glucuronidation at the C7-, C5- and C20-hydroxyls was the primary metabolic pathway of silibinin diastereoisomers in vivo. The present results provide helpful information about the in vivo metabolism and clinical usage of silibinin capsule.