Analysis of silibinin in rat plasma and bile for hepatobiliary excretion and oral bioavailability application.

Silibinin is an herbal ingredient isolated from milk thistle. The aim of this study was to develop a simple liquid chromatographic system to assay silibinin in plasma and bile for pharmacokinetic study. Silibinin was given oral and intravenously. The plasma sample (25 microL) was vortex-mixed with 50 microL of internal standard solution (naringenin 10 microg/mL in acetonitrile) to achieve protein precipitation. Silibinin in the rat plasma and bile was separated using a reversed-phase C18 column (250 mm x 4.6 mm, 5 microm) with a mobile phase of acetonitrile -10 mM monosodium phosphate (pH 5.45 adjusted with orthophosphoric acid) (50:50, v/v) and the flow-rate of 1 mL/min. The UV detection wavelength was 288 nm. The concentration-response relationship from the present method indicated linearity over a concentration range of 0.5-100 microg/mL. Intra- and inter-assay precision and accuracy of silibinin fell well within the predefined limits of acceptability (<15%). An ultrafiltration method was used in this experiment and the protein binding of silibinin was 70.3+/-4.6%. After silibinin administration in rats, the disposition of silibinin in the plasma and bile fluid was due to rapid distribution and equilibration between the blood and hepatobiliary system, and the bile levels of unconjugated silibinin and total silibinin were greater than those in the plasma. The oral bioavailability of silibinin in rats was estimated to be 0.73%.