Absorption, liver first-pass effect, pharmacokinetics and tissue distribution of calycosin-7-O-ß-d-glucopyranoside (C7G) and its major active metabolite, calycosin, following oral administration of C7G in rats by LC-MS/MS.

Previously, we discovered calycosin, an extensively distributed metabolite of Calycosin-7-O-ß-d-glucopyranoside (C7G), elicited stronger anti-virus activity than C7G. However, the pharmacokinetics and tissue distribution of C7G and calycosin remained obscure on C7G treatments. In this study, a liquid chromatography-tandem mass spectrometry method was established and validated for the simultaneous determination of C7G and calycosin, and it was applied to the pharmacokinetics and tissue distribution of C7G and calycosin following oral administration of C7G at 120mg/kg in rats. Consequently, the exposure of C7G and calycosin was both similarly low in the systemic plasma, but the levels of calycosin were 53.5 folds higher than that of C7G in the portal vein plasma, corresponding to the liver extraction ratio (ER) of C7G and calycosin at 0.3% and 98.5% respectively. Therefore, our results revealed that liver first-pass effect played the predominant role in the poor circulating levels of calycosin on C7G treatments, whereas the intestinal first-pass effect was predominant for those of C7G. In contrast to no observation of C7G, the calycosin levels were 212.1, 30.5 and 4.7 folds higher in the liver, kidney and heart than its circulating levels, respectively. The high tissue distribution of calycosin provided new hints and evidences to the pharmacological mechanisms of C7G and Astragali Radix.