Saikosaponins and the deglycosylated metabolites exert liver meridian guiding effect through PXR/CYP3A4 inhibition.

ETHNOPHARMACOLOGICAL RELEVANCE: Radix Bupleuri (RB), traditionally used to treat inflammatory disorders and infectious diseases, represents one of the most successful and widely used herbal drugs in Asia over the past 2000 years. Being realized the role in regulating metabolism and controlling Yin/Yang, RB is not only chosen specifically for treating liver meridian and the corresponding organs, but also believed to have liver meridian guiding property and help potentiate the therapeutic effects of liver. However, the ingredients in RB with liver meridian guiding property and the underly mechanism have not been comprehensively investigated. AIM OF STUDY: Considering the important role of CYP3A4 in first-pass metabolism and the liver exposure of drugs, the present study aimed to determine whether saikosaponins (SSs) and the corresponding saikogenins (SGs) have a role in inhibiting the catalytic activity of CYP3A4 in human liver microsomes and HepG2 hepatoma cells and whether they could suppress CYP3A4 expression by PXR-mediated pathways in HepG2 hepatoma cells. MATERIALS AND METHODS: The effect of SSs and SGs on CYP3A4-mediated midazolam1'-hydroxylation activities in pooled human liver microsomes (HLMs) was first studied. Dose-dependent experiments were performed to obtain the half inhibit concentration (IC50) values. HepG2 cells were used to assay catalytic activity of CYP3A4, reporter function, mRNA levels, and protein expression. The inhibitory effects of SSa and SSd on CYP3A4 activity are negligible, while the corresponding SGs (SGF and SGG) have obvious inhibitory effects on CYP3A4 activity, with IC50 values of 0.45 and 1.30 µM. The similar results were obtained from testing CYP3A4 catalytic activity in HepG2 cells, which correlated well with the suppression of the mRNA and protein levels of CYP3A4. Time-dependent testing of CYP3A4 mRNA and protein levels, as well as co-transfection experiments using the CYP3A4 promoter luciferase plasmid, further confirmed that SSs and SGs could inhibit the expression of CYP3A4 at the transcription level. Furthermore, PXR protein expression decreased in a concentration- and time-dependent manner after cells were exposed to SSs and SGs. PXR overexpression and RNA interference experiments further showed that SSs and SGs down-regulate the catalytic activity and expression of CYP3A4 in HepG2 may be mainly through PXR-dependent manner. CONCLUSION: SSs and SGs inhibit the catalytic activity and expression of CYP3A4 in a PXR-dependent manner, which may be highly related to the liver meridian guiding property of RB.

Pharmacokinetics and oral bioavailability studies of three saikogenins in rats using a validated UFLC-MS/MS method.

Radix Bupleuri (RB) has been widely used in Traditional Chinese Medicine with a long history. Saikosaponins (SSs), the major constituents of RB, are assumed to be transformed into saikogenins (SGs) by human intestinal microflora prior to absorption and then exert pharmacological effects. There have been detailed reports on the deglycosylation of SSs in the gastrointestinal tract. But to date, there is very limited research addressing the further absorption, distribution, metabolism, and excretion of these deglycosylated derivatives in vivo. In this study, a rapid UFLC-MS/MS method was established and fully validated for simultaneously determining four SGs (SGF, SGA, SGD, and SGG) in rat plasma. The developed method was successfully applied to the pharmacokinetics of three SGs (SGF, SGD, and SGG) in rats after oral and intravenous administrations. Finally, the absolute bioavailabilities were calculated at 0.71% for SGF and 0.66% for SGD. However, the oral bioavailability of SGG was not obtained due to the extremely poor absorption in rats.