Absorption, disposition, and pharmacokinetics of saponins from Chinese medicinal herbs: what do we know and what do we need to know more?

Saponins are a group of amphiphilic glycosides containing one or more sugar chains linked to a nonpolar triterpene or steroid aglycone skeleton, which are believed to be responsible for the pharmacological activities of many Chinese medicinal herbs. The purpose of this paper is to summarize the contemporary knowledge of the absorption, disposition, and pharmacokinetics of some important saponins, including ginsenosides, licorice saponins, dioscorea saponins, astragalosides, and saikosaponins. Poor intestinal absorption of saponins is mainly due to their unfavorable physicochemical traits, such as large molecular mass ( > 500 Da), high hydrogen-bonding capacity ( > 12), and high molecular flexibility ( > 10), that underlie poor membrane permeability. Rapid and extensive biliary excretion is another primary factor that limits the oral bioavailability of most saponins. However, several saponins, including ginsenosides Ra3, Rb1, Rc, and Rd, and dioscin, are excreted slowly into the bile and in turn have significantly long elimination half lives (7-25 h in rats). These longcirculating saponins may be used as pharmacokinetic markers to substantiate systemic exposure to the ingested herb extracts. In addition to biliary excretion for elimination of most saponins unchanged, renal excretion may also be important for certain saponins. Saponins can be hydrolyzed by the colonic microflora. After absorption, the deglycosylated aglycones undergo phase I and/or II metabolism by the host. In line with the poor permeability, saponin concentrations in most rat tissues are lower than the concurrent plasma level and the brain level is usually very low. However, the liver concentrations of many saponins, as well as the kidney levels of certain saponins, can be quite high, which involves transporter-mediated uptake mechanisms. Repeated p.o. ingestion of glycyrrhizin appears to be able to induce CYP3A in rodents and humans, while several deglycosylated products of ginsenosides can moderately inhibit CYP activities in vitro with IC50 values of 10-50 μM. More research is required for elucidation of the absorption, disposition, and pharmacokinetics of multiple saponins to enhance understanding which saponins are most likely to exert pharmacological effects in vivo, as well as influence of complex herb matrix. In addition, research is also needed to characterize the microbiotal deglycosylation and the subsequent aglycone metabolism by the host for a broader range of saponins, as well as the hepatobiliary transporter phenotyping for and the interaction with saponins. Furthermore, in vitro and in vivo studies of saponin-based herb-drug interactions are also warranted.