ationdependent manner. However, low concentration of Halofuginone biological activity 20-Rh2 significantly lowered the efflux ratio of digoxin. But, with elevated concentrations of 20-Rh2, the efflux ratio of digoxin were restored. As shown in Fig. 6B, both 20-Ppd and 20-Ppd lowered the efflux ratio of digoxin across Caco-2 cell monolayers concentration-dependently. But the P-gp inhibitory effect of 20-Ppd was more pronounced than that of 20-Ppd. Effects of 20-Rh2 and 20-Rh2 on the sensitivity of MCF-7/Adr cells to adriamycin MCF-7/Adr cell line is an adriamycin resistant human breast cancer cell line. It is derived from the parental human breast cancer cell line MCF-7 by gradual adriamycin selection. Our previous study showed that it is more resistant to adriamycin compared with MCF-7. When series concentrations of adriamycin were added to MCF-7/Adr cells in the presence of 20-Rh2 or 20-Rh2, these cells exhibited differential sensitivities towards adriamycin. As seen in Stereoselective metabolic kinetics of 
ginsenoside Rh2 epimers by rat fecal microflora Deglycosylation contributed greatly to the biotransformation of ginsenoside Rh2 with fecal microflora. As seen in Fig. 5A and 5C, when 20-Rh2 was incubated with rat fecal microflora in anaerobic condition, the level of 20-Rh2 decreased rapidly and the deglycosylation product 20-Ppd appeared as soon as one hour. In addition, a very small amount of 20-Rh2 was also detected throughout the incubation. However, when 20-Rh2 was incubated with rat 12829792 fecal microflora, there was a marked decrease in the level of 20-Rh2, and not only a large amount of 20-Ppd was found but 10455325 also a small amount of 20-Rh2 and 20-Ppd were detected. Furthermore, when the concentrations of 20-Rh2 were raised to 10 mM, the level of 20-Rh2 was decreased rather slowly. In the incubation system, only 20-Ppd could be detected, but not Discussion Chirality is a basic characteristic of biological system. Investigating the stereochemistry of either biomacromolecules or exogenous small molecules plays an important role in exploring the nature of life and promoting the health of people. Especially, since the thalidomide tragedy in 1960s, people have realized that the racemic mixtures and individual stereoisomers could Stereoselective Regulations of P-Glycoprotein exhibit totally different physiochemical and biochemical properties including carcinogenicity and teratogenicity. Developing homochiral drugs has become a demanding tendency of the pharmaceutical industry. Ginsenoside Rh2 is a potential drug obtained from herbal medicines, and its stereoselective properties have also gained much attention. In our previous studies, 20-Rh2 was demonstrated as a potent P-gp inhibitor. This leads us to determine whether 20-Rh2 could also inhibit P-gp. We examined the effects of Rh2 epimers on the oral absorption of P-gp substrate digoxin in rats. In contrast to 20-Rh2 which could promote the oral absorption of digoxin in a dose-dependent manner, 20-Rh2 showed the opposite P-gp inhibitory effect. Then, pharmacokinetic profiles of Rh2 epimers were obtained to elucidate this interesting phenomenon, assuming that different concentrations of Rh2 epimers in vivo might lead to differential Pgp regulations. Actually, our previous studies had shown that the stereoselectivity of Rh2 epimers was one of the factors contributing to the poor oral absorption of Rh2. However, the stereoselective absorptions of Rh2 epimers were only analyzed on models in vitro, without further confirmation i