In vitro stereoselective inhibition of ginsenosides toward UDP-glucuronosyltransferase (UGT) isoforms

We evaluated in vitro, the potential of the six pairs of ginsenoside isomers, stereoisomers at the chiral carbon on position 20, to inhibit the enzymatic activity of several UDP-glucuronosyltransferase (UGT) isoenzymes, major players in the human phase II drug metabolism. The results show that the tested six pairs of ginsenoside isomers exhibited stereoselective inhibitory effects of varying degrees on the ten UGT isoenzymes explored. Of the tested twelve stereoselective ginsenosides, 20(R)-Rg3 had the strongest inhibitory effect on the UGT1A8 isoform with the lowest IC50 value of 5.66±1.04μM. On the other hand, the (S)-isomers of Rg3 and Rh2 also exerted remarkable inhibition on UGT1A8, with IC50 values of 6.89±0.812μM and 5.85±0.821μM, respectively. Although the inhibitory effect was low, both 20(R)-PPT and 20(S)-PPT also inhibited UGT1A8 activity. Considering 1) that the relative contents of 20(R)-Rg3 in processed ginseng are high, 2) that higher exposure to (R)-isomers of ginsenosides occur in the intestine compared to that in the liver, and 3) the inhibitory effects of other ginsenosides on enzymatic activity [20(S)-Rg3, 20(S)-Rh2, 20(R)- and 20(S)-PPT], there may be a potential for herb-drug interactions between processed ginseng and UGT1A8 substrates when concomitantly administered.