Stereoselective formation and metabolism of 20(S)-protopanaxadiol ocotillol type epimers in vivo and in vitro.

(20S,24S)-epoxy-dammarane-3,12,25-triol (24S-epimer) and (20S,24R)-epoxy- dammarane-3,12,25-triol (24R-epimer), a pair of ocotillol type epimers, were identified as the main metabolites of 20(S)-protopanaxadiol (PPD). The aim of this study was to systematically investigate the formation and metabolism of this pair of epimers in vivo and in vitro and to elucidate the isoforms of cytochrome P450 enzymes responsible for the stereoselective metabolism of both epimers. The result showed that 24S-epimer was a more predominant ingredient in rat plasma after oral administration of PPD with higher area under the curve (AUC) values. Both the enzyme kinetic evaluations of the formation and elimination of 24S-epimer and 24R-epimer in rat liver microsomes (RLM) and human liver microsomes (HLM) indicated that 24S-epimer had a higher formation rate and a lower oxygenation metabolism rate than 24R-epimer, and the stereoselective differences were more obvious in HLM than in RLM. The chemical inhibition and recombinant human P450 isoforms assay showed that CYP3A4 was the predominant isoform responsible for the further metabolism of 24R-epimer in HLM. The biliary excretion ratio of the 24S-epimer glucuronide was more than 28-fold higher than that of 24R-epimer glucuronide after intravenous administration to rats, which also indicated 24S-epimer was more preferential to be metabolized as the glucuronide conjugate than 24R-epimer.