Stereoselective oxidation metabolism of 20(S)-protopanaxatriol in human liver microsomes and in rats.

1. In this study, the oxidative metabolites of 20(S)-protopanaxatriol (PPT) were identified in human liver microsomes (HLMs) and in rats using liquid chromatography-electrospray ionization tandem mass spectrometry. 2. Twelve oxidative metabolites were found in HLM, eight of which have not been previously reported. Twenty-four oxidative metabolites were found in rat feces after oral administration and 20 of these, including six found in HLM, were first reported. The results indicated PPT was more extensively metabolized in rats than in HLM. C20-24 epoxides, a pair of epimers (namely, M1-1 and M1-2) were the major metabolites, and other metabolites were derived from their further metabolism. 3. Enzyme kinetics experiments showed that the apparent formation Vmax of M1-1 was 10.4 folds and 2.4 folds higher than that of M1-2 in HLM and in rat liver microsomes (RLMs), respectively. The depletion rate of M1-2 was 11.0 folds faster than M1-1 in HLM, and was similar in RLM. Hence, the remarkable species differences of PPT metabolism mainly resulted from the stereoselective formation and further metabolic elimination of M1-1 and M1-2. 4. Chemical inhibition study and recombinant human P450 isoforms analysis showed that CYP3A4 was the predominant isoform involved in the oxidative metabolism of M1-1 and M1-2.