20S-Protopanaxadiol Inhibits P-Glycoprotein in Multidrug Resistant Cancer Cells

One of the major causes for cancer cells to resist current chemotherapy is attributed to the overexpression of P-glycoprotein (P-gp), resulting in insufficient drug delivery to the tumor sites. Protopanaxadiol ginsenosides Rg3 and Rh2 are known to induce apoptosis and significantly enhance the tumor inhibitory effects of chemotherapeutics in a synergistic fashion. One of the possible mechanisms is by blocking P-gp activity. The final deglycosylation metabolite of protopanaxadiols (PPDs) in vivo is 20(S)-protopapanaxadiol (aglycone PPD, aPPD), which has also shown anticancer activity and synergy with chemotherapy drugs. In the present study, P-gp over-expressing cancer cells were utilized to test whether aPPD also inhibits P-gp activity. We found that aPPD caused similar cytotoxicity in P388adr cells as their parental non-MDR cells, suggesting that aPPD may not be a substrate of P-gp. On the other hand, the calcein AM efflux assay showed that aPPD was able to inhibit P-gp activity as potently as verapamil on MDR cells. The blockage of P-gp activity was highly reversible as wash-out of aPPD resulted in an immediate recovery of P-gp activity. Unlike verapamil, aPPD did not affect ATPase activity of P-gp suggesting a different mechanism of action. The above results indicate that aPPD, unlike its precursor ginsenosides Rg3 and Rh2, is not a substrate of P-gp. It is also the first time that aPPD has showed a reversible nature of its P-gp inhibition. In addition to its pro-apoptotic nature, aPPD may be a potential new P-gp inhibitor for cancer treatment.