The results showed that zhankuic acid A was the most potent P-gp inhibitor compound and (at 20 μM) could reverse drug resistance in human cancer cells, restoring an IC50 of 78.5 nM for doxorubicin, of 48.5 nM for paclitaxel, and of 321.5 nM for vincristine, indicating a reversal fold of 48, 38, and 45 times, respectively. This evidence concerns the gene PGP and cancer.