PGP and cancer: In terms of MDR reversal effects, ZA-A sensitized the P-gp over-expressing cell line (ABCB1/Flp-InTM-293) and MDR cancer cell line (KB/VIN) toward clinically used chemotherapeutic drugs, including doxorubicin, paclitaxel and vincristine, exhibiting the best cytotoxicity enhancing ability among investigated triterpenoids.