PGP and cancer: In terms of MDR reversal effects, ZA-A exhibited the best cytotoxicity-enhancing ability, sensitizing the P-gp over-expressing cell line (ABCB1/Flp-InTM-293) and MDR cancer cell line (KB/VIN) toward several chemotherapeutic drugs, such as doxorubicin, paclitaxel and vincristine.