PGP and cancer: As it is known, chemotherapy is the main therapy for cancer, and its most serious obstacle is multidrug resistance (MDR).81–84 ABCB1 (P-gp)-mediated drug resistance is the most common, and P-glycoprotein is seen as a promising molecular target for overcoming MDR.85–87 In cancer treatment, it may contribute to MDR development as it pumps chemotherapeutic agents out of cells.88 Many drugs are P-gp substrates including anthracycline antibiotics, and taxol derivatives.84,89–91 Accordingly, bivalent securinine mimetics have been investigated as P-gp modulators.