Transporter-Mediated Multidrug Resistance and its Modulation by Chinese Medicines and Other Herbal Products

The main cause of failure in cancer drug therapy is the emergence of cellular resistance to drugs. Cancer cells, after exposure to one drug, can become simultaneously insensitive to mechanistically and chemically unrelated drugs, a phenotype known as multidrug resistance (MDR). Although a number of mechanisms have been proposed to mediate MDR, the classical cellular mechanism involves the overexpression of several members of the ATP-binding cassette (ABC) superfamily of transporters, leading to increased efflux and decreased intracellular drug accumulation. Among these, P-glycoprotein (P-gp, ABCB1), MRP1 (ABCC1) and BCRP (ABCG2) are the main transporters conferring MDR. These transporters are frequently detected in recurrent cancer cells or cancer stem cells. To overcome MDR, various studies have been conducted to investigate potential to discover effective MDR modulators from Chinese medicines (CMs) and other herbal products because many of these have been used for centuries without harmful side effects. This review summarizes: i) The contribution of P-gp, MRP1 and BCRP in cancer drug resistance; ii) known mechanisms of action for MDR modulators; iii) commonly used methods for identification and evaluation of novel modulators of transporter-mediated MDR; and iv) the modulating effects of CMs and other natural products on ABC transporters and MDR. The CM and their active components with potent modulating effects on MDR can be considered as promising lead agents for the design of more effective and less toxic drugs to overcome MDR.