Development of Fourth Generation ABC Inhibitors from Natural Products: A Novel Approach to Overcome Cancer Multidrug Resistance.

Multidrug resistance (MDR) in cancer caused due to overexpression of ABC drug transporters is a major problem in modern chemotherapy. Molecular investigations on MDR have revealed that the resistance is due to various transport proteins of the ABC superfamily which include Phosphoglycoprotein (P-gp/MDR1/ ABCB1), multidrug resistance-associated protein-1 (MRP1), and the breast cancer resistance protein (BCRP). They have been characterized functionally and are considered as major players in the development of MDR in cancer cells. These ATP-dependent transporter proteins cause MDR either by decreased uptake of the drug or increased efflux of the drug from the target organelles. Several MDR-reversing agents are being developed and are in various stages of clinical trials. The first three generations of ABC modulators such as quinine, verapamil, cyclosporine-A, tariquitor, PSC 833, LY335979, and GF120918 required to be administered in high doses to reverse MDR and were associated with adverse effects. Additionally, these modulators non-selectively inhibit ABC and adversely accumulate chemotherapeutic drugs in brain and kidney. Currently, research has stepped up towards reversing MDR by using natural products which exhibitted potential as chemosensitizers. Globally, there is a rich biodiversity of natural products which can be sourced for developing drugs. These products may provide more lead compounds with superior activity, foremost to the development of more effective therapies for MDR cancer cells. Here, we briefly review the status of natural products for reversing MDR modulators, and discuss the long term goal of MDR strategies in current clinical settings.