However, the most common mechanism of resistance is the active efflux of drugs by ATP-binding cassette (ABC) transporters including P-glycoprotein (ABCB1/P-gp), ABCC1/ MRP1 and ABCG2 [6]. These transporters play a key role in the energy-dependent cellular efflux of toxic agents. They are capable of recognizing and extruding a broad range of functionally and structurally unrelated compounds, thereby causing the MDR phenotype in various cancer types. An obvious strategy to restore drug sensitivity in MDR cancer cells caused by ABC drug transporters is to block transporter-mediated drug efflux. Over the past decade, tremendous efforts have been made to discover and synthesize such inhibitors/modulators. Numerous clinical trials have been performed to evaluate the combination of ABCB1/P-gp modulators with standard chemotherapy regimens in enhancing anticancer efficacy [7]. However, none of them has been successfully put into clinical use, partly because of their low potency and lack of specificity