ABCB1 and cancer: Given that the development of MDR remains a major obstacle in cancer treatment [2,17], the use of a selective inhibitor against the activity of ABCB1 or ABCG2 has become an attractive therapeutic option to re-sensitize multidrug-resistant cancer cells to anticancer drugs [18]; however, due to the lack of selectivity or unexpected clinical side effects resulting from new combination therapies [2,19,20,21,22,23], there are currently no U.S. Food and Drug Administration (FDA)-approved agents for clinical application against multidrug-resistant cancers.