ABCB1 and cancer: Given the lack of U.S. Food and Drug Administration (FDA)-approved therapeutic agents for the treatment of multidrug-resistant cancer patients, we and others have been exploring the possibility of repositioning FDA-approved drugs or therapeutic agents that have undergone clinical testing for the reversal of MDR mediated by ABCB1 and/or ABCG2 [16,17,18,19,20,21,22,23,24].