ABCB1 (p-gp, MDR1) and ABCG2 (BCRP) are expressed to a high extent by endothelial cells at the blood−brain barrier (BBB) and other barrier tissues and are involved in drug resistance of tumor (stem) cells. Whereas numerous ABCB1 inhibitors are known, only a few ABCG2modulators with submicromolar activity have been published. Starting from tariquidar (4) analogues as ABCB1 modulators, minimal structural