The efflux pumps 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 ABCG2 modulators with submicromolar activity have been published. Starting from tariquidar (4) analogues as ABCB1 modulators, minimal structural modifications resulted in a drastic shift in favor of ABCG2 inhibition. Highest potency was found when the 3,4-dimethoxy-2-(quinoline-3-carbonylamino)benzoyl moiety in 4 was replaced with a 4-methoxycarbonylbenzoyl moiety bearing a hetarylcarboxamido group in 3-position, e.g., quinoline-3-carboxamido (5, IC(50): 119 nM) or quinoline-2-carboxamido (6, IC(50): 60 nM, flow cytometric mitoxantrone efflux assay, topotecan-resistant MCF-7 breast cancer cells); the selectivity for ABCG2 over ABCB1 was about 100-500 fold and the compounds were inactive at ABCC2 (MRP2). Chemosensitivity assays against MCF-7/Topo cells revealed that the nontoxic inhibitor 6 completely reverted ABCG2-mediated topotecan resistance at concentrations >100 nM, whereas 5 showed ABCG2 independent cytotoxicity. ABCG2 inhibitors might be useful for cancer treatment with respect to reversal of multidrug resistance, overcoming the BBB and targeting of tumor stem cells.