A specific inhibitor of breast cancer resistance protein (BCRP/ABCG2) multidrug efflux transporter; at 5 μM, significantly potentiates the toxicity of chemotherapeutic drugs in colon carcinoma cells; reverses multidrug resistance in cells overexpressing BCRP but not P-glycoprotein (ABCB1) or MRP1 (ABCC1).