In continuing research that led us to identify chromanone derivatives (J. Med. Chem. 2003, 46, 2125) as P-glycoprotein inhibitors, we obtained analogues able to modulate multidrug resistance (MDR) mediated by the breast cancer resistance protein (BCRP). The linkage of 5-hydroxybenzopyran-4-one to piperazines or phenalkylamines affords highly potent inhibitors of BCRP. By using sensitive (HCT116) and resistant colon cancer cells expressing BCRP, we evaluated the effect of 14 benzopyranone (chromone) derivatives on the accumulation and the cytotoxic effect of the anticancer drug, mitoxantrone. At 10 microM, three compounds increased both intracellular accumulation and cytotoxicity of mitoxantrone in HCT116/R cells with a comparable rate as fumitremorgin C and Gleevec used as reference inhibitors. The most potent molecules 5b and 5c are still active at 1 microM, whereas FTC shows weak inhibition. These molecules do not induce cell death as shown by the cell cycle distribution study, which makes them potential candidates for in vivo studies.