Disclosed are compounds that inhibit breast cancer resistance protein (BCRP), of which compound (I-1), ((3S,6S,12aS)-6-isobutyl-9-methoxy-3-methyl-2,3,6,7,12,12a-hexahydropyrazino[1′,2′:1,6]pyrido[3,4-b]indole-1,4-dione) or a pharmaceutically acceptable salt thereof, and the compound 12, ((3S,6S,12aS)-6-isobutyl-9-methoxy-3,10-dimethyl-2,3,6,7,12,12a-hexahydropyrazino[1′,2′:1,6]pyrido[3,4-b]indole-1,4-dione) or a pharmaceutically acceptable salt thereof. Also disclosed are methods of inhibiting BCRP or decreasing BCRP activity and methods of determining potential BCRP substrates.