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.
