Method of inhibiting ABCG2 and other treatment methods

A method of inhibiting ABCG2 in a mammal afflicted with cancer, wherein the cancer over-expresses ABCG2, which method comprises administering to the mammal an effective amount of a compound of formula (I) to inhibit ABCG2 protein:

• wherein R1, R2, and R3 are independently selected from the group consisting of hydrogen, C1-C6 alkyl, and hydroxy C1-C6 alkyl;
• X1 and X2 are independently selected from the group consisting of hydrogen, halo, and C1-C6 alkyl;
• and
• X3 and X4 are independently hydrogen or cyano;
• and
• double bond “a” can be cis or trans and double bond “b” can be Z or E;
• with the proviso that the compound is not botryllamide C, wherein the method further comprises administering to the mammal an effective amount of a chemotherapeutic agent for treating tumor cells in the mammal, wherein the chemotherapeutic agent is selected from the group consisting of mitoxantrone, topotecan, irinotecan, flavopiridol, methotrexate, rhodamine, daunomycin, imatinib, doxorubicin, paclitaxel, docetaxel, and a combination thereof, and wherein the treatment of tumor cells in the mammal with the chemotherapeutic agent is enhanced.