Here, we report the characterization of this novel inhibitor of BCRP, Ac15(Az8)2, which appears to be highly potent, selective for BCRP over P-gp or MRP1, and is effective in an in vivo model of topotecan-resistant human colon cancer xenograft in Balb/c nude mice. This evidence concerns the gene ABCG2 and malignant colon neoplasm.