Disclosed herein are materials and methods for sensitizing multidrug resistant cancer cells that express ABCG2 and related proteins members of a family of ATP-binding transporter superfamily that mediate drug efflux found in some types of multidrug resistant cancer cells. A series of compounds, including (N-(4-chlorophenyl)-2-[(6-{[4,6-di(4-morpholinyl)-1,3,5-tri-azin-2-yl]amino}-1,3-benzothiazol-2-yl)sulfanyl]acetamide), specifically inhibits ABCG2 and can be used to boost the bio-avail-ability of one or more effective cancer killing drugs, making it possible to use certain widely used chemotherapeutic reagent to treat multidrug resistance cancers. Using these compounds in combination with chemotherapeutic drugs that are substrates for ABCG2 and related proteins may also find utility in treating cancer cells that are not currently identified as multi-drug resistant. Additionally, these compounds appear to accelerate the intercellular degradation of ABCG2 and related proteins. They are not toxic to animals at levels at which they effect the activity of ABCG2 expressed in multi-drug resistant cancer lines.