ABCG2 and cancer: Drug-sensitive parental S1 and ABCG2-overexpressing multidrug-resistant S1-M1-80 cancer cells were treated with DMSO (control), 0.5 μM of TP-3654, 5 μM of topotecan, or a combination of 5 μM of topotecan with 0.5 μM of TP-3654 for 48 h before processed as detailed in Section 4.