ABCG2 and cancer: As one of the most prevalent reasons for the lack of drug efficacy in multidrug-resistant cancer cells is due to reduced intracellular drug accumulation caused by active drug efflux mediated by ABCB1 or ABCG2 [21], the intracellular accumulation of HS-173 in parental cancer cell lines and respective sublines overexpressing ABCB1 or ABCG2 was determined using the LC-MS/MS method established previously [39].