ABCG2 and cancer: As one of the most likely reasons for the decreased efficacy of HS-173 is the reduced intracellular concentration of HS-173 caused by ABCB1- and ABCG2-mediated drug efflux, the intracellular accumulation of HS-173 was determined in OVCAR-8, NCI-ADR-RES, S1, and S1-MI-80 cancer cells using the LC-MS/MS method (Figure 4A) as described previously [39].