ABCG2 and cancer: In previous decades, a lot of effort was put towards the design and synthesis of ABC-transporter inhibitors due to the involvement of some of these proteins in cancer multidrug resistance [20]: P-glycoprotein (P-gp; MDR1/ABCB1), breast cancer resistance protein (BCRP; ABCG2), and multidrug resistance proteins (MRP1–4) were such transporters of interest.