Synthesis of methylated quercetin derivatives and their reversal activities on P-gp- and BCRP-mediated multidrug resistance tumour cells

► A series of quercetin derivatives with various O-3 substituents were synthesized. ► They have not any inherent cytotoxicity to cancer and normal mouse fibroblast cells. ► O-3 substituent played a key role for determining P-gp- and BCRP-modulating activity. ► Compound 17 was equipotent against both P-gp and BCRP.