PGP and breast cancer: Quercetin can inhibit the P-gp pump efflux activity dose-dependently and induce apoptosis in resistant human myeloid leukemia and breast cancer cells [29]–[31]; thus, the facilitated DOX-induced apoptosis maybe the result of quercetin-induced P-gp inhibition, followed by an increase in intracellular DOX, and a consequently enhanced apoptosis [32].