PGP and cancer: Additionally, in vitro cytotoxicity studies showed that the common cancer MCF-7 and A549 cell lines, with high expression of P-gp, were highly sensitive to the prodrugs and the IC50 values of Pep-SS-NPs were lower than those of SS-NPs and this effect seems to be related to the presence of Pep that facilitated the TfR-mediated endocytosis.