endent manner but had no effect on MCF10A, even at the 50 mM concentration that killed more than 80% of cancer cells. In addition, apoptotic response measured by Annexin V-positive cells after the MedChemExpress Oleandrin treatment of the Y211F CPPP was observed only in MDA-MB 468 and MDA-MB 231 cells but not in MCF10A. Both the TAT-based scrambled peptide and Y211F CPPP penetrated the cell membrane for entry into the cytosol and nucleus compartments of cells but only the Y211F CPPP specifically targeted EGFR and prevented nEGFR from binding PCNA to compete for PCNA tyrosine Y211 phosphorylation and stability on the chromatin in MDA-MB 468 cells. Y211F CPPP enhances the effect of EGFR TKI and sensitizing EGFR TKI-resistant TNBC cells To determine if CPPP increases the sensitivity of TNBC cells to EGFR TKI, we further treated the EGFR-positive MDA-MB 468 and MDA-MB 231 TNBC cells with a combination of the Y211F CPPP and clinically used 23696131 EGFR TKIs, Tarceva and Iressa. We found that the Y211F CPPP enhanced the effect of both TKIs at more physiological doses in these TNBC 3 Inhibition of p-PCNA Blocks Breast Cancer Growth Inhibition of p-PCNA Blocks Breast Cancer Growth cells. As drug resistance 16365279 of cancer cells is one of the important reasons for therapeutic failure or cancer recurrence, we asked whether the Y211F CPPP is also effective in the EGFR TKI-resistant TNBC cells. To this end, we established two more EGFR TKI-resistant TNBC cell lines: Iressa-resistant MDA-MB 468 and Tarceva-resistant MDA-MB 468. The results showed that the amounts of nEGFR, binding to PCNA, stability of PCNA and Y211 phosphorylated PCNA were enhanced in both the IR and TR cells compared with the parental cells. Importantly, IR and TR cells 5 Inhibition of p-PCNA Blocks Breast Cancer Growth Inhibition of p-PCNA Blocks Breast Cancer Growth 7 Inhibition of p-PCNA Blocks Breast Cancer Growth were attenuated the classical EGFR membrane signaling including downstream ERK1/2 and AKT pathways and were more sensitive to Y211F CPPP treatment than the parental cells, suggesting that the Y211F CPPP has the potential to treat the IR or TR TNBC patients. Tumor targeting of CPPP in vivo To further validate the anti-tumor effect of CPPP in vivo, we established a xenograft animal model by subcutaneous injection of MDA-MB 468 cells into the flanks of nude mice. When the tumors were palpable, mice were randomized into 3 groups and treated with PBS, scrambled peptide, or CPPP by intratumoral injection. Tumor volume was measured at each time interval. As shown in Fig. 5 and S1, mice treated with Y211F CPPP, RF6 CPPP and D-RF6 CPPP had significantly reduced tumor volume compared with those treated with control or scrambled peptides. At the end of the experiment, all mice were sacrificed, and their tumors were isolated and weighed. The CPPP-treated mice had significantly smaller tumor mass than those in the control or scrambled peptide group. The activity of D-RF6 CPPP, on an equimolar basis, appeared to be more potent than Y211F CPPP in 
suppressing the tumor growth of TNBC and reducing tumor volume in mice. Further, in addition to the subcutaneous implantation, we utilize syngeneic orthotopic implantation model to investigate the role of D-RF6 CPPP in suppresses breast tumor growth especially the therapeutic effect against EGFR TKI-resistant TNBC. Indeed, as shown in Fig. 5C and 5D, mice treated with D-RF6 CPPP had significantly reduced tumor volume and mass compared with those treated with control or scramble