Endowment of pH Responsivity to Anticancer Peptides by Introducing 2,3-Diaminopropionic Acid Residues

Endowment of pH responsivity to anticancer peptides is a promising approach to achieve better selectivity to cancer tissues. In this research, a template peptide was designed based on magainin 2, an antimicrobial peptide with anticancer activity, and a series of peptides were designed by replacing different numbers of lysine with the unnatural amino acid, 2,3diaminopropionic acid (Dap), which has a positive charge at weakly acidic pH in cancer tissues, but is neutral at physiological pH 7.4. These Dap-containing peptides are expected to interact more strongly with tumor cells than with normal cells because 1) weakly acidic conditions form in tumors, and 2) the membrane of tumor cells is more anionic than that of normal cells. Although all examined peptides showed potent cytotoxicities to multidrug-resistant cancer cells at a weakly acidic pH (ED₅₀ ≈5 μm), the toxicity decreased with an increase in the number of Dap at pH 7.4 (8 Dap residues resulted in ED₅₀ ≈60 μm). Furthermore, the introduction of Dap reduced cytotoxicity against normal cells. Thus, Dap led to significantly improved cancer targeting due to a pH-dependent charge shift. Fluorescence imaging and model membrane experiments supported this charge-shift model.