Stepwise dual pH and redox-responsive cross-linked polypeptide nanoparticles for enhanced cellular uptake and effective cancer therapy.

The systemic toxicity, reduced cellular internalization, and uncontrollable intracellular drug release of smart nanoparticles (NPs) still need to be overcome for effective cancer therapy. Herein, a series of stepwise dual pH and redox responsive cross-linked polypeptides based on poly(l-lysine-co-N,N-bis(acryloyl)cystamine-co-γ-glutamic acid) (PLBG), were prepared for enhanced cellular uptake and effective cancer therapy. The prepared cross-linked PLBG nanoparticles (PLBG-NPs) exhibit negatively charged surfaces under physiological conditions, and the surface charge of the PLBG-NPs was observed to switch from negative to positive in a slightly acidic tumor extracellular environment at a pH level of ∼6.5. Finally, the endosome escape of the PLBG-NPs was facilitated via a "proton-sponge" effect via protonation of the polymer chain in the endo/lysosome environment of the tumor cell at a pH level of ∼5.0. Moreover, rapid drug release was triggered by a high concentration of reducing glutathione (GSH) in tumor cells via destruction of the disulfide linkages in the PLBG-NPs. Cytotoxicity assays demonstrated that no noticeable cytotoxicity was observed for the PLBG-NPs. However, DOX-loaded PLBG-NPs (PLBG-NPs-DOX) demonstrated remarkable tumor cell killing effects. In HeLa tumor-bearing mice, PLBG-NPs-DOX showed sustained tumor accumulation and enhanced inhibitory effects on tumor growth and angiogenesis. In conclusion, it is suggested that the as-designed novel charge-conversion PLBG-NPs with stepwise pH-responsivity and biodegradability could be used as a potential drug carrier for effective cancer therapy with less systemic toxicity.