Reduction-Responsive Polymer Prodrug Micelles with Enhanced Endosomal Escape Capability for Efficient Intracellular Translocation and Drug Release

High-efficiency endosomal escape of drug delivery nanocarriers for glutathione-based reduction-responsive drug release in cytoplasm can significantly enhance the therapeutic efficacy of the loaded therapeutic drugs. In this report, we develop the polymer prodrug micelles self-assembled from the amphiphilic block copolymer prodrug, PEG-b-P(CPTM-co-ImOAMA), consisting of poly(ethylene glycol) (PEG) and copolymerized segments of disulfide bond-linked camptothecin methacrylate monomer (CPTM) and 1-(1H-imidazole-4-yl)-2-(octylamino)-2-oxoethyl methacrylate (ImOAMA). After cellular internalization through endocytosis, PEG-b-P(CPTM-co-ImOAMA) micelles are trapped in endosomes inside the tumor cells. The endosomal pH can trigger the protonation of the imidazole moieties of PImOAMA segments, which may facilitate endosome escape through the proton sponge effect and the improved interactions between protonated imidazole groups, hydrophobic octyl moieties, and endosomal membranes. Moreover, the high concentration of glutathione in the cytoplasm of cancer cells can trigger the release of active camptothecin (CPT) through cleavage of the disulfide linkers from PCPTM. The in vitro results showed that PEG-b-P(CPTM-co-ImOAMA) micelles could be effectively internalized into cells followed by endosomal escape, which contributed to the significantly improved cancer cell-killing efficacy. Moreover, in vivo studies confirmed that the PEG-b-P(CPTM-co-ImOAMA) micelles realized efficient tumor growth inhibition without obvious side toxicity. Therefore, the proposed reduction-responsive polymeric prodrug micelles with high endosomal escape capability could provide a brilliant potential in a drug delivery platform to achieve enhanced antitumor efficacy.