Polymeric micelles encapsulating pH-responsive doxorubicin prodrug and glutathione-activated zinc(II) phthalocyanine for combined chemotherapy and photodynamic therapy

A series of polymeric micelles encapsulating different ratios of doxorubicin (DOX) and zinc(II) phthalocyanine (ZnPc) have been prepared for dual chemotherapy and photodynamic therapy (PDT). The amphiphilic block copolymers consist of methoxypolyethylene glycol (PEG) and poly(β-benzyl-L-aspartate) (PBLA), in which DOX and ZnPc were conjugated to the aspartate side chain through an acid-labile hydrazone linker and a redox-responsive disulfide linker, respectively. The polymers were self-assembled into spherical polymeric micelles with diameters of about 160–180 nm and their surface charges were found to be nearly neutral due to the outermost PEG layer. These polymeric micelles exhibited excellent stability and silenced fluorescence in aqueous media. The controlled release of DOX and ZnPc was studied in phosphate solution under acidic and reducing environments, respectively. In vitro study demonstrated that these polymeric micelles could be internalized into HepG2 human hepatocellular carcinoma cells, showing the fluorescence of DOX in the nucleus and fluorescence of ZnPc in the cytoplasm. This observation suggested that the acidic and reducing intracellular environments could trigger the release of DOX and ZnPc by cleaving the corresponding linkers. These micelles exhibited different degree of dark- and photo-cytotoxicity on the HepG2 cells due to the chemocytotoxic DOX and the singlet oxygen generated upon irradiation of the ZnPc. With a certain ratio of DOX and ZnPc, they caused a synergistic cytotoxicity as calculated by combination index. The DOX-ZnPc-micelles-2, which has a DOX/ZnPc molar ratio of 3.8, could induce cell death mainly through apoptosis and exhibit preferential tumor retention in tumor-bearing mice via the enhanced permeability and retention effect. The results suggest that these polymeric micelles are promising nanoplatforms for the delivery of anticancer drugs and photosensitizers for dual therapy.