Surface Charge Switchable Polymer/DNA Nanoparticles Responsive to Tumor Extracellular pH for Tumor-Triggered Enhanced Gene Delivery

A tumor-targeted surface charge switchable polymeric gene delivery system with the function of switching surface charge upon reaching the tumor site owing to the tumor extracellular pH (pHe) was developed. The delivery system was fabricated by two steps. First, the positively charged polyplexe nanoparticles were formed between beta-cyclodextrin-oligoethylenimine star polymer (CD-OEI) and plasmid DNA (pDNA). Next, the CD-OEI/pDNA polyplex nanoparticles were coated with a pHe-responsive anionic polymer via an electrostatic interaction to form ternary complexes. The pHe-responsive anionic polymer was block copolymers of poly(ethylene glycol) (PEG) and poly(2-aminoethyl methacrylate) (pAEMA) modified with 2,3-dimethylmaleic anhydride (denoted as PPD). The coating polymer was mixed with a small amount of pHe-insensitive PEG-pAEMA modified with succinic anhydride (denoted as PPS), giving a balanced negatively charged and PEG-shielded surface with a pHe-responsive property for achieving the expected tumor-triggered enhanced gene delivery. At physiological pH 7.4, owing to the charge shielding of anionic surface coating and the PEGylation, the negatively charged CD-OEI/pDNA/PPD+PPS polyplex complexes could avoid the undesirable interaction with serum proteins and nontargeted components. However, the amide bond of PPD was sensitive to pH changes and could be easily hydrolyzed under acidic pHe (<6.8) to expose the primary amine group due to nucleophile catalysis by the carboxylic acid. The PEG block in the copolymers was used to further enhance the surface-shielding effect. Our data showed that excellent particle salt stability and serum tolerance were achieved through the PPD+PPS surface coating. The CD-OEI/pDNA/PPD+PPS complexes achieved lower cellular uptake and transfection efficiency at neutral pH 7.4 while exhibiting comparable cellular uptake and transfection efficiency at acidic pH 6.5 as compared to the uncoated polyplexes, indicating that the surface charge switching worked well.