A new biodegradable cationic copolymer, α,β-poly(N-3-hydroxypropyl)-DL-aspartamide (PHPA) grafted with polyethylenimine (PEI) was synthesized as a nonviral gene delivery vector by conjugating low molecular weight (LMW) PEI onto PHPA backbone. The polycation, termed PHPA-PEI, exhibited good ability to condense plasmid DNA (pDNA) into nanoparticles of around 150 nm with positive charge at a nitrogen/phosphorus (N/P) ratio of 15. The morphology of the nanoparticles observed by atomic force microscopy (AFM) was compact and spherical. PHPA-PEI also showed strong buffering capacity over the pH range 3–10 and protected well the condensed DNA from enzymatic degradation by DNase I over a period of time. pDNA release triggered by the synergistic effect of heparin and degradation demonstrated that PHPA-PEI formulation could continuously release the pDNA over a week. Transfection with pDNA pRL-CMV encoding Renilla luciferase reporter gene (Rluc), mediated by PHPA-PEI/pDNA complexes was carried out in 3T3, HEK293 and COS7 cell lines, and compared with that mediated by PEI (25 kDa)/pDNA complexes. The results showed that PHPA-PEI was not only superior in transfectivity to PEI (25 kDa) but also showed sustained high level luciferase expression. Furthermore, PHPA-PEI exhibited much lower cytotoxicity than PEI (25 kDa) in these cell lines. Therefore, PHPA-PEI may have great potential as a gene delivery vector with low cytotoxicity and high gene transfection efficiency for future gene therapy applications.
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