Gene therapy can be defined as the transfer of genetic materials to specific cells in order to exert a therapeutic effect. It is a promising approach to the treatment of a wide range of diseases by compensating for defective genes or producing beneficial proteins.1) Gene vectors have much important roles in gene therapy. Recently, nonviral vectors have been increasingly proposed as safer alternatives to viral vectors because of their potential advantages such as ease of synthesis, cell/tissue targeting, low immune response, and unrestricted plasmid size.2) Among nonviral systems, cationic polymers have attracted increasing attention because they can easily form self-assembling polyelectrolyte complexes between plasmid DNA and cationic polymers, and mediate transfection via condensing DNA into nanoparticles, protecting DNA from enzymatic degradation, and facilitating the cell uptake and endolysosomal escape.3) Among cationic polymers, polyethylenimine (PEI) and chitosan are widely used as nonviral vectors for gene delivery. They have the same ability to enter cells by binding to proteoglycans on cell surfaces and undergoing endocytosis.4-6) However, after uptake, they have quite different transfection efficiency. PEI is considered to be the most effective cationic polymer for gene delivery.7) Its high proton-buffering capacity results in rapid osmolysis of the endosomes, and the PEI-DNA complexes escape into the cytosol and are subsequently transported into the nucleus.
8)However, PEI is also associated with dose-dependent toxicity, especially at high molecular weight, which probably explains why it has not yet been used in human studies.9) On the other hand, chitosan does not have high proton-buffering capacity, making it unable to escape from the endosomes in time. Chitosan is degraded in the endosome and the material is then released into the cytoplasm after hyperosmotic rupture of the cell membrane caused by the accumulation of degradation products. The material is then transported to the nucleus.5) Therefore chitosan is generally considered less effective in gene delivery systems than PEI in vitro and in vivo. But it is well known as a biocompatible, biodegradable, and relatively non toxic material with high cationic potential.
10)Therefore to design a complex that has both the advantages of PEI and chitosan, in other words, both high transfection efficiency and low toxicity, would be promising.Several groups have reported the pilot research in this area. Kim et al. 11) demonstrated that when PEI (10k) was combined with a water-soluble chitosan (WSC)/DNA complex, the transfection efficiency was enhanced via the proton sponge effect, and cell survival was not markedly decreased. Another group designed a chitosan-graft-PEI (CHI-g-PEI) copolymer using an imine reaction between periodateoxidized chitosan and low molecular weight PEI and obtained good results.12) However, the influence of the N/P ratio (the ratios of moles of the amine groups of cationic polymers to those of the phosphate ones of DNA), which mi...
