Lipid substitution on low molecular weight (0.6-2.0 kDa) polyethylenimine leads to a higher zeta potential of plasmid DNA and enhances transgene expression.

Cationic polymers are desirable gene carriers because of their better safety profiles than viral delivery systems. Low molecular weight (MW) polymers are particularly attractive, since they display little cytotoxicity, but they are also ineffective for gene delivery. To create effective carriers from low MW polymers palmitic acid (PA) was substituted on 0.6-2.0 kDa polyethylenimines (PEIs) and their efficiency for plasmid DNA (pDNA) delivery was evaluated. The extent of lipid substitution was dependent on the lipid/PEI feed ratio and the polymer MW. While the hydrodynamic size of the polymer/pDNA complexes (polyplexes) increased or decreased depending on the extent of lipid substitution, the ζ potential of the assembled complexes was consistently higher as a result of lipid substitution. Lipid substitution generally increased the in vitro toxicity of the PEIs, but it was significantly lower than that of the 25 kDa branched PEI. The in vitro transfection efficiency of the lipid-substituted polymers was higher than that of native PEIs, which were not at all effective. The delivery efficiency was proportional to the extent of lipid substitution as well as the polymer MW. This correlated with the increased uptake of lipid-substituted polyplexes, based on confocal microscopic investigations with FITC-labeled pDNA. The addition of chloroquine further increased the transfection efficiency of lipid-substituted PEIs, indicating that endosomal release was a limiting factor affecting the efficiency of these carriers. This study indicates that lipid substitution on low MW PEIs makes their assembly more effective, resulting in better delivery of pDNA into mammalian cells.