Protein expression after delivery of plasmid DNA to the cell nucleus depends on the processes of transcription and translation. Cytotoxic gene-delivery systems may compromise these processes and limit protein expression. This situation is perhaps most prevalent in current nonviral polycationic gene-delivery systems in which the polycationic nature of the delivery system can lead to cytotoxicity, To approach the problem of creating nontoxic but effective gene-delivery systems, we hypothesized that by optimizing the balance between polymer cationic density with endosomal escape moieties, effective gene transfer with low cytotoxicity could be created, As a model system, we synthesized a series of polymers whose side-chain termini varied with respect to the balance of cationic centers and endosomal escape moieties. Specifically, by polymer-analogous amidation we conjugated imidazole groups to the epsilon -amines of polylysine in varying mole ratios (73.5 mol % imidazole, 82.5 mol % imidazole, and 86.5 mol % imidazole), The primary epsilon -amine terminus of polylysine served as a model for the cationic centers, whereas the imidazole groups served as a model for the endosomal escape moieties. These polymers condensed plasmid DNA into nanostructures <150 nm and possessed little cytotoxicity in vitro, Transfection efficiency, as measured by luciferase protein expression, increased with increasing imidazole content of the polymers in a nonlinear relationship. The polymer with the highest imidazole content (86.5 mol %) mediated the highest protein expression, with levels equal to those mediated by polyethylenimine, but with little to no cytotoxicity.
