Structure-activity relationships of water-soluble cationic methacrylate/methacrylamide polymers for nonviral gene delivery

A number of water-soluble cationic carriers was evaluated as transfectant. Almost all studied cationic methacrylate/methacrylamide polymers were able to condense the structure of plasmid DNA, yielding polymer/plasmid complexes (polyplexes) with a size of 0.1-0.3 mu m and a slightly positive zeta-potential, which can be taken up by cells, e.g., via endocytosis. However, the transfection efficiency and the cytotoxicity of the polymers differed widely: the highest transfection efficiency and cytotoxicity were observed for poly[2-(dimethylamino)ethyl methacrylate], p(DMAEMA). Assuming that polyplexes enter cells via endocytosis, p(DMAEMA) apparently has advantageous properties to escape the endosome. A possible explanation is that, due to its average pK(a) value of 7.5, p(DMAEMA) is partially protonated at physiological pH and might behave as a proton sponge. This might cause a disruption of the endosome, which results in the release of both the polyplexes and cytotoxic endosomal/lysosomal enzymes into the cytosol. On the other hand, the analogues of p(DMAEMA) studied here have a higher average pK(a) value and have, consequently, a higher degree of protonation and a lower buffering capacity. This might be associated with a lower tendency to destabilize the endosome, resulting in both a lower transfection efficiency and a lower cytotoxicity. Furthermore, molecular modeling showed that, of all studied polymers, p(DMAEMA) has the lowest number of interactions with DNA. We therefore hypothesized that the superior transfection efficiency of p(DMAEMA) containing polyplexes can be ascribed to an intrinsic property of p(DMAEMA) to destabilize endosomes combined with an easy dissociation of the polyplex once present in the cytosol and/or the nucleus.