Influence of the media ionic strength on the formation and in vitro biological performance of polycation-DNA complexes

Cationic polymer-DNA complexes, or polyplexes, have been subject to intensive investigation as potentially efficient non-viral systems for gene therapy. Yet the effects of ionic strength, a physiologically relevant parameter, on the formation, physicochemical properties (e.g. size and colloidal stability) and transfection efficiency of polyplexes are still poorly investigated and understood. In this work, we analyze the effect of ionic strength on the formation and transfection efficiency of poly-L-lysine (PLL), branched polyethylenimine (PEI) and bioreducible poly-L-Lysine (bPLL) polycations complexed with plasmid DNA, using different preparation paths. In path I, the polycation and DNA are mixed in water and transferred to saline media afterwards. In path II, the polycation and DNA are mixed already in the presence of salt. Despite that the final compositions are identical, for monovalent salt (NaCI) concentrations >= 70 mM, the two pathways give rise to polyplexes with different sizes and stability. Path I polyplexes are smaller and colloidally more stable than their path II analogues, irrespective of polycation. Regarding the different polycations used, PLL-DNA polyplexes are smaller and more stable than PEI-polyplexes, regardless of path, while bPLL-DNA particles aggregate very easily in saline media. Conversely, when applied to 2D A549 cell cultures, the two assembly pathways do not show significant differences in transfection efficiency, but regarding cellular-uptake, PEI and path I offer better results. Overall, we show that slight differences in ionic strength at the time of polyplex formation strongly influence the size and stability of polycation-DNA complexes, but they do not translate into significant differences in the transfection of 2D in vitro A549 cell-cultures. Notwithstanding, caution should be exercised as the size differences observed could impact transfection in more complex in vivo models. (C) 2021 Elsevier B.V. All rights reserved.