Effect of positive charges on liposome stability, uptake efficacy and impact on cells.

Positively charged nanocarriers usually show higher uptake than neutral or negatively charged ones, but are often associated with poor stability and toxicity on cells. Here, by preparing a series of liposomes with increasing concentration of positively charged lipids, we explored how the amount of positive charges affects liposome stability, toxicity and uptake efficiency. By increasing the amount of positively charged lipids, higher uptake was observed, but accompanied by liposome agglomeration in serum, as well as higher toxicity on cells. With the lower amount of positively charged lipid, a stable liposome could be obtained, without evident toxicity on cell functions. We then compared the uptake of this optimized formulation with that of a liposome with the same amount of charged lipids but negative, and confirmed higher uptake efficiency for the positively charged liposome. Then, the two oppositely charged liposomes were used to understand the mechanisms leading to the different uptake efficiency. We found that despite their different uptake efficiency, similar mechanisms were involved in the uptake of the two liposomes, and, despite their opposite charge, for both liposomes uptake was mediated by the interaction with negatively charged proteoglycans. However, the two liposomes adsorbed different corona proteins in serum and adhesion to the cell membrane was slightly higher for the positive liposomes and these differences are likely explaining their higher uptake efficiency.