Impact of amphiphilic copolymer used as PEG alternative on the formation of a protein corona around coated-liposomes encapsulating siRNA

After a systemic administration, liposomes are quickly covered by blood proteins leading to the formation of a protein corona. This corona is responsible for liposomes with a change of physicochemical properties (as an increase of the size), pharmacokinetics, biodistribution, targeting ability and cellular uptake, complement activation and liposomes rapid clearance. To decrease this phenomenon, polyethylene glycol (PEG) is generally grafted onto liposomes surface to reduce non-specific interactions with blood components. Since PEG faces some issues, alternatives are searched which should optimally protect the liposomes surface from proteins adsorption. The goal of this study was to evaluate the protein corona formation around cationic liposomes (DOTAP/Chol/DOPE 1/0.75/0.5 molar ratio) encapsulating siRNA (N/P ratio of 2.5).
Different formulations were tested: naked liposomes, PEGylated liposomes (15% molar ratio of total lipids) and liposomes grafted with new synthetic amphiphilic N-vinyl pyrrolidone-based copolymers used as PEG alternatives (B15b and B19a differing by the number of monomer units). Liposomes were incubated in Fetal Bovine Serum for two hours and their sizes were compared before and after incubation. An increase in liposomes size was related to the adsorption of proteins around the particles. Measurements were performed with Malvern Zetasizer® and Malvern NanoSight®. Results showed that PEG was already effective at 15% to prevent a significant increase of the size. However, both alternatives used at this concentration were not able to prevent corona formation since both analyses revealed a huge increasing of particles sizes. Higher concentration (40%) was necessary to hinder a significant increase of the size meaning probably that their insertion/coating into lipoplexes are different compared to PEG. These polymers may therefore be a great alternative able to face PEG limitations.
To conclude, through this project, we have proven that our new amphiphilic copolymers were able to prevent protein corona formation around lipoplexes. Further studies are ongoing in order to evaluate the transfection capacity of these coated-vectors carrying siRNA.