Functionalization of Liposomes with Hydrophilic Polymers Results in Macrophage Uptake Independent of the Protein Corona

Liposomes are established drug carriers that are employed to transport and deliver hydrophilic drugs in the body. To minimize unspecific cellular uptake, nanocarriers are commonly modified with poly(ethylene glycol) (PEG), which is known to minimize unspecific protein adsorption. However, to date, it has not been studied whether this is an intrinsic and specific property of PEG or if it can be transferred to hyperbranched polyglycerol (hbPG) as well. Additionally, it remains unclear if the reduction of unspecific cell uptake is independent of the "basic" carrier at which a surface functionalization with polymers is usually applied. Therefore, we studied the protein corona of differently functionalized liposomes (unfunctionalized vs PEG or hbPG-functionalized) using PEGylated and PGylated lipids. Their cellular uptake in macrophages was compared. For all three liposomal samples, rather similar protein corona compositions were found, and also-more importantly-the total amount of proteins adsorbed was very low compared to other nanoparticles. Interestingly, the cellular uptake was then significantly changed by the surface functionalization itself, despite the adsorption of a small amount of proteins: although the PEGylation of liposomes resulted in the abovementioned decreased cell uptake, functionalization with hbPG lead to enhanced macrophage interaction-both in the media with and without proteins. In comparison to other nanocarrier systems, this seems to be a liposome-specific effect related to the low amount of adsorbed proteins.