Uptake and transfection efficiency of PEGylated cationic liposome-DNA complexes with and without RGD-tagging

Steric stabilization of cationic liposome-DNA (CL-DNA) complexes is required for in vivo applications such as gene therapy. PEGylation (PEG: poly(ethylene glycol)) of CL-DNA complexes by addition of PEG2000-lipids yields sterically stabilized nanoparticles but strongly reduces their gene delivery efficacy. PEGylation-induced weakening of the electrostatic binding of CL-DNA nanoparticles to cells (leading to reduced uptake) has been considered as a possible cause, but experimental results have been ambiguous. Using quantitative live-cell imaging in vitro, we have investigated cell attachment and uptake of PEGylated CL-DNA nanoparticles with and without a custom synthesized RGD-peptide grafted to the distal ends of PEG2000-lipids. The RGD-tagged nanoparticles exhibit strongly increased cellular attachment as well as uptake compared to nanoparticles without grafted peptide. Transfection efficiency of RGD-tagged PEGylated CL-DNA NPs increases by about an order of magnitude between NPs with low and high membrane charge density (σM; the average charge per unit area of the membrane; controlled by the molar ratio of cationic to neutral lipid), even though imaging data show that uptake of RGD-tagged particles is only slightly enhanced by high σM. This suggests that endosomal escape and, as a result, transfection efficiency of RGD-tagged NPs is facilitated by high σM. We present a model describing the interactions between PEGylated CL-DNA nanoparticles and the anionic cell membrane which shows how the PEG grafting density and membrane charge density affect adhesion of nanoparticles to the cell surface.