Cationic liposome mediated targeted gene delivery with and without pegylated accessories.

Abstract

As a consequence of safety issues encountered by the use of viral vectors in gene therapy, there has been a steady increase in the development and application of non-viral vectors, especially liposomes. Cationic liposome mediated delivery is one of the most promising nonviral delivery methods. These liposomes are prepared from synthetic lipids, are positively charged and interact favourably with DNA through electrostatic interactions. Cationic liposomes have also shown immense potential in the targeting of specific cell types such as HepG2 (hepatocellular carcinoma) cells, a model in vitro gene delivery system for the study of hepatocyte function. However, these liposomes also have a number of limitations in vivo. In an attempt to overcome these restrictions, a hydrophilic polymer, polyethylene glycol (PEG) is incorporated into the cationic liposome. This covalent attachment of (PEG) to the liposomal surface is thought to sterically stabilise liposomes, promote biological stability, inhibit aggregation, decrease toxicity and immunogenicity, prevent interaction with serum proteins and complement and thus prolonging the circulation time of liposomes in vivo. The versatility and simplicity of cationic liposomes have made them vitally significant non-viral gene delivery vehicles for human gene therapy. In this investigation novel untargeted and targeted glycosylated liposomes with and without PEG were synthesised to evaluate their gene transfer activities in vitro to potentially develop a suitable gene delivery system for future in vivo applications. A constant molar quantity of the cationic cholesterol derivative, 3 [N-(N’, N’-dimethylaminopropane)-carbamoyl] cholesterol (CHOL-T) was mixed with dioleoylphosphatidylethanolamine (DOPE) and a galactose/glucose derivative to produce targeted cationic liposomes. PEG liposomes were prepared in the same way with the addition of distearoylphosphoethanolamine polyethylene glycol 2000 (DPSE-PEG2000), 2% on a molar basis. Supported by transmission electron microscopy characterisation, we present evidence that the pegylation of liposomes affects the DNA binding capability and transfection efficiencies of the cationic liposomes in addition to protecting the plasmid DNA in lipoplexes from serum nuclease degradation. Optimal DNA : liposome binding ratios were obtained from gel retardation studies and confirmed by ethidium bromide intercalation assays. These complexes were then tested on the human hepatoma cell line, HepG2, to determine toxicity and assess transfection efficiencies. From results obtained in this study, it appears that both cationic and pegylated cationic liposomes are well tolerated by cells in vitro. The results further suggest that targeting by use of glycolipids incorporated into the structure of the liposome increases transfection, while pegylation of cationic liposomes marginally decreases the transfection efficiency of the lipoplexes to HepG2 cells in vitro.