Novel siRNA lipoplexes : their targeted and untargeted delivery to mammalian cells in culture.
The high gene knockdown specificity and efficiency of RNA interference (RNAi) provides a potentially viable avenue for the development of a new class of nucleic acid therapeutics for gene-based disease conditions. However, serum instability, inefficient cellular trafficking and non-specific effects of small interfering RNAs (siRNAs), one of the functional mediators of RNAi, has necessitated the development of carriers to facilitate targeted cell delivery. The decline of viral vectors in human gene therapy as a consequence of safety issues has intensified the importance of non-viral vector development. Among the non-viral vectors available for siRNA delivery, cationic liposomes have emerged as an attractive option owing to their simplicity, versatility, relatively low toxicity and potential for cell-specific targeting. Although existing cationic lipids and liposomes traditionally used for DNA delivery have also been used for siRNAs, there still exists a need to develop cationic lipids tailored towards siRNA transfection for improved gene silencing efficiency. Among the cell specific targets available for RNAi therapeutics, hepatocytes expressing the asialoglycoprotein receptor (ASGP-R) are an ideal choice due to the large number of disease targets present for treatment. In this investigation four novel cationic liposome formulations were prepared from equi-molar quantities of either the cationic cholesterol derivative 3β [N-(N’,N’- dimethylaminopropane)-carbamoyl] cholesterol (Chol-T) or 3β [N-(N’, N’, N’- trimethylammoniumpropyl)-carbamoyl] cholesterol iodide (Chol-Q) and DOPE, with and without the hepatotropic ligand, cholesteryl-β-D-galactopyranoside. Electrophoretic gel analysis and SYBR®green displacement assays were employed to determine siRNA binding and condensation efficiencies for all cationic liposomes; while liposome and lipoplex size measurements were made by cryoTEM. SiRNAlipoplex stability in serum was determined by the nuclease protection assay. Cell studies performed on the ASGP-R+ human hepatoma cells, HepG2 and the ASGP-Rembryonic kidney cells, HEK293, to determine lipoplex toxicity and transfection efficiencies were also undertaken. We show that the cationic liposomes formulated for this investigation were able to bind synthetic siRNA optimally at a positive to negative charge ratio of ± 1 : 6. In addition, the cationic liposomes were able to afford siRNA duplexes substantial protection from ribonuclease digestion in serum. From the results obtained in this study, it appears that the cationic liposomes are well tolerated by both the HEK293 and HepG2 cells in vitro. More importantly, the results obtained demonstrated higher transfection efficiencies for the targeted lipoplexes compared with the untargeted controls, strongly supporting the notion that incorporation of the cholestryl-β-D-galactopyranoside into the liposome structure increases transfection efficiency to the targeted HepG2 cells in culture via ASGP receptor mediation. Comparative studies in the HEK293 cell line yielded low transfection effeciences in the order of 20%, with no significant difference being recorded between galactosylated and non-galactosylated lipoplexes.