Delivery of Plasmid DNA into Tumors by Intravenous Injection of PEGylated Cationic Lipoplexes into Tumor-Bearing Mice

For systemic injection of cationic liposome/plasmid DNA (pDNA) complexes (cationic lipoplexes), polyethylene glycol (PEG)-modification (PEGylation) of lipoplexes can enhance their systemic stability. In this study, we examined whether intravenous injection of PEGylated cationic lipoplexes into tumor-bearing mice could deliver pDNA into tumor tissues and induce transgene expression. PEGylation of cationic liposomes could prevent their agglutination with erythrocytes. However, when PEGylated cationic lipoplexes were injected intravenously into tumor-bearing mice, they accumulated in tumor vascular vessels and did not exhibit transgene expression in tumors with both poor and well-developed vascularization. Furthermore, PEGylated cationic lipoplexes of CpG- free pDNA could not increase transgene expression in tumors after intravenous injection. These results suggested that PEGylation could not extravasate cationic lipoplexes from vascular vessels in tumors and abolished transgene expression although it enhanced the systemic stability of cationic lipoplexes by avoiding interactions with blood components such as erythrocytes. Successful delivery of pDNA to tumors by PEGylated cationic liposomes will require a rational strategy and the design of liposomal delivery systems to overcome the issue associated with the use of PEG.