Functional improvement of an IRQ-PEG-MEND for delivering genes to the lung

The targeted delivery of genes to endothelial cells is a potential strategy for curing certain types of disorders including cancer, inflammation and obesity. We previously reported that a liposome (IRQ-LP) modified with the IRQ peptide (IRQRRRR) was taken up by cells via a unique pathway, namely caveolar endocytosis, a cellular uptake pathway that is involved in the blood-to-tissue uptake of macromolecules in vascular endothelial cells. In the present study, we initally investigated the effect of IRQ peptide-modification on the biodistribution of poly(ethyleneglycol) (PEG)-coated liposomes (PEG-LP) after i.v. administration. The IRQ peptide-modified PEG-LP (IRQ-PEG-LP), as well as the PEG-LP were found to be mainly accumulated in the liver. Nevertheless, the fold increase in the lung accumulation of IRQ-PEG-LP, compared to the PEG-LP (approximately 20-folds) was substantially higher than other tissues (<5-fold). Thus, IRQ could function as a target ligand for lungs. We then used the IRQ peptide as a model for a ligand for targeting normal tissue endothelial cells, and then applied it to a gene delivery system. We previously developed a multifunctional envelope-type nano device (MEND), in which plasmid DNA is condensed using a polycation to form a core particle that is encapsulated in a lipid envelope. We modified the IRQ-modified PEG to the MEND (IRQ-PEG-MEND) and marker gene expression was evaluated after i.v. administration. However the transgene expression of the IRQ-PEG-MEND in lungs was low. This is most likely due to the inhibitory effect of the PEG spacer on intracellular trafficking (especially endosomal escape) of the IRQ-PEG-MEND. To overcome the dilemma associated with PEGylation, we improved the MEND system from the point of view of PEG length, lipid chain of the PEG derivative, the polycation and cationic lipid. As a result, transgene expression in lungs was enhanced in stepwise manner, and was finally improved by 5 orders of magnitude compared with the original IRQ-PEG-MEND. Overcoming the dilemma of PEGylation is critical issue for in vivo applications of gene delivery targeting endothelial cells.