The covalent attachment of polyethylene glycol (PEG) to a drug or therapeutic protein (pegylation) can "mask” the agent from the host's immune system (by reducing immunogenicity and antigenicity) and increase the hydrodynamic size of the agent, which prolongs its circulatory time by reducing renal clearance. PEGylation can also provide water solubility to hydrophobic drugs and proteins. Gene therapy vectors (such as viruses) can be PEG-coated to shield them from inactivation by the immune system and to de-target them from organs where they may build up and have a toxic effect. The size of the PEG polymer has been shown to be important, with large polymers achieving the best immune protection.
In the current study, investigators at Tufts University School of Medicine (Boston, MA, USA) used PEG to modify a synthetic peptide (POD), which had been developed to deliver drugs by the ocular route. The new compound, PEG-POD, was used to compact DNA into nanoparticles that were then analyzed using electron microscopy, dynamic light scattering, and fluorescent labeling.
The ability of the PEG-POD-DNA nanoparticles to transfect cells was tested in a mouse model. Transfection efficiency and localization were determined 48 hours after injection of the compound into the subretinal space of the mouse eye. Efficiency of ocular transfection was compared to two other PEGylated peptides: PEG-TAT and PEG-CK30.
Results published in the January 2010 issue of the Journal of Gene Medicine revealed that gene expression in mice injected with PEG-POD was 215 times greater than in controls injected only with DNA. The other two peptides were less potent than PEG-POD, with PEG-TAT being 56-fold and PEG-CK30 being 25-fold more effective than DNA injected alone.
"For the first time, we have demonstrated an efficient way to transfer DNA into cells without using a virus, currently the most common means of DNA delivery. Many nonviral vectors for gene therapy have been developed but few, if any, work in post-mitotic tissues such as the retina and brain. Identifying effective carriers like PEG-POD brings us closer to gene therapy to protect the retinal cells from degeneration,” said senior author Dr. Rajendra Kumar-Singh, associate professor of ophthalmology and neuroscience at Tufts University School of Medicine.
"What makes PEG-POD especially promising is that it will likely have applications beyond the retina. Because PEG-POD protects DNA from damage in the bloodstream, it may pave the way for gene therapy treatments that can be administered through an IV and directed to many other parts of the body,” said Dr. Kumar-Singh.
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