ABSTRACT
The principle of somatic gene therapy is the replacement or correction of a defective or missing gene by means of administering DNA encoding for a specific protein. The use of DNA plasmid as part of a medicine or vaccine quells some of the problems associated with low bioavailability and expensive manufacturing costs of most protein drugs, which, in some disease states, currently provide the only therapeutic option. For gene therapy to be viable, the major barriers to cellular translocation of the plasmid are primarily the cell and nuclear membranes. The large size and anionic nature of plasmids makes their cellular uptake a relatively inefficient event. A variety of nonviral systems have subsequently been identified and developed to enhance cellular delivery of plasmids. Such systems are designed to address the key limiting events in the transport of a plasmid from the administration site to the nucleus of the target cell. The attributes of such delivery systems can be summarized by the acronym DART, where D reflects the distribution of plasmid, A, the access of plasmid to a target cell, followed by recognition (R) of the plasmid by a receptor or other cell sensor and ultimate translocation (T) into the cytoplasm and nucleus. For a clinically viable product, a delivery pathway must be identified that enables DART, ultimately leading to controlled in situ production of a safe and nontoxic protein within specific cell types. Overall findings have suggested that each therapy and target site require a specifically tailored gene delivery system. For instance, efficient delivery of plasmid to skele-
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tal muscle is inhibited by precondensation with cationic polymers, for example, but can be enhanced by protective interactive noncondensing (PINC) polymers such as poly(vinylpyrrolidone) [1]. Current nonviral strategies employing a wide variety of macromolecules such as lipids and polymers in conjunction with a multitude of routes of administration to enhance DNA delivery have not proven to be as efficient as many viral vectors thus far. This is a consequence of their inherent reliability upon the transmembrane carrier mechanisms of the cell for successful uptake. Conversely, viral systems may not be so selective, and although their delivery efficiency may be higher, concerns about genomic integration and potential inability of repeated administrations as a consequence of immune system stimulation can be problematic. Viral vector manufacture and quality control are also typically more problematic than for nonviral systems. Such reliance upon the cell can be bypassed by utilizing physical means to transiently alter cellular permeability. These methods are highly versatile and are generally not limited to specific routes. Transient alteration of cellular membrane integrity provides a doorway for intracellular flux of the macromolecules, such as DNA plasmid. One key requirement is that the balance between efficient delivery and destruction of the cell be maintained.
