An ideal delivery system for in vivo gene transfer should be simple and safe, and provide prolonged expression of transgenes at therapeutic levels. A variety of nonviral delivery systems can be used for gene transfer in different clinical settings (1). Plasmid constructs are attractive candidates for direct injection into organs and tissues because plasmids are well-defined entities that are biochemically stable and have been used successfully for many years (2,3). In a few cases, the relatively low expression levels achieved after simple plasmid injection are sufficient to reach physiologically acceptable levels of bioactivity of secreted peptides (4,5).

One of the most striking recent advances for plasmid delivery in vivo has been that of electropermeabilization, also described as either electroporation or electrokinetic enhancement. This physical process exposes the target tissue to a brief electric field pulse that induces temporary and reversible breakdown of the cell membrane. During the period of membrane destabilization, a variety of molecules, including drugs and plasmids, may gain intracellular access. A scheme of the electroporation process is depicted in Fig. 1. This method of molecule delivery, which is simple, efficient, and reproducible, has become valuable for basic research, with great potential for gene transfer and DNA vaccination (6). Electropora-tion has been used very successfully to transfect tumor cells after injection of plasmid (7,8) or to deliver the antitumor drug bleomycin to cutaneous and subcutaneous tumors in humans (9,10). Electroporation has been extensively used in mice (11-13), rats (14,15), dogs (16), and pigs (17,18) to deliver therapeutic genes that encode for a variety of hormones, cyto-kines, enzymes, or antigens. The numerous tissues and organs that have been targeted include liver, skin, eye, testis, cardiac muscle and smooth muscle, tumors at different locations, and skeletal muscle.

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