Delivery Systems

In many cell types it is feasible to deliver nucleic acids and genes by a variety of methods when the cells are grown in tissue culture (Table 58.1). Nonetheless, some cells, such as pneumocytes and neurons, are not readily isolated from humans and do not grow well in vitro. Furthermore, for many diseases it is essential to alter the phenotype of a significant proportion of the total cell population, making ex vivo gene therapy of limited use.

There is general agreement that no ideal delivery system is available for in vivo gene therapy. Direct or in-tratumoral injection of plasmid DNA or antisense oligomers without a viral vector has been attempted. Expression of genes using traditional nonviral vectors has been low compared to viral strategies. Nonetheless, recent breakthroughs in nonviral delivery systems, including the gene gun, electroporation and naked DNA, suggest that nonviral gene therapy can achieve local expression of therapeutic genes at levels equivalent to those of viral vectors.

Although the mechanism remains undetermined, the injection of naked DNA into skeletal muscle has demonstrated relatively high transfection efficiency. In this setting, DNA is precipitated onto the surface of microscopic metal beads (e.g., gold) and the microprojectiles are accelerated and penetrate intact tissue to several cell layers.

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