Protein Delivery By Gene Transfer

The goal of gene delivery typically involves the transfer of specific genetic material into the target cell populations to trigger a desired outcome. Most ocular gene therapy research has been directed against retinal diseases. Gene transfer can be accomplished with viral or nonviral vectors.

Early gene transfer research in the eye concentrated mostly on the adenovirus systems. Adenovirus vectors caused some inflammatory side effects, and therefore, the main emphasis in the field has shifted to the adeno-associated virus (AAV). Very recently, there have been breakthroughs in this field. Some retinitis patients who suffered from an inherited deficiency of RPE-65 protein were treated with AAV with RPE-65 gene (32). Viral particles were administered to the subretinal site under microscopic observation, and future studies will evaluate the clinical utility of this approach. Viral gene therapy has been reviewed elsewhere, and it is not directly in the scope of this article.

Nonviral gene therapy of the eye has the same basic difficulty as elsewhere: delivery of genes into the nuclei of the ocular target cells is not sufficiently efficient. Gene delivery can be accomplished by using the same routes of administration as described earlier in this text. Toropainen et al. (2007) showed recently that the tight junction barrier of the corneal epithelial surface was overcome by transfecting the surface cells with lipid-DNA complexes (33), with the cells subsequently secreting the marker protein [secreted alkaline phosphatase (SEAP)] into the anterior chamber. This outcome is possible because of the leakiness of the deeper layers of the cornea that allow diffusion of a secreted protein. The proteins, as such, do not permeate across the corneal epithelium. Therefore, protein eyedrops would not be effective in the treatment of intraocular targets.

Since some of the SEAP protein was secreted to the tear fluid (33), it is possible to use the corneal epithelial surface cells as a platform to secrete therapeutic proteins over prolonged times either to the tear fluid or to the anterior chamber. This approach could potentially be used for prolonged delivery of protein therapeutics to the tear fluid (e.g., to treat the dry eye syndrome). The contact period of instilled protein solution would be very short, requiring frequent instillation of expensive protein drug. It remains to be seen if the "gene eyedrops" will be an effective way in the delivery of therapeutic proteins.

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