What Are the Two Most Effective Ways of Administering Gene Therapy to Treat Eye Diseases?
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What Are the Two Most Effective Ways of Administering Gene Therapy to Treat Eye Diseases?

This article outlines two methods for administering gene therapy to the eye. The topics covered are the techniques, the effectiveness, the risk to the patient, and healing process. Intravitreal injections and subretinal injections have both been shown to be effective in allowing the cells to uptake the required genes but subretinal injections are more complicated to undertake, thus the field has moved towards intravitreal injections.

The targeting of certain cell types to treat specific disease conditions is not simply dependent on the gene-carrying viral capsid.  Although the arrangement and type of proteins on the capsid affects whether the virus can transduce the target cells, there are other factors that must be considered to maximize transduction.  For example, the dose of the viral vector must be high enough to achieve the maximum effect without triggering an immune response or affecting untargeted cells.  The health of the cells can also affect the efficacy of the viral vector.  It has been shown that as the cells of the diseased eye degenerate and become disorganized the viral vector can more easily access the cells.  Effective administration technique is often required to ensure the gene reaches its destination.  When using AAVs to target retinal cells for example, the site of delivery is necessary to help steer the virus to its target. 

The two most common types of delivery are intravitreal and subretinal injection.  When intravitreal injection is used, the vector is introduced directly into the vitreous.  This procedure has become very routine ophthalmic clinical situations.  This injection of the vector into the vitreous is relatively safe and achieves efficient transduction of retinal ganglion cells.  The second technique, subretinal injection, deposits the vector below the retina.  The injected fluid induces regional retinal detachment called a bleb.  As the vector diffuses into the retinal tissue, the bleb flattens out and disappears within hours or a few days.  A portion of the vitreous fluid is removed to relieve ocular pressure.  Because this method is also more challenging than intravitreal injections, there is a higher potential for surgical morbidity. 

Excluding the inherent risks of this procedure it is the most effective method in targeting cells associated with inherited blindness.  The risks of subretinal injection seem to outweigh the benefits as future research is focused on improving the efficacy of intravitreal injections by taking a look at the biochemical barriers that prevent transduction from the vitreal side.  When considering the delivery of lentiviruses, intravitreal inections have failed to produce efficient intraocular transduction and expression.  The most promising lentiviral delivery technique is subretinal injection which results in efficient and stable transduction, though the complications surrounding this technique still remain.  Both of the stated methods result in minimal pain and inflammation beyond the initial injection.  This is because the viruses have been designed to minimize inflammatory response in order to be as therapeutic as possible.  Because of this design, the patient will recover quickly and start to see results within weeks that will last years.

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