There are currently dozens of active clinical trials for retinal gene therapy, which is not surprising considering the number of advantages the eye offers for gene therapy. First, the enclosed nature of the eye and that it is relatively immune privileged mean that the immune response against the viral vector will be weaker, increasing the probability of success. Second, retinal cells do not proliferate after birth. This is important as a single injection could potentially offer life-long expression of the therapeutic protein. Third, a number of animal models are available for inherited retinal diseases, which is instrumental for safe and efficient drug development.

Generally, AAV vectors have been found to more effective than lentiviral and adenoviral vectors when administered to the eye. However, different AAV serotypes can yield highly variable results. One study found that the potency of a vector increased 300-fold when a different serotype was used in combination with an optimized gene cassette in a mouse model.

Although much progress has been made in retinal gene therapy, many challenges remain. Many diseases are caused by genes that are too large for standard viral vectors. Promising results have been made by splitting the transgene into two different vectors. Other challenges include situations where the gene mutations result in toxic gain-of-function effects and gene therapy must be used to silence the endogenous gene.

The first gene therapy treatments of retinal diseases have finally become available and many more approved gene therapy drugs can be expected to emerge over the next few years.