617. Identification of Novel AAV Capsid Variants with Enhanced Tropism for the Canine Outer Retina Following Intravitreal Delivery

Abstract

The delivery of AAV vectors across physical barriers to target the outer retina is key to the success of retinal gene therapies. Subretinal injection of AAV vectors is the most common approach to deliver genes to photoreceptors and RPE; however, this route of administration presents surgical challenges and potential complications, as well as mediates transfer to only a limited region of the retinal surface. Intravitreal administration may represent a safer way of targeting larger areas of the outer retina; physical barriers such as the internal limiting membrane significantly limit efficiency of natural AAVs through this route. In a prior murine proof of concept study, we showed that directed evolution is promising method for engineering new AAV capsid variants with enhanced permissivity for the outer retina. Here, we show results from a screen performed in the canine retina, a large animal model with eye structure and size similar to the human eye. A library of >107 distinct capsid variant AAV vectors was injected intravitreally into the eye of a wildtype dog. A subset of the AAV vectors injected was recovered from samples of outer retina by PCR amplification. Isolated variants were then pooled and re-injected in additional wildtype canine eyes, and following 5 total rounds of selection the pool had converged to a relatively small number of variants. High throughput sequencing was used to characterize the convergence of variants over all rounds of selection. The final candidate variants were then evaluated by packaging a GFP reporter controlled by a ubiquitous CAG promoter. Retinal distribution and cellular tropism of the variants was examined by cSLO (AF mode) imaging, histology and immunocytochemistry. Nineteen variants were identified as candidate vectors for targeting the canine outer retina. Intravitreal injection of a mixture of equal amounts of these variants carrying GFP fused to a barcode resulted in efficient targeting of all retinal layers including photoreceptors and RPE in 6 eyes of 3 wildtype dogs. Thus, directed evolution performed in a large animal model, with an eye structure similar to that of humans, resulted in new AAV capsid variants with improved abilities to bypass significant structural barriers and efficiently transduce the outer retina. High throughput sequencing allowed for identification of promising variants and monitoring of convergence. These vectors show promise for gene therapy delivery via the intravitreal route of administration, potentially providing a safer approach for targeting a larger area of outer retina than currently achieved by the more traumatic and focal subretinal injection method.