Dose Range Finding Studies with Two RPGR Transgenes in a Canine Model of X-linked Retinitis Pigmentosa Treated with Subretinal Gene Therapy.

Recombinant adeno-associated viral (rAAV) vector-mediated gene therapy is being developed to treat X-linked retinitis pigmentosa (XLRP) in patients with mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene. In preparation for a clinical gene therapy trial, we conducted dose range-finding (DRF) studies with an AAV2 capsid with three surface tyrosine residues changed to phenylalanine (AAV2tYF) vector administered by subretinal injection in a naturally occurring RPGR-mutant canine model (XLPRA2) to compare two different human RPGR (hRPGR) transgenes and to establish a reasonable starting dose for a clinical trial. Different dose levels of two candidate vectors (0.15 mL at 1.2 x 10¹⁰ - 3.0 x 10¹² vg/mL of rAAV2tYF-GRK1-hRPGRco or 4 x 10¹⁰ - 3.0 x 10¹² vg/mL of rAAV2tYF-GRK1-hRPGRstb), 6.0 x 10¹¹ vg/mL rAAV5-GRK1-hRPGRco reference vector or Vehicle were subretinally administered and the dogs were followed for 8 weeks post dose. Ophthalmic exams, analyses of retinal structure by in vivo imaging using confocal scanning laser ophthalmoscopy (cSLO)/optical coherence tomography (OCT) in the Lower (4.0 x 10¹⁰ vg/mL) and Lowest (1.2 x 10¹⁰ vg/mL) Doses, immunological responses by cell based assays or ELISA, RPGR transgene expression, and reversal of opsin mislocalization by immunohistochemistry (IHC) were performed. No sustained signs of ocular discomfort or ophthalmic complications were noted in any of the injected eyes except some in the High Dose group (3.0 x 10¹² vg/mL), which showed signs of retinal detachment and inflammation. A change in fundus reflectivity suggestive of a rescue effect was seen in the High, Mid (6.0 x 10¹¹ vg/mL), and Low (1.2 x 10¹¹ vg/mL) Dose groups. cSLO/OCT demonstrated qualitative and quantitative evidence of rescue effect in eyes treated with the Lower Dose. Anti-hRPGR antibodies were absent but NAb titers against AAV2 were detected in all animals dosed with rAAV2tYF in an apparent dose-related pattern. RPGR expression was stronger for rAAV2tYF-GRK1-hRPGRco compared to rAAV2tYF-GRK1-hRPGRstb at all dose levels. Subretinal administration of rAAV2tYF-GRK1-hRPGRco and rAAV2tYF-GRK1-hRPGRstb both corrected rod and cone opsin mislocalization, two early markers of disease in the XLPRA2 canine model of RPGR-XLRP. These results support the selection and use of rAAV2tYF-GRK1-hRPGRco (AGTC-501) and guided the initial doses in clinical studies in patients with XLRP caused by RPGR mutations.