Evaluation of the Toxicity and Efficacy of an Adeno-Associated Viral Vector Expressing BEST1 Delivered by Subretinal Injection in a Canine Model of Human Bestrophinopathy

To treat patients affected with bestrophinopathies caused by mutations in the BEST1 gene, a recombinant adeno-associated virus vector (OPGx-BEST1 or rAAV2/2-VMD2- BEST1 ) is being developed. The vector construct includes the human BEST1 cDNA under control of the VMD2/BEST1 promoter and is packaged in an AAV2 capsid. This study evaluated the efficacy and toxicity of OPGx-BEST1 administered by subretinal injection in dogs that were homozygous mutant or compound heterozygotes for 3 naturally occurring mutations in BEST1 . Twelve BEST1-mutant dogs were divided into 4 groups of 3 animals each, and they received in their left eye a subretinal injection of 0.15 mL of OPGx-BEST1 at 1 of 3 concentrations (9.5 × 10 9 vector genome [vg]/mL; 3.0 × 10 10 vg/mL; or 3.0 × 10 11 vg/mL) of OPGx-BEST1, resulting, respectively, in a low (1.4 × 10 9 vg), high (4.5 × 10 9 vg), and highest (4.5 × 10 10 vg) dose or vehicle control. The right eyes were not injected. Subretinal injections were well tolerated and were not associated with any systemic or ocular toxicity. Electroretinography showed improved rod- and cone-mediated responses in eyes treated with OPGx-BEST1. Noninvasive retinal imaging by optical coherence tomography showed improved structural integrity with a reduction or prevention of appearance of vitelliform lesions and reversal of microdetachments in the retinal areas treated with OPGx-BEST1. These results support the use of OPGx-BEST1 in clinical studies with patients affected with bestrophinopathies and define the no-observed-adverse-effect level at 4.5 × 10 10 vg/eye (0.15 mL, 3.0 × 10 11 vg/mL).