Preclinical potency and safety studies of an AAV2-mediated gene therapy vector for the treatment of MERTK associated retinitis pigmentosa

Abstract

Abstract Proof of concept for MERTK gene replacement therapy has been demonstrated using different viral vectors in the Royal College of Surgeon (RCS) rat, a well characterized model of recessive retinitis pigmentosa that contains a mutation in the Mertk gene. MERTK plays a key role in renewal of photoreceptor outer segments (OS) by phagocytosis of shed OS tips. Mutations in MERTK cause impaired phagocytic activity and accumulation of OS debris in the interphotoreceptor space that ultimately leads to photoreceptor cell death. In the present study, we conducted a series of preclinical potency and GLP-compliant safety evaluations of an adeno-associated virus type 2 (AAV2) vector expressing human MERTK cDNA driven by the retinal pigment epithelium-specific, VMD2 promoter. We demonstrate the potency of the vector in RCS rats by improved electroretinogram (ERG) responses in treated eyes compared with contralateral untreated controls. Toxicology and biodistribution studies were performed in Sprague-Dawley (SD) rats injected with two different doses of AAV vectors and buffer control. Delivery of vector in SD rats did not result in a change in ERG amplitudes of rod and cone responses relative to balanced salt solution control-injected eyes, indicating that administration of AAV vector did not adversely affect normal retinal function. In vivo fundoscopic analysis and postmortem retinal morphology of the vector-injected eyes were normal compared with controls. Evaluation of blood smears showed the lack of transformed cells in the treated eyes. All injected eyes and day 1 blood samples were positive for vector genomes, and all peripheral tissues were negative. Our results demonstrate the potency and safety of the AAV2-VMD2-hMERTK vector in animal models tested. A GMP vector has been manufactured and is presently in clinical trial.

FIG. 1.

ERG analysis of RCS rats following AAV2-VMD2-hMERTK delivery. A mixed rod-cone ERG response showed significantly improved ERG amplitude in treated eyes compared with contralateral untreated controls (n=5, p<0.005).

FIG. 2.

Fundoscopic images of treated eyes from the three experimental groups. (A) The treated eye of a group 1 animal (BSS-injected). (B) A treated eye from group 2 (low dose; 4×10⁸ vg/ml). (C) A treated eye from group 3 (high dose; 4×10⁹ vg/ml). These eyes showed no complications from receiving treatment.

FIG. 3.

Retinal morphology of treated eyes from the three experimental groups. (A) BSS-injected eye. (B) A low-dose (4×10⁸ vg/ml) treated eye. (C) A high-dose (4×10⁹ vg/ml) treated eye. All eyes showed normal morphology. ph, photoreceptor cells; ONL, outer nuclei layer; INL, inner nuclei layer; GCL, ganglion cell layer.