Subretinal Gene Therapy

Presentation by Christina Y. Weng, MD, MBA, Summarized by Jonathan F. Russell, MD, PhD

At this year’s meeting, Christina Y. Weng, MD, MBA, delivered an update on subretinal gene therapy. This article summarizes portions of her presentation.

BASICS OF GENE THERAPY

Gene therapy was first suggested as a potential treatment for human disease in 1972, and the era of ocular gene therapy began with two seminal papers in 1994.^1,2 Twenty-five years later, in 2019, the US FDA reported that it had more than 800 applications for cell and gene therapies, reflecting tremendous growth in the field.

Gene therapy involves introducing genes into host cells to treat human disease. Gene therapy encompasses gene augmentation (for autosomal-recessive loss-of-function inherited retinal diseases such as RPE65-associated retinal dystrophy), gene suppression or inactivation (for autosomal-dominant gain-of-function diseases), and use of gene therapy to create a biofactory (for wet age-related macular degeneration [AMD] and other complex disorders).

Current gene therapies use viral vectors to introduce a transgene into host cells. The host cells then produce the protein product of the transgene. Alternative methods that do not require viral vectors are being explored, such as nanoparticles and iontophoresis, but these are in earlier stages of investigation.

SUBRETINAL DELIVERY

Both intravitreal and suprachoroidal approaches to gene delivery are being explored, and there are advantages and disadvantages to each. Subretinal delivery requires vitrectomy, and the gene product is transduced only in the area of the surgically induced bleb.

Intravitreal delivery could potentially induce panretinal gene expression, and it is a safe and familiar procedure. A potential downside, however, is that it may not achieve optimal efficiency of transduction to outer retinal cells. Another potential disadvantage is that the intravitreal approach may result in extraocular biodistribution that could induce an inflammatory response.

The first FDA-approved gene therapy in the United States is voretigene neparvovec-rzyl (Luxturna, Spark Therapeutics). This therapy, approved for treatment of biallelic RPE65 inherited retinal dystrophy, uses an adeno-associated virus (AAV)-2–based vector that encodes the RPE65 transgene. In the phase 3 study of the therapy, the primary outcome was not visual acuity but rather change in performance in a multiluminance mobility test at 1 year after gene therapy. This outcome measure was significantly improved in patients who received treatment.³ Secondary outcomes, which included Goldmann visual fields and full-field stimulus threshold, also improved with treatment.

Other gene therapies in development include NSR-REP1 (Nightstar Therapeutics), an AAV-2–based therapy aimed at treating choroideremia, an X-linked recessive disease. This gene therapy also uses subretinal delivery. In a phase 1/2a trial, the therapy significantly improved BCVA.⁴ A phase 3 trial is in progress.

RGX-314 (RegenxBio) is an AAV-8–based vector carrying a gene encoding a monoclonal anti-VEGF antibody. It is placed in the subretinal space in patients with wet AMD. In a phase 1/2a clinical trial, investigators observed a dose-dependent increase in protein expression with RGX-314, and patients who received the highest dose of gene therapy required no rescue therapy for 5 to 6 months after injection. The therapy was well tolerated. A phase 3b trial in patients with wet AMD will begin soon. The company is also exploring suprachoroidal delivery using the same therapy for treatment of wet AMD and diabetic retinopathy.

Dr. Weng Talks Gene Therapy

SURGICAL PEARLS

Before performing subretinal gene therapy with voretigene, Dr. Weng said, it is essential to confirm the diagnosis with genetic testing. Oral steroids are started 3 days before surgery day and continued for approximately 2 weeks. For those beginning to perform the procedure, an OR practice run may be valuable to ensure that all logistics are in place. This is important because the medication must be compounded and then surgically implanted within a certain time window after compounding.

General anesthesia is preferred in patients receiving voretigene, as most of these patients are children. The surgeon should consider setting up the injection apparatus and priming the syringe at the start of surgery. The apparatus consists of a 41-gauge cannula with a polyamide microtip connected to extension tubing, which is connected to the gene therapy syringe. Two syringes are provided in case there is a problem with the first. Some surgeons use triamcinolone to ensure that the hyaloid is lifted.

To inject voretigene, the subretinal cannula is gently buried along the superotemporal arcade, taking care to avoid vessels or pathology. Once the retina blanches, injection can begin. Optional techniques include bevelling the 41-gauge cannula tip, creating a pre-bleb with balanced saline solution, or using intraoperative OCT. As the bleb crosses the fovea, the surgeon should consider slowing down the rate of injection. Once the injection is complete, it may be best to stay within the bleb for a few seconds before withdrawing the cannula to avoid reflux. One recent study in a porcine model showed that material can reflux out of subretinal injection blebs.⁵

CONCLUSION

After many years of investigation, gene therapy has entered the realm of reality in retinal therapy. Widespread application is currently limited by the technical expertise required to perform subretinal injections and by the high cost of the medication.^6,7 Some of the pointers in Dr. Weng’s presentation may help new users get up to speed on the procedure.