Luxturna (voretigene neparvorec-rzyl; Spark Therapeutics) became the first in vivo gene therapy approved by the US Food and Drug Administration, in 2017.¹ This historic landmark demonstrated that gene therapy is not only safe and effective, but also that it is potentially the answer to a number of medical conditions. This new frontier of precision medicine aims to target disease more directly. These therapies are exciting in that they may be a one-time treatment delivering genes that express therapeutic factors for months or years. The eye is an especially well-suited organ for gene therapy.^2,3 It is a relatively small space that allows for minimal amounts of vector to achieve therapeutic levels of transgenes.

There are a number of vectors and approaches that can be utilized to carry out genomic editing. This can be accomplished by viral or nonviral vectors or even by direct genome editing. Examples of viral vectors include adenoviruses, adeno-associated viruses (AAVs), and lentiviral vectors. Another approach, clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) proteins, have been successfully utilized for genetic modification.⁴ However, this technology is better utilized for conditions such as dystrophies with defined pathologic variants. Disorders with multigenic, epigenetic, and environmental factors are more challenging targets. While many approaches are available for use in gene therapy, AAV viruses have shown significant promise, owing to their safety profile and proven efficacy. The mechanism of delivery can vary as well, whether intravitreally, subretinally, or suprachoroidally. The range of advantages and disadvantages of gene therapy methods has inspired a number of promising clinical trials.

Pathologic angiogenesis is the basis of a number of retinal diseases that are increasing in prevalence each year. Proliferative diabetic retinopathy (PDR) and neovascular age-related macular degeneration (nAMD) are particularly significant causes of morbidity and blindness. Neovascular AMD is a significant cause of blindness and disability in the United States, particularly in individuals over the age of 75.⁵ The abnormal neovascular blood vessels that are the hallmark of the disease invade the subretinal space, leaking, hemorrhaging, and eventually replacing the neural retina with scar tissue.⁶ A separate but related type of neovascular process in PDR leads to the production of abnormal new preretinal blood vessels that lead to decreased vision due to bleeding and retinal detachment. One of the major driving forces in both of these conditions and other causes of angiogenesis is vascular endothelial growth factor (VEGF).⁷

Current treatments are aimed at delivering therapies that can slow or halt this leakage. However, these treatments have limitations. They often require serial injections as often as monthly over the course of years. This places a significant burden on both patients and physicians and that burden is expected to grow as the prevalence of these conditions increases. A novel idea proposed for the management of these conditions has been chronic disease treatment through genome editing. While there are a number of promising approaches and clinical trials, this article will focus on the most advanced investigations.

ACTIVE GENE THERAPY CLINICAL TRIALS FOR RETINAL ANGIOGENESIS

RGX-314

RGX-314 (Regenxbio) is a gene therapy that is delivered by subretinal injection and induces the eye to produce an anti-VEGF A fab. This product is a monoclonal antibody fragment, which is similar to ranibizumab, that binds to and neutralizes VEGF. It is being evaluated in a phase 1/2a, multicenter, dose-escalation study in patients with nAMD (NCT03066258). It is given by a one-time subretinal delivery of the AAV8 viral vector that delivers the encoded gene. This single treatment has the potential to produce constitutive anti-VEGF A and eliminate or drastically decrease the need for additional intravitreal injections. The most recent updates on the phase 1/2a trial revealed that the treatment was well tolerated in the 42 enrolled patients.⁸ There are currently 5 cohorts being analyzed, each with a different initial viral load delivery. Early results have demonstrated that the effects appear to be dose dependent with high viral loads leading to more efficacious treatment. The higher-dose cohorts, 4 and 5, have shown the most significant results. At 1 year, there was a 61% and 85% reduction of anti-VEGF injections in cohorts 4 and 5, respectively, while 73% of patients (8 of 11) in cohort 5 remained anti-VEGF injection free. The phase 2 trial (AAVIATE; NCT04514653) will consist of a new suprachoroidal delivery system that will bypass the need for intravitreal surgery and delivery. This innovative change in delivery could make the treatment safer by avoiding the inherent risks of invasive intraocular surgery and is more easily administered.

Regenxbio has also announced it is investigating a suprachoroidal space microinjector for in-office use. ALTITUDE is a multicenter, open-label, randomized, controlled dose-escalation study that will evaluate the efficacy, safety, and tolerability of suprachoroidal delivery of RGX-314 in patients with diabetic retinopathy.⁹ The trial is planning to enroll 40 patients.

ADVM-022

ADVM-022 (Adverum Biotechnologies) is a gene therapy that was designed to produce anti-VEGF A fusion aflibercept protein via the AAV.7m8 viral vector. In contrast to RGX-314, this treatment is delivered by intravitreal injection, which offers many advantages, such as sparing surgery and allowing the possibility of in-office treatment. ADVM-022 is currently being investigated in an active phase 1 clinical trial (OPTIC; NCT03748784) assessing 30 participants and 2 different dose groups with a single intravitreal injection. The multicenter, dose-ranging trial was designed to assess the safety and tolerability of a single intravitreal ADVM-022 in patients with nAMD who are responsive to anti-VEGF treatment. Recent updates revealed that patients in this highest dose cohort did not require rescue injections as far as 15 months out from initial treatment. More specifically, all 6 patients in the high-dose group and 10 of 15 in the low-dose group were rescue-injection-free at the 18-month time point. Of note, ocular inflammation has been found in some patients receiving treatment. This has been well controlled with steroid eye drops, and there have been no reported incidents of posterior inflammation, vasculitis, retinitis, choroiditis, vascular occlusions, or endophthalmitis. While initial testing utilized oral steroids, current treatment design has successfully managed all inflammation with topical steroids alone.¹⁰ Oral and topical steroids were used prophylactically. Inflammation in all cohorts has been controlled with topical steroids.

Additionally, ADVM-022 is now being investigated in patients with diabetic macular edema (DME) in a phase 2, multicenter, randomized, double-masked, active comparator-controlled trial (INFINITY; NCT04418427) to assess a single intravitreal injection of ADVM-022. This trial is enrolling 33 patients who will be divided across 3 arms: a high-dose injection cohort, a low-dose injection cohort, and an intravitreal aflibercept injection group.¹¹

AAVCAGsCD59

AAVCAGsCD59 (Hemera Biosciences) is an AAV2 gene therapy delivered as an intravitreal injection that directly blocks membrane attack complex (MAC) for the treatment of nAMD. This MAC is a critical and final step of the complement cascade. This is being tested in a phase 1 multicenter, open-label study to assess the efficacy and safety of 2 doses of the AAV serotype 2 (AAVCAGsCD59) expressing sCD59 administered via intravitreal injection 7 days after a single intravitreal injection of anti-VEGF (NCT03585556). There are 25 total participants. Interim results have shown that of 22 patients with at least 6 months of therapy, 4 of 22 (18%) have not required retreatment. Of 11 subjects with at least 12 months of therapy, 2 (18%) have not required retreatment. Additionally, some patients have required both topical and oral steroids for management of inflammation.

FUTURE DIRECTIONS

An exciting area of research has been CRISPR and Cas protein gene editing. This technology is able to directly target angiogenesis at a genetic level and permanently disrupt or reduce the production of pathologic factors. In contrast to other genetic therapies discussed, CRISPR-Cas treatment can potentially be delivered by directly manipulating the genetic code. One particular study of interest is the phase 1/2 BRILLIANCE study (Allergan; Editas Medicine) out of Oregon Health & Science University Casey Eye Institute to assess AGN-151587 (EDIT-101), a CRISPR-Cas-based genome editing treatment for the treatment of Leber congenital amaurosis type 10 (NCT03872479). Approximately 18 participants will be enrolled in up to 5 cohorts to evaluate up to 3 dose levels of AGN-151587 in this study. While this work is looking at an inherited retinal disease, the early results look promising in clinical trials and could one day be applicable to a number of diseases, including those with abnormal vasculature. Again this technology is better suited for conditions such as dystrophies by adding exogenous genes into the genome and tailoring the therapy so that mutations in specific genes are edited out. However, it still may one day be of use in treating angiogenesis. This is the first time this technology has been used for gene therapy in the eye. The medication is administered via a subretinal injection and the first patients were treated this year in the trial.

FUTURE CHALLENGES

There are still a number of questions and concerns regarding gene therapy for the treatment of neovascularization. Firstly, the long-term efficacy of these treatments is still unknown; however, data continue to show longer and longer duration of effect. It is unclear what the long-term impact of constitutively expressed anti-VEGF will be. Theoretical systemic risks are possible, but there is conflicting literature on the topic. Delivery of the vector remains a challenge, with intravitreal approaches often leading to inflammation and subretinal approaches requiring surgery. An in-office suprachoroidal injection device is an exciting idea that may potentially avoid some of the risks associated with intraocular surgery, but further data are needed. The long-term impact of these treatments is still not fully elucidated, but the data thus far are provocative. Additionally, these treatments have the potential to significantly improve the burden on the health-care system and on patients in regards to cutting down the need for intravitreal injections.

CONCLUSION

There is much to be excited about in retina at this time. Many therapies that have the potential to transform the way we deliver care are progressing through clinical trials and showing promising results. There is still a long way to go, but results thus far are encouraging. It is very possible that in the next few years, retina specialists will have gene therapy-based treatments that revolutionize patient care for those with blinding neovascular disorders — breakthroughs for the specialty, and for the lives and quality of life of patients. RP

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8. REGENXBIO announces additional positive interim phase I/IIa trial update and program updates for RGX-314 for the treatment of wet AMD. News release. Accessed October 12, 2020. https://www.biospace.com/article/regenxbio-announces-additional-positive-interim-phase-i-iia-trial-update-and-program-updates-for-rgx-314-for-the-treatment-of-wet-amd/
9. REGENXBIO announces FDA clearance of IND for phase II trial of RGX-314 for the treatment of diabetic retinopathy using suprachoroidal delivery. News release. Accessed October 12, 2020. https://regenxbio.gcs-web.com/news-releases/news-release-details/regenxbio-announces-fda-clearance-ind-phase-ii-trial-rgx-314/
10. Adverum Biotechnologies reports positive 24-week data from first cohort of OPTIC phase 1 trial of advm-022 intravitreal gene therapy to treat wet AMD. News release. Accessed October 12, 2020. https://www.globenewswire.com/news-release/2019/09/12/1914534/0/en/adverum-biotechnologies-reports-positive-24-week-data-from-first-cohort-of-optic-phase-1-trial-of-advm-022-intravitreal-gene-therapy-to-treat-wet-amd.html
11. Adverum Biotechnologies announces new INFINITY phase 2 trial for ADVM-022 in diabetic macular edema, reports recent business progress and first quarter 2020 financial results. Accessed October 12, 2020. http://investors.adverum.com/news-releases/news-release-details/adverum-biotechnologies-announces-new-infinity-phase-2-trial