First-ever retinal gene therapy: Does real-world experience back its use?

Voretigene neparvovec (Luxturna, Spark Therapeutics) is the first causal treatment for biallelic RPE65 mutation-associated retinal disease, which regularly progresses to legal blindness. The one-time gene therapy aims to deliver the correct coding sequence of the human RPE65 to the retinal pigment epithelium and is performed via subretinal injection following vitrectomy.

The therapy was approved by both the European Medicines Agency (in 2018) and United States Food and Drug Administration (in 2017) after the supporting pivotal Phase III study revealed statistically significant functional vision improvement in patients in terms of increased light sensitivity (FST) as well as improved ability to navigate a mobility course at different levels of environmental illumination (MLMT).

New gene therapies like these raise the hope of treating a previously incurable disease with a favourable side effect profile. However, as with any new therapeutic product, there is limited real-world data, and soit is natural that uncertaintiesregarding the durability and benefit-risk ratio exist.

Recent reports of retinal atrophy development in the postoperative course of the disease have led to concerns that voretigene neparvovec could lead to potentially devastating consequences in the long term. It is therefore necessary to closely follow up treated patients with multimodal imaging approaches in order toassess retinal morphology and gain further knowledge on the factors possibly contributing to atrophy development.

RPE65 mutation-associated disease

The RPE65 gene encodes a key enzyme in the retinoid cycleand is responsible for the regeneration of the light-sensitive component of rhodopsin, our visual purple.¹ When light enters the eye, it hits the photoreceptors in the retina leading to a conversion of the light signal into a chemical signal.² 

This so-called photoisomerization of the vitamin A derivative 11-cis-retinal (to all-trans-retinal) cannot take place unlessthere is sufficient functional 11-cis-retinal available. Since 11-cis-retinal decays after initiation of the visual process, it must be perpetually regenerated by specific metabolic processes in the retinal pigment epithelium in order to initiate and maintain the visual process.³

Mutations in the RPE65 gene result in deficiency or severely functionally impaired isomerohydrolase activity, causing a severe rod-cone dystrophy.⁴ The clinical courses of RPE65-associated retinal dystrophy are thought to result from different residual activity of the enzyme.

Clinically, the two most common forms of RPE65-related retinal disease are Leber congenital amaurosis (LCA) and early-onset severe retinal dystrophy (EOSRD). In both forms, visual impairment is first noticed at birth and during the first years of life, respectively, which worsens over time eventually leading to complete blindness.⁵ 

LCA is considered the most severe form of early childhood blindness and was first described by Theodor Leber in 1869.⁶ Affected infants usually show lack of eye contact and nystagmus and/or present with conspicuous pressing of their eyeballs with fingers, fists or toys (oculo-digital phenomenon).

Parents may report that their child frequently trips over objects or bumps into obstacles, especially in dim light. EOSRD which manifests after infancy has also a very poor prognosis and, like LCA, usually leads to blindness in the third to fifth decade of life.

To date, it is known that LCA can be caused not only by alterations in the RPE65 gene, but also by mutations in a further 24 genes (see: https://sph.uth.edu/retnet/sum-dis.htm#A-genes). However, mutations in the RPE65 gene might also manifest in the form of retinopathia pigmentosa (RP20).⁷

In the subtype of RP20, a noticeable deterioration of visual acuity usually occurs in young adulthood or adolescence while concentric loss of visual field is already advanced.⁷ Common to all forms of RPE65-mutation associated retinal disease is a pronounced night blindness which presents as one of the earliest and most characteristic symptoms of the disease, and a progressive irreversible retinal degeneration.

The night blindness can be explained by the functional impairment of the rods, which are already affected at earliest disease stages. Rods, unlike cones, are completely dependent on 11-cis-retinal regeneration through the retinoid cycle of the retinal pigment epithelium. Cones are less affected in early stages of the disease because they can rely on 11-cis-retinal from other sources, such as Müller cells, which explains their better function in early disease stages.^8-10

Gene therapy surgery and mechanism of action

Figure 1. Principle of application of gene therapy with voretigene neparvovec.

Figure 2. Mode of action of AAV-mediated gene therapy. (Images courtesy of LMU Eye Hospital)

Abbreviations: AAV, adeno-associated virus; dsDNA, double-stranded DNA

Voretigene neparvovec consists of the capsid of an adeno-associated viral vector serotype 2 (AAV2) containing a correct coding sequence (cDNA) of the human RPE65 gene and regulatory elements.¹¹ This is provided in the form of frozen concentrate, which must be prepared into a vector solution by trained personnel. 

Subsequently, the therapy is provided in asyringe containing the vector solution which mustbe applied within 4 hours after preparation. Following vitrectomy, delivery of the vector solution is performed by using a small injection cannulaby placing it onto the retina and applying slight pressure to create a retinotomy through which the fluid can pass into the subretinal space (see Figure 1). 

The injection may be performed manually with the help of an assisting surgeon or using a foot-pedal controlled injection device. Patients receive a single dose of 1.5x1011 vector genomes voretigeneneparvovec in each eye; the intended target volume is 300 µl.

The injectionforms one or more fluid-filled bubbles under the retina (blebs) which are reabsorbed within 24–48 hours after subretinal delivery, as the drug is taken up by the target cells, the retinal pigment epithelium. Uptake into the target cells is receptor-mediated.

Once in the nucleus, the single-stranded DNA is transcribed into double-stranded DNA, and the mRNA is subsequently translated in the cytosol into the functional protein, the enzymeisomerohydrolase (illustrated in Figure 2).

Reports of atrophy development following administration

The clinical studies leading to approval of voretigeneneparvovec showed that there are certain risks associated with the gene therapy procedure. However, most of the treatment-related adverse events were transient and mild. These included elevated intraocular pressure (18%), cataract formation (18%), ocular inflammation (8%), retinal tears (8%), dellen (8%) and retinal deposits (8%).^12,13 

The previously undescribed complication of chorioretinal atrophy development following treatment with voretigene neparvovec was recently reported by ophthalmologist Dr William S. Gange and colleagues.¹⁴ Eighteen eyes of 10 patients developed perifoveal chorioretinal atrophy; in 80% of the cases this was seen bilaterally.

Atrophy was first identifiable anywhere between 1 week and 1 year postoperatively at an average of 4.7 months after treatment (the follow-up period ranged from 4–18 months). In 10 eyes, the atrophy occurred within and outside the area of the subretinal bleb, whereas in seven eyes, it formed exclusively within the bleb’s area.

One eye showed atrophy only outside the bleb area. Despite atrophy development, functional resultsremained stable or improved in the majority of patients. Twelve of 18 eyes showed improved visual acuity (VA), whereas in three of the eyes VA did not change.

VA decreased in a further three eyes. After statistical analysis, no significant change in mean VA was found pre- versus postoperatively (P = 0.45). While all 13 eyes with reliable Goldmann visuals showed improvement (expansion or gain of isopters),paracentral scotomas caused by the atrophy were seen in three eyes.

Another recent publication reported progressive atrophy development in 13 eyes of eight patients.¹⁵ All eyes developed atrophy within the bleb area andthree patients additionally developed atrophic changes outside the bleb.

The mean duration of the patients’ follow-up period was 15.3 months (ranging from 6 to 27 months). First signs of developing atrophy as detected by reduced autofluorescence were identifiable in five of eight eyes as early as 2 weeks after treatment which represented the earliest postoperative visit.

At month 3 following therapy, all 13 eyes showed areas of retinal atrophy. Notably, the atrophy area enlarged over time and in six of seven eyes with existing follow-up data after 1 year, atrophy development progressed even after 1 year. Functional improvement shown by FST and perimetry seemed to be overall stable over the observational period despite atrophy development.

What could be the cause of atrophy?

Possible explanations for the development of atrophy include immune reactions against the vector genome (e.g., promotor sequence as the CAG promotor in voretigene neparvovec, the expressed transgene or manufacturing-related impurities) or against the capsid. Manufacturing-related factors could also include subtle deviations in the preparation of the gene therapy shortly before therapy administration at the respective treatment centre.

Surgical factors may also play an important role. Mechanical stress and/or damage at the injection site as well as shear stress within the bleb may directly lead to damage of the retina or may trigger deleterious stress responses. This may be particularly relevant in the setting of RPE65-related retinal disease and other inherited retinal dystrophies in which dystrophic changes and a more fragile, thinned retina may predispose to damage.

Patient-related factors should be considered as well. Age, gender, stage of the disease and immune status of the individual patient could influence the functional outcome and morphological state of the retina after treatment.

Conclusion

Early detection of inherited retinal diseases (IRD) is becoming increasingly important, as earlier diagnoses enable more timely initiations of therapy, and thus potentially lead to a better prognoses for affected patients. Symptoms such as increased sensitivity to light (photophobia), night blindness or nystagmusmay indicate an IRD and require a thorough ophthalmological work-up within a specialised ophthalmogenetic department. 

If there is reasonable suspicion, molecular genetic testing should be carried out. This is crucial to determine whether therapy with voretigene neparvovec is applicable. Gene therapy with this product must only be performed in individuals with confirmed biallelic RPE65-mutation-associated retinal dystrophy and sufficient viable retinal cells.

Limited real-world data initiallyconfirmedthe tolerable safety profile seen in marketing authorisation trials.¹⁶ However, the recent reports of progressive atrophy development following therapy are undoubtedly concerning.

In summary, currently available data is insufficient to draw definite conclusions about the causes of atrophy development and their functional consequences in the long-term. Experiences of other treatment centres, including reports of the exact surgical procedure and patient details as well as longer follow up periods are necessary to reach a more accurate picture on possible long-term effects of the therapy. This could have important implications for the selection of patients and will help predicting the expectable treatment benefit for eligible patients.

Maximilian J. Gerhardt, MD

Stylianos Michalakis, PhD

Günther Rudolph, MD

Claudia Priglinger, MD

Siegfried Priglinger, MD*

E: siegfried.priglinger@med.uni-muenchen.de

Drs Gerhardt, Michalakis,Rudolph,Priglinger andPriglinger are based at the Department of Ophthalmology, Ludwig-Maximilians-University Munich, Mathildenstraße 8, 80336 München, Germany.

*Corresponding author

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