Clinical Trials in Retinal Dystrophies

Abstract

Research development is burgeoning for genetic and cellular therapy for retinal dystrophies. These dystrophies are the focus of many research efforts due to the unique biology and accessibility of the eye, the transformative advances in ocular imaging technology that allows for in vivo monitoring, and the potential benefit people would gain from success in the field – the gift of renewed sight. Progress in the field has revealed the immense complexity of retinal dystrophies and the challenges faced by researchers in the development of this technology. This study reviews the current trials and advancements in genetic and cellular therapy in the treatment of retinal dystrophies and also discusses the current and potential future challenges.

INTRODUCTION

The last few years have shown rapid growth in genetic and cellular therapy advancement for the treatment of retinal dystrophies. Development of effective therapies may revolutionize ophthalmology since there are currently no available treatments for these diseases, and many patients are destined to lose substantial vision during their lifetime. However, extensive research has further revealed the genetic and phenotypic complexity of retinal dystrophies. Causative mutations have been discovered for many different retinal dystrophies affecting multiple new genes, complex biologic pathways, as well as retinal pigment epithelium (RPE), and photoreceptor cell function. These intricate connections and functions offer researchers many challenges in therapeutic development.

The eye represents a unique target organ for gene therapy due to the immune privilege afforded by the blood-ocular barrier, the ability to directly visualize, access and locally treat the target tissue and with regard to clinical trials, there is the benefit of a simultaneous control provided by the other eye. In this review, we discuss the current status of both gene therapy and cell therapy for retinal dystrophies as well as the challenges that researchers face in the development of these therapies for human use.

OVERVIEW OF RETINAL DYSTROPHIES

Retinal dystrophies are a diverse group of disorders, both genetically and phenotypically affecting the function of the retina. Inheritance patterns include X-linked, autosomal dominant and recessive, and mitochondrial forms. Phenotypic categories include retinitis pigmentosa (RP) and related disorders such as Bardet–Biedl and Usher syndromes, Leber's congenital amaurosis (LCA), rod-cone dystrophy, cone or cone-rod dystrophy, Stargardt's macular dystrophy, congenital stationary night blindness, and choroideremia. Currently, 278 genetic loci and 238 genes have been identified affecting a variety of pathways and mechanisms in retinal disease (RetNet, https://www.sph.uth.edu/retnet/sum-dis.htm). The complexity of the neuronal pathways, the structure of each cell, and the diversity of functions of each retinal layer create many challenges in therapeutic development for these diseases. For example, mechanisms affecting diseases of RPE include visual cycle reactions, phagocytic activity, membrane trafficking, and ion transport. Functional rod and cone photoreceptors cells require phototransduction, connecting cilium structure and transport, inner segment protein and vesicle trafficking, chaperone function, lipid metabolism, transcription and RNA splicing, and synaptic transmission. All of these roles must be restored to regain appropriate retinal function.

GENE THERAPY

Until recently, there has been little hope for curing or slowing the progression of these blinding retinal degenerations. Gene therapy offers a therapeutic option for these disorders [Table 1]. In many cases, mouse models have paved the way for our understanding of the major pathologic pathways involved in retinal degenerations. The more recent successful treatment of large animal models marked a turning point in gene therapy and the advent of treatment options in humans.¹,²

Table 1.

Current clinical trials of gene therapy for retinal degenerations

Photoreceptors and RPE are the primary targets for gene therapy as these cells play a key role in the visual cycle and are prone to a large number of potential single gene defects. RPE is a monolayer of epithelial cells between the choroid and the neurosensory retina. It is involved in many aspects of photoreceptor maintenance including the replenishment of 11-cis-retinal and the phagocytosis of rod and cone outer segment discs. The inherent phagocytic ability of the RPE facilitates gene delivery. A significant portion of retinal disease arises from genetic defects affecting the photoreceptors as these are the primary light sensitive cells of retina. Delivering gene therapy to the photoreceptors is more challenging as their lipid-rich discs act as physical barriers between the subretinal space, the site of delivery, and the cell nucleus in the outer nuclear layer.

Various vector technologies have, however, enabled adequate delivery to the photoreceptors. The principle of gene therapy is based on the transfer of a therapeutic gene to the eye using viral or nonviral vectors. Adeno-associated virus (AAV)-based vectors are currently the gold standard for retinal gene therapy and enable cell type specific transgene expression in RPE cells and photoreceptor cells via subretinal injection.³

PREVIOUS AND CURRENT CLINICAL TRIALS

RPE65 for leber's congenital amaurosis type-2

LCA is a severe, early onset form of RP and is defined by severe visual impairment in the first 3 years of life.⁴ Currently, three RPE expressed genes are known to be implicated in LCA and account for up to 17% of LCA cases.⁵

RPE65 is an RPE-specific gene encoding the isomerohydrolase that recycles bleached all-trans-retinal into photosensitive 11-cis-retinal. Mutations in the RPE65 gene account for 5–10% of all LCA cases and cause a severe phenotype called LCA type 2. In this disease, severe visual loss is present at birth or within the 1^st months of life.⁶,⁷

Gene therapy for LCA began in an RPE65 “knockout” mouse model, and the concept was then carried to a canine model of RPE65 deficiency.²,⁸,⁹ The first three human clinical trials investigating gene therapy for LCA due to RPE65 gene mutations were initiated in 2007.¹⁰,¹¹,¹² All three trials used a subretinal injection of recombinant rAAV2 vector to deliver human RPE65 cDNA to target RPE cells. None of the trials showed adverse local or systemic effects, and there were improvements in the perception of light as noted by increased pupillary response to light and self-reported increased visual sensitivity.¹⁰,¹¹ Longer follow-up studies showed that improvement was noted to peak at 6–12 months and then slightly decline, suggesting that perhaps these initial trials did not meet the full demand for RPE65 in affected individuals.¹⁰,¹¹ These groundbreaking studies were the first to demonstrate proof of concept for gene therapy in human retinal disease while improving the lives of many families with children suffering from this devastating disease. The remarkable success of the RPE65 studies has paved the way for other clinical trials, particularly using AAV-mediated gene delivery; however, there are challenges to delivering an effective and enduring dose to halt or inhibit further cell death or degeneration.

MER proto-oncogene tyrosine kinase (RP38) for retinitis pigmentosa

RP is one of the most common types of retinal degenerations and is responsible for vision loss in 1 in 4000 people worldwide.¹³ RP is classically characterized by ophthalmoscopic findings of abnormal dark bone-spicule pigmentation, attenuated blood vessels, and optic atrophy. The most prominent initial symptoms are nyctalopia and peripheral field loss due to photoreceptor dysfunction with the rods being affected first. Eventually, central visual loss occurs due to cone involvement. So far, more than 60 gene defects have been identified.¹⁴,¹⁵,¹⁶

In the Royal College of Surgeons (RCS) rat, a classic model of RP, the MER proto-oncogene tyrosine kinase (MERTK) gene was identified as the causative gene. MERTK is activated by ligands on outer segment disc membranes and triggers outer segment phagocytosis by RPE cells.⁵,¹⁷ Mutations of MERTK cause outer segment debris to accumulate in the subretinal space leading to photoreceptor degeneration. MERTK knockout mice showed impaired ingestion of photoreceptor outer segments by RPE.¹⁸ In one study, an AAV vector was used to transfer MERTK to the subretinal space of RCS rats, leading to restoration of phagocytic function with decrease in outer segment debris.¹⁹ Electroretinogram (ERG) results were transiently improved, and photoreceptor survival was prolonged, for about 12 weeks. A gene therapy trial has recently been started at the King Khaled Eye Hospital in Saudi Arabia to treat patients with RP caused by MERTK mutations (NCT01482195).

ABCA4 for Stargardt's disease

Stargardt's disease, also known as Stargardt's macular degeneration or fundus flavimaculatus, is the most common form of juvenile macular degeneration.²⁰ It is usually inherited as an autosomal recessive disease caused by mutations in the ABCA4 gene.²¹ The ABCA4 protein localizes to photoreceptor outer segments and aides in the translocation of N-retinylidene-phosphatidylethanolamine (PE), an intermediate in the visual cycle, from the lumen of the disc to the photoreceptor cytoplasm.²²,²³

During ABCA4 dysfunction, N-retinylidene-PE accumulates in the outer segment discs and reacts with all-trans-retinal leading to the formation of apolipoprotein 2E (A2E), a major toxic component of lipofuscin. High levels of lipofuscin cause the photoreceptor dysfunction seen in Stargardt's disease.²⁴ A mouse model of the disease was created by Weng et al., which shared several features seen in humans such as delayed rod dark adaptation, delayed clearance of all-trans-retinal, and increased N-retinylidene-PE. There was additionally increased lipofuscin and A2E within cells.²⁵

The size of the ABCA4 gene is too large to be packaged in the more commonly used AAV vector. Currently, a commercial trial led by Oxford Biomedica is using equine infectious anemia virus (EIAV) to deliver ABCA4 (NCT01367444). Beyond the size of the gene and the difficulty of delivering ABCA4 transgene to the photoreceptors, the phenotypic heterogeneity makes Stargardt's disease a challenging condition to treat with gene therapy.

CHM for choroideremia (Rab-escort protein 1 vector)

Choroidermia is a condition leading to the degeneration of the choroid, RPE, and neural retina affecting 1 in 50,000 individuals.²⁶ The disease is X-linked recessive and arises from mutations affecting the CHM gene. CHM encodes Rab-escort protein 1 (REP 1), resulting in a prenylation deficiency and leading to the retinal degeneration seen in choroideremia.²⁷

A current multicenter clinical trial is ongoing (NCT01461213) in which AAV REP1 is administered as a subfoveal injection. The preliminary results of 6 enrolled patients showed that 2 patients gained between 2 and 4 lines of vision while 4 patients recovered 1–3 letters, for a mean visual acuity gain of 3.8 letters at 6 months. Maximal sensitivity measured with dark-adapted microperimetry also increased but not significantly.²⁸

MYO7A for Usher 1B

Usher syndrome is characterized by the dual sensory impairment of the visual and audiovestibular systems.²⁹ It is a heterogeneous disease with 9 associated genes, affecting 1 in 25,000 people.³⁰,³¹ The syndrome is divided into three subtypes based on the severity. Usher I patients are usually born with deafness and vestibular problems.

Mutations in MYO7A causes Usher IB syndrome. The gene encodes myosin VIIA, a protein essential for transport along cilia such as those between photoreceptor inner and outer segments. Usher 1B disease is a good candidate for gene therapy as the syndromic features of deafness and some photoreceptor dysfunction are present at birth, but evidence of retinal degeneration does not manifest until adolescence, thereby giving a therapeutic window to intervene.

There is currently a clinical trial in phase I by Oxford Biomedica (NCT01505062) evaluating MYO7A therapy. One challenge in the treatment of this disease with gene therapy is the large size of the transgene required to express myosin VIIA. An equine lentiviral vector, which can accommodate the larger sized gene, is used in this case rather than the adenovirus vector.

Gene therapy in neovascular age-related macular degeneration

Age-related macular degeneration (AMD) is a complex disease with many potential therapeutic targets. Gene therapy may be used to augment the expression of endogenous antiangiogenic factors to suppress neovascularization (NV) in wet AMD. Pigment epithelium-derived factor (PEDF), vascular endothelial growth factor (VEGF) binding proteins, endostatin, and angiostatin are all potential antiangiogenic therapeutics in this disease and are currently investigated using gene therapy.

PEDF is a serine protease inhibitor, which is both neurotrophic and antiangiogenic.³²,³³,³⁴ Intravitreous or subretinal injection of an adenoviral vector expressing human PEDF suppressed the development of retinal or subretinal NV and caused regression of established NV.³⁵,³⁶,³⁷,³⁸ These results led to a phase I clinical trial in patients with advanced NVAMD. Twenty-eight patients were given various doses of PEDF. About 71% of patients in the high-dose group and 55% in low dose group had improvement or stability in size of NV lesion. This study provided proof of concept for the strategy of gene transfer for antiangiogenesis.³⁹

The secreted extracellular domain of VEGF receptor-1, sFlt-1 is a naturally occurring protein antagonist of VEGF.⁴⁰ Intraocular injections of adenovirus vector associated sFlt-1 in mice and monkeys suppressed subretinal NV and intravenous injections suppressed retinal NV in primates.⁴¹,⁴²,⁴³ Intravitreal AAV2-sFlt01 is under development by Genzyme, and subretinal AAV2-sFlt01 is under development by Avalanche Biotechnologies. Avalanche has completed a phase 1 and phase 2a study.

Endostatin is a cleavage product of collagen XVIII, and angiostatin is a cleavage product of fibrinogen, each one inhibiting tumor angiogenesis. Endostatin plays a role in regulating retinal vascular development and the physiologic regression of the hyaloid vasculature.⁴⁴ In mice models, gene transfer of endostatin blocked vascular leakage, suppressed NV, and inhibited ischemic-induced retinal NV. Inducible expression of VEGF in adult mice causes severe proliferative retinopathy and retinal detachment.⁴⁵ Intraocular expression of endostatin reduces VEGF-induced retinal vascular permeability, NV, and retinal detachment.³⁸,⁴⁶ Oxford Biomedica has currently undertaken a phase I trial for gene transfer of both endostatin and angiostatin in equine infectious anemia viral vectors in patients with advanced NVAMD (NCT01301443).

X-linked retinoschisis (RS1) – adeno-associated virus-RS1

X-linked retinoschisis is a leading cause of hereditary visual loss in young males. This degeneration is caused by mutations in the RS1 gene encoding retinoschisin, a protein implicated in cell adhesion and maintenance of retinal integrity.⁴⁷ It displays an X-linked mode of inheritance and is characterized by foveo-macular schisis and intraretinal cystic degeneration resulting in the progressive loss of macular photoreceptors. The peripheral changes consist of the separation of retinal layers.

Mouse models of retinoschisis showed that subretinal or intravitreal delivery of AAV-mediated gene therapy led to increased expression of RS1 protein, improved retinal integrity, and increased ERG b-wave.⁴⁸,⁴⁹,⁵⁰ Currently, there is an ongoing phase I trial to examine the safety and efficacy of rAAV-hRS1 in patients with X-linked retinoschisis (NCT02416622).

Other than the above mentioned clinical studies, various other studies are currently underway [Table 2]. AAV-derived vectors have led to significant advances in gene transfer therapeutics. However, there are several challenges in the delivery and effectiveness of these treatments.

Table 2.

Promising preclinical gene therapy studies

CHALLENGES IN GENE THERAPY

Vector development and delivery present challenges.⁵¹,⁵² AAV2- and EIAV-vectors currently predominate in retinal gene therapy research due in part to their low immunogenicity and long-term expression.⁵³ Benefits of AAV2-based vectors include their efficiency in transducing postmitotic cells and these vectors are nonintegrating. However, the cargo capacity of AAV2-based vectors is 4.7 kb and its use is restricted to the delivery of small genes. As discussed above, there are many genes that are likely too big for delivery by AAV, including ABCA4 in Stargardt's disease and MYO7A in Usher syndrome. Other delivery options are being pursued to accommodate larger cargo, including dual AAV vectors, EIA-based vectors, and nonviral methods for gene delivery. Dual AAV vectors deliver partial cDNAs that then assemble full-length coding sequences in vivo via trans-splicing. EIA vectors can accommodate cargo up to 8 kb and are currently studied as a potential option in ABCA4-related Stargardt's disease. Nonviral vectors include liposomes, lipocomplexes, polyplexes, nanoparticles, microparticles, or a combination of these and have an unlimited carrying capacity.⁵¹,⁵⁴ Although nonviral vectors may be safer than viral vectors, they transfect less effectively and have shorter lives than the viral vectors.⁵⁵

Delivery of the vectors remains the next challenge. The main locations of delivery are intravitreal, subretinal, and suprachoroidal. Intravitreal delivery is less invasive, can be done in an office setting, and can potentially deliver viral particles across the entire retina.⁵³,⁵⁶ Dilution of AAV once in the vitreous, the necessity for the construct to cross the internal limiting membrane, as well as the relative abundance of AAV receptors that capture vectors after intravitreal administration can affect gene transduction.⁵⁷,⁵⁸,⁵⁹ Subretinal injections are technically challenging and complications have been reported in some of the LCA2 clinical trials. These included foveal thinning, macular hole, choroidal effusion, and ocular hypo- and hyper-tension.¹⁰,⁶⁰ In addition, subretinal injection in extrafoveal regions can generate new “pseudofoveas” from vector transduction.⁶¹

Optimal vectors and method of delivery do not guarantee success. Gene therapy relies on the presence of viable target cells. Some patients with LCA or achromatopsia are suitable patients for gene therapy as they have preserved retinal structure for many years after the diagnosis. In addition, in these diseases, the functional component of the disease can be tested in a reasonable time frame in the context of clinical trials. Instead, many forms of RP have mild phenotypes because of the slow progression of the disease which makes it less suitable from a clinical trial perspective point.

CELL THERAPY

Cell therapy is also a potential landmark regenerative therapy for retinal dystrophies. Especially, in advanced-stage disease, cell or tissue transplantation may provide a potential option for restoration of vision. AMD is the leading cause of blindness in people older than 50 years of age in the Western World. Severe vision loss occurs in patients with the advanced dry form (geographic atrophy) and patients with the wet form. Anti-VEGF injections offer significant visual benefits for patients with the wet form. However, there is no effective treatment for the advanced dry form or for the patients who develop atrophy after repeated anti-VEGF injections. These patients may benefit from cell therapy.

The eye is thought to be an ideal organ for cell transplantation as well. The optical media are clear, allowing for the intraoperative and postoperative view to the area of transplantation, and its immune privilege status make it especially attractive for such treatments. The retinal detail that can be displayed with the state of the art imaging such as optical coherence tomography has improved the evaluation of eyes following cell transplantation.⁶² Research subjects can be more easily monitored for improvements in retinal structure, maintenance of retinal layers, integration of cells in the correct location, and cell and tissue survival in vivo.

Depending on the retinal disease, current cell transplant therapy is directed at replacing the retinal pigment epithelial cells or the photoreceptor cells, as well as implanting cells for the trophic factors that they produce. In diseases primarily affecting the RPE, such as AMD, RPE compromise eventually leads to photoreceptor loss. RPE replacement in such diseases could also prevent secondary photoreceptor degeneration and vision decline.

RPE TRANSPLANTATION

Human RPE cells were first isolated and characterized over 30 years ago.⁶³,⁶⁴ Since then, the understanding of the role of the RPE cell layer in the pathophysiology of retinal disease has shown significant progress. As an attraction in cell transplant research, the RPE cells do not require synaptic connections to perform their role, unlike other cell types in the retina.⁶⁵ However, the ability to perform its essential functions is dependent on the RPE layer being a confluent monolayer with tight junctions and the RPE cells maintaining polarity for ion transport.⁶⁵

Seminal experiments by Li and Turner demonstrated proof of principle of RPE transplantation in an animal model of retinal dystrophy with abnormal RPE phagocytosis function. They demonstrated that subretinal injection of healthy RPE cells allows preservation of the outer nuclear, outer plexiform, and photoreceptor inner/outer segment layers.⁶⁶,⁶⁷

Improvements in surgical strategies for RPE transplantation that have progressed since the Submacular Surgery Trial in 1998 have allowed for innovative methods of RPE transplantation.⁶⁸ Macular translocation in which the neurosensory retina is surgically detached and repositioned to an area of healthy RPE previously showed some improvements in visual acuity and quality of life.⁶⁹,⁷⁰,⁷¹ The procedure was also associated with many complications including torsional diplopia, retinal detachment, proliferative vitreoretinopathy (PVR), and macular hole. Therefore, this procedure has largely been abandoned.⁷⁰ Transplantation of autologous RPE patch grafts on a bed of Bruch's membrane and partial thickness choroid to the submacular space has also been demonstrated by several groups.⁷²,⁷³,⁷⁴ Some studies have shown some slight improvement of vision, but this may just signify the ability to detect a fixation target on the fovea.⁷³ This procedure has also shown significant complications including subretinal bleeding, PVR, and mal-positioned graft. More recently, van Zeeburg et al. published the long-term outcomes of 133 patients who underwent CNV excision followed by autologous RPE-choroid grafting. The postoperative PVR rate was 10% and visual acuity outcomes were modest. However, 5% of the patients did show the best corrected visual acuity better than 20/40.⁷⁵

An alternative method for transplantation is a submacular injection of an RPE cell suspension in the location of an RPE defect. Studies have shown the importance of a healthy Bruchs membrane in the ability of these cells to successfully attach to Bruchs and restore RPE continuity.⁷⁶ Several different cell sources have been studied as potential options in RPE transplant therapy, including autologous RPE cells aspirated from a different location in the retina,⁷⁶ autologous iris pigment epithelium,⁷⁷,⁷⁸ in vitro cultured allogenic cells, human embryonic stem cell-derived (hESC) RPE,⁷⁹,⁸⁰,⁸¹,⁸²,⁸³,⁸⁴ and induced pluripotent stem cell-derived (iPSC) RPE.⁸⁵,⁸⁶,⁸⁷,⁸⁸,⁸⁹ The medium and long-term safety and efficacy of the use of hESC-derived RPE in patients with AMD and Stargardt's macular dystrophy in two open-label phase I/II studies were recently reported (NCT01345006 – Stargardt's macular dystrophy and NCT01344993 – AMD).⁹⁰,⁹¹ None of the eighteen patients showed evidence of adverse cell proliferation, rejection or serious ocular, or systemic safety issues. Complications reported were related to the vitreoretinal surgery and immunosuppression including progression of cataract, localized damage to RPE, and intraocular inflammation, and one patient had staphylococcus endophthalmitis.⁹¹ Best-corrected visual acuity was improved in ten eyes, maintained in seven eyes, and declined in one eye.⁹¹ However, it is emphasized that these studies did not include a control group, and patients and investigators were not masked to treatment.

There are currently eleven clinical trials posted on clinical trials.gov evaluating the use of RPE transplantation in retinal disease [Table 3]. These trials are evaluating the use of RPE cells from several different sources, including transplantation of autologous RPE, translocation of autologous RPE, iPSC-derived RPE, and hESC-derived RPE, and also in the treatment of several different retinal dystrophies – AMD, Stargardt's macular dystrophy, and myopic macular degeneration. The majority of these trials are planning to primarily evaluate the safety and tolerability of RPE transplantation in human subjects while simultaneously evaluating visual outcomes. Future studies will confirm the safety of RPE transplantation from stem cell sources and will further evaluate the efficacy and outcomes while continuing to advance the technology.

Table 3.

Current clinical trials in cell therapy for retinal degenerations

PHOTORECEPTOR CELL TRANSPLANTATION

Progressive loss of photoreceptor cells resulting in decreased vision occurs in RP and other retinal degenerations. Transplantation of these photoreceptor cells and restoration of the function of the outer nuclear layer would provide a treatment for these currently untreatable blinding diseases. Previously blind patients have shown the ability to read with stimulation by subretinal electrodes in advanced macular degeneration suggesting that the downstream circuitry of the human inner retina retains the capacity to be reactivated to restore visual function long after the absence of photoreceptors.⁹²,⁹³ Different from RPE transplantation, in photoreceptor replacement, newly introduced precursors would have to resume their developmental program after transplantation to form a polarized outer nuclear layer with formation of light-sensitive outer segments containing enzymes critical for light sensitivity and then would have to reconnect synaptically with downstream retinal neurons in order for light-evoked signals to send information down the visual pathway.

Protocols for differentiating photoreceptors from ESCs have been extensively optimized by Osakada et al.⁸⁰,⁹⁴ and Lamba et al.⁸⁴,⁹⁵ to allow ESC-derived photoreceptors to be a potential cell source for transplantation.⁸⁴,⁹⁶ Fetal retinal cells or fetal retinal tissue are other potential cell sources.⁹⁷,⁹⁸,⁹⁹,¹⁰⁰ After transplantation, the cells have shown survival and improvement in visual function.⁹⁸,¹⁰¹ However, the major disadvantages of ESCs and fetal cells are ethical restrictions and availability. Therefore, other groups evaluate the use of adult photoreceptor or stem cell sources for transplantation.¹⁰²,¹⁰³ Many of the earlier studies evaluated photoreceptor survival in nondiseased or early diseased retina, but more recent studies have also shown restoration of visual function and survival of photoreceptor cells in retina with more advanced disease as well (which would be more characteristic of its need in cell therapy for human retinal diseases).¹⁰⁴

Some patients suffer from loss of both RPE and photoreceptors. Patients with advanced macular degeneration have RPE disease that later leads to photoreceptor cell death. In the future, these patients may benefit from a combined RPE and photoreceptor transplant. Whether this would be a step-wise approach with RPE replacement and then photoreceptors transplantation or whether the two could be transplanted together is to be discovered. Zhu et al. showed that the survival of photoreceptor progenitor cells was increased when co-cultured with hESC-derived RPE,¹⁰⁵ suggesting that this dual replacement is likely to be a promising strategy. Other recent studies have shown the formation of entire optic cups from both mouse ESC and hESC in minimal media conditions.⁹⁶,¹⁰⁶ Therefore, stratified neural retina and RPE in a single complex may also be a potential route to develop a dual RPE/PR graft that can be used in individuals with end-stage disease with severe retinal RPE and PR loss.

Prosthetic devices are another approach investigated and optimized for the restoration of sight in those blind from retinal degenerations. The Argus II Retinal Prosthesis System (Second Sight Medical Products, Inc., Sylmar, CA) has gained Food and Drug Administration approval and a CE mark in Europe after positive early results from the Argus II clinical trial for patients with RP.¹⁰⁷ This is the first and only retinal implant to have obtained both of these approvals. The 3-year results of the clinical trial evaluating the safety, reliability, and benefit of this retinal implant supported the long-term safety profile and benefit of this system in low vision patients with RP.¹⁰⁷ Twenty-nine of the 30 patients had functioning implants after 3 years and the subjects as a group performed significantly better on vision testing with the system on compared to when it was off. Eleven patients did experience device or surgical complications but these were addressed with standard ophthalmic techniques.¹⁰⁷ These promising results open new and hopeful opportunities for visual function in patients with RP and also other retinal degenerations.

CHALLENGES WITH CELL THERAPY

One of the challenges in cell therapy for treatment of retinal dystrophies is choosing the source of the cells. As discussed above, many different cell sources have been studied for the use in retinal disease, including fetal and adult cells, ESCs, autologous RPE or iris pigment epithelial cells, iPSCs, or other stem cells. ESCs and retinal fetal cells have been used in many studies, but the major disadvantages of these cell sources are ethical restrictions and availability. Autologous cell transplants or more recently developed iPSCs from the individual requiring the transplant are ideal in terms of issues with allogeneic immune rejection and availability; however, there are concerns about differentiation and maturation that could pose a risk of immature teratoma formation.¹⁰⁸

Once the cell source is determined, the mechanism for delivering the cells is the next challenge. Subretinal transplantation is the more commonly used technique. However, it is a more complex surgical procedure requiring a skilled retinal surgeon with experience in subretinal surgery and has a higher risk of surgical complications, including hemorrhage, PVR, graft dislocation, and NV.⁷⁴,⁷⁶,¹⁰⁹ With this technique, the cells are delivered to the intended location and, therefore, better integration and differentiation is observed. Intravitreal injections have also been tried since they are a common procedure in a retina clinic for macular degeneration and are associated with fewer complications.¹¹⁰ The concern is that the host retina may act as a barrier and prevent the transplanted cells from migrating and integrating into the retinal tissue in the correct location. Especially, if using a cell sheet or an RPE-photoreceptor-scaffold complex, a subretinal approach would be necessary since transplants of this size could not traverse the inner retina.

Next, ensuring that the cells integrate into the correct location in the retina and differentiate to establish their natural connections and functions is important. Normally, the photoreceptor inner segments are linked to Müller cells through tight junctions and form synapses with bipolar cells. Fully differentiated photoreceptor cells may not form these necessary connections whereas photoreceptor precursors may differentiate after transplantation and create these important connections. If more than one type of cell is needed to restore the natural retinal cell layers, the question will then be whether the layers should be transplanted sequentially or if a retinal complex including the necessary layers would be optimal for restoration of visual function.

CONCLUSION

Research evaluating potential treatments for retinal dystrophies has progressed significantly over the last few years. For blinding diseases that currently have no available treatments, success in the development of cutting-edge therapies for these diseases would change the lives of many individuals. Mutation-specific genetic therapies for retinal dystrophies are currently being developed. Stem cell transplantation for retinal degenerations has now transitioned from over a decade of preclinical studies to phase I/II clinical trials. The potential benefits are a tremendous motivation to ensure that retinal dystrophy translational research reaches its full potential.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.