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Gene replacement therapy emerging as a novel approach in the treatment of many retinal disorders once deemed incurable

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The range of corporate,
venture and private
finance now available
to commercialise gene
therapy is healthier
than at any time before  Gearóid Tuohy

It has been estimated that one in every 4,000 people suffers from an inherited retinal disease and half of those affected have no family history of the pathology. Individuals diagnosed with diseases such as Leber’s congenital amaurosis (LCA), choroideremia, Stargardt's disease, Usher's syndrome, certain forms of retinitis pigmentosa (RP) as well as other inherited dystrophies typically have a poor visual prognosis and treatment options are currently limited.

In recent years gene replacement therapy has emerged as a novel approach in the treatment of many retinal disorders once deemed incurable. The principle of gene therapy is based on the transfer of a therapeutic gene or part of a gene administered to the eye by use of viral or non-viral vectors.

Gene therapy will be a significant contributor to the future armamentarium of therapies for at least some of the hereditary blinding conditions that are currently incurable Artur V Cideciyan PhD​

After a succession of early clinical failures led many to dismiss the technology as over-hyped, that view has been revised due to notable breakthroughs in gene therapy treatments over the past five years. In the field of ophthalmology there is real hope that the eye can showcase the potential of gene therapy to delay, arrest, reverse or ultimately “cure” a range of blinding diseases. As geneticists have long since noted, the eye is a particularly amenable target for gene therapy. It is one of the few immunologically privileged sites in the body and is therefore less likely to provoke a systemic immune response from gene vectors injected subretinally.

Moreover, the defined volume of the eye means that minute amounts of viral vectors may be all that are necessary to achieve localised therapeutic effects without the risk of systemic toxicity and absorption that comes with intravenous drug delivery. Furthermore, major advances in ocular imaging and the localised nature of retinal pathologies means that treatment effects can be more readily observed and monitored for efficacy and safety than with systemic conditions.

Centre stage

Hence the guarded optimism among researchers and clinicians that gene therapy is now ready to move centre stage in the battle against inherited retinal pathologies. “All the evidence suggests that gene therapy will be a significant contributor to the future armamentarium of therapies for at least some of the hereditary blinding conditions that are currently incurable,” said Artur V Cideciyan PhD, research professor of ophthalmology at the University of Pennsylvania and one of the researchers involved in the groundbreaking human trials of LCA.

Gearóid Tuohy, CEO, Ibla Partners Ltd, a specialty-pharma business advisory group and director, Commercialisation & Technology Transfer, at the Royal College of Surgeons in Dublin, Ireland, pinpoints three areas – vector design, clinical success in LCA and European approval of the first gene therapy product – which are radically changing perceptions of the field. “The range of corporate, venture and private finance now available to commercialise gene therapy is healthier than at any time before, timing which combines with the availability of considerable expertise and know-how across basic biology, medicine, business and regulation – all key elements required to advance these novel therapies from bench to bedside,” he said.

Dr Tuohy stressed the significance of Dutch biotech company uniQure’s achievement in developing the first gene therapy to be approved by the European Medicines Agency (EMA) in July 2012. Its product, Glybera® (alipogene tiparvovec), is designed for the treatment of lipoprotein lipase deficiency (LPL), a rare autosomal recessive disorder affecting about one in a million people. “Aside from demonstrating further clinical proof-of-concept for the AAV delivery system, uniQure's achievement represents a game-changer for the investment community as it heralds the arrival of an entirely new class of therapeutics bringing with it significant medical and business opportunity,” he said.

(a) Control retinal section (X400)
(b) affected retina under-going cell death (X400)
(c) Control retinal section (X1000)
(d) affected retina under-going cell death (X1000)
Normal retina [panel (a) and (c)] and affected retina under-going apoptotic cell death [panel (b) and (d)]. Despite a vast heterogeneity of initial causal factors, a wide range of retinal disorders progress through a process of apoptotic cell death in the retina. The red arrow in (b) indicates disorganised outer segment layers while red arrows in (d) indicate apoptotic cells adjacent to healthy cells. Sections from resin embedded retinal tissues courtesy of Dr Gearóid Tuohy, Trinity College Dublin, Ireland

A quick glance at the clinical trials website also confirms the current flurry of activity in gene therapy for retinal diseases, with several trials already started or recruiting for LCA, choroideremia, Stargardt's disease, achromatopsia and certain forms of RP. Of all the retinal degenerations, LCA is one of the most common cause of inherited blindness in childhood, has the earliest age of onset and can be the most severe, leading to retinal dysfunction and visual impairment at an early age. At present, at least 18 different gene mutations known to lead to different forms of LCA have been identified, including the RPE65 gene, which accounts for 5-10 per cent of all LCA cases.

Building on successful research in animal models, a major breakthrough in LCA research came in 2008 when three groups, two in the US and one in the UK, reported success in RPE65-associated LCA using the adeno-associated viral (AAV2) vector (Bainbridge et al, 2008; Cideciyan et al, 2008; Maguire et al, 2008). In all three trials, patients recovered functional vision without apparent side-effects, sparking renewed interest worldwide in using gene therapies to treat a host of other retinal diseases. In a follow-up study earlier this year, however, the University of Pennsylvania team involved in the initial trials reported that although gene therapy treatment improved eyesight, the photoreceptor cells crucial for vision continued to degenerate. This new finding suggests a need for a combination therapy that restores vision while also protecting the photoreceptor cells, Dr Cideciyan told EuroTimes.

 “Our results in human children and adults with RPE65 form of LCA show that substantial improvements in visual function measured within days to weeks following gene augmentation therapy endures for at least as long as three years. However, the underlying retinal degeneration continues to advance in treated retinas at the same rate as untreated retinas. This result is duplicated in RPE65 mutant dogs when the therapy is initiated at a stage of disease that actually models the human condition,” he said.

Tackling retinal degeneration

While the latest findings are undoubtedly a setback, clinicians are already probing possible methods to overcome the issue of ongoing retinal degeneration after successful gene therapy. “We know now that gene expression following AAV mediated gene transfer is efficient and safe. So the technology has reached a point where substantial cure is possible if intervention takes place in tissue that still contains photoreceptors and RPE cells to be cured,” said Knut Stieger DVM, PhD, of the University Eye Clinic in Giessen, Germany.

“This point cannot be stressed enough – AAV-mediated gene addition therapy only works if there are target cells remaining. Therefore, identification of patients, or more strictly speaking, treatable retinal tissue by OCT and functional tests is the key to successful gene therapy,” he added. Possible solutions exist, however, to tackle the problem of continued neurodegeneration regardless of the successful reactivation of rods and cones in treated areas. “This could potentially be addressed by a combination treatment of a vector containing the specific gene for correcting the disease, for example RPE65, and a second viral vector or encapsulated cells containing complementary DNAs for neuro-protective or anti-apoptotic proteins,” said Dr Stieger.

“Such an approach in non-specific gene therapy has been tested in many animal models and also with the use of ciliary neurotrophic factor in encapsulated holofibres to protect neural cells, including photoreceptor cells, in human patients as developed by Neurotech in the United States,” he said. While promising techniques do exist for gene therapy augmentation with adjuvant treatments, a lot more basic research is needed to assess their safety, cautions Dr Cideciyan. “Such treatments could involve induction of cell-protective pathways or inhibition of cell-death pathways with the use of neurotrophic factors, antioxidants and apoptotic inhibitors. Pre-clinical studies using such approaches will have to demonstrate proof-of-concept and safety before clinical trials can be planned,” he said.

Early results from first choroideremia trials

Other positive news has been emerging from the first clinical trial in humans of a gene therapy for choroideremia, an inherited retinal degenerative disease caused by a defective Rab escort protein-1 (REP1) gene. The disease is X-linked recessive and leads to the degeneration of the choriocapillaris, the retinal pigment epithelium and the photoreceptors of the eye. Starting in October 2011, a team at John Radcliffe Hospital in Oxford led by Prof Robert E MacLaren of the University of Oxford began treatment of the first of six patients using a modified AAV vector to deliver the replacement gene. With the study outcomes currently under review prior to publication, Prof MacLaren told EuroTimes that the six-month data look very promising.

“What we discovered was that most choroideremia patients have subtle defects in retinal function that are similar in some ways to a mild form of LCA. It is not obvious at first because most can read 6/6 or better on a Snellen chart, but it is easily seen on microperimetry. This is helpful for a gene therapy trial because improvements in visual function serve as an early indicator of successful gene transfer. Of course we do not know early on if the degeneration has stopped, but with an improvement in vision we can at least assume that the transgene has been activated in the cells. The first six patients have now reached the full sixmonth follow-up and we will be reporting our results in full shortly, however, we can say that there have been no serious adverse events in any of these patients and the gene therapy appears to be very well tolerated,” he said.

“What is particularly exciting about this trial is that we can also see for the first time the effects of AAV gene therapy in patients who have a near normal foveal structure and in whom vision is not limited by amblyopia. This is the likely scenario for future gene therapy treatments for most retinal diseases, as it will almost certainly be better to intervene to correct the underlying gene defect before the onset of retinal degeneration,” said Prof MacLaren. Another interesting by-product of the choroideremia trial is that it has gone some way towards confirming what Prof MacLaren has advocated for some time – that treatment of the fovea is indeed possible in gene therapy.

“These are early days, but we have noted from our study that the human fovea can be detached before the onset of significant degeneration and exposed to therapeutic levels of AAV vector without significant detrimental effect. There will be complications as with all surgery to the retina, but the principle is there – it can be done. When analysing results from all studies, it is important not to confuse surgical complications with those arising due to properties of the vector,” he said. Although this first trial will hopefully answer some of the key questions relating to the safety of gene therapy, many more questions concerning the benefits of early intervention, optimal dosing and surgical technique remain to be elucidated, added Prof MacLaren.

“Ideally we would like to treat patients as early as possible to preserve their remaining sight. Although the gene therapy appears to replace the missing REP1 protein effectively, we still do not know how long the effect will last. Of course we hope that humans share the same features of retinal gene therapy in the animal models studied so far in which the effects are long lasting, but we cannot be sure until we follow these patients up for years. We also do not know the optimal dose of gene therapy and most importantly we need to develop an effective surgical technique that can be used widely in many centres without complications,” he said.

Other trials on the horizon

As the most common juvenile degenerative retinal disease, Stargardt's disease is caused by a mutation of the ABCR gene that leads to the degeneration of photoreceptors in the retina and vision loss. Up to 28 people are currently being enrolled in a Phase I/IIa clinical trial to be conducted at Oregon Health and Science University in Portland, Oregon, US and at the Centre Hospitalize Nationale D’Opthalmologie des Quinze-Vingts in Paris, France. The trial will assess the safety and efficacy of StarGen™ gene therapy (Oxford BioMedica), which uses LentiVector® technology, a “stripped-down” version of the equine infectious anaemia virus (EIAV), to deliver a corrected version of the ABCR gene.

The same company is also currently recruiting for trials of UshStat®, its gene therapy targeted for the treatment of RP associated with Usher's syndrome type 1B. The first human trials for patients with achromatopsia are also on the horizon. While achromatopsia has often been loosely defined as colour blindness, the disease is actually a lot more debilitating than the term implies, points out Prof Martin Biel, a pharmacologist at Ludwig-Maximilians Universität in Munich, Germany.

“Achromatopsia is a lot more than just colour blindness. It is a strictly autosomal recessive or sporadic disease characterised by a loss of cone function and patients lack colour discrimination, have very poor visual acuity, pendular nystagmus and severe photophobia. Its prevalence is about one in 30,000 and so far at least five genes that have been identified as playing a role in the disease development,” he said. Most cases of achromatopsia are due to a mutation in one or other of the CNGA3 and CNGB3 genes essential for photoreceptive function in cells. Prof Biel in collaboration with Prof Mathias Seeliger of Tübingen University and Dr Tim Gollisch of the Max Planck Institute for Neurobiology showed that it is feasible to rescue fast degenerating cone photoreceptors in mice even though cone degeneration had already started. But will the same treatment work in humans?

“What we need now are robust clinical data,” Prof Biel told EuroTimes. “We simply do not know so far to what extent vision can be restored in patients. We also do not know whether vision can be improved in the long term in patients. These questions can be only answered on the basis of the clinical studies which are about to start in several countries,” he said.

While not wanting to raise expectations to unsustainable levels, Prof Biel remains “very optimistic” that gene therapy of achromatopsia, and to a lesser extent RP, will also deliver positive results in humans. “I am particularly optimistic with regard to the treatment of achromatopsia because the functionally impaired cone photoreceptors survive for quite a long time in these patients. Hence there is a large therapeutic window for the application of viral vectors. RP is more complicated however, since depending on the affected gene neurodegeneration can proceed very fast. A lot will depend on the extent to which the neurodegenerative process can be stopped or slowed down,” he said.

As research continues to advance on all fronts – gene sequencing, diagnostics, patient selection, delivery methods, surgical techniques and so forth – and the trial data continues to accumulate, there is a real sense that gene therapy may be on the cusp of finally achieving its promise. “I am very optimistic with many groups now working on improving retinal gene therapy. I feel that a real breakthrough could happen in a few years,” said Prof Biel. “These are early days, but we now have a second genetic retinal disease showing a promising response to gene therapy. I would urge all clinical retinal specialists to engage with this technology,” said Prof MacLaren. “We are now into the domain of optimising the surgical approach and defining the clinical observations that are needed to select the most appropriate patients and assess treatment effects,” he concluded. 

Identification of patients, or more strictly speaking, treatable retinal tissue by OCT and functional tests is the key to successful gene therapyKnut Stieger DVM, PhD
I would  urge all
clinical retinal
specialists to engage
with this technology therapyRobert E MacLaren
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