Ocular gene therapy ramping up for commercialization

Clinical trials for treating ocular diseases with gene therapy have expanded in scope, from trials addressing rare inherited retinal diseases such as Leber’s congenital amaurosis and Stargardt’s macular dystrophy to more common degenerative-type diseases such as age-related macular degeneration and abnormal blood vessel growth.

 “Gene and stem cell therapies are going to revolutionize the way we treat ophthalmic disorders,” J. Timothy Stout, MD, PhD, MBA, director of the Cullen Eye Institute and chair of the department of ophthalmology at Baylor College of Medicine in Houston, said.

Stout thinks the eye is an ideal organ for gene therapy.

 

“Structural and functional information about the eye can be assayed quickly, painlessly and noninvasively. I can look in the eye and see what is going on, in contrast to gene therapy for the internal organs, which require indirect methods of observation,” he said.

Stout said that ophthalmologists have “exquisite access” to many areas of the eye. Because the eye is a small, relatively immunoprivileged space, it is suited as a model system for gene and stem cell therapies.

“Surgically, I can get to almost every place in the eye and introduce viral vectors that harbor potentially therapeutic genes,” he said.

Stout said that all of the gene therapy methods currently in trial rely on viral vectors.

“These are viruses that have been genetically engineered to act as one-way gene delivery systems,” he said. “They are non-replicating viruses.”

The viral supernatant is injected directly into the eye. Some trials have injected the virus into the vitreous cavity, whereas other trials have injected the virus into the subretinal space. The latter method entails a vitrectomy and “putting a needle through the retina and then injecting the virus-containing fluid into the subretinal space,” Stout said.

As to the preference of one injectable technique over the other, “clearly, an intravitreal injection is a lot less invasive than a subretinal injection and can be done simply in the clinic. It takes 1 minute to do,” Stout said.

However, for the most part, the viruses being studied do not reach the needed cells with an intravitreal injection, which explains the popularity of subretinal injections. “For many of the trials, we need to infect either the retinal pigment epithelial layer or the photoreceptors of the retina,” Stout said.

In the Frederick Verhoeff Lecture on ocular gene therapy, Stout told colleagues at the American Ophthalmological Society meeting, “We believe that the eye does provide a unique opportunity to study gene transfer. We believe that the delivery systems are safe. We believe that we have long-term expression and therapeutic effect. And so our goal is new treatments for disease that we’ve never been able to treat — Leber’s, Usher’s, Stargardt’s — and sustained treatments for diseases that currently require frequent dosing.”

Leber’s congenital amaurosis

Alex V. Levin, MD, MHSc, chief of pediatric ophthalmology and ocular genetics at Wills Eye Institute in Philadelphia, said that advances in ocular gene therapy are coming to fruition.

“We are on the cusp of a very exciting time, whereby the proof of concept and the proof of efficacy has been shown with regard to gene therapy in the eye. The greatest advance has been seen for type 2 Leber’s congenital amaurosis due to RPE65 gene mutation. But we will also see in the very near future a number of clinical trials for other diseases that I believe will make gene therapy a reality in many forms for ocular disease in the not-too-distant future.”

Levin thinks the main focus of gene therapy will be retinal genetic disease, followed by optic nerve disease.

“However, there is also some excitement about using gene therapy for other ocular disorders that are accessible, perhaps corneal disease and glaucoma, but those are a little further down the line,” he said.

Jean Bennett, MD, PhD, a professor of ophthalmology and of cell and developmental biology at the University of Pennsylvania in Philadelphia, predicted that the U.S. Food and Drug Administration could approve gene therapy for Leber’s congenital amaurosis by late 2015, for which a pivotal phase 3 study is currently underway at The Children’s Hospital of Philadelphia and the University of Iowa.

 

“Leber’s congenital amaurosis is an ultra-orphan disease, with an estimated 500 cases in the United States,” Bennett, the scientific director of the phase 3 trial, said. “Although benefiting such a small number of individuals, I think the approval will pave the way for development of a number of other gene therapies from preclinical data into the clinic for much more common blinding diseases, such as age-related macular degeneration or glaucoma or diabetic retinopathy.”

The pending gene therapy for Leber’s congenital amaurosis consists of a single subretinal injection of a recombinant virus called adeno-associated virus.

“This virus delivers the normal copy of the gene, which is defective in the cells of these individuals,” Bennett said. “The subretinal injection allows a local application of this gene therapy reagent to the diseased cells, thus enabling efficient uptake into those cells and limiting access of the reagent to other cells in the eye and in the rest of the body.”

Bennett said this is ideal therapy because the cells in the retina are terminally differentiated soon after birth; hence, the delivered gene is not diluted, as would be the case if cell division occurred.

“As a result, the gene can stay active for at least a decade,” she said. Study results to date have shown no serious adverse events related to the adeno-associated virus vector and that the majority of subjects seem to be responding positively to treatment.

Stargardt’s, Usher’s

Stargardt’s macular dystrophy is one of the few known inherited macular degenerations, affecting approximately 30,000 people in the United States.

“We know that the disease is due to the lack of production of a specific protein in retinal photoreceptors,” Bennett said. “The strategy would be the same as for the rare form of Leber’s congenital amaurosis, in that the normal copy of the missing gene, ABCA4, would be delivered to the photoreceptors.”

A gene therapy clinical trial is ongoing for Stargardt’s disease, consisting of a single injection of an equine-derived lentivirus named E1AV, generated by Oxford Biomedica.

A trial is also ongoing for one form of Usher’s syndrome, which affects about 2,000 people in the United States and results in hearing and visual deficits.

“Usher’s causes degeneration of retinal photoreceptors and cochlear inner hair cells,” Bennett said. “To be able to prevent the blinding component of this disease would be so meaningful to these patients.” Again, a single injection of gene therapy is the treatment protocol.

Bennett said there are three gene therapy clinical trials for choroidal neovascularization due to AMD.

“The results that were reported recently for one of those studies are encouraging,” she said. “Overall, the purpose of gene therapy is to stop the disease in its tracks in a permanent way.”

Glaucoma

At Massachusetts Eye and Ear Infirmary in Boston, genetic testing for glaucoma is conducted in patients who have onset of disease before age 40 years.

“We have found that, in this group of patients, there is at least a 20% chance of finding mutations in a set of genes that is currently known to cause early-onset forms of glaucoma,” Janey L. Wiggs, MD, PhD, an associate professor of ophthalmology at Harvard Medical School, said.

 

These genes include myocilin and a collection of genes, PITX2, FOXC1 and PAX6, that adversely affect the development of the eye.

“We also test for CYP1B1 and LTBP2, which are a known causes of congenital glaucoma,” Wiggs said.

The institute’s CLIA-certified diagnostic laboratory, for which Wiggs serves as director, runs a panel test that evaluates all of these genes at the same time, including a few that contribute to early onset of familial forms of normal tension glaucoma.

“The value of the panel test is that phenotypically many of these patients look the same, even though they may have different genetic causes of their disease,” she said.

Wiggs said it is meaningful to differentiate the genetic causes because some of these genes cause autosomal dominant disease and others cause autosomal recessive disease; therefore, the genetic counseling is different for these two types of inheritance patterns.

For disease caused by some of the early-onset genes, gene-based therapy is “on the horizon,” according to Wiggs.

“However, before such treatment can be entertained for a patient, it is important to show that the disease is actually caused by a mutation in a particular gene. … There has been some really interesting research in the mechanisms that underlie myocilin-related glaucoma,” Wiggs said, adding that there is good evidence to show that myocilin mutations lead to a misfolded protein response, which then causes endoplasmic reticulum stress and loss of cells in the trabecular outflow pathways.

According to Wiggs, there are drugs that can target the misfolded protein response, one of which is sodium 4-phenylbutyrate (PBA).

“In an animal model, PBA has been shown to lower intraocular pressure, both when given orally or topically,” she said. “There are also other agents as well that can target the misfolded protein response, which is key.”

However, a major limitation for both diagnostics and therapy for glaucoma is that, even for the early-onset forms, a molecular cause of the disease can be identified in only about 20% of patients, Wiggs said.

“That means there are a lot of patients who have inherited disease, but we do not know about the [related] genes yet. So gene discovery is really key for improving our diagnostic capabilities. Finding new genes responsible for early-onset glaucoma will make it possible for more patients to benefit from genetic testing and gene-based therapy,” she said.

Neovascular disease

In one of the gene therapy trials for patients with neovascular AMD and active choroidal neovascularization, sponsored by Genzyme, patients are given a single intravitreous injection of an adeno-associated virus-2 (AAV-2) vector that expresses sFLT01, which is a modified form of soluble VEGF receptor 1, according to Peter A. Campochiaro, MD, George S. & Dolores Doré Eccles professor of ophthalmology and neuroscience at Wilmer Eye Institute, Johns Hopkins School of Medicine. sFLT01 is a VEGF-binding protein.

“[sFLT01] is similar to ranibizumab (Lucentis, Genentech) or aflibercept (Eylea, Regeneron). It binds VEGF and, therefore, decreases the stimulus for neovascularization and leakage,” he said.

About 20 patients have been treated over the past 2 years. The therapy expresses sFLT01 “for a very long period of time,” Campochiaro, an investigator in the trial, said. “As part of the protocol we are measuring the amount of sFLT01 that is present in the aqueous at various times after the injection.”

Results are intended to be reported at the annual meeting of the American Academy of Ophthalmology in October, according to Campochiaro.

In a separate trial for neovascular AMD, sponsored by Avalanche Biotechnologies, the native form of sFLT01 is expressed by a subretinal injection of an AAV-2 vector.

A third study for neovascular AMD, sponsored by Oxford Biomedica, entails a subretinal injection of a lentiviral vector that expresses endostatin and angiostatin, which are antiangiogenic proteins. Enrollment is complete, and results will be reported later this year, according to Campochiaro, who is an investigator in the trial.

“There is a great deal of interest now in gene therapy as a sustained delivery approach,” Campochiaro said, and several companies are considering entering this arena.

“One of the big questions is the route of administration: intravitreous vs. subretinal,” he said. “I prefer intravitreous because it is less invasive and does not require an operating room.”

Campochiaro said the Genzyme study will provide the first data with an intravitreous injection of an AAV-2 vector.

“This will be very important data,” he said. “If intravitreous [delivery] allows for sufficient expression, that will certainly have major advantages.”

However, with intravitreous injection, the number of cells that take up the virus and express is small compared with that of subretinal injection, according to Campochiaro.

“We already know that with a subretinal injection there is very strong expression,” he said.

Furthermore, gene delivery approaches will allow measurement of the amount of transgene that is being produced.

“As a result, we will know the duration of expression and the level of expression,” Campochiaro said.

Controlled research

To achieve success in gene therapy, Levin advocated careful, controlled scientific research that is monitored, well-advised and consented.

“This is and always has been the best way to get where we need to go,” he said. The bottleneck, though, is suitable funding because it is expensive to bring these gene therapies to fruition

“We also need collaboration between laboratories and clinical centers, with a reduction in academic competition and property guarding,” Levin said. “Many of these diseases are rare, so bringing patients and groups together to collaborate and move forward will become very important.”

Another criterion for success, according to Levin, is developing necessary patient support services, in particular genetic counseling. Availability and affordability of genetic testing are also critical.

“You cannot have gene therapy unless you know what gene is causing your disease,” Levin said. “Right now, genetic testing itself is very difficult. There are many insurance barriers, for example, to approving genetic testing for an individual.”

Ocular genetics is a specialized area, according to Levin.

“The development of this specialty allows rare disorders to be seen by single individuals, repetitively, which makes it easier for us to recognize these disorders,” Levin said. Among the newer tools to view, analyze and dissect the eye in a noninvasive manner are fundus autofluorescence imaging and optical coherence tomography.

“Our ability to phenotype from a diagnostic testing standpoint has skyrocketed,” Levin said. “Our ability to find the actual genotype that explains the phenotype has also skyrocketed. So our success rate is now quite high in finding the gene that is causing the patient’s disease. But as far as we have come, we still have the limiting factor of who is going to pay for the patient’s gene testing, which can run anywhere from a few hundred dollars to several thousand dollars.”

Treatment has lagged behind the diagnostics, according to Levin, not because of a lack of skill but because of limited funding and research, as well as the need for oversight of institutional review boards.

“It is a hard process,” he said. “However, I believe we are now going to see an explosion in treatment that parallels the explosion in diagnosis. So far, we have every reason to presume that gene therapy will be effective.”

Stout said that the commercial potential of gene therapy will largely depend on the disease burden.

“There are not that many people in the world who have Leber’s congenital amaurosis as a consequence of RPE65 deficiency,” he said. “Contrast that to age-related macular degeneration, which is a very common disease, so the commercial potential there is quite high.”

The downside of current treatment for wet AMD with anti-VEGF injections is that they must be scheduled every 1 to 2 months, according to Stout.

“One of the great things about gene therapy is that it may be a permanent solution,” he said. “You might have a way of treating the growth of blood vessels with one single injection — either a subretinal or intravitreal injection — that would last the lifetime of a patient.”

Ethical considerations

Gene therapy is not without its ethical issues, including resource allocation.

“What labs will be funded?” Levin asked. “But more importantly, who are the patients who will have access to testing? We live in a country where even after enrollment under the Affordable Care Act, 40 million people are still without health insurance. Even for those who have insurance, these tests may remain uncovered or unaffordable.”

Gene therapy will likely cost tens of thousands of dollars per patient for a single ocular indication, Levin said.

“Eventually, injections will be cheaper,” he said. “Still, in the justice equation, should every patient who is going blind from disease have equal access to the treatment that becomes available? Should a 2-year-old have better access than an 82-year-old, or should a patient who is a greater contributor to society and perhaps more wealthy have better access? There are many factors, including whether we should wait until the disease is more severe or should we take a greater risk by treating earlier disease, knowing that the treatment is more efficacious but perhaps associated with a complication that takes away vision in our well-intended process.”

Levin likened such ethical dilemmas to the early days of organ transplantation.

“Who gets the organ?” he said. “Gene therapy is a limited resource with a large cost. It touches upon many of the fundamental issues of the delivery of health care.”

There will also be conflict-of-interest ethics.

“Centers want to become rich and famous, and therefore we do not want to see the barriers pushed too quickly for the wrong reasons,” Levin said. – by Bob Kronemyer