BIO-TECH ADVANCES

A revolution in biological science that began around the middle of the last century has transformed the study of ocular pathology from being a matter of family trees and relatively crude assays to one of DNA analysis and genotyping and detailed bimolecular analysis. However, it wasn’t until this century that these advances had made inroads into ophthalmic medicine and it is only very recently that paediatric ophthalmic patients are beginning to reap the benefits.

Among the newer treatments introduced into paediatric practice in recent years is the use of intravitreal anti-vascular endothelial growth factor (anti-VEGF) agents in infants with retinopathy of prematurity (ROP). A product of decades of research into angiogenesis initiated by Judah Folkman in the 1970s, the agents appear to perform significantly better than laser in terms of recurrence when the retinopathy occurs in zone 1, the small posterior part of the retina surrounding the optic nerve.

Laser treatment, the current gold-standard for ROP, like its predecessor cryoablation, is itself, in a sense, an anti-VEGF treatment. It suppresses the production of VEGF by destroying the peripheral ischaemic retinal tissue that secretes the growth factor, but does not effect the VEGF that has been secreted into the vitreous. Intravitreal anti-VEGF has a more immediate effect in that it directly inhibits VEGF temporarily shutting down the vascularisation process by inactivating VEGF both in the peripheral retina and in the vitreous.

The laser’s advantages include its well-established efficacy and safety, proved through the decades of experience. The laser’s main disadvantage is that if the retinopathy affects a very small area of
the posterior retina, as in zone 1 disease, the patient can end up with significant visual field defects and with significant high myopia.

The advantages of anti-VEGF therapy include the preservation of the visual field, minimising the amount of myopia, especially high myopia (Gelonic et al, JAMA Ophthalmol, epub Aug 7, 2014 and Chen et al, Eye, epub Aug 8, 2014), the simplicity of its administration and the lack of any requirement for expensive laser equipment or expertise in the use of these lasers. The disadvantage of intravitreal anti-VEGF therapy is that there is not yet sufficient data in the published literature to establish the ideal indication, the optimal dosage, or the treatment’s long-term efficacy and long-term safety. Moreover, when recurrences occur after anti-VEGF therapy they tend to occur later than they do after laser treatment. That means that patients need close monitoring for a longer time than is the case with laser therapy.

BEAT-ROP

The BEAT-ROP study is among the most informative of the published studies. It showed that the rate of recurrence for all degrees of ROP Stage 3+ and aggressive posterior ROP in zone I and posterior zone II combined was significantly higher with conventional laser therapy (26 per cent) than it was with intravitreal bevacizumab (six per cent). The mean time to recurrence was 16 weeks following intravitreal bevacizumab therapy, compared with six weeks following conventional laser therapy (Mintz-Hittner et al, N Engl J Med 2011; 364:603-615).

In an interview with EuroTimes, Tim U Krohne MD, FEBO, of University Eye Hospital Bonn, Germany, said that important limitations of the BEAT-ROP study were its short follow-up of only 54 weeks of post-menstrual age and therefore the lack of data regarding late recurrences or complications and functional outcomes. He added that the German ophthalmological and retinal societies are currently recommending restricting the use of intravitreal anti-VEGF therapy for ROP to infants with zone 1 disease and using laser for the more peripheral cases until further data is available.

“The reason is we currently simply don't know what the systemic side effects of the treatment might be. But the drug obviously does leak out of the eye into the systemic circulation and studies show that there is systemic suppression of systemic VEGF activity for weeks in infants who have undergone intravitreal injection of bevacizumab. In contrast to adults, premature babies are still in the process of development, and several organs such as the lung and the brain are known to require VEGF for proper development,” said Dr Krohne.

Dr Krohne added that ranibizumab may have safety advantages in ROP because bevacizumab accumulates to a significantly higher degree in the systemic circulation than is the case with ranibizumab, according to pharmacokinetic research conducted by several teams including his own. This significant difference is illustrated by the systemic half-life of ranibizumab which is only about two hours while that of bevacizumab is about 20 days. A German prospective multicentre trial to investigate the clinical effect of ranibizumab in ROP is ongoing (clinicaltrials.gov: NCT02134457).

Helen Mintz-Hittner MD a co-author of the BEAT-ROP study told EuroTimes that when considering the risks of anti-VEGF in ROP the considerable benefits in terms of preservation of the peripheral retina (visual field enlarged) and allowing the development of the anterior segment (myopia decreased) should not be ignored.

“In severe ROP cases, especially in zone 1, bevacizumab patients develop better visual function ultimately. The destruction of the peripheral retina does not allow it to function. Thus, a restricted visual field develops. Further, the destruction of the peripheral retina that would occur with laser treatment in such severe cases does not allow the development of a normal anterior segment. Thus, high myopia develops. Additionally, laser causes cystoid macular oedema that does not resolve without impacting function. Thus, imperfect visual acuity develops,” she said.

She added that the signs are pretty good so far regarding long-term ocular and systemic side effects of bevacizumab unless the drug is administered prior to the development of Type 1 ROP preventing normal retinal development, especially of the macula (Lepore et al, Ophthalmology, epub, July 4, 2014). Since 2006, when intravitreal bevacizumab was first used for ROP, there have been no reports of local complications, except when injection is performed too early (before Type 1 ROP) or improperly (causing trauma to the lens, retina, etc, or introducing infection), and no systemic complications.

“Thousands of injections for ROP have been given worldwide and no complications in actual human pre-term infants have been reported – only extrapolations and speculations from in vitro retinal cells and in laboratory animals with different doses etc. Specifically the brain, lung, liver etc, develop normally despite anti-VEGF entering the systemic circulation,” she said.

Therefore, ranibizumab is unlikely to have any real safety benefit over bevacizumab. In addition, bevacizumab is much less expensive and is available in almost any hospital that treats cancer patients, she pointed out.

She added that the primary drawback to anti-VEGF treatment for ROP is the longer time required for monitoring for ROP recurrences. She and her associates are preparing another paper that will discuss the risk factors for recurrence, the timing and appearance of recurrence in ROP Stage 3+ and in APROP and suggest a recurrence follow-up schedule.

Gene therapy

Gene therapy in paediatric eye disease has picked up pace since the publication in 2008 of three trials that confirmed that gene therapy can increase cone sensitivity in eyes of young adults with Leber’s congenital amaurosis type 2 (LCA2) (Bainbridge JW et al. N Engl J Med 2008; 358: 2231-2239; Hauswirth WW et al. Hum Gene Ther 2008; 19: 979-990; Maguire AM et al. N Engl J Med 2008; 358: 2240-2248).

Up until now, the published trials have been mainly restricted to adult patients in whom the retina has already degenerated, with a considerable and irreversible loss of photoreceptor cells. Moreover, the disease continues to progress in patients who receive gene therapy.

Better results are likely to be achieved in younger patients, who still retain the bulk of their photoreceptors, and who therefore have a greater potential for visual gains and who may also be less prone to retinal degeneration following treatment, said Robin Ali, (pictured above), PhD, Moorfields Eye Hospital and University College London, UK, a co-author of one of the first gene therapy trials for LCA2.

“The key to the most effective treatment will always be to treat as early as possible. And that's what we see in all our animal studies where we have genetic models of disease. The earlier we treat, the better the outcome,” he said.

He noted that there are about 15 types of LCA. All are caused by autosomal recessive mutations and they share the characteristic of an absence of, or a severe reduction in, retinal function at birth that is followed by a degeneration of the photoreceptors.

In some types of LCA there are defects in rod function, in others there are defects in both rod and cone function. In the case of LCA2, there is a complete absence of rod function from birth. The disease is caused by defects in the gene for RPE65, a retinoid isomerase expressed in the retinal pigment epithelium. RPE65 plays a critical role in the regeneration of the 11-cis retinal chromophore in the visual cycle.

Patients with LCA2 have problems with night vision and peripheral vision, but pretty good central vision conditions to start with. But then, when patients are about eight or nine years old – and for reasons as yet not fully understood but probably related to the lack of function – the rod photoreceptors begin to die. As the retinal degeneration continues the cones start to die and central vision deteriorates.

On the basis of the results achieved in the first trials – all of which indicated patients receiving the therapy achieved some visual and quality-of-life benefits – investigators are beginning to carry out gene therapy in paediatric patients with LCA2 and the results so far appear to agree with the findings obtained in animal models of the disease that earlier treatment yields better visual outcomes.

“We have not yet published on children – we are about to write up our findings – but we have enrolled eight children in our trial, some as young as five and they received the adeno-associated virus vector. And we've seen improvements in their retinal sensitivity,” Dr Ali said.

He noted that the different centres carrying out the LCA2 gene therapy trials have used slightly different variants of the adeno-associated virus vector and have also used different promoters to promote expression of the gene in the target cells.

He added that he and his associates are planning a second trial, with a new, optimised AAV vector and promoter in the hopes that by enhancing delivery and expression of the gene they may also be able to prevent the retinal degeneration.

Meanwhile, research is continuing at numerous centres around the world on gene therapy for the other types of LCA and other hereditary retinal disorders. A phase I/IIa study of gene therapy for Stargardt’s Macular Degeneration is currently recruiting patients. In addition, Dr Ali and his associates have approval, for instance, to start a trial for achromatopsia, a macular disorder.

Future developments may include viral vectors that are effective in the retina when injected intravitreally rather than beneath the retina. The exciting possibilities of gene editing may also play an important role in gene therapy in the future. Gene editing involves the use of CRISPR/Cas9 enzyme to replace a bad gene with a good one right in the chromosome where it belongs.

“That would be the ultimate, because you would not have to worry about getting the right levels of expression as you would have appropriate functioning of the corrected gene. I think that might be the future,” Dr Ali said.

Tim U Krohne: krohne@uni-bonn.de

Helen A Mintz-Hittner:
helen.a.mintz-hittner@uth.tmc.edu

Robin Ali: r.ali@ucl.ac.uk