Genetic Link to Dry AMD Sparks Hope
The recent identification of the first gene linked specifically to geographic atrophy in age-related macular degeneration holds promise for the development of effective therapies. But the discovery also heightens concerns about potential adverse effects from experimental drugs based on double-stranded RNA molecules called small interfering RNAs (siRNAs).
In the study, which was published online in The New England Journal of Medicine,1 a multi-institutional team of researchers found that a mutation in the functional toll-like receptor 3 (TLR3) gene confers protection against geographic atrophy. The mutation—TLR3 variant rs3775291—involves substitution of phenylalanine (Phe) for leucine (Leu) at amino acid 412.
The researchers examined the role of the mutation in human, in vitro and mouse studies. In a case-control study of Americans of European descent, they genotyped 271 patients with geographic atrophy, 179 with choroidal neovascularization and 421 healthy controls. Their findings confirmed previously published reports of a link between TLR3 activation and AMD. But the results also revealed that individuals with the less active TLR3 variant have a reduced risk of developing geographic atrophy. The variant had no effect on choroidal neovascularization.
In vitro studies showed that the prototypic TLR3 ligand polyinosine-polycytidylic acid (a synthetic double-stranded RNA that activates TLR3) induced cell death in a dose-dependent fashion in primary human retinal pigment epithelial cells homozygous for the 412Leu variant. Cell deaths were 50 percent lower in 412Leu-Phe human retinal pigment epithelial cells.
Finally, in mouse studies, researchers injected polyinosine-polycytidylic acid into the vitreous humor of wild-type mice and Tlr3-knockout mice. Forty-eight hours later, retinal pigment epithelial cell loss was approximately 61 percent greater in the wild-type mice than in the Tlr3-knockout mice. Fundus examinations conducted two weeks after the injection revealed that wild-type mice had features consistent with geographic loss of photoreceptors and retinal pigment epithelial cells, whereas the Tlr3-knockout mice did not.
These findings provide insight into the causes of geographic atrophy and make the development of preventive therapies possible, according to Jayakrishna Ambati, MD, professor and vice chairman of ophthalmology and visual sciences at the University of Kentucky and a senior author of the study. His team has developed experimental drugs that block TLR3 and prevent geographic atrophy in animal models. They are seeking regulatory approval to begin a clinical trial next year.
Dr. Ambati noted that results from the current study amplify concerns about possible adverse effects from the siRNA drugs currently in advanced-phase clinical trials for AMD.
At the very least, he said, researchers should examine whether these siRNAs activate the TLR3 receptor, and they should specifically look for potential adverse effects among patients in the ongoing trials.
“It’s a question of doing what is in the best interest of patients,” he said. “We do not know for a fact that these drugs are causing harm, but there is tremendous evidence now that cannot be ignored—in animal models and in humans—that siRNA drugs could potentially be very harmful. I don’t think anyone, regardless of where one stands on this issue, wants to find out about this as we found out about Vioxx—after the fact.”
1 N Engl J Med 2008;359(14):1456–1463. Also published at www.nejm.org on Aug. 27, 2008 (10.1056/NEJMoa0802437).
Glaucoma Patients Take IOP Monitoring Home
Scientists at Greifswald University Hospital in Germany have developed a telemedicine program for glaucoma, utilizing a home-based tonometer, and the program has been used by health insurance company Techniker Krankenkasse since June 1. Patients hold a tonometer with an electronic pressure sensor to the surface of the eye to measure intraocular pressure, and the data are transmitted via modem to an electronic medical record, which can be accessed by the patient’s ophthalmologist, who can then customize medications to the patient’s needs.
Plans call for Techniker Krankenkasse patients to be seen once by an ophthalmologist at Greifswald University Hospital, and then to receive a small suitcase with a tonometer and modem. Each patient receives an individual schedule for IOP measurements and also measures arterial blood pressure regularly. Using these parameters, ocular perfusion pressure (OPP) is calculated automatically. A number of epidemiologic studies have shown that OPP is a risk factor for glaucoma and that it may be a more appropriate predictor of progression than arterial blood pressure or IOP alone, according to Frank Tost, MD, vice director of ophthalmology at Greifswald University Hospital. Once a month, a diurnal profile of IOP and arterial blood pressure is created with seven measurements between 6 a.m. and midnight.
“Self-measurements at home represent a feasible method to record and detect intraday IOP fluctuations and explore the influence of blood pressure on glaucoma progression. The result is that we can adapt therapy options to a patient’s individual situation,” Dr. Tost said.
How does the home-based monitoring by patients compare to assessments done in an ophthalmology office?
In a study of 25 patients published in the German journal Klinische Monatsblätter für Augenheilkunde,1 Dr. Tost and fellow researchers compared IOP measurements obtained with the home-based telemedical device with those obtained in eye specialists’ practices. The diurnal profile showed more frequent circadian IOP variation with the home-based tonometer. The IOP values were an average of 18.9 mmHg for right eyes and 18.2 mmHg for left eyes compared with 16.3 mmHg for both eyes as measured during ambulatory care.
“We’ve found that there is a low incidence of measuring errors with the home-based self-tonometry, and they don’t have an effect on the overall time pattern and course of IOP values,” Dr. Tost said.
1 Klin Monatsbl Augenheilkd 2008;225:1–6.
Ed: Validation of the tonometer has yet to be published.
Novel Therapeutic for Corneal Inflammation
Researchers from Case Western Reserve University and Penn State, Hershey, are the first to report the anti-inflammatory effect of a novel therapeutic for the cornea in vivo. They’ve demonstrated similar results with in vitro studies on human corneal epithelial cells as well.1
Delivered in nanoliposomal bilayer formulations, short-chain ceramides may provide a highly desired alternative to particle steroids, said Eric Pearlman, PhD, professor and director of research at Case Western Reserve University’s department of ophthalmology and visual sciences, who collaborated with Mark Kester, PhD, at Penn State. “Steroids have the unfortunate side effect of increasing intraocular pressure and can lead to the development of glaucoma,” said Dr. Pearlman.
Dr. Pearlman’s lab has focused largely on understanding corneal inflammation as it relates to contact lens wear, especially extended wear lenses. His lab has developed an animal model to better understand the etiology of this type of corneal inflammation.
“We think bacterial or other microbial products stimulate corneal epithelial cells, which activate and release a series of cytokines,” said Dr. Pearlman. This leads to recruitment of highly inflammatory neutrophils, which release their granular content, causing corneal damage.
Reporting in the Journal of Leukocyte Biology, Drs. Pearlman and Kester found that the nanosized packages of C-6 ceramide reduced corneal haze and corneal thickening, in addition to inflammation. The nanoliposomal formulation allows for both relatively long-term stability and the use of ceramide in aqueous solutions, they said.
A family of sphingolipids, ceramides are known to stimulate differentiation, inhibit proliferation and induce apoptosis, such as with cancer cells, and is a major subject of Dr. Kester’s research. However, in this study, they did not cause apoptosis of corneal epithelial cells or inhibit wound-healing responses.
Further studies by both the Case Western and Penn State labs will identify key signaling pathways mediating these inflammatory responses. This should lead to a better understanding of the entire mechanism.
Dr. Pearlman predicts these ceramides will be used together with antibiotics in a combined anti-inflammatory, antimicrobial therapy. He and Dr. Kester also believe that short-chain ceramides in nanoscale-pegylated liposomes may have far-reaching potential, including with other eye diseases, such as diabetic retinopathy and macular degeneration.
1 Sun, Y. et al. J Leukoc Biol 2008;83:1511–1520.
DME Trial Shows Strength of Scientific Method
Laser trumps corticosteroids in the treatment of diabetic macular edema. This finding, from a randomized, clinical trial conducted by the Diabetic Retinopathy Clinical Research Network DRCR.net, shattered the pretrial presumption by some ophthalmologists that intravitreal triamcinolone may be better than focal/grid photocoagulation. In the long run, it is not.
Not only will this finding change current clinical practice (Michael S. Ip, MD, chairman of the protocol for this clinical trial, predicts that treatment with IVT alone for DME will be abandoned), but also it demonstrates that anecdotal evidence is no match for the strongest scientific evidence.
The trial, which pitted 1-mg and 4-mg doses of preservative-free IVT against focal/grid photocoagulation, reaffirmed the lesson of the 1985 Early Treatment Diabetic Retinopathy Study, that laser reduces the risk of vision loss from DME.
That earlier lesson didn’t stop doctors from using Kenalog (off label), following reports that began circulating in 2001.
“The improvement that we saw with this treatment modality, at least in the short term, was so above and beyond anything we’d seen, including laser treatment,” Dr. Ip said.
Doctors jumped on the bandwagon. By 2005, 91 percent of retina specialists reported using corticosteroids to treat DME1 despite pseudoendophthalmitis and the formation of cataracts and glaucoma in some patients, and despite a lack of long-term clinical data.
Now there are such data. The DRCR.net reported in Ophthalmology2 that 28 percent of the corticosteroid group experienced substantial vision loss compared with 19 percent in the laser group. Also, one-third of the laser group experienced substantial improvement in vision, a benefit never before attributed to laser.
But laser was the tortoise to steroid’s hare. At four months, mean visual acuity was better in the 4-mg triamcinolone group than in either the laser or 1-mg group. By one year, there were no significant differences among groups.
Then at 16 months, and through the primary outcome visit at two years, mean visual acuity was better in the laser group. “The effect of laser is slower than IVT, but is steady and more sustained,” Dr. Ip said. Laser also was associated with far fewer adverse effects.
Still, the study found that IVT is effective. The role that it might play, in combination with laser, is being studied in a current DRCR.net protocol.
In the meantime, this study should settle the pretrial division within the ophthalmologic community over the use of steroids to treat DME. “This is a definitive study,” Dr. Ip said. “A well-designed, randomized controlled trial, although never perfect, can overcome many of the shortcomings of small, initial studies to provide us with guidelines to follow for clinical practice.”
1 American Society of Retina Specialists, Preferences and Trends Survey, 2005.
2 Ophthalmology 2008;115:1447–1459.
Trypan Blue May Reduce Capsule Elasticity
For 20 years, continuous curvilinear capsulorhexis has been the gold standard to successfully open the anterior capsule in cataract surgery, providing protection against capsular tears, as well as low complication rates.
Dye-aided cataract surgery is very important for complex cases—patients with mature cataracts known to have diaphanous, fragile anterior and posterior capsules, patients with poor retroillumination from the fundus, or patients in whom clear viewing of the capsule is compromised, said H. Burkhard Dick, MD, clinical professor at the Center for Vision Science, Ruhr University Eye Hospital in Bochum, Germany.
Stains, including fluorescein or indocyanine green, are routinely used for these cases, but the more intense and persistent stain obtained from trypan blue is preferable, according to Prof. Dick. Yet there have been reports of impaired elasticity of the lens capsule after application of trypan blue, and decreased elasticity could increase the risk for capsular tears and result in a higher incidence of intraoperative complications, he said.
To assess the mechanical effects of trypan blue on the lens capsule, Prof. Dick and two colleagues designed a prospective study using 19 anterior capsules obtained from human eyes (with a mean age of 75.3 years) at the time of cataract surgery.1 Two identically sized strips of each capsule were prepared: One was exposed to trypan blue stain for 10 seconds, the other was a control. Sixty-three elasticity measurements were taken on all the specimens using a modified rheometer.
“In pediatric surgery, I routinely stain the capsule with trypan blue and have found that the capsule elasticity is tremendously decreased. This we know. But the studies that have already been done to clinically document biochemical changes in elasticity due to trypan blue have used animal or cadaver eyes. Not only did we use human eyes in an intraindividual comparison, avoiding the inherent differences between the capsule of one individual and another, our sophisticated technology allowed us to take immediate measurements, critical to accurate results,” said Prof. Dick.
While the German study confirms that trypan blue staining leads to significant reductions in elasticity and an increase in stiffness, the conclusions raise other pertinent issues.
“Our study confirms clinical changes in the capsule by dye, and this should be taken into consideration in dye-enhanced cataract surgery,” said Prof. Dick. But more work remains, he said.
“We want to know if the effect of staining to obtain a successful outcome may outweigh the difficulties of staining on these eyes. We have yet to find out whether we can decrease exposure time to the dye and by how much. We don’t know yet what the optimal time would be. And we must to look closely at the concentrations of the dye. Future studies will allow us to obtain a greater margin of safety, while still allowing adequate staining of the anterior capsule.”
1 J Cataract Refract Surg 2008;34:1367–1373.
EyeNet thanks Steven I. Rosenfeld, MD, FACS, Brian A. Francis, MD, Christopher J. Rapuano, MD, and Anne E. Fung, MD, for their help with this issue’s News in Review.