Among the multitude of age-related conditions we face is age-related macular degeneration (AMD). The risk of AMD increases directly with age, and AMD is more common in white Americans and others of European descent, affecting more than 14% of those older than 80 years in this population.
Certain precautions can help mitigate the progression of AMD. Because obesity can increase the risk of AMD progression, good nutrition (including leafy vegetables, fruits, and foods rich in omega-3 fatty acids), and weight control are suggested, along with nutritional supplementation.
Although these precautionary measures can help to decelerate AMD progression, often some form of therapy is ultimately required. This article describes therapeutic options in development that could pave the way for more effective management of dry AMD.
Cellular damage that accompanies aging often causes incomplete phagocytosis and breakdown of the outer segments of retinal pigment epithelium (RPE) cells. This can result in a buildup of the waste product lipofuscin in RPE cell granules. Lipofuscin is composed of membrane phospholipids, membrane proteins, visual cycle proteins, lipid-protein complexes, and retinoids. N-retinylidene-N-retinylethanolamine (A2E), one of the main chromophores residing in the lipofuscin granules, is responsible for lipofuscin-mediated phototoxicity.
Phospholipids of plasma membranes are rich in docosahexaenoic acid (DHA), a multisaturated omega-3 lipid acid. When eyes are exposed to sunlight in an environment rich in oxygen, DHA combines with phosphatidyletanolamine PE (PE-DHA) or phosphatidylcholine (PC-DHA) and by peroxidation splits into carbonic fragments, which, when combined with protein, make up carboxyethylpyrrole-protein adducts (CEPs). CEPs play a role in the inflammatory process associated with AMD.
Malondialdehyde (MDA) is another molecule involved in AMD progression. Complement factor H binds MDA and can block MDA-induced proinflammatory effects.
During the visual cycle, a reaction between all-trans-retinal and membrane phospholipids causes the release of A2E, the photosensitization of which leads to the production of reactive oxygen species (ROS). This excessive oxidative stress can result in RPE cell dysfunction and, ultimately, apoptotic cell death.
The development of drusen, or yellow amorphic deposits in the space in between RPE and Bruch membrane, signals AMD progression. Drusen may be nodular (hard and discrete) due to a focal thickening of the RPE basement membrane or may be limited in separation of the RPE basement membrane from the Bruch membrane.
The disease can then progress into geographic atrophy (GA), a devastating complication of dry AMD. Drusen-associated GA can result in severe irreversible central vision loss due to the formation of dense scotomas, resulting in deficits in reading and other visual tasks. The decline in vision may be minor in the early stages if the foveas are spared, but later with central involvement it becomes severe. Visual acuity as measured on eye charts does not correlate well with GA lesions or progression, prompting the development of alternative end points for clinical research to test therapeutic agents.
No treatment options for dry AMD are currently in the market, and patients are usually advised to follow the precautionary measures outlined above to help slow disease progression. Many companies and researchers are developing products to target dry AMD progression by exploiting a variety of modes of action. The table “Therapeutics Under Clinical Development for Dry AMD/GA” outlines small molecules, cell therapies, devices, and aptamers/peptides/antibodies in development for dry AMD treatment.
Intensive doses of statins, such as atorvastatin (Lipitor, Pfizer), rosuvastatin calcium (Crestor, AstraZeneca), and simvastatin (Zocor, Merck), were found to potentially clear the lipid debris that can lead to vision impairment in a subset of AMD patients studied in an early stage clinical trial. Additionally, controlling lipid cholesterol levels may be helpful in maintaining vision.
Visual cycle modulators such as emixustat HCl (Acucela) function by reducing the toxic accumulation of A2E during the visual cycle, potentially slowing the progression of AMD. This is brought about by targeting RPE65 isomerase, a key enzyme for the isomerization process of all-trans-retinol to 11-cis-retinol.
Inhibition of the complement system has been studied as a therapeutic approach, as local inflammation and complement activation are implicated in drusen formation.
A number of agents targeting the complement system are in development. Lampalizumab (Genentech) is a monoclonal antibody fragment targeting complement factor D.
GSK933776 (GlaxoSmithKline) is a humanized mouse IgG1 monoclonal antibody directed against the N-terminus of the A β peptide in drusen, with the aim of reducing the pool of toxic species in the retina. Its Fc region has been engineered to reduce FC-receptor binding and complement activation to weaken the antibody’s effector function and minimize the risk of side effects.
Restoration of choroidal blood flow is the proposed method of action of MC-1101 (MacuClear), using a combination of vasodilation, antiinflammation, and antioxidant properties; this could help to maintain the integrity of RPE cells and the Bruch membrane, thus preventing dry AMD from progressing to the wet form.
Some centers are exploring the repurposing of existing drugs for the treatment of AMD. Researchers at the University of Virginia are exploring whether the tetracycline antibiotic doxycycline, under development as Oracea, could help prevent photoreceptor cell loss and slow the progression of dry AMD.
Stem cell transplantation is another treatment modality being explored by numerous investigators. It is hoped that transplanted stem cells could help to regenerate the damaged RPE layer. Investigations under way include the following:
Several devices are also under development for the treatment of dry AMD. The photobiomodulation treatment protocol of the LT300 Light Delivery System (LumiThera) uses exposure to low levels of laser light or to light-emitting diodes emitting visible or near infrared light applied to the eye tissue to stimulate cellular processes, potentially leading to beneficial clinical effects.
Other devices exploit electrical stimulation in different ways. The Argus II (Second Sight Medical Products) bypasses defunct retinal cells and stimulates the remaining viable cells, inducing visual perception by converting images captured by a miniature video camera mounted on a patient’s glasses into a series of small electrical pulses. These signals are transmitted wirelessly to an array of electrodes implanted on the surface of the retina. The pulses are intended to stimulate the retina’s remaining cells, resulting in the perception of patterns of light in the brain. The patient then learns to interpret these visual patterns, thereby regaining some visual function. The system is controlled by software and is upgradeable, which may provide improved performance as new algorithms are developed and tested.
The Nova Oculus (The Eye Machine Canada) uses externally applied microcurrent electrical stimulation to administer precise amounts of electrical current. A battery-operated device delivers low-intensity electricity that increases blood flow to the surrounding tissue. As a result, the area being treated is better nourished, more oxygen gets to the tissue, and waste products are more efficiently removed.
The Rheohemapheresis device (University Hospital Hradec Kralove) is designed to improve microcirculation in order to halt the activity of dry AMD. The technique is based on constant flow separation of plasma through a cell separator and secondary filtration of plasma through a hollow-fiber membrane.
No currently available treatments can prevent vision loss in individuals with advanced AMD. Appropriate nutritional supplementation may delay or prevent the development of AMD. Numerous therapeutic options for dry AMD are under development.