There are a variety of chronic intraocular disorders, which, if untreated, may lead to partial or even complete vision loss. One prominent chronic intraocular disorder is age-related macular degeneration, which is the leading cause of blindness amongst elderly Americans affecting a third of patients aged 75 years and older (Fine et al. (2000) NEW ENGL. J. MED. 342: 483-492). There are two forms of age-related macular degeneration, a dry form and a wet (also known as a neovascular) form.
The dry form involves a gradual degeneration of a specialized tissue beneath the retina, called the retinal pigment epithelium, accompanied by the loss of the overlying photoreceptor cells. These changes result in a gradual loss of vision. The wet form is characterized by the growth of new blood vessels beneath the retina which can bleed and leak fluid, resulting in a rapid, severe and irreversible loss of central vision in the majority cases. This loss of central vision adversely affects one's everyday life by impairing the ability to read, drive and recognize faces. In some cases, the macular degeneration progresses from the dry form to the wet form, and there are at least 200,000 newly diagnosed cases a year of the wet form (Hawkins et al. (1999) MOL. VISION 5: 26-29). The wet form accounts for approximately 90% of the severe vision loss associated with age-related macular degeneration.
At this time, current diagnostic methods cannot accurately predict the risk of age-related macular degeneration for an individual. Unfortunately, the degeneration of the retina has already begun by the time age-related macular degeneration is diagnosed in the clinic. Further, most current treatments are limited in their applicability, and are unable to prevent or reverse the loss of vision especially in the case of the wet type, the more severe form of the disease (Miller et al. (1999) ARCH. OPHTHALMOL. 117(9): 1161-1173).
Currently, the treatment of the dry form of age-related macular degeneration includes administration of antioxidant vitamins and/or zinc. Treatment of the wet form of age-related macular degeneration, however, has proved to be more difficult.
Several methods have been approved in the United States of America for treating the wet form of age-related macular degeneration. Two are laser based approaches, and include laser photocoagulation and photodynamic therapy using a benzoporphyrin derivative photosensitizer known as Visudyne. Two require the administration of therapeutic molecules that bind and inactivate or reduce the activity of Vascular Endothelial Growth Factor (VEGF), one is known as Lucentis (ranibizumab), which is a humanized anti-VEGF antibody fragment, and the other is known as Macugen (pegaptanib sodium injection), which is an anti-VEGF aptamer.
During laser photocoagulation, thermal laser light is used to heat and photocoagulate the neovasculature of the choroid. A problem associated with this approach is that the laser light must pass through the photoreceptor cells of the retina in order to photocoagulate the blood vessels in the underlying choroid. As a result, this treatment destroys the photoreceptor cells of the retina creating blind spots with associated vision loss.
During photodynamic therapy, a benzoporphyrin derivative photosensitizer known as Visudyne and available from QLT, Inc. (Vancouver, Canada) is administered to the individual to be treated. Once the photosensitizer accumulates in the choroidal neovasculature, non-thermal light from a laser is applied to the region to be treated, which activates the photosensitizer in that region. The activated photosensitizer generates free radicals that damage the vasculature in the vicinity of the photosensitizer (see, U.S. Pat. Nos. 5,798,349 and 6,225,303). This approach is more selective than laser photocoagulation and is less likely to result in blind spots. Under certain circumstances, this treatment has been found to restore vision in patients afflicted with the disorder (see, U.S. Pat. Nos. 5,756,541 and 5,910,510).
Lucentis, which is available from Genentech, Inc., CA, is a humanized therapeutic antibody that binds and inhibits or reduces the activity of VEGF, a protein believed to play a role in angiogenesis. Pegaptanib sodium, which is available from OSI Pharmaceuticals, Inc., NY, is a pegylated aptamer that targets VEGF165, the isoform believed to be responsible for primary pathological ocular neovascularization.
Despite these efforts, there is still an ongoing need for methods of identifying individuals at risk of developing age-related macular degeneration so that such individuals can be monitored more closely and then treated to slow, stop or reverse the onset of age-related macular degeneration. In addition, there is still an ongoing need for new methods of preventing the onset of age-related macular degeneration, and, once established, the treatment of age-related macular degeneration.
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