While the cells seem to survive for months in mice, it’s not yet clear how long they will survive in humans, who live much longer lives, or whether they will affect the long-term function of the retina in other ways. For example, they might interfere with the interaction between the photoreceptors and the retinal pigment epithelium, says Reh.
Because the stem-cell treatment doesn’t replace lost cells, it most likely needs to be administered early in the course of a disease. “This would not be something for advanced macular degeneration, where the receptors are already damaged,” says Lund. “We would want to spot patients who are seriously as risk and hopefully slow or stop process of disease.” A number of genetic factors have been identified that boost risk for the disease, and genetic testing is available.
Taken together, retinitis pigmentosa and macular degeneration are the most common causes of blindness in people 40 and older. No treatments yet exist for the most common form of macular degeneration, which accounts for about 90 percent of cases. A number of novel therapies are under development, including drugs, cell transplants, and implanted devices. Advanced Cell Technology, based in Worcester, MA, uses human embryonic stem cells to grow retinal pigment epithelium, often the first cell type to die off in age-related macular degeneration and other eye diseases. The company filed for permission to begin clinical trials of the cells last November.