While this reaction is vital for sight, researchers believe that slowing the cycle in the subset of photoreceptors responsible for night vision, known as rods, could slow damage without having a large impact on daytime vision. (Preliminary results suggest it can affect dark-adaptation--when our eyes adjust to low-light conditions.) "During the daytime, the rods are spinning like crazy, wasting vitamin A for no good use," says Ryo Kubota, an ophthalmologist and founder of Acucela, a Seattle-based startup that is developing treatments for macular degeneration. "It's like a CCD camera pointed at the sun."

One compound developed by Acucela that is in clinical trials inhibits the enzyme that converts the photopigment in photoreceptors from one form to another. This process happens only in the eye, allowing the drug to be administered systemically without affecting other tissue, says Kubota. The company has finished initial safety testing in humans and plans to begin a clinical trial assessing the compound's effectiveness in patients with late-stage dry macular degeneration in a few weeks. Kubota also aims to test the compound in diabetic retinopathy and Stargardt disease, a rare, genetically inherited form of macular degeneration.

A second drug that acts by a slightly different mechanism is being evaluated for macular degeneration by Sirion Therapeutics, a Florida-based pharmaceutical company. The compound is a synthetic vitamin A derivative that is thought to reduce toxin buildup by binding to one of the proteins involved in the reaction. According to preliminary results from tests of the drug in patients with late-stage dry macular degeneration, it can slow the scarring that is characteristic of the disease by 45 percent. However, scientists won't know if the results are statistically significant until completion of the study next year. Because no treatments have been approved for dry AMD, the U.S. Food and Drug Administration has fast-tracked the drug, speeding the review process.