By Christopher Wanjek, Columnist
Published: 10/15/2013 07:06 AM EDT on LiveScience
A drop a day might soon keep blindness away. Researchers say they have found a possible treatment for age-related macular degeneration (AMD) -- the leading cause of blindness among the elderly -- that could be delivered via eye drops.
There currently is no cure for AMD, nor is there a treatment for its most common form, the so-called dry AMD, which affects 90 percent of AMD suffers. The new research, which was conducted in animals, could lead to treatment for people with AMD in the future, the researchers said.
The findings were published Wednesday (Oct. 9) in the journal PLoS One.
There are two forms of AMD: a "dry" early-stage form characterized by a slow and progressive blurring of central vision, and a "wet" advanced-stage form characterized by further vision loss and the development of blood vessels in the back of the eye that can leak and damage surrounding tissues.
Nearly 2 million Americans ages 40 years and older have poor vision caused by AMD, according to the Centers for Disease Control and Prevention. Worldwide, as many as a third of all people over age 65 have at least some early form of AMD, according to a study published in 2012 in the journal Lancet. Almost all cases of wet AMD develop from dry AMD. [9 Healthy Habits You Can Do in 1 Minute (Or Less)]
Certain antioxidant dietary supplements, such as lutein, initially showed promise in treating AMD, but several large studies found no support for this. So, people with dry AMD can only wait and hope the disease doesn't progress into debilitating vision loss.
Wet AMD is treated with repeated monthly or bimonthly injections, in the eye, of medication designed to inhibit the formation of new blood vessels, such as the cancer drug bevacizumab (known by its brand name Avastin).
In the new findings, the researchers at Tufts University in Massachusetts led by associate professor of ophthalmology Rajendra Kumar-Singh describe their work as a "proof of concept" study. They demonstrated, in mice, that a chemical called PPADS (short for pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid) repairs AMD-induced damage to the eye.
Previous research has shown that AMD is caused in part by high levels of the membrane attack complex (MAC), which is a part of a normal, healthy immune system. The MAC typically forms on the surface of invading bacteria, poking holes through them and destroying them. In people with AMD, however, for reasons not entirely clear, the MAC also targets cells in the retina, killing them and causing a loss of vision.
In the new study, the researchers experimented with PPADS because it is thought to interfere with both MAC formation and new blood vessel growth.
Working with anesthetized mice, the researchers induced tissue damage and blood vessel growth characteristic of AMD. They then applied PPADS daily and, essentially, watched the drug heal the eye damage.
Kumar-Singh told LiveScience that the eye drops that ultimately could be used on people likely wouldn't use PPADS, but rather a more refined drug.
This research is the first demonstration that a drug can slow the features of dry and wet AMD by topical application -- that is, something that could be self-administered as eye drops.
"An ideal therapy would be one that can be self-administered daily by patients," so that they can avoid uncomfortable injections, Kumar-Singh said.
Robert Mullins, an AMD expert and associate professor of ophthalmology and visual sciences at the University of Iowa, Iowa City, who was not part of the new research, said he was intrigued by the study.
"There is very strong support for the idea that MAC contributes to AMD, and that attenuating MAC could be helpful," Mullins said.
However, he said that whether MAC is involved in AMD "is still an area of intense study." If MAC injury is the source of the blood vessel degeneration seen in wet AMD, then local "small-molecule inhibition" as demonstrated with PPADS "holds exciting possibilities," he added.
Christopher Wanjek is the author of a new novel, "Hey, Einstein!", a comical nature-versus-nurture tale about raising clones of Albert Einstein in less-than-ideal settings. His column, Bad Medicine, appears regularly on LiveScience.