Human embryonic stem cells will be tested as a treatment for blindness, a US company announced in the second such clinical trial to examine how the controversial process works in people.
Just 12 adult patients will take part in the trial to see how the treatment using retinal cells derived from human embryonic stem cells affects patients with a common form of vision loss that takes hold in children as young as six.
The process has been tested on rats and mice and has been found to halt the progressive disease without causing tumors or other side effects, said chief scientific officer Bob Lanza at the biotech company Advanced Cell Technology.
"These cells have been really performing quite spectacularly in the animals. If we can see that in the human patients we will hit a home run here," said Lanza on Monday.
The trial, run and funded by the California-based company ACT, was allowed to go ahead after the US Food and Drug Administration cleared its application to start a phase I study.
Another company, Geron Corp, began a similar trial in October, the first of its kind to test human embryonic stem cells in patients with spinal cord injuries.
Scientists say the use of embryonic stem cells as a treatment for cancer and other diseases holds great promise, but the process has drawn fire from religious conservatives and others who oppose it.
Embryonic stem cell research is controversial because human embryos are destroyed in order to obtain the cells capable of developing into almost every tissue of the body.
Using retinal cells derived from human embryonic stem cells, the latest treatment involves injecting the stem cells into the eyes of patients with Stargardt's Macular Dystrophy, a disease which affects around 25,000 people in the United States.
The 12 people in the initial study will have an advanced form of Stargardt's disease and do not expect to have their vision restored, since the test's main objective is to gauge safety.
If ACT's trial shows that the treatment is safe, the process would be eventually be tested on younger patients to see if it can prevent blindness.
The company has also applied for FDA permission to begin a wider trial on people with age-related macular degeneration, or vision loss, which affects as many as 30 million people in the United States and Europe.
Stargardt's disease causes blindness by destroying the pigmented layer of the retina, called the retinal pigment epithelium (RPE). After that follows degradation of photoreceptors, which are cells in the retina that detect light.
Patients often experience blurry vision, difficulty seeing in low-light and eventually most lose their ability to see at all. The disease can be inherited by a child when two parents carry the gene mutation that causes it.
The treatment process being tested by ACT worked in animals by creating an abundance of new RPE cells, Lanza explained.
"Using stem cells, we can generate a virtually unlimited supply of healthy RPE cells, which are the first cells to die off in (Stargardt's) and other forms of macular degeneration," Lanza said.
The process showed 100 percent improvement in rats' visual performance and "near-normal function" was also achieved in mice, both without negative side effects, he said.
Researchers expect to see results in real-time due to the use of high resolution instruments that allow them to track the cells in the eye, with an assessment of the progress possible within weeks.
"We should be able to know what's going on along the way," said Lanza. "It is quite conceivable we could see improvement within six weeks or so."
"But we don't know. This has never been done to know how this is going to play out."