An experimental therapy using human embryonic stem cells to treat degenerative eye diseases has proved safe and effective in animal studies, and may begin early human trials in the next few months if it receives approval from the Food and Drug Administration. If granted approval, the therapy will be the second embryonic-stem-cell-based treatment to progress to human trials, and it will provide a test case for further applications of stem cells.

While scientists have made huge advances using stem cells to treat diseases in animal models, testing these experimental therapies in humans poses some unique challenges. One is proving that the cells are safe: embryonic stem cells, which can develop into any tissue type in the body, carry the risk of forming tumors. Another challenge is the threat of immune rejection of the transplanted cells; in most cases, introducing foreign cells would require a patient to take powerful drugs for life to suppress the immune system, as is the case with organ transplants. For that reason, the first stem-cell therapies have focused on the eye and nervous system, so-called immune-privileged sites that do not experience this response to foreign cells. Geron, a biotech company based in Menlo Park, CA, received FDA approval in January for a trial to treat patients with acute spinal-cord injuries with cells derived from embryonic stem cells.

This latest treatment for eye disease, developed by Advanced Cell Technology (ACT), based in Worcester, MA, uses human embryonic stem cells to re-create a type of cell in the retina that supports the photoreceptors needed for vision. These cells, called retinal pigment epithelium (RPE), are often the first to die off in age-related macular degeneration and other eye diseases, which in turn leads to loss of vision. Several years ago, scientists found that human embryonic stem cells could be a source of RPE cells, and subsequent studies found that these cells could restore vision in mouse models of macular degeneration.

In a recent study published online in the journal Stem Cells, researchers from ACT and Oregon Health Sciences University show that their stem-cell therapy provides a long-term benefit in animal models of vision loss. A second experiment tested the long-term safety of the cells in mice--an important requirement for moving into human testing--and found no evidence that the cells cause tumors.

To test the efficacy of the cell transplants, the researchers injected RPE cells derived from embryonic-stem-cell lines into the eyes of rats with a genetic defect in their RPE that causes their vision to gradually deteriorate. After three months, the retinas of treated rats had many more photoreceptors than those of untreated diseased rats, and the treated animals performed better in vision tests; however, their performance in the tests diminished with time. The transplants were also able to improve vision in a mouse model of Stargardt's disease, a rare but untreatable illness that causes blindness early in life.