Gene Therapy for Eye Diseases

The pharmaceutical giant Genzyme has started a clinical trial to see whether a drug to treat macular generation could be delivered via long-lasting gene therapy rather than monthly injections.



Eye colors: Drusen, the yellow flecks in this image of the retina, are common in people with age-related macular degeneration. These flecks are made up of proteins involved in the part of the immune system called the complement system, which has also been implicated in the disease by genetic studies.


A drug called Lucentis, made by Genetech, has proved effective at treating the wet form of age-related macular degeneration, which can lead to blindness. Some 200,000 Americans a year are diagnosed with the disease. But Lucentis has to be injected into the eye every month or two, a burden for patients and doctors.
Lucentis binds to and neutralizes a wound-healing growth factor known as VEGF. This binding action stalls the excess growth of blood vessels in the eye that characterizes age-related macular degeneration. Genzyme's gene therapy drug, officially called AAV2-sFLT01, would insinuate itself into the patient's retinal cell to produce the same VEGF-binding protein as Lucentis over far longer periods--up to several years.


A phase 1 clinical trial of Genzyme's gene therapy treatment began at the end of May. Three patients received the treatment, according to Sam Wadsworth, a Genzyme group vice president in charge of gene and cell therapy. Preliminary results should be available in about a year.


The trial is one of a handful worldwide seeking to prove the effectiveness of gene therapy for eye diseases. The Genzyme trial also involves using new type of virus as the delivery mechanism. Early results of a federally funded trial to deliver normal-functioning genes to patients with a rare retinal disease known as type 2 leber congenital amaurosis, or LCA, have confirmed that this "viral vector" has merit for eye treatments, several researchers say.


The LCA trials "demonstrated success both in terms of safety and ability to introduce the gene and have efficacy and success," said Jeffrey S. Heier, an assistant professor at Tufts University School of Medicine and director of retinal research at Ophthalmic Consultants of Boston, a private practice group, who is involved in the Genzyme research. "This study is taking the virus vector that they used, and [Genzyme has] taken what has really been the success of the anti-VEGF story and they've packaged the two together."


Eyes have been an early target for gene therapy because they are small--meaning they require relatively little active dose, they are self-contained, and because the tools of eye surgery have advanced enough to make the treatments possible. The drug has to be delivered to the retina, a thin film lining the inner wall of the eye. Instrumentation has improved in recent years to allow injections through the retina without piercing it, said Shalesh Kaushal, chairman of ophthalmology at University of Massachusetts Memorial Medical Center and UMass Medical School.


To Kaushal, who is involved in the Genzyme study as well as the LCA research, the big challenge will be broadening the use of gene therapy to dozens more diseases, and using that understanding to eventually reach beyond the eye. "If one could understand those fundamental cellular, biochemical events and identify targets, you might have the chance to treat many diseases with a single gene-therapy construct," Kaushal said.


Earlier gene therapy programs used a type of virus called adenovirus to target genes, but both the LCA and Genzyme trials are using adeno-associated virus, which is far less inflammatory and which expresses itself over longer periods than adenovirus, therefore making the treatment last longer, Wadsworth said. Viruses are used to deliver gene therapies because they are adept at getting through cell walls.


VEGF is involved in vascular cell growth throughout the body, and its expression increases in the presence of a wound. Studies have shown that with Lucentis, virtually all the VEGF-binding protein stays within the eye, and does not significantly affect VEGF levels elsewhere in the body, Wadsworth says. Genzyme's drug will provide even lower levels of the VEGF-binding protein, so it's expected that the drug will not have any adverse affects throughout the body, he said.


The trick will be getting the cells to produce enough VEGF-binding protein to help patients, said Peter Campochiaro, a professor at the Wilmer Eye Institute at Johns Hopkins Medicine, who is involved in the research. In addition to establishing safety, the current phase 1 trial will explore four different doses of the study