The diameter of the human eye is only about an inch, but for people with retinal degenerations, it can seem like a million miles.
That’s because the human eye isn’t amenable to absorbing substances that might save or restore vision. As Dr. Alan Weiner, president of DrugDel Consulting, pointed out during the Friday afternoon VISIONS session on retinal drug delivery, evolution has led to the development of an eye that protects itself well by washing things (i.e., eye drops) away from its surface. Tears can remove as much as 95 percent of an eye drop.
Also, the vitreous, the jelly-like substance that fills the interior of the eye, serves as a formidable barrier to getting sustained delivery of therapies from the front of the eye to the retina. Furthermore, Bruch’s membrane is a layer of tissue that can make it difficult to deliver treatments from the other direction, underneath the retina. This means that a treatment taken orally is often less than ideal, because so little of it makes its way to the retina.
However, as Dr. Weiner noted, there are emerging tiny implantable devices, such as those being developed by pSivida, which can provide sustained, long-term release of drugs to the retina. The company is currently conducting clinical trials of its products for the treatment of dry and wet age-related macular degeneration (AMD) and diabetic macular edema. The company believes the implants, the size of a rice grain, can deliver therapeutic substances for up to three years.
Human engineered viruses, which are designed to deliver corrective or protective genes to cells in the retina, have also performed well thus far in gene therapy clinical trials for Leber congenital amaurosis, and have recently moved into human studies for a number of types of retinal degenerations, including Stargardt disease, Usher syndrome and choroideremia. Researchers believe a single treatment, injected underneath the retina, will last several years or even a lifetime.
During the session, Dr. James McGinnis, of Oklahoma University Health Sciences Center, reported on his promising work with cerium oxide nanoparticles called nanoceria — tiny manmade particles less than 1/12,000th of an inch that devour the harmful byproducts common to virtually all retinal degenerations. The nanoparticles work like Pacmen, gobbling up the vision-robbing byproducts.
Dr. McGinnis has demonstrated this technology successfully in several rodent models of retinal disease, including retinitis pigmentosa, Usher syndrome and wet AMD. While he has additional lab research to perform, he is focused on moving nanoceria into a clinical trial.
Nanoparticles are also showing promise for delivering large disease-causing genes, which exceed the cargo capacity of viral systems, to the retina. Dr. Muna Naash, also of Oklahoma University, has made impressive progress in developing nanoparticle-based gene delivery systems.
The Foundation funds both Drs. Naash and McGinnis, and we are continually on the lookout for innovative and effective ways to get vision-saving treatments to the back of the eye. A treatment doesn’t do much good if we can’t get it to where it is needed.
– Dr. Steve Rose
Pictured above: Dr. James McGinnis