Photodynamic Therapy Targets Eyes

Robert C. Pini

Doctors at Massachusetts Eye and Ear Infirmary recently began preclinical trials of photodynamic therapy treatments for age-related macular degeneration and ocular melanoma. Patients are injected with a photosensitive chemical that accumulates in vascularized tissues of diseased and damaged areas. When laser light shines on those areas, it activates the chemical and destroys the target cells, sparing the surrounding healthy tissue.

Using a standard ophthalmic slit lamp, a physician identifies the treatment site for ocular melanoma, then demarcates it with a low-power laser diode aiming beam for delivery of femtosecond, multiphoton energy with a Microlase DPM-1000. Courtesy of Photogen Inc.

Dr. Lucy Young, an associate professor at Harvard Medical School who is conducting the trial for ocular melanoma, said the same type of photodynamic therapy is used successfully to treat some lung and esophageal cancers. But these new trials must prove the procedure can work in eyes without damaging vital vision structures such as the retina, optic nerve and macula.

To increase precision, Photogen Inc. in Knoxville, Tenn., developed treatment methods for the trials using a multiphoton approach and ultrafast lasers. Photogen CEO and President John Smolik explained that delivering high peak power in a femtosecond time frame stimulates the photodynamic chemistry and limits damage to adjacent tissue.

Photocoagulation, which involves longer pulses, is currently used for treating wet age-related macular degeneration, a condition in which tiny blood vessels behind the retina begin to leak, leading to severe vision loss. But the treatment can burn tissue adjacent to the damaged area. Eric Wachter, a senior scientist with Photogen, said the trials for the condition will use a variety of photosensitizers.

In January, QLT PhotoTherapeutics Inc. in Vancouver, British Columbia, Canada, announced preliminary, positive results from a Phase III study using the photosensitizer verteporfin to treat the wet form of macular degeneration.

In the trial for ocular melanoma, Wachter said, researchers will look for the effect of photodynamic therapy on a tumor and on me- tastasis away from the primary site. They will use either drugs or melanin precursors present in the cancer cells as possible photosensitizers. The researchers hope to improve on the current treatment of removing a cancerous eye.

The trials use lasers from the Coherent Laser Group in Santa Clara, Calif. A Vitesse Ti:sapphire at 800 nm activates the drug photosensitizers, and a Microlase DPM-1000 Nd:YLF at 1047 nm activates the melanin precursors. An ophthalmic slit lamp used to examine the eye is from Carl Zeiss in Oberkochen, Germany.

After preclinical trials in early summer, Photogen hopes to move into human clinical trials within a year.