Novel Treatment for Ocular Squamous Cell Carcinoma Looks Promising
Although typically slow to spread, squamous cell carcinomas (SCCs) aggressively invade local tissue, causing tissue damage and dysfunction. SCC remains the most common neoplasia (tumor) of the equine eye, occurring in, on, or around the eye, and is the second most common tumor in horses.

Depending on the location and how invasive the tumor is at the time of diagnosis, treatment can be challenging. In addition to surgical excision, which may not be feasible, other treatment options include cryotherapy, topical/injectable chemotherapy, and photodynamic and radiation therapy.

At the 2019 annual convention of the American Association of Equine Practitioners (AAEP), held Dec. 7-11 in Denver, Kathryn Wotman, DVM, Dipl. ACVIM (Large Animal), Dipl. ACVO, assistant professor of comparative ophthalmology at Colorado State University’s College of Veterinary Medicine and Biomedical Sciences, described an option from human medicine that was recently applied to horses.

“Topical and intra-tumoral immunotherapy using interferon alpha-2b was recently evaluated as an alternative therapy in people with ocular SCC,” Wotman said.

Interferon is a proinflammatory cytokine that, after injection, stimulates the body’s immune system to destroy the cancer cells and promote programed cell death (apoptosis) of the abnormal tumor tissue.

“Interferon alpha-2b has not been studied specifically for the treatment of SCC in horses,” she said. “However, recombinant human interferon alpha-2b has been used safely in horses for treatment of other disease processes.”

Wotman and colleagues therefore explored the use of interferon alpha-2b in nine equine eyes with confirmed eyelid SCC. Each eye was treated with 10 million units interferon alpha-2b injected around the tumor every two weeks for a total of four to six treatments. The team followed the horses for up to one year after injection.

One horse was removed from the study due to owner concerns regarding disease progression, and one was lost to follow-up.

“For the remaining eyes, results showed either a reduction in tumor size or complete regression,” said Wotman.

For example, one tumor was originally 30.5 mm², and it regressed completely following the third injection. A second tumor measured 129 mm2 and was reduced by 74%. A third tumor measuring 264 mm² achieved 100% tumor regression, and a fourth tumor measuring 17 mm² had a 32% reduction following treatment.

Three of these tumors reduced enough in size following interferon treatments that veterinarians could perform surgical excision to achieve tumor-free margins.

“The results of this pilot study revealed that perilesional injection of interferon alpha-2b effectively reduced tumor volume and had minimal side effects such as local swelling,” Wotman summarized.