Melanomas are among the most common skin tumors in horses. They are particularly common in gray horses because the gene responsible for the gray coat color is also responsible for melanomas. In fact, less than 6% of gray horses over the age of 16 are free from melanomas, which occur most frequently around the tail, anus, eyes, and lips.
Because current treatment strategies are limited and many are not curative, a small handful of researchers in this field have explored the potential for developing vaccines that can treat affected horses. One example is a vaccine directed against “tyrosinase,” a molecule present on melanoma cells.
Jeffrey Phillips, DVM, MS, PhD, Dipl. ACVIM, assistant professor of veterinary medicine at Lincoln Memorial University’s College of Veterinary and Comparative Medicine, said one melanoma vaccine (trade name Oncept) has had a good response in their clinical trial. Specifically, the researchers inject 15 horses “with a series of four biweekly injections along with a six-month booster,” Phillips and colleagues wrote in an abstract summarizing their work, supported by the Morris Animal Foundation. In that study Phillips found that the tyrosinase vaccine “appears to be safe and well-tolerated in tumor-bearing horses and appears to result in both clinical activity and a measureable immune response in treated patients.”
More recently, Lawman and colleagues have begun testing a new vaccine (trade name ImmuneFX) that “uses a bacterial gene that primes and educates the immune system to attack tumor cells throughout the body without damaging healthy cells or tissues.”
They have tested the technology in three horses with melanomas so far, and Lawman said the results have been positive: “In one horse, within six months of vaccine initiation the tumor burden was reduced by 40% for lesions that were injected using a needleless injector. The tumor burden was reduced by 48% for the lesions that were not directly injected in the same horse.”
Kris Hennessy, PhD, DVM, of Hennessy Research, in Shawnee Mission, Kansas, will conduct the ImmuneFX field study. Participating horses will receive six doses of the vaccine, and the study is anticipated to last about one year.
A successful study outcome will allow Morphogenesis to seek a commercial license from the USDA to sell the vaccine for treatment of equine melanoma.