Melanomas in Horses: New Topical Treatment Shows Promise

With their motley, ever-changing shades and degrees of dappling, gray coats attract much attention to the horses that sport them, yet one curse of the color is the likelihood for melanomas. While many melanomas in horses are benign, some tumors advance to malignancy, occasionally ending in widespread metastasis that includes the lymph nodes, liver, spleen, and lungs.^1,2

Researchers recently identified a plant-based substance that might help battle malignant melanomas in horses.³

In horses, melanomas are typically firm, black, and raised, and appear externally under the tail, around genitalia, and on the head, though they may erupt anywhere on the body. Internally, the tumors can invade organ systems and potentially trigger significant health problems, such as ataxia and colic. Ulcerated melanomas may bleed and cause hygiene difficulties, especially during fly season.

Treatment of melanoma is often not pursued because of the usually benign, slow-growing nature of the tumors, though all melanomas should be considered as potential neoplasms. Therapies for melanoma reduction are either ineffective or difficult to use. Surgical excision of tumors has been performed successfully, though location precludes removal at times. Chemotherapeutic drugs, like intralesional cisplatin, have been used but exposure to a toxic drug may make some caretakers reluctant to use it. Cimetidine slows growth in some melanomas, as reported anecdotally.

The substance featured in this recent study, betulinic acid, occurs in many plants, including white-barked birch trees. As a biological agent, betulinic acid has antiparasitic, antiretroviral, and anti-inflammatory effects. Human in vitro studies have revealed benefits of betulinic acid on various forms of cancer and specifically as a selective inhibitor of melanoma.⁴

In this in vitro study, researchers evaluated the potential of betulinic acid as a topical therapy for malignant melanomas in horses. In the end, the researchers found betulinic acid showed antiproliferative effects on equine melanoma cells, with high concentrations of the compound reaching the required skin layers, leading them to believe that betulinic acid is “a promising substance for the topical melanoma treatment.”

This trial gives researchers more confidence in the use of betulinic acid in further preclinical and potentially clinical trials in horses diagnosed with cancerous melanoma.

¹Sundburg, J.P., T. Burnstein, E.H. Page, W.W.R.F. Kirkham. 1997. Neoplasms of Equidae. Journal of the American Veterinary Medical Association 170:150-152.

 ²MacGillivray, K.C., R.W. Sweeney, and F. Del Piero. 2002. Metastatic melanomas in horses. Journal of Veterinary Internal Medicine 16:452-456.

³Weber, L.A., J. Meibner, J. Delarocque, J. Kalbitz, K. Feige, M. Kietzmann, A. Michaelis, R. Paschke, J. Michael, B. Pratscher, and J-M.V. Cavalleri. 2020. Betulinic acid shows anticancer activity against equine melanoma cells and permeates isolate equine skin in vitro. BMC Veterinary Research 16:44.

 ⁴Pisha, E., H. Chai, I.S. Lee, T.E. Chagwedera, N.R. Farnsworth, G.A. Cordell, C.W. Beecher, H.H. Fong, A.D. Kinghorn, D.M. Brown, et al. 1995. Discovery of betulinic acid as a selective inhibitor of human melanoma that functions by induction of apoptosis. Nature Medicine 1:1046–1051.