A New Weapon in the Fight Against EQUINE MELANOMA
By Patti Schofler
Sport horses of all breeds and genders, particularly those with dark skin and grey coats, are potential targets of the deadly form of skin cancer, melanoma. Just as immunotherapy has demonstrated promise in the treat- ment of human cancers, hope has arisen as to its effective- ness with equine cancer, particularly melanoma, and in January the possibility of an effective melanoma vaccine coming on the market came one step closer to reality.
control rate (a metric for the percentage of subjects responding positively to the vaccine) was 100 percent without any side effects. Morphogenesis, Inc., the creator of the vaccine, has submitted an analysis of more than 15,000 data points from the trial to the U.S. Department of Agriculture, a major step toward receiving a commer- cial license to produce and sell the vaccine.
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MELANOMA BASICS Melanomas are skin lesions that occur in dark-skinned horses (usually grey or black) over five years of age of any sex or breed. Up to 80 percent of all cases have grey coats. One study claims that less than six percent of grey horses over sixteen are free from melanomas. The lesions are produced from melanocytes, the cells that produce pigment in the skin and hair. Their occurrence is genetic, not caused by sun or UV light. They can be slow grow- ing and benign, or particularly aggressive and malignant with a fast growth rate. Generally, horses with melanomas have a long time
before the cancer becomes life threatening. The tumors are usually found on the surface under the tail, around or in the eye, on the external genitalia, at the ear base, on the lips and in the guttural pouch. They can take the form of singular nodules or coalescing masses, resembling heads of cauliflower. Melanomas can be locally invasive and metastatic and can vary from superficial lesions on the skin to obstructive internal gastrointestinal lesions causing weight loss, constipation and colic to neurologic involvement resulting in lameness and lack of muscle control. In fact, the continual development of individual tumors makes melanoma a moving target.
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n the clinical trial of the vaccine IFx-VetDirect, of the 22 horses who received the treatment, 73 percent showed tumor regression and the overall disease
Dr. Kris Hennessy, responsible for conducting the clinical trial of 22 horses for the vaccine developed by Morphogenesis, Inc.
TreaTmenT OpTiOns – Old and new Treatment has taken the form of surgery by laser or cauterization, neither of which guarantees complete removal. Cimetidine, an antihistaminic drug, has been used to at best reduce tumors by 50 percent. Radiation may be useful for very small tumors. A canine melanoma vaccine called Oncept has
attracted the interest of many in the equine community. This vaccine targets tyrosinases, an enzyme expressed by the tumor. If the enzyme is killed, tumor growth is slowed or reversed. The goal of this study conducted at Lincoln Memorial University in Tennessee is to demon- strate whether this vaccine currently used to treat mela- nomas in dogs is also safe and effective for treating them in horses. German researchers are studying a treatment with betulinic acid, which comes from the bark of white birch trees and is used for treating human melanomas. It attacks cancer cells by breaking down the membranes of the mitochondria which produces energy in the cell; if it malfunctions, the cell dies. “The options for treating this disease are certainly
few. Despite the ability to remove subcutaneous mela- nomas, it is likely they will recur and spread to internal organs,” explains Dr. Patricia Lawman, co-founder and CEO of Morphogenesis. “We’ve seen some horrific cases. One horse had 26 lesions. And sometimes you can’t even measure them because they’ve coalesced into these huge masses. Or an internal lesion may grow large enough to block a horse’s defecation, causing him to stop eating.”
Photos courtesy of Dr. Kris Hennessy
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