We’ve been talking a lot about vaccines lately, but only about those vaccines that are used to prevent disease. Today’s topic is a little different — a vaccine that treats a preexisting disease: oral malignant melanomas in dogs.
Oral melanomas are nasty. Because they are hidden within the dog’s mouth, they tend not be noticed and diagnosed until the tumor is large, at least relative to its location. Melanomas originate from the soft tissues of the mouth (gums, tongue, etc.) but grow rapidly and can invade underlying bone. Oral melanomas also metastasize quickly. One study found that 80% of dogs diagnosed with oral melanoma had metastatic tumors in their regional lymph nodes and/or lungs.
These characteristics have made treating oral melanomas in dogs very difficult. In some cases, radical surgeries (e.g., removal of part of the jaw) are necessary to control the disease locally, but at the same time the chances are very high that the cancer has already spread to other parts of the body. Chemotherapy can help, but in general, it is not all that effective for this type of cancer.
A few years ago, a canine oral melanoma vaccine hit the market. It’s called a vaccine (or more properly, immunotherapy) because it works by stimulating an immune response against a disease. But unlike traditional, preventative vaccines, it is given to animals who are already suffering from the disease in question.
The vaccine contains DNA containing the gene that codes for human tyrosinase, a protein that is produced in higher than normal amounts by melanoma cells. After being injected with the vaccine (through the use of a transdermal device), the dog’s muscle cells in the area take up this DNA and then start to produce the human tyrosinase protein. According to the vaccine’s label: “The human tyrosinase protein is different enough from the canine tyrosinase protein that it will stimulate an immune response, yet similar enough to the canine tyrosinase that the immune response is effective against canine melanoma cells which express tyrosinase.”
Dogs initially receive the vaccine every two weeks for a total of four doses and then need a booster every six months. It should be noted that the vaccine is labeled for use after local disease has been controlled to the greatest extent possible (through surgery and/or radiation) and metastasis is not evident or has been dealt with (e.g., affected lymph nodes were surgically removed). Veterinarians have tried the vaccine when these parameters have not been met and some anecdotal reports have been positive, but owners should not expect the vaccine to be recommended or to be especially effective under these conditions.
This product is so new that giving owners an idea of what to expect regarding improvements in survival times is difficult. It was released under a conditional USDA Veterinary Biological Product License, which means the USDA was convinced that it was safe and had a “reasonable expectation of efficacy based on initial trials.” The vaccine’s manufacturer reports that “During this period of conditional license, additional research will be conducted to further support the safety and efficacy of the vaccine.” So far, studies have shown somewhat mixed results. Veterinary oncologists report lots of success stories (dogs living for longer than expected) as well as dogs that died before their condition had a chance to improve as a result of the vaccine.
With time, this new treatment option will hopefully prove to be a significant advance in the treatment of oral melanoma in dogs and potentially pave the way for similar therapies in people.
Dr. Jennifer Coates