There are typically two main avenues of chemotherapy for treating mast cell tumors in dogs: the more “traditional” chemotherapy drugs (e.g., CCNU, vinblastine, prednisone), and the newer class of drugs called tyrosine kinase inhibitors (Palladia and Kinavet).
Traditional chemotherapy drugs work by causing damage to DNA within cells, without regard to whether the cell is a tumor cell or a healthy cell. This is the reason for some of the side effects seen with chemotherapy, including adverse gastrointestinal signs and lowered white blood cell counts.
The mechanism of action of tyrosine kinase inhibitors (TKIs) is very different. These drugs work primarily by inhibiting the action of a receptor on the surface of mast cells that is mutated in about 20-30 percent of tumors. When the receptor is mutated, it causes uncontrolled cell division, leading to tumor growth.
TKIs can also work by inhibiting the growth of blood vessels to tumor cells (this is called anti-angiogenesis therapy). This mechanism of action is separate from the previously mentioned mechanism, which means tumors without the specific receptor mutation may still have a good response to treatment.
TKIs are orally administered medications given continually at home. Dogs need to have “steady state” levels of these drugs in their blood stream to keep the receptor continuously turned off. The receptor is present on other cells in the body, so side effects can occur with TKIs as well, but are generally fairly limited in their spectrum.
The take home messages for canine mast cell tumors are:
- They are very unpredictable in their behavior.
- The biggest predictor of behavior is the grade of the tumor, which can ONLY be determined via biopsy.
- Staging tests are important to look for spread of disease and should include labwork, regional lymph node aspirates, an abdominal ultrasound, and in some cases, a bone marrow aspirate.
- Surgery is the mainstay of treatment for most dogs.
- Radiation therapy and chemotherapy play roles for dogs with mast cell tumors — consult a veterinary oncologist to be sure you know all the options available for treating your dog!
Dr. Joanne Intile