Mast cell tumor treatment: chemotherapy

In my last blog, I discussed surgery and radiation for MCT. Today we will focus on chemotherapy.

Which dogs need chemo?

Your dog may not even need chemotherapy. In many dogs that I see with MCT, I do not recommend chemotherapy at all. This is because chemotherapy is not as effective as surgery and radiation for treating the primary tumor, especially if the cancer is of a lower grade and/or confined to one local area.

The exception to this is when I use chemotherapy (instead of radiation) for large MCT tumors which need to be shrunk prior to surgical removal. I also consider palliative chemotherapy for non-resectable (inoperable) tumors, if radiation therapy is not available in your area, or if you have decided against radiation.

If I’m worried about metastasis, I may recommend chemotherapy in addition to surgery and radiation. This is most common for grade III MCT, which is prone to metastasize.

In addition, I sometimes also use chemotherapy for grade II MCT with high proliferation marker scores or for multiple skin MCTs. Some of these cases are complicated, so I decide this on a case-by-case basis. Remember, these are not one size fits all tumors, so consult with a specialist to decide on the best options for YOUR dog.

Commonly used drugs include vinblastine, prednisone, Lomustine, and the new tyrosine kinase inhibitors, Palladia and masitinib (Kinavet CA-1). You can read more about all these drugs in the chemotherapy chapter in the Guide, but let’s talk about the new class of drugs called tyrosine kinase inhibitors (TKI), Palladia and Kinavet.

One-third of dogs with MCT have a genetic mutation in the c-Kit gene that leads to a cellular protein called tyrosine kinase to be stuck in the “on” position. This stimulates tumor growth and leads to increased survival for mast cell tumors.

The TKI drugs, Palladia and Kinavet target this genetic mutation specifically. They work in two ways to fight MCT: cut off blood supply to MCT by blocking angiogenesis, and inhibit tyrosine kinase. Because the drug targets the protein and blood vessel receptors, it is considered “targeted therapy.”

Both Palladia and Kinavet are available in the U.S. and are FDA-approved specifically for dogs with recurrent, Grade II and Grade III MCT, with or without regional lymph node involvement.

When do I use the TKIs?  I think they are important tools for dogs with the c-kit mutation, recurrent tumors, inoperable tumors, and can even be considered for incompletely resected MCT. I know in my previous blog, I recommended radiation after surgery to prevent recurrence, but the way TKIs inhibit blood vessels (anti-angiogenesis) make them an alternative for preventing recurrence as well. We still need more studies on this approach.  Because complete responses with many of the MCT drugs are uncommon, I often use the TKIs in combination protocols.

Overall, these are relatively new drugs, and I’m excited about its results both in clinical studies and in my patients.  Like all chemotherapy drugs, there is the potential for side effects with TKIs. I personally find them well-tolerated in my patients. In my practice, I work with Guardians to monitor them closely, adjust the protocol as needed, and maintain a great quality of life while taking the medication.  Don’t be scared of these drugs. It helps to find a specialist with a lot of experience using these. Not every dog needs chemotherapy including the TKIs.

Additional Considerations for Mast Cell Tumors

In addition to surgery and radiation, and possibly chemotherapy, you may want to consider supportive medications to counteract the effects of possible degranulation (when the mast cell tumor breaks open). The following are especially helpful for dogs with large tumors, detectable spread, and systemic signs such as vomiting: anti-histamines (such as Benadryl), antacids (such as Pepcid) and proton pump inhibitors (such as Prilosec). These can often be stopped after surgery, if the tumor has been successfully removed and spread has not occurred; some dogs may need these drugs long-term to control their symptoms.

Finally don’t forget the “non-conventional” approaches like Apocaps as discussed in the Guide.