Masitinib Mesylate Treatment of Mast Cell Tumor in a Dog

Mast cell tumors are one of the most common tumors I see on the skin and subcutaneous tissues of dogs although they can occur anywhere. Mast cells are part of the immune system and are found all over the body. Mast cell tumors occur when the inhibitory signal is lost and a mast cell replicates without any control. These tumors grow fast and can be difficult to remove. Malignant cells creep into the surrounding tissues from a tumor making it difficult to get clean margins.

Pictured below is a 9 year old spayed female Australian shepherd mix named Molly. She was always a healthy girl until 4 months ago when she started having respiratory problems. X-rays of her abdomen and chest revealed large tumors.  She needed furosemide therapy to treat the fluid that accumulated in her lungs and chest from the cancer. Tumors also appear on her skin including a small pink mass on her lower right eyelid. The tumor grew quickly into a large ulcerated mass pictured below. Microscopic analysis of her lump revealed a mast cell tumor.

Molly pic09132015

Molly 2 eye pic 09132015

Since surgical removal is not an option for Molly, her family has decided to treat her with masitinib mesylate (Kinavet-CA1) made by AB Science, Chatham, New Jersey. Masitinib is approved for the treatment of nonresectable grade II and III cutaneous mast cell tumors in dogs. It is a tyrosine kinase inhibitor that targets the c-Kit receptor. In dogs who have the c-Kit mutation, the results have been remarkable.

Molly started her treatment on Monday. The tablets are given once a day with food. So far, she is tolerating the medicine well. Please keep your fingers crossed for Molly.


-AB SCIENCE, Kinavet-CA1: Canine Mast Cell Tumor Targeted Therapy, Promotional Materials.

-Shell, Linda. ‘Mast Cell Tumor’ Associate,, last updated 06/06/2011.

Published by kristennelsondvm

Dr. Kristen Nelson grew up on a farm in Watertown, Minn., where she developed a deep love for animals of all kinds. She received a Doctor of Veterinary Medicine degree from the University of Minnesota, College of Veterinary Medicine. Kris then completed a small-animal internship at the prestigious Animal Medical Center in New York City. In addition to writing and speaking, she cares for small and exotic animals in Scottsdale, Az. Dr. Nelson is widely quoted in the media. Her credits include Ladies’ Home Journal, USA TODAY, the Los Angeles Times and numerous radio and television interviews. Dr. Nelson has written two books, Coated With Fur: A Vet’s Life and Coated With Fur: A Blind Cat’s Love. Kris and her husband Steve share their home with rescued cats, birds and a dog.