The tyrosine kinase inhibitor masitinib is used for the chemotherapeutic treatment of mast cell tumors (MCTs). Masitinib targets canine c-Kit, a stem cell factor receptor implicated in the regulation of mast cell proliferation and differentiation for physiologically normal conditions and processes involved in MCT development. A previous clinical trial concerning safety and efficacy demonstrated that masitinib success rates for MCT treatment in dogs may be more accurate when data have been analyzed for longer than 6 months. Therefore, this multicentric, placebo-controlled, randomized, double-blind prospective study evaluated the long-term survival of dogs with MCTs (n = 132) treated with masitinib. Eligible client-owned dogs with measurable nonresectable or recurrent postsurgical grade 2 or 3 MCTs without lymph node or visceral metastasis were included in the study. Dogs received either an oral placebo or masitinib until tumor progression was detected. Survival was confirmed in 90.9% of dogs with nonresectable tumors at 12 months and 78% at 24 months, a significant improvement compared with placebo treatment. Median overall survival time was 617 days (range, 433–937 days) for masitinib-treated dogs. Long-term follow-up data in this report supported previous evidence that masitinib is effective in the treatment of nonresectable MCTs and was beneficial in terms of length of survival. Short-term response was not predictive of long-term survival. Control at 6 months was highly predictive of long-term survival. Masitinib treatment improved long-term survival in dogs with nonresectable tumors regardless of c-Kit status, although treatment was particularly efficacious in c-Kit positive dogs.

Commentary: Masitinib (Kinavet, and toceranib phosphate (Palladia, are the first chemotherapy drugs approved by the FDA for use in veterinary patients, and both have been approved for use in dogs with MCTs. These drugs provide excellent new options for dogs with aggressive MCTs; however, it is important to realize that chemotherapy is not indicated for all dogs with MCTs. These drugs are typically recommended for dogs with nonresectable primary tumors, tumors that have metastasized, or high-grade tumors considered at high risk for metastasis. Masitinib and toceranib phosphate have a unique mechanism of action, targeting specific cell surface receptors that play an integral role in triggering cell division. Future studies will likely examine use of these drugs in combination with conventional chemotherapy drugs used to treat MCTs (such as lomustine and vinblastine) as well as treatment for other types of cancers.—Dennis Bailey, DVM, Diplomate ACVIM (Oncology)

Evaluation of 12- and 24-month survival rates after treatment with masitinib in dogs with nonresectable mast cell tumors. Hahn KA, Legendre AM, Shaw NG, et al. AM J VET RES 71:1354-1361, 2010.