Masitinib is safe and effective for the treatment of canine mast cell tumors.

BACKGROUND Activation of the KIT receptor tyrosine kinase is associated with the development of canine mast cell tumors (MCT). HYPOTHESIS/OBJECTIVE To evaluate the efficacy of masitinib, a potent and selective inhibitor of KIT, in the treatment of canine MCT. ANIMALS Two hundred and two client-owned dogs with nonmetastatic recurrent or nonresectable grade II or III MCT. METHODS Double-blind, randomized, placebo-controlled phase III clinical trial. Dogs were administered masitinib (12.5 mg/kg/d PO) or a placebo. Time-to-tumor progression (TTP), overall survival, objective response at 6 months, and toxicity were assessed. RESULTS Masitinib increased overall TTP compared with placebo from 75 to 118 days (P = .038). This effect was more pronounced when masitinib was used as first-line therapy, with an increase in the median TTP from 75 to 253 days (P = .001) and regardless of whether the tumors expressed mutant (83 versus not reached [P = .009]) or wild-type KIT (66 versus 253 [P = .008]). Masitinib was generally well tolerated, with mild (grade I) or moderate (grade II) diarrhea or vomiting as the most common adverse events. CONCLUSIONS AND CLINICAL IMPORTANCE Masitinib is safe and effective at delaying tumor progression in dogs presenting with recurrent or nonresectable grade II or III nonmetastatic MCT.

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