Clinical Response to Imatinib Mesylate and Toxicity Profile in 35 Dogs With Mast Cell Tumours

Mast cell tumours (MCTs) are common in dogs. Treatment options include surgery, radiation therapy, and cytotoxic chemotherapy; however, in cases of inoperable or metastatic tumours, tyrosine kinase inhibitors (TKIs) can be used. Imatinib mesylate has been used in the treatment of solid tumours in both human and veterinary medicine. Previous studies have shown some efficacy in dogs with MCTs; however, additional data are needed to better define the optimal dosage, toxicity profile, and clinical outcomes associated with its use. Dogs with a cytological or histopathological diagnosis of mucosal, cutaneous or subcutaneous MCTs were included. Medical records from 2017 to 2024 were reviewed for clinical presentation, results of staging procedures, diagnostic tests, and treatment. Inclusion required the presence of macroscopic disease and administration of imatinib, either as a sole treatment or in combination with surgery. Thirty-five cases were included, all receiving medical treatment with or without surgical excision. Imatinib was administered as first-line treatment in 8 dogs (22.8%) and as a rescue treatment in 27 dogs (77.1%), achieving an overall clinical benefit, including complete response, partial response, and stable disease of 77%. Median progression-free survival was 37 days (range 13-770 days), while the overall median survival time (MST) was 270 days (range 83-1396 days). Following imatinib treatment, the MST was 105 days (range 22-1145 days). Gastrointestinal and haematological adverse events were recorded in 2 (5.7%) and 3 (8.6%) dogs, respectively, and were self-limiting. Imatinib treatment was generally well tolerated, with measurable clinical responses observed and only a limited spectrum of toxicities. Further studies are warranted to better characterise its safety and efficacy in dogs with MCTs.