Vinblastine/prednisolone chemotherapy leads to hematological toxicity in dogs with high-grade or metastatic mast cell tumors.

Objective: To determine the myelosuppressive effects/hematological toxicities, other general toxicities, and when these occur during vinblastine/prednisolone chemotherapy in dogs bearing high-grade or metastatic cutaneous/subcutaneous mast cell tumors (MCTs).

Methods: Medical records were retrospectively reviewed between November 1, 2016, and March 1, 2023. Thirty client-owned dogs with histopathologically confirmed cutaneous high-grade MCTs/metastatic subcutaneous MCTs and that subsequently completed a 12-week vinblastine/prednisolone chemotherapy protocol were included. Hematology was assessed before commencing chemotherapy and before each vinblastine treatment. The effect of each treatment upon hematological values was evaluated. Measured outcomes included the type, frequency, and severity of hematological and other more general toxicities.

Results: 24 of 30 dogs experienced at least 1 hematological toxicity, 6 experienced gastrointestinal toxicity, and 4 experienced lethargy. The most common toxicity was anemia (15/30 [50%]), with 93.3% (14/15 dogs) classified as Veterinary Cooperative Oncology Group-Common Terminology Criteria for Adverse Events grade I and 6.6% (1/15) classified as grade II. The second most common toxicity was neutropenia (14/30 [46.6%]), with 71.4% (10/14) classified as grade I and 28.6% (4/14) as grade III. The least common hematological toxicity was thrombocytopenia (4/30 [13%]), all grade I. Neutropenia mainly occurred during weeks 2 and 3; however, there was no significant decrease in neutrophil count relative to baseline. Neutrophil count increased and Hct decreased during weeks 6 to 12 of treatment when compared to baseline. No change in platelet count was observed.

Clinical Relevance: Vinblastine/prednisolone chemotherapy leads to hematological toxicity; however, this was mostly low-grade and did not require major intervention. Vinblastine/prednisolone chemotherapy is well tolerated in dogs bearing high-grade or metastatic MCTs.