Efficacy of Vinblastine for Treatment of Canine Mast Cell Tumors

Background: The optimal dosage and clinical efficacy of vinblastine (VBL) for treatment of mast cell tumors (MCTs) in dogs has not been established. Hypothesis: Single‐agent VBL has antitumor activity against MCTs in dogs. Animals: Fifty‐one dogs with nonresectable grade II or III cutaneous MCTs. Methods: Prospective, open clinical trial. Dogs were systematically allocated (by hospital record number) to receive IV treatment with VBL at a dosage of 2.0 mg/m² (weekly for 4 treatments then biweekly for 4 treatments; VBL 2.0) or treatment with VBL at a dosage of 3.5 mg/m² (biweekly for 5 treatments; VBL 3.5). The primary outcome measure was reduction in tumor size. Results: Twenty‐five dogs were allocated to the VBL 2.0 group and 26 were allocated to the VBL 3.5 group. In the VBL 2.0 group, 3 (12%) had a partial response (PR) for a median of 77 days (range, 48–229 days). Overall response rate in the VBL 3.5 group was 27%. One dog (4%) had a complete response for 63 days and 6 dogs (23%) had a PR for a median of 28 days (range, 28–78 days). Toxicoses were uncommon in the VBL 2.0 group. Twelve (46%) dogs in the VBL 3.5 group had <500 neutrophils/μL 7 days after treatment; 2 dogs with neutropenia developed concurrent fevers. Conclusions and Clinical Importance: VBL, when used as a single‐agent, has activity against MCTs in dogs although the response rate is lower than those reported for VBL‐containing combination protocols. Further, findings suggest VBL at a dosage of 3.5 mg/m² should be considered for use in future phase II/III trials.