Phase II open-label study of single-agent hydroxyurea for treatment of mast cell tumours in dogs

This prospective study evaluated the efficacy and safety of hydroxyurea (HU) in dogs with measurable mast cell tumours (MCTs). Dogs were treated with HU at 60 mg kg⁻¹per os q24h for 14 days then 30 mg kg⁻¹ q24h thereafter or until MCT recurrence. Forty-six dogs were enrolled. The overall response rate was 28%. Two dogs had a complete response (CR) for 256 and 448 days, respectively. Eleven dogs had a partial response for a median duration of 46 days (range, 28-189 days). Grade 2 to 4 neutropenia occurred in eight dogs and grade 4 thrombocytopenia in two. Grade 3-4 anaemia occurred in seven dogs; overall, there was a significant decrease in haematocrit after treatment with HU. The median drop in haematocrit was 10%. This study demonstrated that HU has activity in the treatment of MCTs with mild anaemia being the primary adverse event.