Combination of Hypofractionated Radiation Therapy and Gilvetmab for the Treatment of Cutaneous or Subcutaneous Mast Cell Tumours in Dogs.

Gilvetmab is a caninised monoclonal antibody against canine PD-1. Concurrent or sequential administration of anti-PD-1 antibodies and radiation therapy (RT) improves survival time in various species. Eighteen dogs with mast cell tumours (MCTs) were prospectively treated with 24 Gy of RT over Days 0, 7 and 21 and gilvetmab every 2 weeks beginning on the second fraction of RT (Day 7) for up to 10 doses. Best tumour response was complete in one (6%), partial in eight (44%) and stable disease in nine dogs (50%) for an ORR of 50%. Median time to best response was 7 days after the first fraction of RT. Thirteen dogs (72%) developed progressive disease with a median progression free interval (PFI) of 116 days after the first fraction of RT. Nine adverse events (AEs) attributed to gilvetmab were recorded in seven dogs (39%) consisting of infusion reactions in four, metabolic changes in three and gastrointestinal signs and pancreatitis in one each. Overall, five dogs (28%) experienced Grade 3 AEs. Radiation AEs were limited to Grade 1 for both acute and late effects in seven dogs (39%). There was a significant difference in PFI and MST for dogs with and without lymph node metastasis with a median PFI of 64 versus 292 days and MST of 103 versus 587 days (p = 0.02 and 0.005, respectively). This sequence, or combination, of therapies did not provide durable tumour control for canine MCT and the AE profile warrants further evaluation.