ALVAC‐fIL2, a feline interleukin‐2 immunomodulator, as a treatment for sarcoids in horses: A pilot study

BACKGROUND: Sarcoid tumors are common in horses and may negatively impact the performance and value of the horse. No known treatment is reliably successful. HYPOTHESES/OBJECTIVES: To determine tolerability, overall response rate, time to response, and progression‐free survival of horses with biopsy‐confirmed or suspected sarcoids treated with ALVAC‐fIL2. ANIMALS: Client‐owned horses with measurable, presumed‐ or biopsy‐confirmed sarcoid tumors. METHODS: Prospective pilot study. One milliliter of ALVAC‐fIL2 was injected into 4 to 5 areas of the sarcoid(s) in each horse (week 0); this treatment was repeated in weeks 1, 3, and 7. Sarcoids were measured at each visit, and response to treatment was determined according to the Response Evaluation Criteria in Solid Tumors for dogs (v1.0). After the final treatment, horses were reassessed and sarcoids remeasured every 3 months until tumor progression or for a minimum of 1 year if progression was not documented. RESULTS: Fourteen horses were included. Tumor size decreased in 86% of the horses, and the median time to first response was 89 days (range, 34‐406 days). Median time to best response was 211 days (range, 56‐406 days), but 3 of the sarcoids still were decreasing in size at the time of final evaluation. The median progression‐free interval was not reached. Adverse events were minimal and included transient focal inflammation in 2 horses. CONCLUSIONS AND CLINICAL IMPORTANCE: Intratumoral injection of ALVAC‐fIL2 has promise as a well‐tolerated and effective, tissue‐sparing treatment for horses with sarcoid tumors.