Drug Repurposing in Veterinary Oncology: Myth or Reality?

: Drug repurposing, that is, the identification of new therapeutic indications for existing medications, has been shown to be a cost-effective and time-efficient alternative to de novo drug development. This review provides a comprehensive overview of repurposed drugs in veterinary oncology, describing their mechanisms of action, current evidence of clinical benefit, and translational relevance. The therapeutic agents discussed include non-steroidal anti-inflammatory drugs (e.g., piroxicam), metabolic modulators (e.g., metformin), anti-parasitic drugs (e.g., fenbendazole), immunomodulators (e.g., thalidomide, oclacitinib), cardiovascular agents (e.g., propranolol, statins, losartan), and other compounds such as auranofin and disulfiram. A critical evaluation of the extant evidence-based data from preclinical research, naturally occurring tumor models, and clinical studies is provided, with particular emphasis on both the therapeutic potential and the current limitations. The present review also focused on combination strategies and multimodal protocols, where repurposed drugs may enhance the efficacy of chemotherapy, targeted therapies, or immunotherapy. Challenges to clinical implementation, including limited funding, regulatory and ethical considerations, and the need for well-designed, multi-institutional clinical trials, are discussed. Ultimately, drug repurposing represents a practical and translationally valuable approach to broaden therapeutic options, improve quality of life in companion animals, and advance comparative oncology by promoting progress that benefits both veterinary and human patients.