PROJECT SUMMARY Naturally occurring osteosarcoma in dogs has been proposed as a promising translational platform to serve as an intermediary between mouse studies and human clinical trials because it develops in the presence of a fully intact immune system and demonstrates a similar clinical progression to the human disease. Furthermore, canine osteosarcoma is a naturally occurring, heterogenous disease that recapitulates the spectrum of histopathologic, immunologic, and genomic complexity of the disease in humans. However, despite the promise of the dog model, the true translational potential of this model remains unproven and important species differences in overall outcomes exist that are often not accounted for in the traditional design of translationally focused dog trials. To optimize the design and translational impact of canine osteosarcoma trials moving forward, this proposal seeks to establish the breadth and range of natural outcomes for this disease together with deep characterization of the tumor microenvironment and immune landscape. To attain these objectives, we propose the following two aims, 1) Determine natural disease progression and outcomes in a contemporaneous prospectively enrolled population of dogs treated surgically for naturally occurring osteosarcoma, and 2) Inform optimal design and establish biologically based outcome assessments for translational canine osteosarcoma studies. This work will enroll client-owned dogs with naturally occurring osteosarcoma through the NCI Comparative Oncology Trials Consortium (COTC), a well-established trials network of veterinary academic institutions. In addition to primary clinical outcome measures for dogs treated with surgery alone, deep characterization of the tumor microenvironment and immune landscape will allow us to better understand the impacts of inherent biology and chemotherapy resistance, define the role of the immune system in the natural progression of disease, and to compare molecular and immune signatures between pre- existing human and canine osteosarcoma datasets. Hence, this canine trial paired with correlative bio-marker studies are aimed to expand and improve the utility of the canine osteosarcoma model by ensuring that future dog trials can be optimally designed to provide reliable information for the benefit of human patients with chemotherapy resistant osteosarcoma.