Summary Osteosarcoma (PI: Sita Withers, PhD). This proposal will identify novel treatment strategies for osteosarcoma which has a 5-year survival rate of 70%. The expression of epidermal growth factor receptor HER2 is upregulated in 40% of human and canine osteosarcomas and has correlated with a poor outcome in some human studies. Using murine models of osteosarcoma and lung metastasis, Dr. Withers will evaluate EGFR:HER2 inhibitors of signaling pathways (eg, PI3K-Akt and ERK/MAPK) that have shown efficacy in a xenograft mouse model of breast cancer. The hypothesis of this proposal is that peptidomimetic HER2 heterodimerization inhibitors of EGFR-family signaling will lead to novel tools for treating metastatic OSA. Dr. Withers' approach includes evaluation in a naturally-occurring canine osteosarcoma model, which mimics more closely lung metastasis in humans, for translational information in the treatment of human osteosarcoma. Combinations of small molecule inhibitors will be identified which show in vivo efficacy against the development of osteosarcoma and also limit metastasis to the lungs. 3D printing of osteosarcoma and immune cells will be used to characterize the effects of HER2 inhibition in the context of exposure to typical features of the tumor microenvironment. Two specific aims are proposed: Aim 1 To define the effects of EGFR-family signaling inhibition on OSA cell function and signaling pathways. Aim 2 To detail the efficacy and toxicity of optimal EGFR signaling inhibition in preclinical in vivo models of OSA.