Both inside the central nervous system and outside in the peripheral nervous system, cancer cells grow along nerves as routes of invasion and metastasis called neural invasion. This growth is common in several carcinomas including breast cancer and is associated with poor prognosis. Proteolysis of cell adhesion molecules (CAMs) occurs in development, and growing evidence suggests this post-translational modification may promote tumor migration and invasion on nerves that ultimately leads to metastasis to the brain in various tumor types including breast cancer. The receptor protein tyrosine phosphatase PTPµ is a CAM that is proteolyzed in cancer cells to generate an extracellular fragment that is a unique imaging biomarker of the tumor microenvironment. The PTPµ- targeted agents we developed bind to this biomarker and recognize human brain tumors as well as invasive primary breast cancer and breast cancer that has metastasized to the brain. Systemic delivery of the PTPµ- targeted agent results in binding to tumor cells within minutes in xenograft models. Using a 3D cryo-imaging system we analyzed the extent of cell migration and dispersal within the brain. We found that the PTPµ-targeted agent labels 99% of all dispersing tumor cells far away from the main tumor mass on nerves in mouse models. This proposal represents the convergence of our expertise in neuroscience, cell adhesion, imaging and cancer to test if the PTPµ biomarker can be used to detect tumor growth along nerves leading to brain metastases. Gold nanoparticles (AuNPs) have shown outstanding versatility in biomedical applications including imaging diagnostics, drug delivery, and radiation therapy. In this proposal, we describe the development of theranostic AuNPs for the detection and treatment of breast cancer metastases. We will achieve more sensitive detection and treatment of invasive and metastatic lesions through the use of a three component theranostic nanoparticle containing: 1) a highly specific targeting agent of the PTPµ biomarker in the tumor microenvironment; 2) a protease-sensitive quenched near infrared fluorophore for fluorescent imaging; and 3) a gold nanoparticle (AuNP) for sensitization to radiotherapy. We will test whether the PTPµ-targeted agents detects nerve associated growth using 3D single cell resolution cryo-imaging that precisely tracks migration of individual cancer cells on nerves. We will utilize our established human patient-derived xenograft models of metastatic breast cancer and models that metastasize from the breast to the brain. Metastatic tumors are resistant to almost all chemotherapeutics so “physical” killing strategies like radiation must be improved and employed for better therapeutic outcomes. By delivering PTPµ-targeted conjugated AuNPs directly to primary and metastatic breast cancer we will exploit the radiosensitization of AuNP to reduce the required dose of radiation needed for radiotherapy thereby reducing collateral damage to normal...