Project Summary Despite early detection and adjuvant therapy, breast cancer remains the leading cause of cancer mortality in women, largely due to distant, incurable recurrences arising years, or even decades, after treatment of the primary tumor. Recurrent, metastatic tumors arise from the pool of residual local and disseminated tumor cells (DTCs) that survive primary treatment and persist in the host in a presumed dormant state. Indeed, the presence of bone marrow DTCs following treatment is independently associated with a substantially increased risk of recurrence. At present, however, the mechanisms enabling residual tumor cells to maintain dormancy and ultimately recur are poorly understood, and DTC-directed treatment approaches are non-existent. Consequently, the ability to therapeutically target survival mechanisms utilized by DTCs would constitute a transformational new approach to preventing breast cancer recurrence and the mortality associated with it. Increasingly, treatment for cancer patients involves targeted therapies. In particular, targeted inhibition of HER2 in breast cancer patients in the adjuvant setting represents a clear example in which residual tumor cells can be kept at bay for extended periods of time. Using genetically engineered mouse models that faithfully recapitulate tumor dormancy and recurrence, we have developed an hypothesis that activation of the c-MET pathway may be essential for the survival of human and mouse breast cancer cells subjected to chemotherapy or targeted anti-HER2 therapy. Importantly, whereas c-MET appears to play only a limited role in primary breast tumorigenesis, our preclinical studies suggest the possibility that activation of the c-MET pathway may play a critical role in enabling dormant residual tumor cell survival and tumor recurrence. We hypothesize that effectively disabling the survival mechanisms by which DTCs persist in breast cancer patients following treatment will deplete this critical reservoir of cells, reduce tumor recurrence, and thereby improve survival. Specifically, we hypothesize that c-MET activation contributes to the survival and recurrence of DTCs, and that these mechanisms are recapitulated in patients. The specific aims of this proposal are to: (1) Determine the impact of c-MET pathway inhibition on residual disease and recurrence in mice; and (2) Determine the impact of c-MET inhibition on residual disease and recurrence in patients, by leveraging patient samples from a novel clinical trial initiated based on findings from the initial study period for this R01 application. These aims will be accomplished using a novel flow cytometry approach to isolate and evaluate c- MET in DTCs in mouse models and in breast cancer patients, by functionally evaluating the impact of c-MET pathway inhibition on the survival and recurrence of DTCs following targeted therapy or chemotherapy in mouse models, and evaluation of primary and metastatic patient samples. Results of the proposed studies have the potential to transform treatment options for breast cancer survivors by enabling efforts to target the unique vulnerabilities of dormant DTCs, thereby preventing metastatic breast cancer recurrence.