Despite therapeutic advances, the 5-year survival rate from metastatic triple negative breast cancer (TNBC) has not improved significantly for 30 years. As such, significant benefits to patients with advanced breast cancer will result by developing novel therapies, which target advanced metastatic breast cancer. To address this need we screened metastatic cancers for genes which could contribute to metastasis. From this screen we identified MDA-9/Syntenin-1/Sydecan Binding Protein (MDA-9) as a regulator of cancer cell metastasis. Published and preliminary studies using several mouse models of metastasis confirm that inhibiting MDA-9 prevents metastasis and sensitizes progressive metastatic tumors to chemotherapies leading to improved survival. These results support the hypothesis: that targeting MDA-9 with inhibitors will lead to a combinatorial therapeutic approach that simultaneously inhibits both the dissemination and growth/survival of progressive TNBC. We developed an effective small molecule inhibitor of MDA-9 called PDZ1i. Administering PDZ1i in several mouse models of metastatic cancers effectively prevents metastasis and sensitizes metastatic tumors to chemotherapies, supporting our hypothesis. In this application. we propose experiments to better understand how PDZ1i exerts anti-tumor/metastatic effects and if this information can be used to make PDZ1i a better chemotherapy. In Specific Aim 1, we seek to formulate PDZ1i for clinical applications, building upon our preliminary work with a novel nanocrystal technology (PDZ1i-Nano), and to characterize non-cell-autonomous mechanisms by which PDZ1i suppresses tumor growth. Focus will be on the immune system, particularly the infiltration of effector immune cells in response to PDZ1i-Nano treated tumor cells. Experiments will employ TNBC metastatic mouse models, and human patient-derived (PDX) tumors and humanized mice. The precise impact of this recruitment on tumor growth control will be evaluated in this aim. In Specific Aim 2, genome wide datasets will be used to define the signaling pathways, regulatory transcription factors and down-stream gene targets regulated by MDA- 9 to promote cell motility in vitro and metastasis in vivo. In Specific Aim 3, PDZ1i-Nano will be combined with a standard of care (SOC) chemotherapy, paclitaxel (Taxol), to improve the therapy of mouse models of TNBC. These studies will include using PDZ1i-Nano in both an adjuvant and neoadjuvant setting as a means of preventing the appearance of metastases. Experiments will study how PDZ1i-Nano sensitizes TNBC cells or the host cells to the effects of chemotherapies to improve tumor growth control, which are required for PDZ1i to progress into clinical trials. Because the PDZ1i small molecule inhibitor has initial favorable drug characteristics (ADME, pharmacokinetics and toxicity testing), performs well in mouse tumor models, can be combined with FDA approved SOC chemotherapies, the results of this research program c...