The vast majority of breast cancer-related deaths are due to secondary tumors after metastasis. There is currently no effective therapy to limit metastasis. Our long term objective is to develop new therapies that reduce cancer invasion, a critical first step in metastasis. This proposal is based on our observations that a novel extracellular form of the molecular chaperone hsp90a is required for cancer invasion by the activation of the matrix metalloproteinase MMP2. Hsp90 has been implicated in cancer and hsp90 inhibitors with anti-tumor activity are currently in clinical trials. These drugs may be problematic in that they interfere with the many intracellular functions of hsp90. Our findings suggest our main hypothesis that inhibiting extracellular hsp90a will decrease invasion and thus limit metastasis. This presents an opportunity for novel anti-cancer therapy by inhibiting invasion without interfering with the myriad intracellular functions of hsp90. To support this idea, we will address three specific aims. We will determine the mechanism of how hsp90a functions on the outside of cancer cells (Aim 1). We will then use this information to develop and test extracellular hsp90 inhibitors (Aim 2). We already have one impermeant hsp90 inhibitor in hand and several candidates for neutralizing single chain antibodies from our collaborators at NCI and Xerion Pharmaceuticals. Finally, we will test the best of these inhibitors of extracellular hsp90a for their ability to limit metastasis in a new model developed by our collaborators at Tufts using human breast cancer cells metastasizing to human bone explants in immunocompromised mice (Aim 3). Thus, these experiments take us from an initial discovery of hsp90a function with cell-based assays to in vivo animal models taking an interdisciplinary approach to address a key issue of human health: limiting metastasis to improve breast cancer prognosis. These studies aim to expedite the translation of our basic research into a potential therapy. If successful, the proposed work would provide data for developing future clinical studies and thus impact human health.