PROJECT SUMMARY/ABSTRACT The majority of breast cancers express estrogen receptor a (ER+), a molecule that typically promotes cell proliferation when activated by its ligand estrogen. Despite successes seen in treating patients with ER+ breast tumors with endocrine therapies, approximately 33% of these treated patients will develop recurrent metastatic tumors after endocrine treatment. These recurrent tumors primarily metastasize to bone and do not respond to current treatment options. Bone is the most common and one of the most dangerous sites for breast cancer metastatic tumor growth. At death, roughly 73% of women with breast cancer have bone metastasis. The bone architecture and the ubiquity of vascular sinusoids provide accessibility to and easy exit from bone to increase the spread through the body from the metastatic bone tumor. The bone microenvironment releases cytokines, chemokines, and growth factors that inhibit colonization of cancer cells in healthy bone or support colonization in cancer. These factors also can create an immunosuppressive environment that prevents a normal immune response or response to immunotherapy. In the proposed research, we will investigate how the chemokine Cxcl5, its decoy receptor Ackr1, and its receptor Cxcr2 contribute to ER+ breast cancer metastatic colonization of bone. We also will examine CXCR2 inhibitors for their efficacy as single agents and as a combination therapy (with bisphosphonates or immunotherapy) to inhibit breast cancer metastasis to bone. CXCR2 inhibitors are attractive therapeutics against metastatic breast cancer that may have efficacy in treating the formation of metastatic bone tumors that are dependent on the CXCL5:CXCR2 signaling axis and resistant to current therapies. Since patients with these tumors currently have few treatment options and often are incurable, this study could have significant translational potential. Significantly, if targeting or inhibiting these factors can reduce bone metastasis in preclinical animal models, then we will be in a position at the end of the grant period to propose that these compounds be used in a clinical trial in breast cancer patients with ER+ tumors metastasized to bone. Since these recurrent tumors typically do not respond well to current therapies, this treatment strategy would be significantly impactful and bring significant hope for patients with this disease. Future development of additional optimized small molecules or peptide inhibitors of CXCL5/CXCR2 will expand the therapeutic options available to clinicians in the care of breast cancer patients with metastatic disease.