Approximately 70% of breast cancers express the estrogen receptor (ER). Although ER inhibitors such as tamoxifen have saved the lives of millions of breast cancer patients, development of resistance to tamoxifen occurs in ~40-50% patients receiving hormone therapy. Thus, there is an unmet need to develop novel therapies targeting hormone refractory breast cancers to improve patient survival. One of the major mechanisms of developing hormone refractoriness is protein kinase-driven signaling pathways becoming pathologically activated and circumventing the dependence on ER signaling. Given the availability of pharmacological inhibitors targeting a large spectrum of protein kinases, kinases have become most attractive therapeutic targets in many diseases particularly in cancer. In this proposed study, we will use advanced high- resolution and high-accuracy proteomic approaches to systematically identify novel protein kinases that are activated in hormone-refractory breast cancers. To keep our study relevant to the clinical setting, instead of using in vitro cultured cell lines, our discovery efforts will instead employ breast tumor samples from patients with different responses to hormonal therapy. We will validate the therapeutic potential of candidate kinases discovered in these studies in xenograft tumors directly derived from hormone refractory metastatic tumors. We will also validate our discoveries on breast cancer tumor microarrays (TMA) comprising >800 breast cancer tumor cores with clinical outcomes. This strategy will help us discern kinases that are required for hormone refractoriness observed in ER-positive breast cancer. In order to shorten the path from bench to bedside, we have recruited a highly qualified multidisciplinary team of investigators capable of ultimately translating these pre-clinical discoveries to clinical trials. A successful outcome of or proposed studies will not only identify kinases with therapeutic potential but also set the stage for initiating clinical trials to test promising drugs in patients with hormone refractory breast cancer. PUBLIC HEALTH RELEVANCE: Development of resistance to hormonal therapy is the most common cause of breast cancer death. In this proposed study, we will use advanced proteomic approaches to identify pathologically activated kinases that drive development of hormone refractoriness. Our clinical resources and experienced multidisciplinary team increase the likelihood that our discoveries will be translated into novel therapeutic options for patients.