Abstract Breast cancer is currently the most commonly diagnosed and second leading cause of cancer-related deaths among women in the Unites States. For the past several years, research has focused on the role of inflammatory cytokines such as interleukin-6 (IL-6) and their role in cancer. Increasing evidence suggests that breast cancer patients with high levels of proinflammatory modulators have an overall worse prognosis. Research performed by our lab and others has also shown that oncostatin M (OSM) promotes breast cancer progression and metastasis by inducing circulating tumor cell (CTC) numbers, and metastasis to distant sites in vivo. While efforts have been made to develop FDA approved therapeutics for OSM, previous studies have all utilized monoclonal antibodies and none have survived clinical trials. For the first time, our research group has developed a stepwise approach towards designing and testing novel small molecule inhibitors (SMIs) against OSM. To determine viable candidates, in silico computational screening of compound libraries towards OSM revealed top potential compounds that interact with the receptor binding region and inhibit biological function. These top candidate inhibitors were then analyzed for direct interaction with cytokines using fluorescent quenching and chemical shift perturbation NMR (CSP-NMR). In vitro efficacy of each SMI was then analyzed by measuring inhibition of all signaling cascades activated by OSM using enzyme-linked immunosorbent assays (ELISA) and immunoblot assays. From these in silico and in vitro methods, SMIs were then structurally modified with effective functional groups to increase efficacy of binding interaction and reduce toxicity for living systems. Finally, our most promising small molecule inhibitor that has been sufficiently tested for efficacy was examined in vivo utilizing a novel estrogen receptor-positive (ER+) mouse model of metastatic breast cancer, and treatment with our SMI proved to decrease tumor burden and metastasis, particularly to the lung when compared to vehicle control. In conclusion, our lab has generated the first ever SMI aimed to inhibit OSM-mediated cancer progression. Our continued efforts aim to develop a novel FDA approved therapeutic to improve outcomes for breast cancer patients. Citation Format: Cody L Wolf, Clyde Pruett, Darren Lighter, Cooper McGrath, Andrea Feci, Joseph Tuccinardi, Simion Dinca, Ken Tawara, Matthew King, Lisa Warner, Don Warner, Cheryl L. Jorcyk. Novel Small Molecule Inhibitors Targeting Proinflammatory Cytokines in Metastatic Breast Cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr B045.