Between 10% and 20% of patients diagnosed with breast cancer will have a type called triple-negative breast cancer (TNBC). TNBC is characterized by lack of receptors for estrogen, progesterone, and epidermal growth factor. The cancer is therefore no longer dependent on these hormones (i.e., estrogen or progesterone) or pathways (i.e., epidermal growth factor) for growth, and therapeutic strategies that target these receptors, which are used in other types of breast cancer, will not work. Thus, TNBC patients are currently treated with a combination of surgery and non-specific treatments such as chemotherapy and radiation. While advances in the diagnosis of early-stage breast cancer may result in substantially improved outcomes, approximately one-third of TNBC patients will develop metastatic disease that has spread to other parts of the body. Treatment options for patients with metastases are few and have limited efficacy. The 5-year survival rate for these patients is <25% with ~40,000 deaths expected this year. The high rate of mortality due to metastatic TNBC is the overarching challenge that our research addresses. New therapeutic strategies that harness the patient’s own immune system, such as the use of agents called immune checkpoint inhibitors (ICIs), to kill cancer cells has revolutionized cancer treatment. This strategy has considerable promise for the treatment of metastatic TNBC. However, a barrier to the success of ICI therapy in metastatic TNBC is that the tumor causes the expansion of cells called myeloid-derived suppressor cells (MDSCs). Ordinarily, MDSCs are a small and transient population that work to reset the immune system after an injury, such as infection. However, in cancer, MDSCs considerably expand their numbers and are persistent, resulting in the suppression of anti-tumor immune responses. Thus, any attempt to use the immune system to kill cancer cells would be enhanced by the simultaneous elimination of MDSCs. We propose to test a novel strategy to eliminate MDSCs by targeting them at their source, which is a type of cell called a myeloid progenitor that resides in the bone marrow. The cancer causes these myeloid progenitors to produce MDSCs by altering their ability to generate (or “differentiate”) into normal myeloid blood cells. There are similarities between our strategy and strategies used in the treatment of acute myeloid leukemia (AML), which is a cancer caused by the expansion of myeloid progenitors; albeit in this case, such myeloid progenitors also accumulate additional alterations making them cancerous. Inspired by this concept, we have repurposed a compound currently being tested in clinical trials for AML called Brequinar (BRQ). We treated a preclinical model of TNBC with BRQ and an ICI and were excited to observe that the combination of BRQ and ICI profoundly reduced tumor growth; in contrast, neither agent alone had any significant effect on tumor growth. To further understand how BRQ was working, we performed an experiment in tissue culture dishes where we found that BRQ reduced the formation of MDSCs. Based on these findings, we hypothesize that BRQ induces the differentiation of myeloid progenitors, suppresses the generation of MDSCs, and improves ICI therapy in TNBC. We will test this hypothesis in two Specific Aims: 1) To determine the mechanisms by which BRQ enhances responses to ICIs in preclinical models of metastatic TNBC. 2) To test the hypothesis that BRQ inhibits MDSC generation through the induction of myeloid progenitor differentiation. Our research will benefit patients with TNBC (and potentially other types of breast cancer) by investigating the therapeutic potential of a new combination of drugs that enhance the ability of the immune system to kill cancer cells. BRQ is currently being tested in patients, namely those with AML, and has demonstrated no significant toxicities, suggesting that the benefits of combining of BRQ and an ICI may outweigh the risks. Importantly, because BRQ is already in clinical trials, we anticipate that the outcomes of this research can be rapidly translated to the clinic for use in TNBC patients at the end of the 3-year research period. Thus, the overall impact of this research, which aligns with the mission of the BCRP, will be the discovery of a novel combination immunotherapy for metastatic TNBC that can replace or complement current treatment regimens and ultimately reduce patient mortality and increase survival.