Leflunomide in Previously Treated Metastatic Triple Negative Cancers

Inclusion Criteria: * Women with histologically confirmed HER2-negative metastatic and/or locally advanced, inoperable breast cancer on a prior biopsy performed as a component of standard of care. The biopsy site may include the primary tumor, regional lymph node, or metastatic site if accessible to biopsy. * Age ≥ 18. * Prior treatment for metastatic breast cancer: * ≤ 3 prior chemotherapies for metastatic disease and up to 2 prior antibody drug conjugate regimens (eg, sacitizumab govitecan, trastuzumab deruxtecan). Patients with ER-positive breast cancer (ER\>10%) must have had progressive disease after at least 1 prior line of CKD4/6 inhibitor, and also a PIK3CA inhibitor if known to have a somatic PIK3CA activating mutation (by tumor or ctDNA assay) sensitive to the PIK3CA inhibitor alpelisib. * Prior immunotherapy is permitted and does not count as chemotherapy. * The use denosumab or zoledronic is permitted. * History of previously treated brain metastases with ≥ 4 weeks after definitive surgery and gamma knife/whole brain radiation and not taking steroids. * ≥ 4 weeks from last oral or IV chemotherapy, small molecule inhibitor, a biologic agent, surgery or radiation. * Performance status 0-2. * Adequate organ and marrow function as defined below: * leukocytes ≥ 3,000/mcL * Absolute neutrophil count ≥ 1,000/mcL * platelets ≥ 100,000/mcl * total bilirubin within institutional upper limit of normal. (≤ ULN) * AST (SGOT)/ALT (SPGT) ≤ 3 x ULN (3xULN if liver met…