Background: Emerging studies suggest that breast cancer, particularly triple negative breast cancer (TNBC), may be sensitive to immunotherapy. However, the response rate of single agent immune checkpoint blockade agent in TNBC is rather low. Previous genomic study in residual tumor after neoadjuvant chemotherapy showed inverse correlations between MEK activation signature and the amount of tumor infiltrating lymphocytes (TILs) in residual disease samples as well as poor outcome. Preclinical study also showed that the combination of MEK inhibitor and anti-PD-L1 antibody in mouse model can eradicate TNBC tumors. Methods: This is a single arm, Phase I/II trial of Pembrolizumab (P) in combination with Binimetinib (B) in patients with unresectable locally advanced or metastatic TNBC. This trial is currently opened for accrual at Mayo Clinic in Florida and Minnesota. Patients with TNBC defined as ER ≤ 10% and PR ≤ 10% who received ≤ 3 prior lines with measurable disease will be enrolled. The primary objective of the Phase I part is to determine the maximum tolerated dose of B in combination with P and for the Phase II part is objective response rate (ORR) by RECIST criteria. The secondary endpoints include ORR by irRECIST, progression free survival, and overall survival. The total sample size is 15-38 patients with 6-12 patients in Phase I with 2 dose levels and 9-26 patients in Phase II. Simon’s Two-Stage Optimal Design is used to test the null hypothesis that this two-drug combination has an ORR of at most 15% vs. the alternative hypothesis that it has an ORR of at least 35%. Patients will receive single agent B for 2 weeks prior to starting P. A mandatory biopsy will be performed before starting B and an optional biopsy will be performed after 2 weeks of B. Tumor tissue will be evaluated for the amount and phenotypes of TILs, PD-L1 expression, and gene expression analysis using PanCancer Immune Profiling Panel, and PDJ amplification. Peripheral blood will be evaluated for circulating immunoregulatory cells, cytokine profiling, circulating tumor cells (CTCs), as well as p-ERK and PD-L1 expression on CTCs. Clinical trial information: NCT03106415