BReast CAncer gene 1 (BRCA1) encodes for an 1863 amino acid protein. The C-terminus domains of BRCA1 (BRCT) interact with a variety of other proteins to facilitate cellular functions such as checkpoint regulation, DNA damage response and DNA repair. Mutations found in the BRCT domains of BRCA1, result in the formation of tumors in the breast and ovarian tissues. These mutations genetically predispose women to breast and ovarian cancers. However, retrospective clinical studies show that patients with mutations in the BRCT domains respond better to chemotherapy. M1775R is a cancer causing mutation found in the BRCT domains of BRCA1. The molecular basis for the cancer caused by the M1775R mutant protein has been attributed to the loss of BRCTM1775R binding to phosphorylated proteins, such as the carboxy helicase BACH1 (pBACH1). Biochemical and preclinical studies have shown that the BRCT-pBACH1 interaction is essential for check point regulation and DNA repair and cells carrying BRCT mutants are sensitive to DNA damaging chemotherapeutic agents. Taken together, these studies indicate that cancers with the BRCT mutations respond better to therapy due to the loss of BRCT DNA repair function. Using a combination of cell-free and cell-based assay we identified a small molecule inhibitor (BI-94) of the BRCT-pBACH1 interaction. We also show that BI-94 sensitizes breast cancer cells to growth inhibition and apoptosis induced by Etoposide and Bleomycin. In this application, we propose to continue these studies by 1) exploring if the resistance induced by the BRCT-pBACH1 interaction towards Bleomycin / Etoposide treatment extends to other breast and ovarian cancer therapeutics in specific aim 1 and 2) advance BI-94 / or a close analog as an anti-cancer agent through specific aims 2-4.PROJECT NARRATIVE In this application we propose to develop small molecule inhibitors of protein-protein interaction. The successful completion of this project will 1) provide valuable tools to understand the genesis and progression of cancers due to BRCA1 mutation and 2) provide lead compounds that can be developed as chemotherapeutic agents to treat sporadic breast and ovarian cancer patients.