Triple negative breast cancer (TNBC) is a heterogeneous group of clinically aggressive breast cancers. TNBC patients have a high risk of recurrence and metastasis, and current treatment options remain limited. There is strong evidence supporting the involvement of Notch signaling in TNBC progression. Expression of Notch1 and its ligand Jagged1 correlate with poor prognosis. Emerging evidence suggests that cancer stem-like cells (CSCs) that escape chemo or radiation therapy in TNBC are often Notch-dependent. At the same time, there is evidence that active tumor immunity predicts good response to neo-adjuvant chemotherapy in TNBC. Notch inhibitors, including Gamma Secretase Inhibitors (GSIs) are quite effective in preclinical models of TNBC, where they eliminate CSCs resistant to chemotherapy. However, the success of GSIs in clinical trials is limited by their intestinal toxicity and adverse immunological effects. CD4 and CD8 T-cells, necessary to adaptive tumor immunity, require Notch1 for activation. Our overarching goal is to replace GSIs with agents that lack their systemic toxicity and adverse immunological effects. We identified Sulindac Sulfide (SS), the active metabolite of FDA-approved NSAID Sulindac, as a potential candidate to replace GSI. SS has Gamma Secretase Modifier (GSM) activity. We confirmed that SS inhibits Notch1 cleavage in TNBC cells. SS significantly inhibited mammosphere growth in all human and murine TNBC models we tested: 1) human MDA-MB-231 cells; 2) murine TNBC model C0321, from targeted conditional knockout of Lunatic Fringe (LFng-/-); and 3) Two TNBC patient-derived xenograft models, 2K1 and 4IC. In contrast, SS did not inhibit Notch expression or cleavage in murine T cells. In C0321 tumors, which recapitulate human mesenchymal TNBC, we found that SS had remarkable single-agent anti-tumor activity and virtually eliminated Notch1 expression in tumors. SS caused an increase in intra-tumoral CD11c+ dendritic cells, but decreased CD4 cells, which in this model are largely PD-1 positive (exhausted). CD8 cells were modestly increased. SS did not affect the number of tumor infiltrating macrophages or myeloid-derived suppressor cells (MDSC). However, SS blocked the immunosuppressive function of bone marrow-derived MDSC. We are currently investigating the mechanisms of this anti-tumor activity. Our data support further investigation of SS for the treatment of TNBC, with standard of care or with immunotherapy agents. Repurposing an FDA-approved, safe agent for the treatment of TNBC would be significantly easier and more cost-effective than developing unproven investigational agents.

Citation Format: Hossain F, Ucar D, Majumder S, Xu K, Ran Y, Minter L, Xi Y, Burow M, Golde T, Osborne B, Miele L. Repurposing sulindac sulfide as a notch inhibitor to target cancer stem-like cells in triple negative breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-22-01.