108600, a novel CK2/DYRK1/TNIK inhibitor, targets and inhibits cancer stem cells (CSC) in triple negative breast cancer (TNBC), inhibiting tumor growth and metastases in patient-derived xenograft models. CSC’s have been shown to promote immune evasion of several types of cancer. Specifically, over-expression of CK2 has been shown to promote intratumoral recruitment of myeloid derived suppressor cells. We investigated the effects of 108600 treatment on the immune microenvironment of triple negative breast cancer since targeting CSC’s could promote favorable anti-tumor immune responses and mechanistically contribute to tumor growth inhibition. Methods:C57BL/6 mice bearing murine triple negative E0771 tumors were generated by orthotopic injection and treated either with vehicle control or 108600 for 5 days. Tumor growth was assessed by daily caliper measurement All tumors were recovered at endpoint, and processed for RNA sequencing, flow cytometry or Western blot analysis. Results:108600 treatment significantly inhibited growth of E0771 tumors in vivo, demonstrating for the first time efficacy of 108600 against TNBC in immunocompetent models. 108600 treatment decreased intratumoral phosphorylation and expression of 108600 targets AKT1 (Ser 129) and Cyclin D1, which are substrates of CK2α and Dyrk1, respectively. To further explore nature of transcriptional and signaling pathways affected by 108600 treatment in vivo, RNA sequencing was performed. DGE and subsequent annotation analysis showed that various immune (GO:0006955, GO) and inflammatory (GO:0006954, GO) signaling pathways associated with Stat1 (mmu04062, KEGG) and IFNγ (mmu04060, KEGG) were down-regulated in 108600-treated E0771 tumors. GSEA (Gene set enrichment analysis) indicated that the regulatory T cell (Treg) population (GSE42021, GSE40685) was suppressed by 108600 treatment. To validate these findings, tumor infiltrating leukocytes (TIL) were isolated from vehicle or 108600-treated tumors and analyzed by flow cytometry. CD4, CD25, and FOXP3 expression was used to gate the Treg population. The Treg population was significantly suppressed by 108600 treatment, confirming the RNA sequencing results. 108600 also likely regulates expression of immunomodulatory molecules in TNBC tumor cells since 108600 treatment increased PD-L1 surface expression on E0771 cells in vitro. Conclusions:Our study supports 108600, an inhibitor that targets breast cancer stem cells, as a modulator of the immune microenvironment of TNBC. 108600 suppresses the Treg population among the TIL population and increases tumoral expression of PD-L1. Tregs have been associated with disease progression and metastases of TNBC. Further studies are ongoing to clarify the functional consequences of these changes in the setting of tumor growth inhibition. Synergies between 108600 and checkpoint inhibition are also under investigation and could lead to novel combination therapies for TNBC. Citation Format: Katsutoshi Sato, Stacey J. Baker, E. P. Reddy, Hanna Y. Irie. Novel cancer stem cell inhibitor 108600 modulates tumor immunomicroenvironment of triple negative breast cancer (TNBC) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD3-08.