Abstract 4754: discovery of a casein kinase 2 small-molecule inhibitor that antagonizes wildtype and mutant estrogen receptor signaling in breast cancer

Abstract Introduction: Activating somatic mutations in ESR1, the gene encoding estrogen receptor (ER) α, are frequently found in metastatic breast cancers. The somatic mutations Y537S and D538G mutants drive estrogen-independent activities and diminish the potency of the current standard-of-care agents. Consequently, there is an unmet clinical need to develop a novel drug that inhibits ER mutant signaling. Casein kinase 2 (CK2) is a serine/threonine protein kinase that is abnormally elevated in various cancers, including breast cancer. CK2 activates ER signaling and is considered a promising therapeutic target. Here, we seek to investigate mechanisms of CK2-mediated activation of ER signaling in breast cancer and to use DNA-encoded chemical libraries (DELs) to identify and validate potent, selective, and efficacious CK2 inhibitors that will serve as lead compounds for novel therapeutics in ESR1 mutant breast cancer. Methods: The catalytic subunits of CK2 were screened with our in-house collection of 3.98 billion barcoded compounds. Hits enriched with CK2 were synthesized off DNA and tested in a biochemical assay. The most potent hits were analyzed for intracellular target engagement using NanoBRET tracer displacement assays. The top hit was extensively studied in WT, Y537S, and D538G mutant breast cancer cells. Metabolic stability was determined in mouse and human liver microsomes, and pharmacokinetics studies were performed in mice. Primary and metastatic tumors were harvested and used to develop ex vivo organoids and a WHIM20 patient-derived xenograft (PDX) model carrying the ESR1 mutation Y537S. The antitumor activity of the top hit was tested in the WHIM20 PDX model. Results: Leveraging our multibillion small molecule collection of DELs against catalytic subunits of CK2, we identified compound CDD-2007, which has low nanomolar cellular potency and good metabolic stability. CDD-2007 abolishes estrogen-induced gene programs in both wild type and mutant (Y537S and D538G) ER-positive breast cancer cell lines. In cellular assays, CDD-2007 inhibits Ser129 phosphorylation of AKT with double-digit nanomolar potency in WT and mutant breast cancer cells, but no significant loss of AKT Ser129 phosphorylation is seen in normal human breast epithelial cells (MCF10A). CDD-2007 potently inhibits primary and metastatic tumor-derived organoid growth in an ESR1 Y537S PDX model. CDD-2007 inhibits primary tumor growth in PDX models of breast cancer and synergizes with the ER antagonist fulvestrant in the ESR1 Y537S model without toxicity. Conclusion: Together, our data show that CDD-2007 is a selective and potent inhibitor of CK2 with good drug-like properties that inhibit ERα signaling in endocrine-resistant metastatic breast cancer. Therefore, blocking CK2 signifies a promising therapeutic strategy for ESR1 mutant breast cancer. Citation Format: Anil Kumar Devakrishnan, Yong Wang, Suzanne AW Fuqua, Martin Matthew Matzuk, Murugesan Palaniappan. discovery of a casein kinase 2 small-molecule inhibitor that antagonizes wildtype and mutant estrogen receptor signaling in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4754.