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Background Medullary thyroid cancer (MTC) is a rare tumor that originates from the parafollicular C-cells of the thyroid. While localized MTC can sometimes be cured surgically, there is presently no known cure for metastatic MTC. In the setting of progressive metastatic MTC, TKIs (selpercatinib, cabozantinib and vandetanib) can prolong survival for some patients, at the expense of side effects and cumulative toxicities. 1–3 In particular, daily selpercatinib administration resulted in a robust overall survival (OS) benefit in MTC patients harboring RET mutations.2 However, no randomized trial has demonstrated any OS benefit in RET wild-type patients. Thus, there is a medical need to move the field forward, and explore additional therapeutic strategies with the moonshot aim of curing metastatic MTC. T cell engagers (TCEs) are an emerging class of biologics that simultaneously bind to tumor cell surface antigens and to CD3 expressed on T cells, forming an immunological synapse, resulting in T cell activation and tumor cell lysis.4–9 MTC-targeting TCEs (MTCEs) were engineered and their activity was extensively characterized in vitro.Methods Analysis of a private RNASeq dataset in tandem with a literature review was used to identify CEA, DLL3 and GFRA4 as promising MTC-associated tumor antigens to target. 10–15 Interestingly, a post-hoc ChatGPT query identified the same target antigens. MTCEs targeting CEA, DLL3 and GFRA4 were synthesized and binding to T cells and target-transduced CHO cells was characterized using flow cytometry (figure 1). Cytotoxicity was measured against MTC (MZ and TT) and non-MTC target cell lines with MTCEs in patient peripheral T cell : tumor cell co-cultures at varying E:T ratios using an XTT assay and live cell imaging. Cytotoxicity and IFNg secretion were measured with MTCEs as monotherapy and in combination with selpercatinib or cabozantinib.Results All MTCEs mediated potent cytotoxicity against target-transduced CHO cells, while exhibiting no measurable cytotoxicity against antigen-negative cell lines. MTCEs targeting CEA and DLL3 mediated potent cytotoxicity against both MTC cell lines, whereas GFRA4 was only effective against the TT cell line ( figure 2), consistent with the lack of GFRA4 expression on MZ cells. Total eradication of tumor cells was observed after 48 hours at E:T ratios as low as 1:8 in conjunction with MTCE-mediated IFNg secretion. Interestingly, enhanced in vitro tumor control was observed when combining MTCEs with TKIs, highlighting the potential of a novel therapeutic treatment regimen.Conclusions MTCEs exhibited robust in vitro activity, warrant further preclinical investigation and may one day revolutionize the treatment of MTC.Acknowledgements The author would like to acknowledge Dr. Eric Tran (EACRI) and Jena French (UC-Denver) for many thoughtful discussions over the years. They have both gone out of their way to support and inspire.References Kaliszewski, Krzysztof, et al. Cancers. 2022;14:15:3643.Hadoux, Julien, et al. New England Journal of Medicine. 2023;389:20:1851–1861.Schlumberger M, et al. Annals of Oncology. 2017;28:11:2813–2819.Huehls, Amelia M, Tiffany A Coupet, Charles L Sentman. Immunology and cell biology. 2015;93:3:290–296.Przepiorka, Donna, et al. ‘FDA approval: blinatumomab.’ Clinical Cancer Research. 2015;21:18:4035–4039.Ahn, Myung-Ju, et al. New England Journal of Medicine. 2023;389:22:2063–2075.Clarke, Hannah. ‘JANX007 demonstrates encouraging clinical activity in metastatic CRPC.’ Urology Times. 2025;53:1.Segal, Neil H, et al. Nature Communications .2024;15:1:4091.Giffin, Michael J, et al. Clinical Cancer Research. 2021;27:5:1526–1537.Uhlén, Mathias, et al. Tissue-based map of the human proteome. Science. 2015;347:6220(2015):1260419.Salaun, Pierre-Yves, et al. Journal of Nuclear Medicine. 2012;53:8:1185–1192.Lindahl, Maria, et al. J. Biol. Chem. 2000;14:14.Ingenwerth, Marc, et al. Endocrine connections. 2021;10:3:283–289.Erickson, Tim Andrew, et al. Thyroid. 2022;32:7:789–798.McCue, Amelia C, et al. Mabs. 2024;16(1). Taylor & Francis.Abstract 969 Figure 1Identification of MTC surface antigens to target. RNAseq and confirmatory flow data Identifying CEA, GFRA4 and DLL3 as addressable surface antigensAbstract 969 Figure 2MTCEs eradicate MTC TT cells in vitro. MTCEs targeting DLL3, GFRA4 and CEA mediate potent cytotoxicity against the MTC TT cell line and require presence of T cells. Neither T cells alone or T cells in combination with the anti-CLDN18.2 TCE gresonitamab mediate observable cytotoxicity