Abstract 261: A mRNA-encoded trispecific CD19xBCMAxCD3 T cell engager to treat cancer and autoimmunity.

Abstract Background: B-cell-directed therapies have transformed treatment for B-cell-driven cancers and autoimmune diseases, yet current T-cell-based modalities, including CAR-T cells and T-cell engagers (TCEs), are limited by safety concerns such as cytokine release syndrome (CRS)—a particular risk for autoimmune patients requiring wide safety margins. In addition, plasma cells drive disease progression and relapse but are not efficiently eliminated by CD19-targeted treatments. To overcome these limitations, we developed a first-in-class mRNA-encoded trispecific TCE targeting CD19, BCMA, and CD3 to achieve broad B-cell and plasma-cell depletion with improved safety. Methods: The TCE was constructed by fusing a CD3-binding scFv with VHH domains targeting CD19 and BCMA. The encoding mRNA was optimized through codon and structural engineering to balance Codon Adaptation Index (CAI) and Minimal Free Energy (MFE) across untranslated regions and the coding sequence. The optimized mRNA was formulated in a novel lipid nanoparticle (LNP) designed for preferential biodistribution to secondary lymphoid organs, including spleen, bone marrow, and lymph nodes. B-cell cytotoxicity was evaluated using PBMCs from healthy donors, autoimmune patients, or in BCMA+ multiple myeloma co-culture assays. In vivo B-cell depletion, pharmacokinetics, biodistribution, and safety were tested in humanized immune-deficient mice reconstituted with CD34+ HSCs or systemic lupus erythematosus (SLE) PBMCs, in human CD19/CD3 transgenic mice, and in non-human primates (NHPs). Activity against BCMA+ tumors was assessed in PBMC-reconstituted multiple myeloma xenografts. Results: The mRNA-encoded TCE induced potent B-cell killing in human PBMC assays, achieving an EC50 of ∼0.1 pM. Robust B-cell depletion was confirmed across multiple mouse models. In the SLE-PBMC model, treatment significantly reduced autoantibody levels. In OPM2 tumor-bearing mice, the TCE induced profound CD19+ B-cell depletion in blood and lymphoid tissues and complete regression of BCMA+ tumors. In NHPs, ultra-low mRNA-LNP dosing (5 µg/kg) produced complete peripheral B-cell depletion within 6 hours, with broad depletion of naïve, memory, and plasma cell compartments in spleen, lymph nodes, and bone marrow by day 15. B-cell reconstitution began ∼3 weeks post-dose and was dominated by naïve phenotypes, suggesting immune resetting. Subcutaneous dosing achieved efficacy comparable to intravenous delivery but with markedly reduced cytokine release, consistent with the favorable pharmacokinetic profile of mRNA translation. No toxicity or clinical pathology abnormalities were observed at doses up to 100 µg/kg. Conclusion: This mRNA-encoded trispecific CD19×BCMA×CD3 TCE demonstrates potent and broad B-cell and plasma-cell depletion, anti-tumor activity, and an improved safety profile, supporting ongoing First-in-Human clinical evaluation. Citation Format: Beibei Cao, Bingxu Zhang, Shuting Yao, Qiang Zhang, Shanshan Zhou, Lun Zhang, Lei Li, Chenxing Ni, Jianqi Zhang, Ya Zhou, Xiaoyun Ma, Xiaoju Zhang, Hongya Han, Wei Xu. A mRNA-encoded trispecific CD19xBCMAxCD3 T cell engager to treat cancer and autoimmunity abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 261.