Abstract 6726: Costimulatory IgM T-cell engagers with enhanced and durable cytotoxicity

Abstract T-cell engagers (TCEs) have established therapeutic effect in treatment of various cancers by harnessing the power of the immune system against tumors in a targeted manner. IgM-based TCEs may offer high avidity, specificity, and safety advantages over other modalities because of their multivalent architecture and unique positioning of the CD3 binding domain on the J-chain. We have built a costimulatory IgM TCE platform that engages both signal 1 and signal 2 on T-cells, with the goal of enabling their optimal activation and survival for more robust and durable cytotoxic activity. Multiple antigen targeting costimulatory IgM TCEs were generated and activity was evaluated both in vitro and in vivo to assess the role of CD28 co-stimulation. Costimulatory IgM TCEs engaged both CD3 and CD28 on the T-cell surface, enhancing in vitro cytotoxicity and T-cell activation. Levels of IL-2 increased significantly with dual signal engagement promoting proliferation and survival of T-cells. In long-term culture assays, costimulatory IgM TCEs promoted better T-cell proliferation and survival compared to IgM TCEs with only a CD3 binder, and was even more pronounced at low E: T ratios. In the absence of CD3 engagement, CD28 binding alone did not induce any T-cell activation, cytotoxicity, or cytokines. Activation of T-cells was also fully dependent on the presence of target cells, underscoring the safety profile of the platform. In Hu-PBMC xenograft tumor models, costimulatory IgM TCEs exhibited stronger in vivo anti-tumor activity compared to TCEs only engaging CD3. Tumor growth inhibition was maintained for longer after dosing was stopped with costimulatory IgM TCEs, indicating the durability of responses. Costimulatory IgM TCEs suppressed tumor growth in xenograft studies using cell lines that expressed both high and low target copy numbers suggestive of their broad utility against multiple tumor types. Enhanced in vivo anti-tumor activity of costimulatory IgM TCEs was associated with significant increases in intra-tumoral CD8/CD3+T-cells. Both peripheral and intra-tumoral T-cells exhibited enhanced anti-apoptotic and cytotoxic phenotypes indicative of the costimulatory activation. IgM-based costimulatory TCEs provide enhanced cytotoxicity through optimal T-cell activation and proliferation/survival. This dual engagement of T-cell activation signals, along with the high avidity target binding offered by IgM platform, could enable the creation of effective therapeutics targeting pathogenic cells in autoimmune diseases and solid tumors that will maintain activity even in conditions with low T-cell counts. Citation Format: Jie Xue, Keyu Li, Poonam Yakkundi, Palak Chadasama, Tina Mao, Zee Malik, Vidhya Rao, Madhura Joglekar, Kristene Mai, Gene Li, Deepal Pandya, Rodnie Rosete, Zhongde Ye, Leyla Tahrani, Jinqiu Wang, Nardeen Hanna, Elizabeth Perez, Yue Wang, Sachi Ahmed, Lusiana Widjaja, Paul Hinton, Krzysztof Bzymek, Bruce Keyt, Miho Oyasu, Liqin Liu, Angus Sinclair, Umesh S. Muchhal. Costimulatory IgM T-cell engagers with enhanced and durable cytotoxicity abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6726.