Abstract 1239: SAIL66, a next generation T cell engager targeting CLDN6, potentiates efficacy by binding to CD3/CD137

Abstract Background: Claudin-6 (CLDN6), one of over 20 known human CLDN family members, is upregulated in many solid tumors but shows minimal or no expression in healthy adult tissues. This makes it an ideal target for cancer therapy. Although several therapeutic modalities targeting CLDN6 are currently under investigation—including T cell engagers (TCE), CAR-T, and antibody drug conjugates—a more potent therapeutic option is still desirable. TCEs are a promising avenue within cancer immunotherapy, but their current efficacy and safety must be improved, particularly against solid tumors. Methods and Results: To enhance immune activation and achieve successful clinical outcomes against solid tumors expressing CLDN6, we generated SAIL66, a CLDN6-targeting next generation TCE, using a new technological format called Dual-Ig®. SAIL66 is a tri-specific monoclonal antibody designed to bind to CLDN6 on cancer cells with one Fab arm and to both CD3 and CD137 with the other Fab arm. The preclinical characterization of SAIL66 was performed in a series of in vitro and in vivo studies which included comparisons to a conventional TCE targeting CLDN6 and CD3. SAIL66 demonstrated high specificity for CLDN6 despite its similarity to CLDN3, CLDN4, and CLDN9, suggesting a lack of off-target toxicity in patients. Using a Jurkat cell system harboring NFAT or NF-κB reporter genes, we confirmed that SAIL66 exerts CD3 and CD137 signal induction activity depending on CLDN6 expression. SAIL66 induced activation of T cells, release of cytokines, and lysis of CLDN6-positive cancer cells. In vivo studies with a humanized mouse (huNOG) model showed that SAIL66 had better anti-tumor efficacy in eliminating CLDN6-positive cancer cells than a conventional TCE. Flow cytometry analysis in the huNOG model revealed that SAIL66 also more significantly increased CD3+ T cell infiltration of tumor tissue and resulted in a smaller percentage of exhausted T cells in tumor. Moreover, transcriptome analysis in the huNOG model showed that both SAIL66 and the conventional TCE increase expression of genes associated with immune activation, with a durable increase observed especially with SAIL66. Further, we clarified that T cells repeatedly stimulated with SAIL66 had a greater ability to inhibit the growth of CLDN6-positive cancer cells than T cells stimulated with the conventional TCE in vitro. Conclusion and Discussion: Our study shows that SAIL66 has better anti-tumor efficacy against CLDN6-positive cancer cells than a conventional TCE. This is due to effective immune activation by SAIL66 which triggers CD3 signaling (signal 1) combined with CD137 co-stimulatory signaling (signal 2) in T cells. Thus, SAIL66 represents a promising therapeutic option for solid tumors expressing CLDN6. A clinical trial for CLDN6-positive cancer patient is now ongoing (NCT05735366). Citation Format: Moe Yoshimoto, Naoki Kimura, Takayuki Kamikawa, Shinya Ishii, Masaru Muraoka, Kenji Taniguchi, Ryo Uchikawa, Momoko Okuda-Miura, Sho Akai, Tatsushi Kodama, Hirofumi Sakumoto, Shigeto Kawai, Mei Shimada, Mika Kamata-Sakurai, Hiroyuki Aburatani, Takehisa Kitazawa, Tomoyuki Igawa. SAIL66, a next generation T cell engager targeting CLDN6, potentiates efficacy by binding to CD3/CD137 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 1239.