Abstract 7326: ATG-201, a novel T-cell engager (TCE) effectively depletes B cells with reduced risk of CRS for the treatment of B cell malignancies and B cell related autoimmune diseases

Abstract Background: CD19-targeted therapies, such as chimeric antigen receptor (CAR)-T or T-cell engagers (TCE), have been approved for the treatment of B cell malignancies. By depleting autoreactive B cells, CD19-targeted CAR-T have shown early yet promising efficacy in treating patients with B cell-driven autoimmune diseases. However, the clinical application of TCE continues to be greatly hindered by the unfavorable pharmacokinetics and toxicity associated with cytokine release syndrome. Here we developed a "2+1" CD19 x CD3 TCE, ATG-201, which effectively depletes B cells with minimal risk of CRS and IgG-like PK profile. It demonstrates potent anti-disease in vivo efficacy in mouse models for lymphoma and autoimmune diseases. Methods: ATG-201 was constructed by introducing a high affinity, novel conformational epitope-targeted anti-CD3 single chain fragment variable (scFv) to the hinge region of one of the heavy chains of a CD19 monoclonal antibody. It was evaluated in a series of in vitro studies for binding affinity, T cell dependent cytotoxicity (TDCC) cytokine release, and drug developability. The in vivo anti-lymphoma efficacy and pharmacodynamic effect were evaluated in Raji xenograft model. The tissue reside B cell depletion was assessed in CD34+ hematopoietic stem cells humanized NDG-hIL-15 mice. Pharmacokinetic parameters of ATG-201 was evaluated in normal Balb/c mice. Results: ATG-201 showed limited CD3+ T cell binding before CD19-crosslinking. It activated T cells and only in the presence of CD19+ cells. It induced potent TDCC against CD19 positive cells with pM range IC50. ATG-201 demonstrated stronger naïve B cell depletion activity with much lower cytokine release compared to benchmark TCEs in vitro. ATG-201 showed potent anti-lymphoma activity in PBMC-humanized subcutaneous Raji xenograft model with significant augment of infiltrated CD8+ T cells in tumor microenvironment and limited proinflammatory cytokines detected in plasma. Single dose ATG-201 completely and deeply depleted B cells in blood, bone marrow and spleen of CD34+ cells humanized NDG-hIL-15 mice. ATG-201 exhibited a mAb-like pharmacokinetic profile in Balb/c mice with a T1/2 of ∼300h. ATG-201 was stable at high concentration (125mg/ml) and under stress conditions. The surrogate CD19xCD3 TCE demonstrated potent efficacy in mouse systemic lupus erythematosus model, inhibiting the lymph node swelling and auto-antibody producing. Conclusion: ATG-201 demonstrates CD19-dependent CD3 binding and activation, inducing effective B cell depletion in vitro and in vivo with low cytokine release, which provides potential for the treatment of B cell malignancies and B cell related autoimmune diseases. Citation Format: Gang Bian, Huiling Liu, Tengteng Li, Zaoshun Hu, Peng Chen, Jay Mei, Bing Hou. ATG-201, a novel T-cell engager (TCE) effectively depletes B cells with reduced risk of CRS for the treatment of B cell malignancies and B cell related autoimmune diseases [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 7326.