Preclinical Characterization of CLSP-1025, a First-in-Class, Mutation-Specific T-Cell Engager Targeting a Neoantigen Derived from a Common p53 Mutation

Abstract Purpose: Targeting intracellular oncogenic mutations with T-cell engagers (TCE) has the potential to expand precision immunotherapy beyond surface antigens. In this study, we describe the preclinical characterization of CLSP-1025, a mutation-specific TCE that targets the p53R175H neoantigen presented by HLA-A*02:01. Experimental Design: CLSP-1025 binding, selectivity, and functional activity were evaluated using biochemical assays, T-cell activation and cytotoxicity assays, primary human cells, patient-derived tumor organoids, and humanized mouse models. Off-target activity was assessed using broad human leukocyte antigen (HLA) cross-reactivity screens, primary tissue panels, and cytokine release assays. Pharmacokinetic (PK) and pharmacodynamic properties were characterized in rats and human CD3 transgenic mice. Results: CLSP-1025 bound with nM affinity to both CD3 and the HLA-A*02:01–p53R175H complex, with structural modeling suggesting spacing consistent with a physiologic immune synapse. The molecule mediated potent and mutation-dependent cytotoxicity across tumor cell lines and patient-derived organoids expressing p53R175H, with minimal activity against p53 wild-type targets. Extensive cross-reactivity screening demonstrated a narrow HLA recognition profile and limited off-target T-cell activation. In humanized mouse models, CLSP-1025 induced robust tumor growth inhibition, accompanied by increased intratumoral CD8+ and CD4+ T-cell infiltration. PK analyses showed dose-proportional exposure with an IgG-like half-life, and CD3 receptor occupancy correlated with systemic exposure. Conclusions: These data establish CLSP-1025 as a highly selective, mutation-specific TCE and support the feasibility of targeting shared intracellular neoantigens with peptide–HLA–directed immunotherapies. This favorable preclinical activity and specificity support ongoing clinical investigation of CLSP-1025 in HLA-A*02:01+ patients with cancers harboring p53R175H and outline a framework for developing next-generation TCEs against additional driver mutations.