Abstract 3507: Discovery and preclinical characterization of novel TCR-mimetic T-cell engagers targeting TERT peptide-HLA complex for the treatment of solid and hematologic malignancies

Abstract TERT (telomerase reverse transcriptase) is highly expressed in 85-95% of all tumors, making it a compelling target for cancer therapy. Although several TERT-targeting approaches are being investigated in clinical trials, only one therapeutic (RYTELO®, imetelstat) has received FDA approval for the treatment of low-risk myelodysplastic syndrome. Targeting HLA-restricted peptides (pHLA) derived from intracellular cancer antigens allows for broader exploration of the cancer proteome. The HLA-A*02:01-restricted TERT540 peptide is presented by human tumors and can be a target for T cell-mediated tumor lysis. TCR mimicking (TCRm) antibodies recognize pHLA complexes and are capable of re-directing T cells once incorporated into a T Cell Engager (TCE) format. Here we report the preclinical characterization of potent and specific TCRm-based TCEs targeting TERT540 pHLA. TERT540-targeted TCEs consist of a single humanized CD3 binding domain and either one or two fully human TCRm domains, thus recognizing TERT540/HLA-A*02:01 in either monovalent or bivalent forms. Monomeric affinity for TERT540 pHLA was found to be in sub-nM range, and target binding was greatly enhanced through avidity effects in the presence of two TCRm binding domains. Alanine/glycine scan identified multiple amino acid residues in the TERT540 peptide sequence that are required for TCE binding. Extensive specificity profiling revealed no meaningful activity of TERT540 TCEs against cells pulsed with supra-physiological levels of computationally predicted HLA-A*02:01 cross-reactive peptides or multiple unrelated HLAs (alloreactivity). Furthermore, both TERT540 TCEs showed no interactions with a large panel of normal primary cells from multiple vital tissues and organs in vitro. A modest IFNγ response was noted against bone marrow (BM) CD34+ hematopoietic stem cells at high concentrations of TERT540 TCEs. In contrast, both compounds induced potent, target-dependent T cell activation and T cell-mediated killing of multiple TERT- and HLA-A*02:01- positive leukemia, lung, and ovarian cancer cell lines, achieving sub-nM EC50 values in vitro. Consistent with the specificity profile of TERT540 TCEs, no activity was observed against TERT-negative or HLA-A*02:01-negative cells. In vivo efficacy studies with TERT540-targeted TCEs are currently in progress. Comprehensive analytical and biophysical assessments revealed favorable developability profiles for both TCEs. In summary, these data provide strong preclinical evidence of potency, specificity, safety, and developability of novel, TCRm-based TCEs for TERT-targeted cancer therapy. Citation Format: Yu Huang, Bhupal Ban, Jessica Jimenez, Preethi Sankaran, Christine A. Devlin, Melissa Bikowitz, Qunyan Yu, Ricard Masia, Delainey O’Connor, Emily McNally, Sarah Jaffe, Tanzila Rahman, Jennifer Helble, Alona Kulesha, Andrew Wolpert, Brendan Whalen, Shawn O’Malley, Yue Li, Michael Jennings, Galina Gabriely, Amanda Mak, Tao Wang, Benjamin Lee, Michi Schebesta, Michael Princiotta, Geraldine L. Paulus, Dmitri Wiederschain. Discovery and preclinical characterization of novel TCR-mimetic T-cell engagers targeting TERT peptide-HLA complex for the treatment of solid and hematologic malignancies [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 3507.