Abstract 44: A novel, codon-based method to modulate off-tumor toxicity of chimeric antigen receptor (CAR) T cells

Abstract Chimeric antigen receptor (CAR) T cell therapies for hematologic malignancies, while promising and efficacious, can also result in a variety of adverse effects. Of note, on-target, off-tumor toxicity is one adverse effect that continues to limit the safety and efficacy of CAR-T cells and has even led to life-threatening or fatal outcomes in patients (Morgan et al. 2010.). To address this, we have developed a unique, codon-based approach by replacing strategically selected codons within CAR non-antigen binding domains with their rare synonymous isoforms. Hoekema et al. (1987) reported the ability of synonymous rare codons to alter gene and protein expression in other model systems. In CAR-T cells, we aim to use this novel, codon-based approach to fine tune and modulate CAR-T cell expression and function to elicit full on-tumor activity while lessening off-tumor toxicity. In our approach, we replaced specific endogenous codons within the non-antigen binding domains of Hu8F4-CAR, a TCR-like 2nd generation CAR developed by our group which targets the leukemia-associated antigen PR1/HLA-A2 (Ma et al. 2016.). Briefly, we created variants of Hu8F4-CAR containing rare codon substitutions in two subtypes: intracellular-based (type 1) and combined extracellular- and intracellular-based (type 2). Each variant subtype (type 1 or 2) had either between 6 and 30 or 27 and 57 rare codons introduced, respectively. Expression profiling and functional assessment of the Hu8F4-CAR variants was assessed in the Jurkat mutant cell line (J.RT3-T3.5) or human primary healthy peripheral blood mononuclear cells (PBMCs) that were enriched for T cells, respectively. Expression profiling results show that surface expression is reduced in J.RT3-T3.5 cells when assessed by flow cytometry by 1-1.5-fold compared to unmodified Hu8F4-CAR. When evaluated by immunoblotting, novel lower molecular weight bands emerge only in rare codon-modified variants, suggesting that rare codons may have altered translational or post-translational processing leading to truncated CAR protein isoforms (30, 45, or 60kDa vs 73kDa for unmodified Hu8F4-CAR), which may contribute to reduced cell surface CAR expression. Furthermore, we have conducted in vitro functional testing of the rare codon-modified variants and show reduced activity against non-malignant off-tumor targets that express PR1 and/or HLA-A2 including healthy donor PBMCs, mobilized hematopoietic cells, and bone marrow. Importantly, in addition to reduced off-tumor activity against healthy myeloid cells, rare codon-modified variants preserve their activity against on-tumor target cells, including the U937-A2+ cell line and primary acute myeloid leukemia. Altogether, these results suggest that our approach can modulate CAR expression and functionality creating a novel methodology to reduce the likelihood of off-tumor adverse effects typically associated with CAR therapies. Citation Format: Rolando A. Vedia, Sijie Lu, Anne V. Philips, Anna Sergeeva, Lisa S. St John, Karen Clise-Dwyer, Gheath Alatrash, Jeffrey J. Molldrem. A novel, codon-based method to modulate off-tumor toxicity of chimeric antigen receptor (CAR) T cells [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(6_Suppl):Abstract nr 44.