Abstract 2891: Enhanced anti-tumor responses with a novel dual pMHC T-cell engager bispecific antibody

Abstract Intracellular tumor antigens presented as peptides on MHC (pMHC) class I molecules are attractive targets for more tumor-selective immunotherapeutic approaches with promising data already emerging from clinical trials. pMHCs have been targeted by TCR-engineered T cells or soluble recombinant T-cell receptors (TCRs) fused to an anti-CD3 fragment. Naturally occurring cancer-reactive TCRs have weak affinity and require substantial affinity enhancements for their cognate pMHC. However, the outcome of this process is difficult to predict and bears the risk for off-target cross reactivities in normal tissues, which may lead to severe adverse events in the clinic. Here, we describe a highly potent dual pMHC T-cell engager (TCE) antibody format with high specificity towards tumor-specific pMHCs utilizing an HLA-A*02:01 restricted MAGE-A4 epitope to validate the concept. A series of monovalent and bivalent antibody constructs composed of anti-MAGE-A4 binding arms, ranging in affinities from 30 nM to 100 pM, were fused to an anti-CD3 Fab fragment with lower affinity compared to that commonly used for TCR-fusions. The different antibody constructs were evaluated for selective killing of MAGE-A4/HLA-A*02 positive human U2OS osteosarcoma and A375 melanoma cancer cells versus a panel of different MAGE-A4-negative/HLA-A*02-positive human cell lines. Bivalent bispecific antibody variants mediated a 7-fold greater degree of cancer cell killing and similarly increased T cell activation compared to their monovalent bispecific counterparts. EC50 values ranged as low as single digit picomolar, while the overall cross reactivity against MAGE-A4-negative/HLA-A*02-positive cells was not substantially affected. These results prove that dual targeting of pMHCs on cancer cells provides selective and efficient T cell-mediated target cell killing and T cell activation, even at very low levels of pMHC on the cell surface, highlighting the pivotal roles played by the affinity of the individual arms, valency, and epitope densities. We further analyzed the dual pMHC TCE for potential off-target effects by recognition of similar and physiologically relevant non-MAGE-A4 peptides. T2 cells pulsed with similar peptides and co-cultured with PBMC effector cells showed no significant T cell activation or IFNg release in the presence of the dual pMHC TCE in comparison to MAGE-A4 peptide-pulsed T2 cells. Finally, we compared the dual pMHC TCE against a recombinant TCR fused to an anti-CD3 scFv, a construct that is currently in clinical development. Interestingly, the dual pMHC TCE resulted in a 3-fold more potent cancer cell killing while having significantly lower effect on cytokine production. In conclusion, dual pMHC targeting with TCE antibodies is an attractive alternative to soluble affinity-enhanced TCR-based cancer immunotherapies as they facilitate potent tumor targeting without the need for extensive affinity enhancements. Citation Format: Stephanie Jungmichel, Fabian Scheifele, Anna Sobieraj, Severin Wendelspiess, Thomas Schleier, Philip Knobel, Camilla Winnewisser, Melissa Vrohlings, Philipp Richle, Hannes Merten, Jacqueline Blunschi, Christian Leisner, Leonardo Borras. Enhanced anti-tumor responses with a novel dual pMHC T-cell engager bispecific antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2891.