Abstract 2357: T cell receptor beta chain-directed Tri-specific antibody molecules retarget V-beta T cell subsets to tumors to promote potent and durable antitumor activity

Abstract Background We have previously shown that novel bifunctional “STAR” antibody-fusion molecules comprising germline β chain TCR-targeting antibodies fused to costimulatory cytokines co-engage the TCR and T cell cytokine receptors, thereby promoting activation and expansion of subsets of T cells expressing distinct germline variable β (Vβ) chain TCRs. Here we describe a new class of T cell engager (TriSTAR) that utilizes a tri-specific format comprising the STAR TCR β chain antibody-IL-2 fusion with an additional Fab arm targeting tumor-associated antigens (TAA). By design, TriSTAR molecules promote the activation and expansion of Vβ T cell subsets that adopt a novel memory phenotype and are subsequently re-targeted to tumors expressing specific TAAs. TriSTAR molecules induce potent and durable anti-tumor activity which in refractory murine solid tumor models appear to be superior to established anti-CD3 T cell engager (TCE) formats. Methods Using the gp75 (Tryp1) melanoma antigen as a model TAA, gp75-targeting TriSTARs were tested in vivo in the B16F10 syngeneic mouse tumor model, compared with a conventional gp75-targeting CD3 bispecific TCE. Cured mice were re-challenged with tumors to assess the potential of TriSTARs to induce immunological memory responses. The pharmacodynamics and pharmacokinetics of gp75-targeting TriSTARs and anti-CD3 TCEs were assessed in blood and tumor using flow cytometry, NanoString and other immunoassays. This work was further extended to testing human TAA-targeting TriSTARs using in vitro human cytotoxicity assays and a transgenic MC38 model. Results In the B16F10 murine melanoma model, gp75-targeting TriSTARs induced potent anti-tumor responses that were superior to control constructs and gp75-targeting CD3 TCEs. The superior anti-tumor activity of the gp75-targeting TriSTARs was attributed to a significant increase and accumulation of CD8+, CD25+ and Granzyme B+ T cells in tumors that was due to the expansion of the target Vβ T cell subset. Further immune profiling by flow cytometry and NanoString in tumors and blood isolated from treated B16F10 mice highlighted significant remodeling of tumor-infiltrating lymphocytes (TILs) and other immunological changes associated with immune activation. These observations with murine gp75-targeting TriSTAR molecules were broadly confirmed with TriSTARs targeting human TAAs in both human in vitro studies and human TAA transgenic mouse tumor models. Conclusions Vβ TCR-targeting TriSTAR molecules represent a new class of T cell engager with the potential to deliver improved anti-tumor activity compared with existing anti-CD3 T cell engagers in immunologically “cold” solid tumors with low levels of T cell infiltration and antigen presentation and may therefore offer a new therapeutic approach for the treatment of patients with refractory solid tumors. Citation Format: Madan Katragadda, Jacques Moisan, Gurkan Guntas, Manuel Sequeria, Roya Servattalab, Jessica Lowry, Robert Ruidera, Kent Nybakken, Erik Gerson, Ke Liu, Kevin Chin, Zhen Su, Andrew Bayliffe. T cell receptor beta chain-directed Tri-specific antibody molecules retarget V-beta T cell subsets to tumors to promote potent and durable antitumor activity [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 2357.