Abstract 556: Novel anti-TACI single and dual-targeting CAR T cells overcome BCMA antigen loss in multiple myeloma

Abstract

Abstract Anti-BCMA CAR T cell therapy has had success clinically against multiple myeloma, recently leading to FDA approval. However, the median progression free survival is 8.8 months, suggesting that targeting BCMA alone may not be sufficient. Reports of BCMA-negative relapse have occurred, highlighting the need for therapeutics that can overcome antigen loss. We have generated a novel antibody against a second target in multiple myeloma, transmembrane activator and CAML interactor (TACI). This antibody specifically recognizes TACI-positive cells (with either endogenous or exogenous expression) and has no recognition of non-B cell peripheral blood mononuclear cells. We designed novel second-generation CAR T cells based on the single chain variable fragment (scFv) from this antibody. We observe that anti-TACI CARs are cytotoxic in vitro at comparable levels to anti-BCMA CAR against multiple myeloma cell lines MM1S and RPMI-8226. Anti-TACI CARs are also functional in vivo and expand in peripheral blood of xenograft multiple myeloma models. We generated BCMA-knockout MM1S cells and observe that while anti-BCMA CAR treated mice have outgrowth of tumor, anti-TACI CAR treated animals retain anti-tumor activity. To overcome potential antigen loss of either BCMA or TACI, we designed tandem bispecific CAR T cells based on both single-targeting CAR designs. These BCMA and TACI dual-targeting CARs are functional in vitro and in vivo, even in the context of single antigen loss. We monitor the activation (CD69), exhaustion (PD-1, TIM-3, LAG-3), and memory (CCR7, CD45RA) phenotype over time to investigate the differences of single versus dual-targeting CAR T cells. We also study the ability of these CAR T cells to bind soluble antigen, as well as their binding avidity to tumor cells. We observe that the anti-TACI scFv more proximal to the CD3ζ signaling domain has a stronger effect on CAR functionality than when this scFv is more distal. Our studies provide a potential superior therapeutic option which remains efficacious in the context of BCMA antigen loss. Citation Format: Rebecca C. Larson, Ana Castano, Amanda A. Bouffard, Michael C. Kann, Andrea Schmidts, Kathleen M. Gallagher, Marcela V. Maus. Novel anti-TACI single and dual-targeting CAR T cells overcome BCMA antigen loss in multiple myeloma [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 556.