Abstract 4811: Enhanced potency of BCMA/CD19 dual-targeting allogeneic CAR-T cells for relapsed/ refractory multiple myeloma with 4-1BB/TACI intracellular domains

Abstract B-cell maturation antigen (BCMA)-directed chimeric antigen receptor (CAR) T cell therapy has shown high initial response rates in relapsed/refractory multiple myeloma (RRMM), but most patients eventually relapse with progressive disease. Previous studies have identified a less differentiated, CD19-positive progenitor subpopulation of MM cells that contributes to resistance and poor survival. Furthermore, anti-CD19 CAR-T cells given after high-dose melphalan and stem cell transplantation have shown to improve patient survival and can eliminate MM progenitor cells that are resistant to BCMA-directed CAR-T cells. P-BCMACD19-ALLO1 is a fully allogeneic CAR-T therapy designed to target BCMA and CD19. Additionally, P-BCMACD19-ALLO1 is a T stem cell memory (TSCM)-rich product derived from healthy donor T cells using a nonviral, transposon-based system for transgene delivery, with knockout of TRBC1/2 and B2M to prevent graft-versus-host disease and reduce host T-cell mediated rejection, respectively. TSCM -rich CAR-Ts, including our BCMA-targeting allogeneic P-BCMA-ALLO1 CAR-T for RRMM (NCT04960579), have shown favorable efficacy and safety in the clinic. To develop P-BCMACD19-ALLO1, we constructed the BCMA CAR using two fully human, single-domain, heavy-chain variable domains (VHs) that independently bind BCMA and are joined together by a G4S linker in tandem. The tandem anti-BCMA VHs binds to all clinical escape mutants, including teclistamab-resistant R27P mutation. The anti-CD19 CAR binder comprises of a fully human VH against CD19. Both CARs are expressed from a single, multi-cistronic transgene that also contains an inducible Caspase-9 safety switch and a dihydrofolate reductase mutein. Herein, we show enhanced in vivo potency of P-BCMACD19-ALLO1against BCMA/CD19-double positive RPMI-8226 MM xenograft model using an optimized ICD combination that incorporates a novel co-stimulatory domain derived from the tumor necrosis factor receptor family member transmembrane activator and CAML interactor (TACI) in the CD19 CAR and the standard 4-1BB in the BCMA CAR. P-BCMACD19-ALLO1 demonstrates target-specific, in vitro cytotoxicity against tumor cells expressing BCMA, CD19, or both antigens but lacks activity against antigen-negative tumor cells. P-BCMACD19-ALLO1 also demonstrates potent, in vitro killing of tumor cells expressing R27P, P33Del, P33S and S30Del escape mutants. Lastly, P-BCMACD19-ALLO1 effectively eliminates primary bone marrow CD81+CD19+ progenitor cells as well as colony-forming cells derived from MM patient samples, suggesting that P-BCMACD19-ALLO1 may increase the depth and the durability of response. Taken together, our data underscore the therapeutic potential of P-BCMACD19-ALLO1 and support its application for treating RRMM to be further evaluated in clinical trials. Citation Format: Steven Wang, Iris Pang, Claudia Chang, Michelle Burrascano, Nick DeMarco, Danny Mendoza-Reyes, Sonia Reyes, Arturo Barcenas, Pastor Nieto, Jose Diaz, Garret Arauz, Chris Lynn, Tony Nguyen, Connor Reed, Mona Connerney, Quy Le, Julia Coronella, Devon J. Shedlock. Enhanced potency of BCMA/CD19 dual-targeting allogeneic CAR-T cells for relapsed/ refractory multiple myeloma with 4-1BB/TACI intracellular domains [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 4811.