Enhanced Potency of BCMA/CD19 Dual-Targeting Allogeneic CAR-T Cells for Relapsed/ Refractory Multiple Myeloma with 4-1BB/TACI Intracellular Domains

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 a majority of 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, it has been reported that anti-CD19 CAR-T cells given after high-dose melphalan and stem cell transplant improve patient survival and can eliminate MM progenitor cells that are resistant to BCMA-directed CAR-T cells in ex vivo assays.

P-BCMACD19-ALLO1 is a fully allogeneic CAR-T therapy expressing two full-length CARs targeting BCMA and CD19, each with an optimized intracellular domain (ICD) for enhanced potency. P-BCMACD19-ALLO1 is a T stem cell (TSCM)-rich product derived from healthy donor pan-T cells using a nonviral transposon-based system for transgene delivery, with knockout of TRBC1/2 and B2M. Clinical data from Poseida and others have demonstrated safety and efficacy advantages associated with TSCM CAR-T, including for P-BCMA-ALLO1, our BCMA-targeting allogeneic CAR-T for RRMM (NCT04960579).

We show that replacing the standard 4-1BB co-stimulatory domain with an ICD derived from the tumor necrosis factor (TNF) receptor family member transmembrane activator and CAML interactor (TACI) enhances in vivo potency against a CD19 +/BCMA+ xenograft model when TACI is placed in the CD19 CAR. In contrast, the configuration of CD19 CAR with 41BB + BCMA CAR with TACI underperformed compared to the dual 4-1BB configuration.

The BCMA CAR was constructed using two unique fully human single-domain heavy-chain variable domains (VHs) against BCMA joined by a GS4 linker in tandem, independently binding BCMA. The binding of two unique BCMA epitopes may prevent the emergence of BCMA escape mutants. The anti-CD19 CAR binder comprises a fully human single-domain heavy-chain VH against CD19. Both CARs are expressed from a single transgene containing BCMA and CD19 CARs, an inducible Caspase-9 safety switch, and a dihydrofolate reductase mutein, separated by T2A sequences for independent expression. P-BCMACD19-ALLO1 CAR-T cells demonstrated potent in vitro cytotoxicity against tumor cells expressing BCMA, CD19, or both antigens, while single BCMA and CD19 CAR-T cells only recognized tumor cells that are BCMA and CD19 positive, respectively.

In this study, we report the enhanced in vivo activity of P-BCMACD19-ALLO1 CAR-T cells using 4-1BB/TACI ICD combination. These findings underscore the therapeutic potential of P-BCMACD19-ALLO1 CAR-T cells to be evaluated in clinical trials.

Wang:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Pang:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Chang:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Burrascano:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. DeMarco:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Mendoza:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Reyes:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Barcenas:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Nieto:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Diaz:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Arauz:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Lynn:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Nguyen:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Reed:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Connerney:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. LE:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Coronella:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company. Shedlock:Poseida Therapeutics: Current Employment, Current equity holder in publicly-traded company.