Preclinical Evaluation of ALLO-605, an Allogeneic BCMA Turbocar T^TM Cell Therapy for the Treatment of Multiple Myeloma

Chimeric antigen receptor (CAR) T cells have demonstrated unprecedented efficacy in heavily pretreated relapsed and/or refractory multiple myeloma (MM) patients but may require further engineering to achieve their greatest potential. Enhancing cell expansion and persistence by providing cytokine support has been shown to improve the long-term antitumor activity of adoptively transferred CAR T cells in preclinical models. Combining CAR T cells with systemically-administered cytokines or cytokine mimetics can however result in toxicities and adverse events. Alternatively, cytokine signaling can be provided in a constitutive, CAR T cell-intrinsic fashion, by exogenous expression of a Constitutively Active Chimeric Cytokine Receptor (CACCR). CACCRs can be engineered by combining a membrane-tethered dimerization and JAK-binding domain derived from the thrombopoietin receptor (TpoR) fused to an intracellular signaling domain derived from a cytokine receptor. We investigated the impact of CACCR expression on the phenotype, functionality, persistence, and safety profile of allogeneic CAR T cells targeting BCMA to produce a second generation allogeneic BCMA "TurboCAR T™" candidate (ALLO-605). BCMA CAR T cells were generated from healthy donor T cells by lentiviral transduction with a CAR construct followed by genetic inactivation of the TRAC and CD52 loci using TALEN® gene editing. Allogeneic BCMA TurboCAR™ T cells, engineered for stoichiometric expression of the CAR and a CACCR via a self-cleaving peptide, were produced similarly. Constitutive expression of the CACCR during manufacturing had no negative effects on CAR T cell phenotype or yield and resulted in a product with over 60% stem cell memory/central memory T cells. In vitro, BCMA TurboCAR T™ cells showed enhanced cytokine secretion, polyfunctionality and improved serial killing activity. In a disseminated mouse model of multiple myeloma, BCMA TurboCAR T™ cells exhibited at least a 2-fold increase in peak expansion and enhanced survival and persistence compared to BCMA CAR T cells, resulting in prolonged antitumor responses and delaying relapses. Despite this enhanced persistence, we found that exposure to target cells was absolutely required for the expansion and long-term activity of BCMA TurboCAR T™ cells and no evidence of target- and cytokine-independent proliferation was observed. Since adoptive cell therapies have the potential to elicit toxicities in some patients, two alternative approaches to modulate BCMA TurboCAR T™ cell activity were investigated. First, we tested the ability of a CD20-based off-switch incorporated within the CAR to sensitize cells to rituximab and found effective depletion of BCMA TurboCAR T™ cells by complement or effector cells in vitro and in vivo in the presence of the antibody. In addition, we confirmed rapid inhibition of BCMA TurboCAR T™ cells by the protein tyrosine kinase inhibitor dasatinib, which has been shown to interfere with LCK activity. The prolonged persistence and antitumor responses seen in preclinical models along with a favorable safety profile of BCMA TurboCAR™ T cells support clinical investigation of ALLO-605 in relapsed or refractory multiple myeloma.