CD19CAR T Cell Products of Defined CD4:CD8 Composition and Transgene Expression Show Prolonged Persistence and Durable MRD-Negative Remission in Pediatric and Young Adult B-Cell ALL

Introduction: Multiply recurrent pre-B-cell ALL, and particularly relapse following allo-HSCT, has dismal outcomes due in large part to ineffectual therapies. The primary objectives of the Phase 1 portion of the PLAT-02 study (NCT02028455) were to determine the feasibility of manufacturing products of defined composition and transgene expression, the safety of the cryopreserved T cell product infusion, and to describe the full toxicity profile, including development of clinically significant GVHD in the post-allo-HSCT cohort. Methods: Subjects on the PLAT-02 study undergo apheresis, with their CD4 and CD8 T cell subsets prepared immunomagnetically. Following anti-CD3xCD28 bead stimulation, T cell lines are transduced with a SIN lentiviral vector that directs the co-expression of the FMC63scFv:IgG4hinge:CD28tm:4-1BB:ζ CAR and the selection/tracking/suicide construct EGFRt. Transduced cells are propagated using recombinant human cytokine cocktails to numbers suitable for clinical use over 10-22 days, during which time they are subjected to EGFRt immunomagnetic positive selection. Shortly following lymphodepleting chemotherapy, cryopreserved CD4/EGFRt+ and CD8/EGFRt+ T cell products are thawed and infused at the bedside such that patients receive a 1:1 ratio of EGFRt+ CD4 and CD8 T cells at the protocol-prescribed dose level. Results: 45 subjects have been enrolled and 43 have been treated from dose level 1 (5 x 105 CAR-T cells/kg) through 4 (10 x 106 CAR-T cells/kg). Therapeutic T cell products were released on all 45 enrolled subjects, with 1 subject requiring a second apheresis. Two subjects died of disease prior to their infusion. All 43 infused subjects received lymphodepletion chemotherapy prior to T cell infusion (cyclophosphamide, n=27; fludarabine/cyclophosphamide; n=14 cyclophosphamide/etoposide n=1; fludarabine n=1,). 91% (39/43) of subjects received infusions at the desired 1:1 CD4:CD8 ratio and their infusions were well tolerated with only 1 related AE >grade 2. 93% (40/43) of subjects had a documented MRD-negative CR within 21 days following CAR-T cell therapy. The 12 month event-free survival (EFS) is 50.8% (95% CI 36.6, 69.9) and 12 month OS is 69.5% (95% CI 55.8, 86.5). All responding subjects exhibited in vivo expansion of CAR-T cells. The % of CAR T cell expansion over time is not impacted by dose level or lymphodepletion but is impacted by disease burden (p=0.004) and total CD19 antigen burden (p=0.001) at the time of lymphodepletion. The median duration of functional CAR-T cell persistence as measured by ongoing B-cell aplasia (BCA) is impacted by the total CD19 antigen burden in the bone marrow at time of lymphodepletion (>15% vs Conclusions: Infusions of defined composition CD4:CD8 CD19 CAR/EGFRt+ T cells/kg produce high rates of MRD-negative CR in pediatric and young adult B-cell ALL patients. Based on intent to treat analyses without a proliferation screen, we have found it is feasible to generate CAR products from each of the enrolled subjects. Despite the high rates of MRD-negative CR, the durability of remission is highly influenced by the functional persistence of CAR-T cells. Strategies to enhance persistence are currently being investigated, including episodic antigen stimulation through subject derived tAPC engineered to express truncated CD19. Disclosures Gardner:Amgen: Honoraria. Li:Juno Therapeutics: Employment, Equity Ownership. Jensen:Juno Therapeutics, Inc: Consultancy, Equity Ownership, Membership on an entity9s Board of Directors or advisory committees, Research Funding.