Safety and Antitumor Effects of CD19-Specific Autologous Chimeric Antigen Receptor-Modified T (CAR-T) Cells Expressing the Inducible Caspase 9 Safety Switch (iC9-CAR19 T Cells) in Adult Acute Lymphoblastic Leukemia (ALL)

Introduction: CAR-T cells targeting the CD19 antigen are approved to treat children and young adults with relapsed and refractory B-cell ALL, in whom response rates are >80%. Acute toxicities, including cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS) complicate CAR-T therapy in most patients. Though most patients with CRS or ICANS experience toxicities that can be managed with supportive care including corticosteroids and tocilizumab, 46% experience grade 3-4 CRS and 13% experience grade 3 ICANS (Maude SL et al N Engl J Med 2018; 37:439). Thus, novel approaches to address the management of high grade toxicities are needed. Donor T lymphocytes engineered to express human caspase 9 fused to a modified human FK-binding protein that induces caspase-dependent apoptosis when exposed to the dimerizing drug rimiducid (Di Stasi A et al. N Engl J Med 2011; 365:1673). We hypothesized that the inducible caspase 9 safety switch (iC9) coupled with CAR19 could mitigate severe CRS or ICANS in patients treated with CAR-T cells. We initiated a phase I trial to test the safety and efficacy of autologous T lymphocytes, genetically modified to express both iC9 and CAR19 administered to patients with relapsed and refractory B-ALL. Methods: Subjects with B-cell ALL in 2nd or greater bone marrow (BM) relapse, relapse >100 days after allogeneic stem cell transplant, disease refractory to ≥2 induction therapies, or with measurable residual disease (MRD) persistence/recurrence were enrolled in a phase I dose escalation trial. Autologous T-lymphocytes were collected, and CAR-T cell products generated by gene modification with a γ-retroviral vector encoding for iC9, ΔNGFR (for selection and tracking purposes) and CAR.CD19 (encoding 4-1BB) genes (Diaconu I et al Mol Ther 2017; 25:580). Subjects underwent lymphodepletion with fludarabine and cyclophosphamide and CAR-T cells were subsequently infused at one of two dose levels (DL1: 5 x 105 CAR-T cells/kg; DL2: 1 x 106 CAR-T cells/kg). Toxicities were graded by CTCAE v5 or ASBMT consensus grading for CRS and ICANS. Dose limiting toxicities (DLT) were grade 3-4 CRS or ICANS lasting >7 days despite standard of care intervention or grade 3 or higher autoimmune or non-CRS/ICANS organ toxicity. CAR-T cell expansion in peripheral blood (PB) was determined by flow cytometry (FC) and Q-PCR. Leukemia response was determined by NCCN criteria at 4 and 8 weeks after CAR-T infusion. Results: Nine prod