S1635 ACADEMIC, PHASE1 TRIAL ON T CELLS EXPRESSING BOTH CD19 CHIMERIC ANTIGEN RECEPTOR AND INDUCIBLE CASPASE 9 SAFETY SWITCH FOR TREATMENT OF CHILDHOOD ACUTE LYMPHOBLASTIC LEUKAEMIA AND NON‐HODGKIN LYMPHOMA

Background: Survival rates of children affected by relapsed/refractory (r/r) B‐Cell Precursor Acute Lymphoblastic Leukemia (BCP‐ALL) or relapse of Non‐Hodgkin lymphoma of the B lineage (B‐NHL) remain largely unsatisfactory. Targeted immunotherapy with T‐cells genetically modified to express a CD19‐directed chimeric antigen receptor (CAR) revealed unprecedented antitumor efficacy, leading to the FDA and EMA approval of two CD19‐CAR products. Relevant toxicities have, however, been reported, being mainly related to the development of severe Cytokine Release Syndrome (CRS) and/or of neurotoxicity Aims: At Ospedale Pediatrico Bambino Gesù (OPBG) in Rome, we opened an academic, phase I/II clinical trial on the use of autologous T cells transduced with a 2 nd generation CD19‐CAR and the suicide gene inducible caspase‐9 (iC9‐CD19‐CAR), for treatment of pediatric patients and young adults (aged 1 – 25 years) affected by BCP‐ALL or B‐NHL. We now report the results of the phase I portion of the study Methods: We developed a clinical‐grade, retroviral, CD19‐specific CAR construct, including 4.1bb as costimulatory domain and iC9, as safety switch. The phase I, dose‐escalation, portion of the study consecutively enrolled patients with r/r BCP‐ALL or B‐NHL. Three dose levels (DL) have been tested, namely: DL1, 0,5; DL2, 1,5; DL3, 3,0 × 10 6 CAR + T cells per kg of recipient body weight. Autologous T cells collected by leukoapheresis were engineered through a 14‐day manufacturing process. All patients received a lymphodepleting regimen consisting of fludarabine and cyclophosphamide and iC9‐CD19‐CAR T cells were subsequently administered as single infusion. Patients were monitored for toxicity, expansion and persistence of the CAR T cells Results: A total of 9 children were enrolled in the trial and received iC9‐CD19‐CAR T cells between January and August 2018 (characteristics of the patients in Fig1A). The designed dose concentration was successfully produced for all the patients (details in Fig1A). No dose limiting toxicities (DLTs) have been recorded, defining the MTD as 3,0 × 10 6 CAR + T cells/kg. The treatment was overall tolerated and all the toxicities were fully reversible, the most severe being grade 3–4 cytopenias, occurring in all the patients; in 6/9 patients (66,7%) the hematological toxicity developed before CAR T‐cell infusion and persisted after. CRS occurred in 6/9 patients and was mild, reaching grade 2 in 1 patient only. Notably, none of the patients developed neurotoxicity and no activation of iC9 was required. Although beyond the primary objective of the phase I, all patients were assessed for response and 7/8 patients with BCP‐ALL (87,5%) achieved complete remission, including 2/3 patients receiving the DL1 and 5 patients who had failed a previous haematopoietic stem‐cell transplantation. One CD19‐negative relapse occurring 3 months after infusion was recorded. The iC9‐CD19‐CAR T cells expanded in vivo and were detectable in blood, bone marrow and cerebrospinal fluid of the responders (figure 1B). The patient with longest follow‐up maintains CR at 14 months and iC9‐CD19‐CAR T cells have been detectable by flow cytometry, in blood, for up to 4 months after infusion Summary/Conclusion: iC9‐CD19‐CAR T cells in an academic setting is feasible, safe and largely effective in treating highly resistant/relapsed BCP‐ALL. In our trial, no major or life‐threatening toxicities were recorded and despite the mild CRS recorded, high rates of CR were achieved, suggesting that a mild activation of retroviral, 4.1bb –including, iC9‐CD19‐CAR T cells is sufficient to mediate a potent antitumor effect