LAUSR

Ninety percent of relapse/refractory B‐cell acute lymphatic leukemia (R/R B‐ALL) patients can achieve complete remission (CR) after CD19‐targeting chimeric antigen receptor T (CAR‐T) cell therapy. However, around 50% of them relapse in 1 year. Persistent CAR‐T cell engraftment is considered as the key to remain durable remission. Here, we initiated a phase I study to treat 10 pediatric B‐ALL patients using a CD19‐targeted second generation CAR with a 4‐1BB intracellular costimulatory domain. All patients received a standard fludarabine and cyclophosphamide (FC) preconditioning regiment, followed by a CAR‐T infusion with a median number of 0.5 (0.3‐1.58) × 106 CAR+ T cells/kg. The pretreatment tumor burdens were high with a median bone marrow (BM) blasts percentage of 59.2% (7.31%‐86.2%), excluding one patient only with brain infiltration of leukemia cells (0% BM blasts). The initial CR rate was 80% (n = 8/10). Four patients (40%) experienced serious (grade > 2) cytokine release syndrome (CRS) and three patients (30%) with obvious neurotoxicity. Monthly assessments of CD19+ minimal residual disease (MRD) and CAR‐T engraftment demonstrated the anti‐CD19 activity of long‐term engrafted CAR‐T cell clones in one patient for more than 2 years.