Phase 1 Study of CD19/CD22 Bispecific Chimeric Antigen Receptor (CAR) Therapy in Children and Young Adults with B Cell Acute Lymphoblastic Leukemia (ALL)

Chimeric Antigen Receptor (CAR) therapy targeting CD19 achieves complete remission (CR) rates of 70%-90% in relapsed/refractory B-ALL. Relapse due to loss of the CD19 targeted epitope presents a therapeutic challenge as evidenced by the largest global pediatric CD19-CAR experience which showed 15 of 16 relapses to be explained by CD19 downregulation (Maude et al, NEJM 2018). Alternatively targeting CD22 using CD22-CAR therapy has demonstrated a CR rate of approximately 70% in both CD19+ and CD19- B-ALL, however relapse due to CD22 downregulation limits the curative potential of singularly-targeting CD22 (Fry et al, Nat Med. 2018). We hypothesized that simultaneous targeting of CD19 and CD22 via a bispecific CAR-T cell would be a safe and tolerable treatment strategy in relapsed/refractory B-cell ALL and address immune evasion. Here, we report the first clinical experience in pediatric patients using bispecific CD19-CD22 CAR T cells. We describe a single institution phase I dose escalation study in pediatric patients with relapsed or refractory B cell ALL. We utilized lentiviral transduction of a bivalent CAR construct incorporating the fmc63 CD19 and m971 CD22 single chain variable fragments (scFvs) used in clinically tested CAR constructs and a 41BB costimulatory endodomain (Fry et al, Nat Med. 2018). Our primary objectives are feasibility of production of this bivalent CAR and safety at 3 dose escalation levels (1x106, 3x106 and 1x107 CAR T cells/kg). Clinical response assessment is evaluated as a secondary aim. All patients described received lymphodepletion with fludarabine (25mg/m2 x 3 days) and cyclophosphamide (900mg/m2 x 1) followed by fresh or cryopreserved CAR T cell infusion after a 7-9 day production time. Patients were prospectively monitored at predefined intervals for disease response and correlative assessments. Four pediatric patients with precursor-B ALL, age 2-17, have been enrolled and treated with CD19/CD22 bispecific CAR T cells at dose level 1 (1x106) [Table 1]. Three patients entered CAR therapy with low disease burden detected by minimal residual disease (MRD) alone and 1 patient initiated therapy with 12% bone marrow blasts. All patients were CNS1 at time of treatment. The toxicity profile mirrored that of the singular CD19 and CD22 CAR experience with 3 patients experiencing reversible CRS (2 Grade I, 1 Grade II), onset day 3-8, and 2 patients experiencing grade I neurotoxicity, onset day 3-9. In our cohort, we experienced lower