Phase I study of CD19 CAR T-cell therapy harboring a fully human scFv in CAR-naïve adult B-ALL patients

CD19-directed chimeric antigen receptor-engineered (CD19 CAR) T-cell therapy elicits high response rates but fails to induce durable responses in most adults with relapsed or refractory (R/R) B-cell acute lymphoblastic leukemia (B-ALL). In a previous clinical trial (NCT01865617), we observed anti-CAR immune responses associated with impaired in vivo CAR T-cell expansion after second infusions. Because these CD8+ T-cell responses were predominantly directed at peptides derived from the murine single chain variable fragment (scFv) in the CAR, we conducted a clinical trial investigating the safety and efficacy of CD19 CAR T-cells engineered with a CAR incorporating a fully human scFv (JCAR021) in adults with R/R B-ALL (NCT03103971). Twenty-three patients received lymphodepletion chemotherapy and JCAR021 infusion. Nineteen patients developed CRS (any grade, 83%; grade 2, 61%) and 12 developed neurotoxicity (52%; grade ≥3, 35%). The overall response and CR/CRi rates were 82% and 64%, respectively. We observed MRD-negative marrow responses in 82% of those with marrow disease and extramedullary responses by PET-CT in 79% (CR, 50%) of those with measurable FDG-avid disease. The median duration of remission (DOR) was 10 months with a 4-year DOR probability of 29%. Four patients underwent allo-HCT while in CR/CRi after JCAR021. Durable remissions were observed in patients with low marrow disease burden. In contrast, the DOR was limited in those with high marrow burden, highlighting a remaining critical need to identify new strategies to prolong remissions. We observed similar outcomes in CAR-naïve adult B-ALL patients receiving CD19 CAR T-cells expressing a fully human or murine scFv-containing CAR.