Outcomes of PLAT-02 and PLAT-03: Evaluating CD19 CAR T-Cell Therapy and CD19-expressing T-APC Support in Pediatric B-ALL

• A phase 2 academic clinical trial of CD19 CAR T cells demonstrates durable remissions with a 1-year LFS in low disease burden groups of >90%

• Exogenous CD19 antigen using engineered autologous T cells is safe and may extend CAR T-cell persistence with further study warranted.

This study reports outcomes of PLAT-02, a phase 2 trial of SCRI-CAR19, a second-generation CAR T-cell product with FMC63 scFv and 41BB costimulation, in pediatric and young adult patients with B-cell acute lymphoblastic leukemia; and PLAT-03, a companion study evaluating exogenous CD19 antigen stimulation with serial infusions of T cells expressing truncated CD19, T-cell antigen presenting cells (T-APCs). The efficacy cohort of PLAT-02 (n=72 patients, median age 12.5 years) received fludarabine/cyclophosphamide lymphodepletion followed by a dose of 1X106 CAR+ T cells/kg. MRD-negative complete remission rate was 89%. Leukemia free survival (LFS) with 95% CI at 1 and 2 years was 0.71 (0.58, 0.81) and 0.64 (0.51, 0.75). Patients with low disease burden had significantly higher 1-year LFS (0.91 vs. 0.42). Rapid in vivo contraction of CAR T cells after infusion was associated with CAR loss within six months compared to those without rapid contraction (57% vs. 19%, respectively). Most common grade 3/4 adverse events included cytokine release syndrome in 13% and neurotoxicity in 16%. The companion pilot, PLAT-03, enrolled 26 patients, and 19 received T-APCs. Neither cytokine-release syndrome nor neurotoxicity were observed after T-APC infusion. T-APC infusion in patients improved persistence (P=0.03) with rapid CAR T-cell contraction was associated with decreased early CAR loss (20% with T-APC vs. 57% without). Further exploration of serial artificial CD19 antigen exposure is warranted based on these pilot results. PLAT-02 (NCT02028455) and PLAT-03 (NCT03186118)