Intent-to-treat leukemia remission by CD19 CAR T cells of defined formulation and dose in children and young adults

Abstract

Transitioning CD19-directed chimeric antigen receptor (CAR) T cells from early-phase trials in relapsed patients to a viable therapeutic approach with predictable efficacy and low toxicity for broad application among patients with high unmet need is currently complicated by product heterogeneity resulting from transduction of undefined T-cell mixtures, variability of transgene expression, and terminal differentiation of cells at the end of culture. A phase 1 trial of 45 children and young adults with relapsed or refractory B-lineage acute lymphoblastic leukemia was conducted using a CD19 CAR product of defined CD4/CD8 composition, uniform CAR expression, and limited effector differentiation. Products meeting all defined specifications occurred in 93% of enrolled patients. The maximum tolerated dose was 10⁶ CAR T cells per kg, and there were no deaths or instances of cerebral edema attributable to product toxicity. The overall intent-to-treat minimal residual disease-negative (MRD^-) remission rate for this phase 1 study was 89%. The MRD^- remission rate was 93% in patients who received a CAR T-cell product and 100% in the subset of patients who received fludarabine and cyclophosphamide lymphodepletion. Twenty-three percent of patients developed reversible severe cytokine release syndrome and/or reversible severe neurotoxicity. These data demonstrate that manufacturing a defined-composition CD19 CAR T cell identifies an optimal cell dose with highly potent antitumor activity and a tolerable adverse effect profile in a cohort of patients with an otherwise poor prognosis. This trial was registered at www.clinicaltrials.gov as #NCT02028455.

Figure 1.

Product manufacturing and product phenotype. (A) Manufacturing schema of PLAT-02–formulated CD19 CAR products. (B) Flow cytometric phenotype of input apheresis T cells and final products (FPs). (C) Frequency of transgene-expressing T cells in formulated CD4 and CD8 products based on EGFRt flow cytometric enumeration. (D) EGFRt transgene expression levels of formulated products based on mean fluorescence intensity (MFI). PBMC, peripheral blood mononuclear cell; SP, starting products.

Figure 2.

Magnitude and duration of functional CD19 CAR engraftment. (A) Representative engraftment and detection of CD19 CAR⁺/EGFRt⁺ T cells in peripheral blood (PB), bone marrow (BM), and cerebrospinal fluid (CSF) 21 days after infusion. (B) Representative functional activity of CD19 CAR product as measured by clearance of B cells (leukemic and normal) in PB, BM, and CSF. (C) Magnitude and duration of CAR engraftment by flow cytometric quantitation of EGFRt⁺CD3⁺ T cells. (D) Durability of functional CD19 CAR engraftment in treated patients based on BCA. Flu/cy, fludarabine and cyclophosphamide.

Figure 3.

Antileukemia response durability. Kaplan-Meier curves of (A) EFS and (B) OS. (C) Waterfall plot of individual research participant remission duration, BCA duration, and timing of allogeneic (allo) HSCT and/or relapse. (D) Effect of CD19 burden on BCA durability. (E) Effect of flu as component of lymphodepletion regimen on BCA durability.

Figure 4.

Severity of cytokine release syndrome and neurotoxicity. (A) Severity of CRS in treated patients as a function of cell dose, disease burden, CD19 burden, and lymphodepletion regimen. (B) Severity of neurotoxicity based on parameters described in (A) as well as severity of CRS.