CAR T-Cell Kinetics, Persistence and Clinical Outcomes in Adult Patients with Relapsed/Refractory B-Cell ALL Treated with Obecabtagene Autoleucel in the FELIX Study

Assessments of chimeric antigen receptor (CAR) T-cell pharmacokinetics by flow cytometry (FC) and a droplet digital PCR (ddPCR) assay were compared in adult patients with relapsed/refractory B-cell acute lymphoblastic leukemia following treatment with obecabtagene autoleucel (obe-cel) in the Phase Ib/II FELIX study (NCT04404660). CAR T-cell persistence and B-cell aplasia (BCA) were then correlated with event-free survival (EFS). Peripheral blood (PB) samples collected from 127 obe-cel-infused patients were tested by FC and ddPCR. The Spearman correlation coefficient was used to measure the correlation between the number of CAR T-positive cells by FC and ddPCR. The impact of CAR T-cell persistence and BCA (B-cells <20 cells/µL by FC in PB) on EFS was assessed using Cox proportional hazards regression. ddPCR was observed to statistically correlate with both the surface (0.60, P < 0.0001) and intracellular FC assays (0.74, P < 0.0001). A higher sensitivity for detecting CAR T-cell positive samples was observed with ddPCR, with 58.8% and 41.3% of samples negative by surface and intracellular FC, respectively, being positive by ddPCR. Loss of CAR T-cell persistence (HR: 2.7; 95% CI: 1.4–5.4), and to a lesser degree B-cell recovery (HR: 1.7; 95% CI: 0.7–3.8), as time-dependent variables and at Month 3, were associated with poorer EFS. ddPCR demonstrated enhanced sensitivity over FC methods for detection of obe-cel persistence. Additionally, ongoing persistence and BCA were associated with longer EFS and may be taken into consideration, together with clinical parameters, in informing decision making.