Optimal fludarabine lymphodepletion is associated with improved outcomes following CAR T-cell Therapy.

Chimeric antigen receptor (CAR) T-cells provide a therapeutic option in hematologic malignancies. However, treatment failure after initial response approaches 50%. In allogeneic hematopoietic cell transplantation, optimal fludarabine exposure improves immune reconstitution, resulting in lower nonrelapse mortality and increased survival. We hypothesized that optimal fludarabine exposure in lymphodepleting chemotherapy prior to CAR T-cell therapy would improve outcomes. In a retrospective analysis of relapsed/refractory B-cell acute lymphoblastic leukemia patients undergoing CAR T-cell (tisagenlecleucel) infusion after cyclophosphamide/fludarabine lymphodepleting chemotherapy, we estimated the fludarabine exposure as area-under-the-curve (AUC;mg*hr/L) using a validated population-pharmacokinetic model. Fludarabine exposure was related to overall survival (OS), cumulative incidence of relapse (CIR), and a composite endpoint (loss of B-cell aplasia (BCA) or relapse). Eligible patients (n=152) had a median age of 12.5 years (range <1-26), response rate of 86% (131/152), 12-month OS of 75.1% (95%-CI: 67.6-82.6%), and 12-month CIR of 36.4% (95%-CI: 27.5-45.2%). Optimal fludarabine-exposure was determined as an AUC≥13.8mg*hr/L. In multivariable analyses patients with an AUC<13.8mg*hr/L had a 2.5-fold higher CIR (HR=2.45 [1.34-4.48]; P=0.005) and a twofold higher risk of relapse or loss of BCA (HR=1.96 [1.19-3.23]; P=0.01) compared to those with optimal fludarabine exposure. High preinfusion disease burden was also associated with an increased risk of relapse (HR=2.66 [1.45-4.87]; P=0.001) and death (HR=4.77 [2.10-10.9]; p<0.001). Personalized PK-directed dosing to achieve optimal fludarabine exposure should be tested in prospective trials and based on this analysis may reduce disease relapse after CAR T-cell therapy.