Modeling Effects of Dose, Schedule, and Pharmacokinetics of Fludarabine Lymphodepletion Therapy on CAR T-cell and Cytokine Dynamics
Authors: Panetta John , Talleur Aimee , Naik Swati , Gottschalk Steven , Leggas Mark 
Published: February 2025

Objectives: Lymphodepletion of host T-cells is thought to improve the ability of CAR T-cells to expand and persist. Recent studies have shown higher Fludarabine (FLU) cumulative systemic exposure was associated with lower risk of relapse and improved leukemia-free survival [1, 2] and its pharmacokinetics (PK) are highly variable with increased clearance in younger children [3]. We developed a model of the dynamics between lymphodepletion, CAR T-cells, and cytokines and used simulations to test hypotheses on how changes in lymphodepletion affected CAR T-cell persistence and cytokine dynamics.Methods: This study used data from 24 patients ≤21 years old with relapsed/refractory B-ALL treated with CD19.41 BBς-CAR T-cells (NCT03573700) [4]. Data included FLU dosage, serial CAR T-cell concentrations and 38 different cytokine levels for at least 28 days post infusion. FLU PK used for the analysis were from HCT conditioning regimens in pediatric populations [3] since it was not available in the above study. We defined a FLU target cumulative AUC range of 13.8 to 25 mg×hr/L based on studies that related FLU exposure to survival and toxicities. A system of ordinary differential equations was developed to describe the kinetics of FLU PK, host lymphocytes, cytokines, and CAR T-cells. We used simulations to assess the effects of FLU dosing along with other covariates including age and baseline cytokine levels on CAR T-cell persistence defined as CAR T-cell AUC.Results: Using clinical trial simulations we show that age adjusted dosing regimens increase the median percentage of individuals in the FLU target exposure range relative to the current fixed dosing regimen of 25 mg/m2X3 QD from 22&#37; to 72&#37;. This increased exposure in the target range led to a predicted increase in the median overall survival (35&#37;) and decrease in the median cumulative incidence of relapse (58&#37;) relative to fixed dosing. Next, we used simulations to evaluate the effects of FLU cumulative AUC on CAR T-cell exposure using the median expected FLU cumulative AUC of three age groups: 10 mg×hr/L for age &#60; 6; 15 mg×hr/L for age between 10 and 15; and 25 mg×hr/L for age >15, given a FLU dose of 25 mg/m2X3 QD. The simulations showed an increased CAR T-cell AUC of 70&#37; when the FLU exposure increased from 10 to 15 mg×hr/L and an additional 54&#37; when increased from 15 to 25 mg×hr/L. Additionally, simulations shows that an increase in the baseline cytokine IL15 level from their 25th to 75th percentile increased the CAR T-cell AUC by as much as 19X.Conclusions: These simulation studies showed that adjusting FLU dose by age increases the number of individuals achieving exposure within the target range. In addition, these data and model provided insights into how changes in lymphodepletion affect CAR T-cell exposure. The significance of this model is that it provides us with an efficient method to test hypotheses on how changes in the model parameters (e.g. lymphodepletion or cytokine related parameters) affect CAR T-cell exposure and persistence.Citations: [1] Fabrizio, V.A., et al., Blood Adv, 2022. 6(7)[2] Dekker, L., et al., Blood Adv, 2022. 6(7)[3] Langenhorst, J.B., et al., Clin Pharmacokinet, 2019. 58(5) [4] Talleur, A.C., et al., Blood Adv., 2022. 6(21)