Rapid Peak Cilta-cel Expansion is Associated with Delayed Neurotoxicity in Multiple Myeloma.

The impact of chimeric antigen receptor (CAR)-T cell expansion and persistence on clinical outcomes and treatment-related morbidity in patients with relapsed/refractory multiple myeloma (RRMM) remains incompletely defined, in part due to limited availability of standardized CAR-T cell quantification assays. We evaluated CAR-T cell kinetics and their association with efficacy and toxicity in RRMM patients treated with idecabtagene vicleucel (ide-cel) or ciltacabtagene autoleucel (cilta-cel). Using a uniform flow cytometry-based platform (N=90; cilta-cel, n=54; ide-cel, n=36), we observed significantly greater CAR-T cell expansion with cilta-cel than with ide-cel (median 106 vs 49 cells/uL). Peak CAR-T cell expansion was associated with clinical response in the ide-cel cohort but not with cilta-cel, where rapid and excessive expansion was instead associated with an increased risk of delayed neurotoxicities (DNTs), a complication with potential long-term functional consequences (median peak 1,009 vs 96 cells/uL). To identify clinically accessible biomarkers of CAR-T cell expansion, we analyzed absolute lymphocyte count (ALC) as a surrogate biomarker in a larger multicenter cohort (N=532; cilta-cel, n=256; ide-cel, n=276). Higher peak ALC was significantly associated with the development of DNTs, particularly Parkinsonism after cilta-cel. A peak ALC ³3000/uL - or ³2500/uL following a daily twofold increase - predicted elevated DNT risk (sensitivity 81%, specificity 59%). Together, these findings delineate distinct expansion-toxicity relationships in cilta-cel and ide-cel therapy, establish ALC as a practical, uniformly available surrogate for CAR-T cell expansion, and define quantitative thresholds that may enable early recognition of patients at risk for DNT, informing preemptive strategies to mitigate morbidity following cilta-cel.