Assessment of long-term persistence and phenotypic features of BCMA CAR-T cells in patients with relapsed or refractory multiple myeloma treated with ciltacabtagene autoleucel.

e19521 Background: Ciltacabtagene autoleucel (Carvykti, cilta-cel) is a BCMA-directed CAR-T associated with deep and durable responses in relapsed or refractory multiple myeloma (RRMM). CAR-T cells rapidly expand, peaking 10–14 days post-infusion, after which circulating levels decline and frequently fall below the detection limits (LoD) of spectral flow cytometry (SFC) and next-generation sequencing (NGS). While early expansion correlates with initial response, the relationship between remission durability, and long-term persistence and state of therapeutic cells after response remains poorly understood. This retrospective study evaluated long-term circulating persistence and phenotypic characteristics of CAR-T cells in RRMM patients treated with cilta-cel using CB-Scout, a novel AI-driven nanophotonic imaging ultra-sensitive single-cell assay (Limit of Blank (LoB) < 4.7×10⁻⁸; LoD < 6.1×10⁻⁷). Methods: Five RRMM patients treated with cilta-cel at Yale Cancer Center with available pre-infusion and at least 2 post-infusion (range 2-8) cryopreserved peripheral blood mononuclear cell (PBMC) samples were included in this study. All 5 patients had at least a very good partial response to therapy. Median duration of follow up was 211 days (17-883) with 3 patients having at least 6 months of follow up (211-883 days). CAR-T cells were evaluated using CB-Scout (CellsBin; Vega 1.2 system) and compared to SFC. A shared immunophenotyping panel included CAR detection, memory/stemness markers (CD45RA, CCR7, CD27, CD28, CD95, CD57), activation (CD25, 4-1BB), and exhaustion markers (PD-1, TIGIT, LAG-3, CD39). Results: No CAR-T cells were detected in pre-infusion samples, consistent with the CB-Scout LoB. During early post-infusion time points, median peak CAR-T expansion was 1.3% of PBMCs, concordant with SFC, including ≥80% agreement across key phenotypic metrics. By Day 33, CAR-T frequencies declined below 0.01% of PBMCs in 4/5 patients, falling below SFC reliability limits. In patients with long-term follow-up, CAR-T cells were detected at rare frequencies (~0.0001%–0.01% of PBMCs) using CB-Scout but not by SFC and despite low abundance, phenotypic characterization remained feasible. Detailed CAR-T phenotypic depiction of all 5 patients will be shared during the meeting. Conclusions: Longitudinal cell-therapy tracking with CB-Scout enables assessment of CAR-T phenotypic persistence beyond SFC and NGS. The ability to quantify and phenotypically characterize CAR-T cells at extended time points post infusion may be useful in predicting remission durability and guiding subsequent therapy selection.