P175 | CAR-T CELL EXPANSION AND CLINICAL OUTCOMES IN RELAPSED/REFRACTORY MULTIPLE MYELOMA: A SINGLE-CENTER EXPERIENCE WITH IDE-CEL.

Idecabtagene vicleucel (ide-cel), a BCMA-directed CAR-T cell therapy, has shown superior progression-free survival compared to standard regimens in relapsed/refractory multiple myeloma (RRMM)1. However, the impact of disease biology on CAR-T expansion and treatment outcomes is still not fully understood. We conducted a retrospective observational study involving 22 RRMM patients treated with ide-cel at Humanitas Research Hospital (July 2024–April 2025). CAR-T expansion was assessed in 20 patients by flow cytometry on peripheral blood samples at serial time points (day +3, +7, +10, +14, +21, +30). CRS and ICANS were graded according to ASTCT criteria2. Statistical analysis used Mann–Whitney U and Kruskal–Wallis tests to compare CAR-T expansion across subgroups, and correlation analysis assessed associations with response. The median age of patients was 64 years. High-risk cytogenetic features3 were present in 55% of the cohort, extramedullary disease (EMD) was present in 33% of the cohort, and 50% had received five or more prior lines of therapy. At the time of CAR-T cell infusion, 80% of patients had either progressive or stable disease. Additionally, 55% of patients had an intermediate or high MyCARE score4. Disease reassessment at day 60 was performed in 16 patients, 7 results in CR + VGPR, 3 results in PR, 6 results in SD or PD. The median follow-up was 4 months. CAR-T expansion peaked on day 3 post-infusion, with persistence at day 30 in 30% of patients. EMD was associated with delayed, but not increased, expansion. Patients with high-risk cytogenetics showed a trend toward higher peak expansion, though this was not statistically significant. Patients without relapse showed significantly higher CAR-T expansion compared to those with relapse (P=0.009, Figure 1A). Similarly, responders at day 60 showed increased CAR-T expansion vs non-responders (P<0.01, Figure 1B). No significant differences in CAR-T expansion were observed in patients with high-risk cytogenetics (Figure 1C), presence of EMD (Figure 1D), or CRS grade. In our study, in RRMM patients treated with ide-cel, CAR-T cell expansion is strongly associated with depth of response but not with CRS grade or unfavorable biological features. Limitations of the study include its retrospective design and the small sample size. Future investigations are warranted to develop effective therapeutic strategies targeting EMD and high-risk cytogenetic abnormalities persist as significant predictors of relapse.