7537 Background: Ciltacabtagene autoleucel (cilta-cel) is an FDA approved chimeric antigen receptor (CAR) T cell therapy for relapsed/refractory multiple myeloma (RRMM). Cilta-cel is associated with gastrointestinal (GI) toxicities including immune effector cell (IEC)-associated enterocolitis. The pathophysiology of IEC-associated enterocolitis is poorly understood and may represent a clonal CAR T cell lymphoproliferative disorder. Methods: Retrospective review of patients with RRMM treated with standard of care (SOC) cilta-cel at a single center between 07/21/2022-10/31/2025. Baseline characteristics, outcomes, and GI toxicities were collected. CAR transgene was detected via digital droplet polymerase chain reaction (ddPCR) in GI tissue and peripheral blood samples. CAR T cell clonality was assessed via T cell receptor (TCR) fragment analysis and TCR Beta Constant Region 1 (TRBC1) assessment via IHC or flow cytometry. Results: In total 201 patients were included of whom 20 (10%) exhibited GI toxicity. Diarrhea accounted for most cases (15/20). CAR T cells were detected in 5/20 (25%) GI biopsies (Table 1). A clonal CAR T-cell lymphoproliferative disorder affected predominantly the duodenum, evidenced by TRBC1-restricted CD4- or CD8-positive T cells in lamina propria. The cells were positive for CD3 and TCRalpha while negative for CD103 and TCRdelta. MUM1, CD138 and BCMA demonstrated absence of plasma cells in all samples. TCR fragment analysis identified clonal TCR rearrangements in 3/5 evaluable cases. The remaining 2 cases showed equivocal fragment analysis with prominent but subthreshold peaks, likely reflecting low tumor burden. Symptoms resolved in 13/15 patients without detectable CAR T cells compared to 1/5 patients with detectable CAR T cells. Of the remaining 4/5 patients, 2 died and 2 had ongoing symptoms at time of last follow-up. Conclusions: GI toxicity was observed in 10% of patients treated with cilta-cel. CAR T-cell infiltration, predominantly duodenal, was detected in 25% of symptomatic patients consistent with IEC-associated enterocolitis. TRBC1-restricted CAR T cell populations were detected in all cases supporting the presence of a clonal CAR T cell lymphoproliferative disorder. At the meeting we will present additional translational studies from GI and peripheral blood CAR T cells including whole genome sequencing, spectral flow cytometry, and single cell RNA sequencing. Clinical and pathologic features of IEC-associated enterocolitis. ID Onset Outcome Site CAR ddPCR TRBC1/CD3 IHC TCR Fragment Analysis (Day) Tissue Blood 1 60 Death Duodenum 25.12% NA 7.30% Clonal 2 42 Ongoing Duodenum, Jejunum 6.60% 1.30% 84.70% Borderline clonal 3 169 Death Duodenum 14% 0.25% 31.60% Clonal 4 111 Ongoing Duodenum, Antrum, Ileum, Colon 22.40% 1.32% 88.40% Clonal 5 39 Resolved Duodenum 1.70% 0.03% 26.20% Borderline clonal
Jurgens et al. (Thu,) studied this question.