883 | SILTUXIMAB IN IDIOPATHIC MULTICENTRIC CASTLEMAN DISEASE: REAL‐WORLD EXPERIENCE FROM ROMANIA AND GREECE

Background: Castleman disease (CD) is a very rare, non-malignant lymphoproliferative disorder that can beclassified as unicentric or multicentric (MCD). MCD is associated with systemic symptoms, including organdysfunction due to cytokine dysregulation, primarily interleukin-6 (IL-6). The anti-IL-6 monoclonal antibodysiltuximab is recommended as a frontline treatment for idiopathic MCD (iMCD), but real-world data on its usein routine clinical practice are limited. This study aimed to assess disease response and survival outcomes inpatients with iMCD treated with siltuximab therapy in real-world settings in Greece and Romania. Methods: This retrospective cohort study included adult patients with iMCD treated with siltuximab in clinicalpractice across Greece and Romania between January 2017 and December 2022. The primary endpoint wasoverall response rate and secondary endpoints included survival and safety outcomes. Responseassessments were performed according to the Castleman Disease Collaborative Network guidelines. Patientswere followed until death, loss to follow-up or study conclusion (October 2023). Results: Forty-eight patients with iMCD were included in the study. Mean age at baseline was 65 years, with significant age differences between patients from Greece (74 years) and Romania (54 years). The majority ofpatients were male (68.8%) and received one prior line of therapy (75%). Patients included in the study received a median of nine cycles of siltuximab. Response data were available for 38 patients. The overall response to siltuximab was 71.1%, with 55.3% of patients achieving a complete response, and 15.8% apartial response. The estimated overall survival rate at 3 years was 74% and the median survival was 123 months. The most common adverse events (> 5%) included elevated liver enzymes, anxiety, allergic reactions and nausea/diarrhea. Serious adverse events were experienced by 16.7% of the patients. Conclusions: Our results suggest that siltuximab-based therapy is effective in treating iMCD in real-world settings in Greece and Romania. To our knowledge, this study represents the largest real-world analysis of siltuximab in European patients with iMCD so far. Keywords: other lymphoid cancers; patient safety and standards of care; immunotherapy No potential sources of conflict of interest.