Siltuximab as a first-line therapy for idiopathic multicentric Castleman disease: a retrospective analysis based on the SiMuLa study of the Italian regional network.

Background: Idiopathic Multicentric Castleman Disease (iMCD) is a rare, heterogeneous lymphoproliferative disorder characterized by polyclonal lymphoid hyperplasia, systemic inflammatory symptoms, and generalized lymphadenopathy that can result in multiorgan dysfunction. The interleukin-6 (IL-6)-mediated hyperinflammatory state represents the key pathogenic mechanism of iMCD. Currently, first-line therapy is based on Siltuximab, with or without corticosteroids. Siltuximab (SYLVANT) is a chimeric monoclonal antibody that binds soluble human IL-6, forming stable, high-affinity complexes that neutralize its biological activity. Approximately 40-50% of patients fail to achieve a complete response during first-line Siltuximab therapy. This study aimed to retrospectively evaluate real-world outcomes and safety in patients with iMCD treated with first-line Siltuximab outside clinical trials across the Lazio region of Italy. Methods: Real-world data were retrospectively collected from patients with iMCD who received at least one dose of Siltuximab as first-line therapy (11 mg/kg every 3 weeks, until disease progression) between 2018 and March 2025 at Hematology Units across the Lazio region, Italy. Treatment responses were assessed according to Castleman Disease Collaborative Network (CDCN) criteria, based on biochemical, radiologic, and clinical parameters. Safety was evaluated using NCI-CTCAE version 5.0. Results: Fourteen patients were included. The median age was 54 years (range, 18-81), and the median treatment duration was 28 months (range, 3-55). The complete remission rate (CRR) was 29%, while the overall response rate (ORR) reached 86%. At data cutoff, 12 of 14 patients (85.7%) were alive. No infusion-related reactions occurred, and the overall safety profile was favorable. Conclusion: This preliminary real-world experience confirms that first-line Siltuximab is effective and well tolerated in patients with iMCD, achieving durable disease control in the majority of cases. These findings support the continued use of Siltuximab as the standard of care and underscore the importance of expanding real-world registries to better define the epidemiology and treatment outcomes of iMCD in Italy.