Long-term safety and efficacy of filgotinib in patients with idiopathic multicentric Castleman disease: 52-week results from a Phase Ib clinical trial

Objective: To evaluate the long-term safety and exploratory efficacy of filgotinib, a JAK1-preferential inhibitor, in patients with idiopathic multicentric Castleman disease (iMCD) over 52 weeks in a small, single-arm extension study. Methods: This Phase Ib single-arm extension study enroled five patients with iMCD who completed an initial 8 weeks’ treatment with filgotinib. Patients received filgotinib 200 mg once daily for an additional 44 weeks (total 52 weeks). The primary endpoints assessed the comprehensive safety parameters. The secondary endpoints were efficacy. Results: All five patients completed the full 52-week treatment period without permanent discontinuation. No serious adverse events or deaths were observed. All adverse events were mild-to-moderate (Grade 1-2), with upper respiratory infections in four patients and headache in three of five patients. Herpes zoster occurred in one patient. At Week 52, three of five patients had ≥1-point CHAP score reduction (one additional patient compared with Week 8). Median changes from baseline included: C-reactive protein −1.85 mg/dl, haemoglobin +0.50 g/dl, albumin +0.30 g/dl, and ECOG-PS zero points. Conclusion: Filgotinib was well-tolerated over 52 weeks with no serious adverse events reported. Numerical changes in disease activity markers were observed over time, providing preliminary support for the therapeutic concept of JAK inhibition in iMCD.