Siltuximab In Siltuximab-RElapsed/REfractory Multicentric CAstleman Disease

Inclusion Criteria: * Documented history of consensus histologic, laboratory, and clinical diagnostic criteria of iMCD. * Archival and/or baseline incisional/excisional biopsy for retrospective central histologic confirmation of iMCD. * CDCNRC-defined disease progression on or after prior treatment with siltuximab at 11 mg/kg q3w without unacceptable toxicity within 12 weeks between the last dose of siltuximab and the date of signed patient informed consent form (ICF). * At least 1 measurable abnormal lymph node mass that is ≥1 cm in its longest transverse diameter as assessed by computerized tomography (CT) scan that has not been previously irradiated. * Elevated (\>10 mg/L) and rising serum CRP in the absence of additional iMCD treatment. * Evidence of at least an additional one of the following laboratory or clinical signs of iMCD per international, evidence-based consensus diagnostic criteria for HIV or HHV 8-negative iMCD: * Anemia, thrombocytopenia, hypoalbuminemia, renal dysfunction, or polyclonal hypergammaglobulinemia. * Constitutional symptoms (night sweats, fever (\>38°C), weight loss, or fatigue (CTCAE lymphoma B-symptoms score ≥2), large spleen and/or liver, fluid accumulation, eruptive cherry hemangiomatosis/violaceous papules, or lymphocytic interstitial pneumonitis. * Adequate clinical laboratory measurements within 3 weeks prior to study entry in all parameters below: * Absolute neutrophil count ≥1.0 × 109/L, hemoglobin \<17 g/dL, and platelets ≥50 × 109/L without transfusion, hematopoietic growth factors, or both for \>7 days prior to measurement. * AST, ALT, total bilirubin, and alkaline phosphatase ≤5 × ULN. * Fasting cholesterol \<300 mg/dL and fasting triglyceride \<400 mg/dL. * Age ≥12 years. Exclusion Criteria: * Documentation of HIV or HHV-8 infection or presence of other infection-related disorders that resemble clinical or histological features of iMCD * Diagnosis of any malignant/benign lymphoproliferative disorders * Diagnosis of autoimmune/autoinflammatory disease * Treatment with corticosteroids (prednisone dose-equivalent \>1 mg/kg/day) within 7 days prior to study entry. * History of solid organ transplant, allogeneic bone marrow transplant, or allogeneic peripheral blood stem cell transplant. * Previous malignancy with the following exceptions: * Past malignancy with treatment that was completed at least 2 years before signing informed consent and the patient has no evidence of disease, or * Concurrent malignancy that is clinically stable and does not require tumor-directed treatment (eg, nonmelanoma skin cancer and carcinoma in situ)