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 ×…