The purpose of this study is to evaluate the long-term safety of siltuximab in patients with multicentric Castleman's disease (MCD).

This is an open-label (all people know the identity of the intervention), multicenter (study conducted in multiple sites), non-randomized (patients are not assigned by chance to treatment groups), Phase 2b study. Up to 75 patients with MCD will be eligible for the study, the majority of whom will be on active therapy with siltuximab at the time of enrollment. Patients will be either siltuximab-naive or have not progressed on siltuximab in the opinion of the investigator. Duration of disease control and survival will be assessed. Data collection for patients who discontinue treatment will be limited to survival, occurrence of malignancies, and subsequent therapies for MCD, which will be assessed twice per year until the patient has been lost to follow up or has withdrawn consent for the study, whichever occurs first. An interim analysis will be conducted (no later than 2 years after the start of enrollment) to further evaluate the benefit and safety of long-term treatment with siltuximab in patients with MCD. A data will occur at 6 years after the start of enrollment and for those patients remaining on treatment after the data cutoff, data collection will be limited to pregnancies and serious adverse events (SAEs), including information on study agent administration and concomitant medications associated with an SAE. Safety evaluations for adverse events, clinical laboratory tests, vital signs, and physical examination will be performed throughout the study. The end of study is the date of the last assessment for the last patient.

Inclusion Criteria:

• Has multicentric Castleman's disease
• Have previously been enrolled in Study C0328T03 or CNTO328MCD2001 (either treatment arm)
• Have had their last administration of study treatment (siltuximab or placebo) less than 6 weeks (window of plus 2 weeks) prior to first dose
• Patients must not have had disease progression while receiving siltuximab. For those patients originally assigned to placebo in the CNTO328MCD2001 study, patients who have received less than 4 months of siltuximab following crossover will also be eligible
• Have adequate clinical laboratory parameters within 2 weeks prior to the first dose of siltuximab for this study

Exclusion Criteria:

• Unmanageable toxicity, an adverse event, progression of disease, or withdrawal of consent as reason for discontinuing treatment from previous sponsor-initiated siltuximab study
• Vaccination with live, attenuated vaccines within 4 weeks of first dose of this study
• Known unmanageable allergies, hypersensitivity, intolerance to monoclonal antibodies, to murine, chimeric, human proteins or their excipients