Sirolimus in Previously Treated Idiopathic Multicentric Castleman Disease

Inclusion Criteria: * Male or female, age 2-80 * Documented disease history consistent with the diagnostic criteria for iMCD * Failed/refractory (patient did not achieve sufficient disease control with anti-IL-6 therapy, as determined by the site investigator), relapsed (return of symptoms while on therapy), or inability to tolerate anti-IL-6 or anti-IL-6 receptor therapy * Evidence of active disease, defined as at least two abnormalities in the criteria comprising the CBR criteria, including at least one objective measurement (hemoglobin, weight loss, or lymph node size) * Ability to consume oral medication in the form of a tablet * Ability to provide, or for a legally authorized representative to provide on their behalf, informed consent prior to any study-specific activities Exclusion Criteria: * Subjects cannot be pregnant or nursing females * Except for anti-IL6 blockade therapy (siltuximab or tocilizumab), the last dose of which must be ≥ 14 days prior to enrollment (unless subjects cannot or are unwilling to undergo a 14 day washout period), subjects cannot have received any systemic therapy(ies) intended to treat iMCD other than corticosteroids within 28 days of enrollment * Subjects cannot have previously received sirolimus monotherapy to treat iMCD * Subjects cannot have any of the following: ECOG \>3 (or Karnofsky/Lansky score ≤ 60 in children); Estimated glomerular filtration rate (eGFR) \< 30 mL/min/1.73 m2 or creatinine \> 3.0 mg/dL; Absolute neutrophil count (ANC) \< 1000 x 109/L ((\< 500 x 109/L in children); Hemoglobin ≤ 6.5 g/dL (transfusion independent, defined as not receiving a red blood cell transfusion for ≥ 7 days prior); Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) laboratory values greater than three times the upper limit of normal; Albumin \< 2 g/dL (transfusion independent, defined as not receiving intravenous albumin for ≥ 7 days prior); Platelet count ≤ 40 x 109/L (transfusion independent, defined as not receiving platelet transfusion for ≥ 7 days prior); Pulmonary involvement or interstitial pneumonitis with dyspnea (adequate pulmonary function is defined as pulse oximetry \> 94% on room air if there is clinical indication for determination (e.g. dyspnea at rest, history of interstitial pneumonitis, etc.)); Fasting cholesterol \> 300 mg/dL or fasting triglyceride \> 400 mg/dL * Subjects cannot have uncontrolled infection or infectious disease(s) that is/are exclusionary for / mimickers of iMCD * Subjects cannot have rheumatologic disease(s) that is/are exclusionary for / mimickers of iMCD * Subjects cannot have a prior malignancy except for: (1) adequately treated basal cell or squamous cell skin cancer, (2) in situ cervical cancer, or (3) other cancer for which the subject has not received treatment within one year prior to enrollment * Subjects cannot have a documented history of human immunodeficiency virus (HIV) or HHV-8 infection, or severe combined immunodeficiency syndrome * Subjects cannot have a history of liver or lung transplantation * Subjects cannot have ongoing or planned participation in another clinical trial involving iMCD directed treatment or that involves immunomodulatory or anti-neoplastic treatment * Subjects cannot have prior sensitivity / allergy to any formulation of sirolimus, its components or its analogues * Subjects cannot have serious medical illness, or psychiatric illness or disorders that could potentially interfere with the completion of treatment according to this protocol or participation in the trial * Subjects cannot have psychiatric disorders that compromises the ability to provide informed consent * Subjects cannot have any other condition or finding that in the opinion of the investigator would make participation in this trial inappropriate