Safety and Efficacy Study of Romiplostim to Treat Immune Thrombocytopenia (ITP) in Pediatric Patients

Inclusion Criteria: * Diagnosis of primary ITP according to the American Society of Hematology (ASH) guidelines at least 6 months prior to screening, regardless of splenectomy status * Subject must be refractory to a prior ITP therapy, having relapsed after at least 1 prior ITP therapy, or ineligible for other ITP therapies; prior therapy includes first-line therapies * Age ≥ 1 year and \< 18 years at the time of providing informed consent * The mean of 2 platelet counts taken during the screening period must be ≤ 30 x 10\^9/L with neither count \> 35 x 10\^9/L * A serum creatinine concentration ≤ 1.5 times the laboratory normal range (for each age category) during the screening period * Adequate liver function; serum bilirubin ≤ 1.5 times the laboratory normal range during the screening period; aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 3.0 times the laboratory normal range during the screening period * Hemoglobin \> 10.0 g/dL during the screening period * Subject and/or subject's legally acceptable representative has provided informed consent prior to any study-specific procedure; subject has provided assent, where required Exclusion Criteria: * Known history of a bone marrow stem cell disorder; any abnormal bone marrow findings other than those typical of ITP must be approved by Amgen before a subject may be enrolled in the study * Known active or prior malignancy except adequately treated basal cell carcinoma * Known history of congenital thrombocytopenia * Known history of hepatitis B, hepatitis C, or human immunodeficiency virus (HIV) * Known history of H. pylori by urea breath test or stool antigen test within 6 months of enrollment or successfully treated with no evidence of infection * Known history of systemic lupus erythematosus, evans syndrome, or autoimmune neutropenia * Known history of antiphospholipid antibody syndrome or positive for lupus anticoagulant * Known history of disseminated intravascular coagulation, hemolytic uremic syndrome, or thrombotic thrombocytopenic purpura * Previous history of venous thromboembolism or thrombotic events * Previous use of romiplostim, pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF), Eltrombopag, recombinant human thrombopoietin (rHuTPO) or any platelet producing agent * Rituximab (for any indication) or 6-mercaptopurine (6-MP) within 14 weeks before the screening visit, or anticipated use during the time of the proposed study * Splenectomy within 4 weeks of the screening visit * All hematopoietic growth factors including interleukin-11 (IL-11) (oprelvekin) within 4 weeks before the screening visit * Alkylating agents within 8 weeks before the screening visit or anticipated use during the time of the proposed study * Vaccinations known to decrease platelet counts within 8 weeks before the screening visit * Known hypersensitivity to any recombinant E coli-derived product (eg, Infergen, Neupogen, Somatropin, and Actimmune) * Other criteria may apply