The purpose of this study is to evaluate the efficacy of romiplostim in the treatment of thrombocytopenia in pediatric patients with Immune thrombocytopenia purpura (ITP) as measured by durable platelet response.

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