Study of Romiplostim(AMG531) in Subjects With Aplastic Anemia

Inclusion Criteria: * Diagnosis of Aplastic Anemia (AA) confirmed by peripheral blood and bone-marrow examinations, etc. * Refractory to at least one course of immunosuppressive therapy including horse or rabbit anti-human thymocyte immunoglobulin (ATG); or ineligible for ATG treatment and refractory to cyclosporin (CyA) * Thrombocytopenia defined as a platelet count of ≤ 30 × 10\^9/L * Preserving main organ function as a result of screening as follows; * Total bilirubin: \< 1.5 times the upper limit of the laboratory normal range * Alanine aminotransferase: \< 3.0 times the upper limit of the laboratory normal range * Aspartate aminotransferase: \< 3.0 times the upper limit of the laboratory normal range * Creatinine value: ≤ 2.0 mg/dL * An Eastern Cooperative Oncology Group performance status score of 0 to 2 at screening * ≥ 20 years of age at the time of obtaining informed consent * Patients who have provided written informed consent of their free will to participate in this study Exclusion Criteria: * Concurrent active infection not adequately responding to appropriate therapy * Bone marrow reticulin grade of ≥ 2 based on the grading scale for reticulin indicated in Bone Marrow Pathology (2nd edition) * Proportion of blasts in bone marrow \> 2% * Previous or concurrent active malignancies, other than localized tumors diagnosed more than one year previously and treated surgically with curative intent (basal cell carcinoma; or surgically resected in situ carcinoma of the cervix with an apparent success of ≥ 12 months prior to enrollment; as well as other cancers which have not been treated and remained disease-free for at least 5 years before enrollment are eligible) * Clinically significant cardiac disease (class III or IV of the New York Heart Association classification; unstable angina pectoris; myocardial infarction within 6 months before enrollment; cardiac disease accompanied by angioplasty or stenting within 6 months before enrollment; or clinically significant cardiac arrhythmias) or uncontrollable hypertension * Arterial or venous thrombosis within one year before enrollment * Positive for anti-human immunodeficiency virus antibodies, hepatitis B surface antigen, or hepatitis C virus-RNA at screening * Thrombocytopenia due to any other cause (e.g., myelodysplastic syndrome, idiopathic thrombocytopenic purpura, or liver cirrhosis) * Patients with acute myeloblastic leukemia or chronic myelomonocytic leukemia * Concurrent occurrence of hemolytic predominant paroxysmal nocturnal hemoglobinuria (Hemolytic predominant is defined as lactate dehydrogenase \> 1.5 times the upper limit of the laboratory normal range) * Uncontrolled diabetes mellitus * Receiving other investigational products within 16 weeks before romiplostim treatment initiation * Receiving any agent to treat AA, including the following agents before romiplostim treatment initiation; * ATG treatment within 6 months before romiplostim treatment initiation * CyA or anabolic steroid treatment within 6 weeks before romiplostim treatment initiation: However, the patients who are treated with a CyA or anabolic steroid for at least 6 months before romiplostim treatment initiation may be enrolled if their blood cell count are stable at screening, and their dosage and administration will be kept for 6 weeks before romiplostim treatment initiation and during romiplostim dosing period. * A history of polyethylene glycol-conjugated recombinant human megakaryocyte growth and development factor, recombinant human thrombopoietin (TPO), romiplostim, or other TPO-receptor agonists * Having a plan to undergo hematopoietic stem cell transplantation within 1 year * Having hypersensitivity to any recombinant protein E. coli derivative protein * Lactating or pregnant women or women of child-bearing potential who have no intention of using oral contraceptives or birth control * Having abnormalities by the cytogenetic test in bone marrow cells * Patients who are considered to be ineligible for the study by the investigator or subinvestigator for reasons other than above