The purpose of this study is to evaluate the efficacy and safety of AMG 531 compared with placebo in thrombocytopenic Japanese subjects with immune (idiopathic) thrombocytopenic purpura (ITP) .

Inclusion Criteria:

• Japanese patients with diagnosis of ITP according to the diagnostic criteria proposed by Research Committee for Idiopathic Hematopoietic Disorders of the Ministry of Health, Labour and Welfare [MHLW] (revised in 1990) at least 6 months before the first screening visit
• The mean of the 3 scheduled platelet counts taken at the scheduled visits during the screening period must be ≤ 30 x 10^9/L, with no individual count > 35 x 10^9/L
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2
• Subjects must be ≥ 20 years of age at the time of obtaining the informed consent
• Have received at least 1 prior treatment for ITP
• If known Helicobacter pylori positive, having completed one course of Helicobacter pylori eradication therapy at least 12 weeks before the first screening visit
• A hemoglobin value taken at scheduled visit during the screening period must be ≥ 10 g/dL
• A serum creatinine concentration taken at scheduled visit during the screening period must be ≤ 2 mg/dL
• Adequate liver function, as evidenced by a total bilirubin taken at scheduled visit during the screening period ≤ 1.5 times of the upper limit of the normal range (except for patients with a confirmed diagnosis of Gilbert's Disease) or an alanine aminotransferase and aspartate aminotransferase taken at the screening visit ≤ 3 times of the upper limit of the normal range

Exclusion Criteria:

• Any known history of bone marrow stem cell disorder. Any abnormal bone marrow findings other than those typical of ITP.
• Any active malignancy. If prior history of cancer other than basal cell carcinoma or cervical carcinoma in situ, no treatment or active disease within 5 years before the first screening visit.
• Documented diagnosis of arterial thrombosis (eg, stroke, transient ischemic attack, or myocardial infarction); history of venous thrombosis (eg, deep vein thrombosis, pulmonary embolism) and receiving anticoagulation therapy at the first screening visit.
• Documented diagnosis of anti phospholipid antibody syndrome
• Currently receiving any treatment for ITP except oral corticosteroids, azathioprine and/or danazol administered at a constant dose and schedule from at least 4 weeks prior to the first screening visit
• Received intravenous immunoglobulin, anti D immunoglobulin, or any drug administered to increase platelet counts (eg, immunosuppressants except azathioprine) within 2 weeks before the first screening visit
• Have had a splenectomy for any reason within 12 weeks before the first screening visit
• Past or present participation in any study evaluating pegacaristim (polyethylene glycol-conjugated recombinant human megakaryocyte growth and development factor, KRN9000), Eltrombopag (SB 497115), recombinant human thrombopoietin, AMG 531, or other Mpl stimulation product
• Received hematopoietic growth factors (eg, granulocyte colony stimulating factor, macrophage colony stimulating factor, erythropoietin, interleukin 11) for any reason within 4 weeks before the first screening visit
• Received any anti malignancy agents (eg, cyclophosphamide, 6 mercaptopurine, vincristine, vinblastine, Interferon alfa) for any reason within 8 weeks before the first screening visit
• Received any monoclonal antibody drugs (eg, rituximab) for any reason within 14 weeks before the first screening visit
• Less than 4 weeks since receipt of any therapeutic drug or device that is not MHLW approved for any indication before the first screening visit
• Pregnant or breast feeding
• Subjects of reproductive potential who are not using adequate contraceptive precautions, in the judgment of the investigator
• Known severe drug hypersensitivity
• Concerns for subject's compliance with the protocol

12 weeks of treatment plus follow-up period up to 12 weeks

The table of Other Adverse Events summarizes the most frequent non-serious occurrences of adverse events.