The purpose of this study is to assess the safety and efficacy of long term dosing of AMG 531 in thrombocytopenic Japanese subjects with ITP.

It is anticipated that AMG 531 will be a safe and well tolerated in long term treatment and that AMG 531 will effectively raise and maintain platelet counts to a desired therapeutic range, when individual dose adjustments based on platelet counts are permitted.

This study is available to subjects who have completed any previous AMG 531 ITP study in Japan and meet the eligibility criteria of this study.

Romiplostim was administered by subcutaneous (SC) injection once per week. If subjects entered the extension study within 12 weeks from the last investigational product administration in the previous study and had shown an increase in platelet counts ≥ 20 x 109/L from baseline at least once during the 13-week treatment period (excluding 4 weeks after receiving rescue medication), they were treated with romiplostim at the same weekly dose (last dose on study) received in the previous study. Otherwise, subjects were treated with romiplostim at a starting dose of 3 μg/kg. Dose adjustment based on platelet counts was allowed throughout the treatment period to allow subjects to maintain platelet counts in the target range of ≥ 50 to ≤ 200 x 109/L, up to a maximum permitted dose of 10 μg/kg.

Inclusion Criteria

• Subjects must have previously completed an AMG 531 ITP study in Japan.
• Platelet count taken at the screening visit must be < 50 x 109/L.
• Before any study-specific procedure, the appropriate written informed consent must be obtained.

Exclusion Criteria

• Any significant change in medical history since completion of the previous AMG 531 ITP study including bone marrow stem cell disorders or new active malignancies
• known positive result from a test for neutralizing antibodies to AMG 531 in the previous AMG 531 ITP study
• 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 screening visit
• received intravenous immunoglobulin, anti-D immunoglobulin, or any drug administered to increase platelet counts (eg, immunosuppressants etc) within 1 week before the screening visit
• received anti-malignancy agents (eg, cyclophosphamide, 6-mercaptopurine, vincristine, vinblastine, Interferon-alfa etc) within 4 weeks before the screening visit
• received any monoclonal antibody drugs (eg, rituximab etc) within 8 weeks before the screening visit
• Less than 4 weeks since receipt of any therapeutic drug or device that is not Ministry of Health, Labor and Welfare (MHLW) approved for any indication before the screening visit (excluding AMG 531)
• 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

The duration of treatment in this study ranged from 12 to 243 weeks; the median (Q1, Q3) duration of treatment in this study was 146.0 (119.5, 166.0) weeks.