The purpose of this study is to describe the number of months with a platelet response over a 12 month treatment period and to describe ITP remission rates in adults with ITP receiving romiplostim.

The study includes a 4-week screening period, a 12-month romiplostim treatment period, and a romiplostim dose-tapering period.

During the 12-month treatment period romiplostim doses could be increased or decreased to maintain a platelet count between ≥ 50 x 10^9/L and ≤ 200 x 10^9/L. Participants who dose reduce such that they no longer require treatment with romiplostim during the 12-month treatment period will continue with all required study procedures up to 12 months and will be monitored for ITP remission for at least 6 months.

At the completion of the 12-month treatment period, participants receiving only romiplostim and with a platelet count ≥ 50 x 10^9/L will enter the tapering period, during which the romiplostim dose will be decreased by 1 µg/kg every 2 weeks, for up to 19 weeks. If a participant maintains a platelet count of ≥ 50 x 10^9/L in the absence of romiplostim and all medications for ITP (concomitant or rescue), the participant will be followed for at least 6 months to confirm the incidence of ITP remission. If a participant's platelet count falls below 50 x 10^9/L and the participant has tapered off treatment with romiplostim, the participant will enter the stabilization period and reinitiate romiplostim for up to 8 weeks.

Inclusion Criteria:

- Subject has been diagnosed with primary ITP according to the American Society of Hematology (ASH) guidelines (George et al, 1996) and previously received only 1st line therapies.

First line therapy is defined as corticosteroids, immunoglobulin G (IVIG), anti-D and vinca alkaloids (used for the treatment of ITP related thrombocytopenia only). A platelet transfusion at any time during the six month period since the original diagnosis would not exclude the subject from study participation

• Initial diagnosis of primary ITP within 6 months of enrollment
• Age ≥ 18 years at screening
• A single platelet count ≤ 30 x 10⁹/L at any time during the screening period
• Subject or subject's legally acceptable representative has provided informed consent

Exclusion Criteria:

• Known history of a bone marrow stem cell disorder
• Surgical resection of the spleen
• Subject has a history of cancer or current malignancy other than basal cell carcinoma or cervical cancer in-situ with active treatment or disease within 5 years of screening
• Known history of congenital thrombocytopenia
• Known history of hepatitis B, hepatitis C, or human immunodeficiency virus
• Positive H. pylori by urea breath test or stool antigen test at screening
• 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 recurrent venous thromboembolism or thrombotic events or an occurrence within 5 years of enrollment.
• 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 mercaptopurine (6-MP) or anticipated use during the time of the proposed study
• All hematopoietic growth factors including interleukin-11 (IL-11) (oprelvekin) within 4 weeks before the screening visit
• Alkylating agents use at any time before or during the screening visit or anticipated during the time of the proposed study
• Known hypersensitivity to any recombinant E. coli-derived product (eg, Infergen, Neupogen, Somatropin, and Actimmune)
• Currently enrolled in another investigational device or drug study, or less than 30 days since ending another investigational device or drug study(s), or receiving other investigational agent(s)
• Subject will have any other investigational procedures performed while enrolled in this clinical study
• Subject is pregnant or breast feeding, or planning to become pregnant within 5 weeks after the end of treatment
• Female subject of child bearing potential is not willing to use, in combination with her partner, highly effective contraception during treatment and for 4 weeks after the end of treatment
• Subject has previously enrolled into a romiplostim study
• Subject will not be available for protocol required study visits, to the best of the subject's and investigator's knowledge
• Subject has any kind of disorder that, in the opinion of the investigator, may compromise the ability of the subject to give written informed consent and/or to comply with all required study procedures