Inclusion Criteria:

- Diagnosis of primary ITP according to the 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;AST and 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 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, PEG-rHuMGDF, Eltrombopag, rHuTPO or any platelet
producing agent

- Rituximab (for any indication) or 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 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