The purposes of this study are: to determine a weekly dose that demonstrates a satisfactory safety profile and sustains elevated platelet counts within a targeted therapeutic level (a doubling of baseline platelet counts and within the range of greater than or equal to 50 x 10^9/L and less than or equal to 450 x 10^9/L in thrombocytopenic subjects with ITP; to assess Amgen megakaryopoiesis protein 2 [AMP2 (AMG 531)] pharmacokinetics (PK) in thrombocytopenic subjects with ITP.

Ages Eligible for Study:  18 Years   -   65 Years,  Genders Eligible for Study:  Both

Criteria

Inclusion Criteria:

• Diagnosis of ITP according to American Society of Hematology (ASH) guidelines at least 3 months before enrollment
• Have completed at least 1 prior treatment for ITP
• The mean of the 2 platelet counts taken during the screening and pre-treatment periods must be: *less than 30 x 10^9/L for those subjects not receiving any ITP therapy, with no count greater than 35 x 10^9/L; less than 50 x 10^9/L for those subjects receiving a constant dose schedule of corticosteroids, with no count greater than 55 x 10^9/L
• Hemoglobin greater than 10.0 g/dL
• Written informed consent

Exclusion Criteria:

• Any known history of bone marrow stem cell disorder
• 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 randomization
• Unstable or uncontrolled disease or condition related to or impacting cardiac function (e.g., unstable angina, congestive heart failure [New York Heart Association (NYHA) greater than class II], uncontrolled hypertension [diastolic greater than 100 mmHg] or cardiac arrhythmia)
• Have 3 or more of the following predisposing factors for thromboembolic events: *diabetes; *smoker using oral contraceptives; *hypercholesteremia (greater than 240 mg/dL); *treatment for hypertension
• Known positive test for human immunodeficiency virus (HIV) infection or hepatitis C virus
• Received any treatment for ITP (except for a constant dose schedule of corticosteroids) within 4 weeks before the screening visit
• Received IVIg or WinRho within 2 weeks before the screening visit
• Received hematopoietic growth factors, including IL-11 (Neumega®) within 4 weeks before the screening visit
• Past or present participation in any study evaluating PEG-rHuMGDF, recombinant human thrombopoietin (rHuTPO) or related platelet product
• Received any alkylating agents within 8 weeks before the screening visit or anticipated use during the time of the proposed study
• Received any monoclonal antibody (eg, rituximab) within 16 weeks before the screening visit or anticipated use during the time of the proposed study
• Less than 4 weeks since receipt of any therapeutic drug or device that is not FDA approved for any indication before the screening period
• Less than 2 months since major surgery (including laparoscopic splenectomy)
• Pregnant or breast feeding
• Subjects of reproductive potential who are not using adequate contraceptive precautions, in the judgment of the investigator