This is a dose and schedule finding study of AMG 531 designed to assess the activity of AMG 531 to reduce the rate of clinically significant bleeding and blood transfusions in subjects with myelodysplastic syndrome (MDS) receiving lenalidomide. Subjects with MDS that are planned to receive at least four cycles of lenalidomide for treatment of their disease are appropriate to screen for this study.

All subjects meeting the eligibility criteria will receive lenalidomide 10 mg capsule by mouth daily every day of each 28-day cycle. Subjects will receive AMG 531 or placebo once a week by subcutaneous injection for 16 weeks.

Inclusion Criteria:

• Diagnosis of MDS by bone marrow biopsy based on the World Health Organization (WHO) classification
• Low or Intermediate-1 risk category MDS using the IPSS
• Planned to receive lenalidomide 10 mg capsule by mouth daily for all 28 days of each cycle for at least 4 cycles
• Eastern Cooperative Oncology (ECOG) performance status of 0-2
• Subjects must be at least 18 years of age or older

Exclusion Criteria:

• Prior exposure to >3 cycles of lenalidomide
• Exposure to lenalidomide within the last 30 days
• Prior history of leukemia or aplastic anemia
• Prior history of stem cell transplantation
• Prior malignancy (other than in situ cervical cancer or basal cell cancer of the skin) unless treated with curative intent and without evidence of disease for 3 years before randomization
• Active or uncontrolled infections
• Unstable angina, congestive heart failure [NYHA > class II], uncontrolled hypertension [diastolic > 100 mmHg], uncontrolled cardiac arrhythmia, or recent (within 1 year) myocardial infarction
• History of arterial thrombosis ( eg, stroke or transient ischemic attack) in the past year
• History of venous thrombosis in the past year
• Received IL-11 within 4 weeks of screening
• Less than 4 weeks since receipt of any investigational drug or device
• Have previously received any other thrombopoietic growth factor
• Pregnant or breast feeding
• Subjects of reproductive potential who are not using adequate contraceptive precautions, in the judgment of the investigator
• Known hypersensitivity to any recombinant E coli-derived product

Up to 21 weeks

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

Two subjects who were randomized to Placebo arm received one dose of Romiplostim inadvertently and were summarized in Romiplostim 750 mcg and 500 mcg arm, respectively.