Study AOP 03-007 was designed as a pivotal study to test, if Anagrelide (Thromboreductin®)was not inferior to HU with respect to efficacy in patients with ET. This approach to demonstrate non-inferiority was based on the following decision points:

• ET is a rare disease and recruitment of large patient number (> 1600) to prove superiority was not considered possible.

. It was decided to recruit only treatment naïve high risk patients to avoid pre-treatment bias, which further limited the number of patients eligible for the study.

Inclusion Criteria:

• Presence of essential thrombocythaemia with high-risk profile.

Exclusion Criteria:

• previous treatment with cytoreductive drugs or Anagrelide
• pregnant women or women in childbearing age with inadequate contraception
• patients with contraindications for study drugs due to anaphylactoid reactions to either active or non-active ingredients
• known lactose intolerance
• cardiovascular diseases grade III-IV (Toxicity Criteria of the South West Oncology Group, 1992) - with a negative benefit/risk ratio
• severe renal disease (Creatinin Clearance < 30 ml/min)
• severe liver disease (AST or ALT > 5-times normal)
• coexisting, malignant, systemic diseases