Polycythemia vera (PV) and essential thrombocythemia (ET) are classical Philadelphia-negative myeloproliferative neoplasms (MPN) that are characterized by an excess of cells in the peripheral blood, clonal bone marrow hyperplasia, and extramedullary hematopoiesis. The symptoms of these patients may range from asymptomatic disease to symptomatic disease that may significantly affect their activities of daily living, such as severe generalized pruritus, night sweats and fevers, erythromelalgia, bone and muscle pain, weight loss, and fatigue. Moreover, the patients may develop thromboembolic and hemorrhagic complications, transition to myelofibrosis (MF), and transformation to acute leukemia. In principle, the only potentially curative therapy for MPNs is allogenic stem cell transplantation (allo-SCT). However, due to significant transplant-associated morbidity and mortality, this therapeutic option is only applied in exceptional cases of ET or PV. The majority of patients do not qualify for allo-SCT since the risks of this treatment clearly outweigh the potential benefits. Moreover, even with a non-transplantation approach, patients with ET and PV have a life expectancy comparable to or close to healthy age-matched control persons. For patients with standard risk PV, phlebotomy and acetylsalicylic acid are standard of care (target hematocrit below 45 %), while patients with standard risk ET should receive either no specific treatment or acetylsalicylic acid (provided that no microvascular symptoms or secondary acquired von Willebrand syndrome are present).

However, in patients who are at high risk to develop thromboembolic or hemorrhagic complications (high-risk patients), cytoreductive treatment is generally indicated to prevent these potentially life-threatening complications. In PV and ET, high risk patients are characterized by advanced age (\> 60 years) and / or a history of thromboembolic or hemorrhagic events {1,2,3}. In ET, a platelet count \> 1500 x 109/l is associated with an increased risk of bleeding, and thus should result in a platelet lowering treatment {2}. In PV, in addition to the risk-score based therapy, cytoreduction is also required in patients with progressive or marked myeloproliferation (leukocytosis, thrombocytosis, symptomatic splenomegaly, increase of frequency of phlebotomy requirement), or devastating constitutional symptoms {1,2,4}. In Germany, best available therapy (BAT) includes approved drugs such as hydroxyurea (HU; approved for both PV and ET) and anagrelide (approved for second-line treatment of ET) and non-approved options such as alpha-interferon, pipobroman, busulfan (in elderly patients), and radioactive phosphorus (32P). In rare cases, patients may also benefit from splenic irradiation or splenectomy.

Ruxolitinib is a JAK1/2-specific tyrosine kinase inhibitor (TKI) which has been approved for the treatment of symptomatic myelofibrosis. The compound was shown to be superior to hydroxyurea in reducing splenomegaly and constitutional symptoms. Ruxolitinib is currently studied in phase 2 and phase 3 clinical trials for HU-resistant or HU-intolerant PV and ET. The aim of the present study is to assess the feasibility, efficacy, and safety of ruxolitinib treatment vs. BAT in patients with high-risk PV or -ET.