Study to Determine the Safety and Efficacy of INCB018424 in Patients With Polycythemia Vera or Essential Thrombocythemia

Inclusion Criteria: * Confirmed diagnosis of polycythemia vera or essential thrombocythemia as determined by treating physician * Disease refractory to hydroxyurea or for whom treatment with hydroxyurea is contraindicated or have refused further treatment with hydroxyurea due to side effects. * Patient meets baseline clinical lab parameters Exclusion Criteria: * Treatment with interferon alpha or anagrelide within 7 days and hydroxyurea within 1 day of starting INCB018424. * Patients diagnosed with another malignancy unless the malignancy was cervical intraepithelial neoplasia or basal or squamous cell skin cancer and the patient has been disease free for \> 3 years * Patients receiving therapy with intermediate or high dose steroids greater than the equivalent of 10 mg prednisone per day * Clinically significant cardiac disease (New York Heart Association (NYHA) Class III or IV)