• Determine whether a specific reduction in the JAK2 V617F allele has been indicated in this study. [ Time Frame: 18 weeks + ] [ Designated as safety issue: No ]
  

• - improvements in hemoglobin values, neutrophil count, and platelet count. - reduction in dose of hydroxyurea - reduction in splenic enlargement - rate of phlebotomy [ Time Frame: 18 weeks + ] [ Designated as safety issue: No ]
  

Drug: lestaurtinib
60 mg bid - 120 mg bid for an 18 weeks (126 days) treatment duration

This is an 18-week open-label, multicenter study to evaluate the efficacy and tolerability of CEP-701 (lestaurtinib) treatment at a dosage of 80 mg bid for 18 weeks (126 days) in patients with Polycythemia Vera (PV) who have abnormal baseline neutrophil counts or require hydroxyurea therapy and patients with Essential Thrombocytosis (ET) who require hydroxyurea therapy for disease control.

Inclusion Criteria:

• The patient has polycythemia vera (PV) or essential thrombocytosis (ET).
• The patient has a detectable JAK2 V617F mutation.

• Patients with PV have at least 1 of the following risk factors:

  1. neutrophil count greater than 7000/mm3
  2. receiving hydroxyurea treatment
• Patients with ET are receiving concomitant hydroxyurea.
• The patient has an ECOG performance score of 0, 1, or 2.

Exclusion Criteria:

• The patient has bilirubin levels or aspartate transaminases (AST) levels within exclusionary ranges.
• patient has serum creatinine concentrations within exclusionary ranges.
• patient has an untreated or progressive infection.
• patient has any physical or psychiatric condition that may compromise participation in the study.
• has a history of venous or arterial thrombosis within 6 months.
• use of hydroxyurea has been initiated or escalated in the month prior to screening.
• has active gastrointestinal ulceration or bleeding.
• patient has used an investigational drug within the past 30 days.
• patient is being treated with anagrelide.
• patient has previously taken CEP-701 (lestaurtinib).
• patient has hypersensitivity to CEP-701 (lestaurtinib) or any component of CEP-701 (lestaurtinib).
• patient has received interferon within the past 30 days.