Study With SAR302503 in Patients With Polycythemia Vera or Essential Thrombocythemia

Inclusion criteria: * Has had a diagnosis of hydroxyurea resistant or intolerant polycythemia vera (PV) or essential thrombocythemia (ET) documented at Screening. * Polycythemia vera or essential thrombocythemia defined according to the revised WHO criteria. * Polycythemia vera resistance or intolerance to hydroxyurea is defined as polycythemia vera patients on hydroxyurea with a hematocrit \>45%, or phlebotomy twice in the last 6 months and at least once in the last 3 months. * Essential thrombocythemia resistance or intolerance to hydroxurea is defined as essential thrombocythemia patients on HU with platelet count \>600 x 10x9/L. Dose Expansion Phase (polycythemia vera) and 600 mg/day group (essential thrombocythemia): * Has had a diagnosis of polycythemia vera or essential thrombocythemia according to the revised WHO 2008 criteria. * PV patients must be resistant or intolerant to hydroxyurea. * ET patients must be resistant or intolerant to hydroxyurea. * Provide written informed consent to participate. Exclusion criteria: * Less than 18 years of age. * Participation in any study of an investigational agent (drug, biologic, device) within 30 days prior to initiation of study drug, unless during non-treatment phase. (Prior treatment with another JAK2 inhibitor is allowed.) * Unwilling to comply with scheduled visits, treatment plans, laboratory assessments, and other study-related procedures. * Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 3 or 4 at study entry. * Splenectomy. * Active malignancy other than polycythemia vera or essential thrombocythemia, except adequately treated basal cell carcinoma and squamous cell carcinoma of the skin, cervical carcinoma in situ, or other malignancies that have been stable and off therapy for ≥5 years. * Major surgery within 28 days or radiation within 3 months prior to initiation of study drug. * Active acute infection requiring antibiotics. * Known human immunodeficiency virus or acquired immunodeficiency syndrome-related illness. * Uncontrolled congestive heart failure (New York Heart Association Classification 3 or 4), angina, myocardial infarction, cerebrovascular accident, coronary/peripheral artery bypass surgery, transient ischemic attack, or pulmonary embolism within 3 months prior to initiation of study drug. * Any severe acute or chronic medical, neurological, or psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or study drug administration, may interfere with the informed consent process and/or with compliance with the requirements of the study, or may interfere with interpretation of study results and, in the Investigator's opinion, would make the patient inappropriate for entry into this study. * Inadequate organ function. * Known active (acute or chronic) Hepatitis A, B, or C; and Hepatitis B and C carriers. * Prior history of chronic liver disease (eg, chronic alcoholic liver disease, autoimmune hepatitis, sclerosing cholangitis, primary biliary cirrhosis, hemachromatosis, non-alcoholic steatohepatitis \[NASH\]). * Concomitant treatment with or use of drugs or herbal agents known to be at least moderate inhibitors or inducers cytochrome P450 3A4 (CYP3A4). * Presence of any gastric or other disorder that would inhibit absorption of oral medication. * Known hypersensitivity to any excipients in the study drug formulation. * Women of childbearing potential, unless using effective contraception while on study drug. * Men who partner with a woman of childbearing potential, unless they agree to use effective contraception while on study drug. The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.