Myeloid Splenomegaly

Primary Objective:
- To evaluate the efficacy of daily oral doses of 300 mg, 400 mg, and 500 mg SAR302503 and combined for the response rate defined with the ≥35% reduction of spleen volume as determined by magnetic resonance imaging (MRI or computed tomography scan [CT] in patients with contraindications for MRI).
Secondary Objectives:
* To evaluate the safety of SAR302503 for both pooled (300, 400, and 500mg) and individual doses population.
* To evaluate the pharmacokinetics (PK) of SAR302503 after single and repeat-dose.
* To evaluate the effect on Myelofibrosis (MF)-associated symptoms (Key MF symptoms) as measured by the modified Myelofibrosis Symptom Assessment Form (MFSAF).
* To evaluate the durability of splenic response.
* To evaluate the effect of SAR302503 on bone marrow with regard to changes on reticulin fibrosis.

Primary Objective:
- To evaluate the efficacy of daily oral doses of 300 mg, 400 mg, and 500 mg SAR302503 for the reduction of spleen volume as determined by magnetic resonance imaging (MRI).
Secondary Objectives:
* To evaluate the safety of SAR302503.
* To evaluate the pharmacokinetics (PK) of SAR302503 after single and repeat doses.
* To evaluate the pharmacodynamics of SAR302503 as measured by changes in JAK2V617F allele burden in those patients with JAK2V617F mutation, changes in substrate phosphorylation in the JAK-STAT signal transduction pathway, and the expression of cytokines.
* To measure improvement in baseline Myeloproliferative Neoplasm (MPN) associated symptoms, as well as overall impact in quality of life (QOL), through serial administration of the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF).
* To measure generic health-related quality of life (HRQL) and utility values using the EQ-5D questionnaire.