Purpose

Primary Objective: * To evaluate the efficacy of daily oral doses of 400 mg or 500 mg of SAR302503 (Investigational Medicinal Product, IMP) compared to placebo in the reduction of spleen volume as determined by magnetic resonance imaging (MRI) (or computed tomography scan in patients with contraindications for MRI). Secondary Objectives: * To evaluate the effect on Myelofibrosis (MF)-associated symptoms (key MF symptoms) as measured by the modified Myelofibrosis Symptom Assessment Form (MFSAF) diary. * To evaluate the Overall Survival of patients treated with either 400 mg/day or 500 mg/day of IMP as compared to placebo. * To evaluate the Progression Free Survival of patients treated with either 400 mg/day or 500 mg/day of IMP as compared to placebo. * To evaluate the durability of splenic response. * To evaluate the safety of IMP.