An Open Label Study of Itacitinib Administered Orally in Patients With Myelofibrosis

Inclusion Criteria: * Must be diagnosed with PMF, PPV-MF or PET-MF as confirmed by bone marrow biopsy. * Must score at least 1 point on the Dynamic International Prognostic Scoring System (DIPSS) for prognostic risk factors and have peripheral blast count \<10% at both Screening and Baseline hematology assessments. * Subjects must discontinue all drugs used to treat underlying MF disease no later than Day -14. * Subjects must have hemoglobin value \>/= 8.0g/dL and be willing to receive blood transfusions, have a platelet count \>/=50x10\^9/L and absolute neutrophil count (ANC) \>/= 1x10\^9/L. * Subjects must have palpable spleen or history of splenectomy * Active symptoms at the screening visit Exclusion Criteria: * Women who are pregnant or breastfeeding, and men and women who cannot comply with requirements to avoid fathering a child or becoming pregnant, respectively. * Subjects with impaired liver function, end stage renal disease on dialysis or clinically significant concurrent infections requiring therapy. * Subjects with unstable cardiac function or invasive malignancies over the previous 2 years except treated basal or squamous carcinomas of the skin, completely resected intraepithelial carcinoma of the cervix and completely resected papillary thyroid and follicular thyroid cancers.