Combination of Fedratinib and Decitabine for Myeloproliferative Neoplasms (MPN)- Accelerated Phase (AP)/Blast Phase (BP)

Inclusion Criteria: * Subjects must have MPN-AP as defined by 10%-19% blasts in the peripheral blood or bone marrow and evidence of dysplastic marrow features with a concomitant diagnosis of essential thrombocythemia (ET), polycythemia vera (PV) or primary myelofibrosis (PMF) or a diagnosis of MPN-BP as defined by 20% blasts in the blood or bone marrow following a previous diagnosis of ET, PV or PMF. * Subjects must have adequate organ function documented within 14 days of study entry as follows: 1. Estimated creatinine clearance (by Cockcroft-Gault Equation) of ≥ 50 mL/min 2. Serum total bilirubin ≤ 1.5 × ULN (unless attributable to Gilbert's disease or hemolysis, in which case the direct bilirubin level must be ≤ 1.5 × upper limit of normal (ULN)). 3. Alkaline phosphatase, serum aspartate aminotransferase (AST) and alanine transaminase (ALT) ≤ 2.5 × ULN. * ≥ 18 years of age. * Eastern Cooperative Oncology Group (ECOG) Performance status of 0-2. Patients with ECOG performance status of 3 will be eligible if the lower performance status is deemed by the investigator to be due entirely to MPN-AP/BP and not due to another comorbidity. Exclusion Criteria: * Receipt of chemotherapy or investigational therapy, with the exception of hydroxyurea, within 4 weeks of study entry. Previous treatment at any time with decitabine, fedratinib or ruxolitinib as single agents will not exclude eligibility. Previous stem cell transplant will not exclude eligibility as long as other inclusion/exclusion criteria have been met and subjects do not have Grade ≥ II graft-versus-host disease (GVHD) requiring systemic immunosuppressive therapy. * Subjects with an Human leukocyte antigen (HLA)-compatible donor or stem cell source who are immediate candidates for allogeneic hematopoietic cell transplantation (HCT). * Subjects who are receiving any concurrent treatment for acute myeloid leukemia (AML), including other investigational agents. * Diagnosis of acute myelofibrosis. * Uncontrolled intercurrent illness including, but not limited to hepatitis, human immunodeficiency virus (HIV)-positive subjects receiving combination antiretroviral therapy, ongoing or active infection, symptomatic congestive heart failure (New York Heart Association Class 3 or 4), unstable angina pectoris, ventricular arrhythmia, Child-Pugh Class C cirrhosis, or psychiatric illness/social situations that would limit compliance with study requirements. * Subjects with a prior history of Wernicke's encephalopathy (WE) will be excluded. If a subject has signs or symptoms of encephalopathy, including Wernicke's encephalopathy (e.g. severe ataxia, ocular paralysis or cerebellar signs), thiamine deficiency must be excluded and a brain MRI should be obtained prior to study initiation to evaluate for WE. * Other medications, severe acute/chronic medical or psychiatric conditions, or laboratory abnormalities that may increase the risk associated with study participation or study drug administration, or may interfere with the interpretation of study results, that in the judgment of the Investigator would make the subject inappropriate for entry into this study. * Pregnant women are excluded because of the potential for teratogenic or abortifacient effects.