A Trial of Fedratinib in Subjects With DIPSS, Intermediate or High-Risk Primary Myelofibrosis, Post-Polycythemia Vera Myelofibrosis, or Post-Essential Thrombocythemia Myelofibrosis and Previously Treated With Ruxolitinib

1. Subject is at least 18 years of age at the time of signing the informed consent form (ICF)

2. Subject has an Eastern Cooperative Oncology Group (ECOG) Performance Score (PS) of 0, 1 or 2

3. Subject has diagnosis of primary myelofibrosis (PMF) according to the 2016 World Health Organization (WHO) criteria, or diagnosis of post-ET or post-PV myelofibrosis according to the IWG-MRT 2007 criteria, confirmed by the most recent local pathology report

4. Subject has a DIPSS Risk score of Intermediate or High

5. Subject has a measurable splenomegaly during the screening period as demonstrated by spleen volume of ≥ 450 cm3 by MRI or CT-scan assessment or by palpable spleen measuring ≥ 5 cm below the left costal margin.

6. Subject has been previously exposed to ruxolitinib, while diagnosed with MF (PMF, post-ET MF or post-PV MF), and must meet at least one of the following criteria (a or b)

1. Treatment with ruxolitinib for ≥ 3 months

2. Treatment with ruxolitinib for ≥ 28 days complicated by any of the following:

* Development of a red blood cell transfusion requirement (at least 2 units/month for 2 months) or

* Grade ≥ 3 AEs of thrombocytopenia, anemia, hematoma, and/or hemorrhage while on treatment with ruxolitinib

7. Subject must have treatment-related toxicities from prior therapy resolved to Grade 1 or pretreatment baseline before start of last therapy prior to fedratinib treatment.

8. Subject must understand and voluntarily sign an ICF prior to any study-related assessments/procedures being conducted

9. Subject is willing and able to adhere to the study visit schedule and other protocol requirements

10. Participants must agree to use effective contraception

2. Subject is pregnant or lactating female

3. Subject with previous splenectomy

4. Subject with previous or planned hematopoietic cell transplant

5. Subject with prior history of encephalopathy, including Wernicke's

6. Subject with signs or symptoms of encephalopathy including Wernicke's (eg, severe ataxia, ocular paralysis or cerebellar signs)

7. Subject with thiamine deficiency, defined as thiamine levels in whole blood below normal range according to institutional standard and not corrected prior to enrollment on the study

8. Subject with concomitant treatment with or use of pharmaceutical, herbal agents or food known to be strong or moderate inducers of Cytochrome P450 3A4 (CYP3A4), or dual CYP2C19 and CYP3A4 inhibitors

9. Subject on any chemotherapy, immunomodulatory drug therapy (eg, thalidomide, interferon-alpha), anagrelide, immunosuppressive therapy, systemic corticosteroids \> 10 mg/day prednisone or equivalent. Subjects who have had prior exposure to hydroxyurea (eg, Hydrea) in the past may be enrolled into the study as long as it has not been administered within 14 days prior to the start of fedratinib treatment

10. Subject has received ruxolitinib within 14 days prior to the start of fedratinib

11. Subject on treatment with myeloid growth factor (eg, granulocyte-colony stimulating factor \[G-CSF\]) within 14 days prior to the start of fedratinib treatment

12. Subject with previous exposure to Janus kinase (JAK) inhibitor(s) for more than 1 cycle other than ruxolitinib treatment

13. Subject on treatment with aspirin with doses \> 150 mg daily

14. Subject with major surgery within 28 days before starting fedratinib treatment

15. Subject with diagnosis of chronic liver disease (eg, chronic alcoholic liver disease, autoimmune hepatitis, sclerosing cholangitis, primary biliary cirrhosis, hemochromatosis, non-alcoholic steatohepatitis)

16. Subject with prior malignancy other than the disease under study unless the subject has not required treatment for the malignancy for at least 3 years prior to enrollment.

However, subject with the following history/concurrent conditions provided successfully treated may enroll: non-invasive skin cancer, in situ cervical cancer, carcinoma in situ of the breast, incidental histologic finding of prostate cancer (T1a or T1b using the tumor, nodes, metastasis \[TNM\] clinical staging system), or is free of disease and on hormonal treatment only

17. Subject with uncontrolled congestive heart failure (New York Heart Association Classification 3 or 4)

18. Subject with known human immunodeficiency virus (HIV), known active infectious Hepatitis B (HepB), and/or known active infectious Hepatitis C (HepC)

19. Subject with serious active infection

20. Subject with presence of any significant gastric or other disorder that would inhibit absorption of oral medication

21. Subject is unable to swallow capsule

22. Subject has any significant medical condition, laboratory abnormality, or psychiatric illness that would prevent the subject from participating in the study

23. Subject has any condition including the presence of laboratory abnormalities, which places the subject at unacceptable risk if he/she were to participate in the study

24. Subject has any condition that confounds the ability to interpret data from the study

25. Subject with participation in any study of an investigational agent (drug, biologic, device) within 30 days prior to start of fedratinib treatment

26. Subject with life expectancy of less than 6 months.

What this study is about