Inclusion Criteria:

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, and must meet at least one of the
following criteria (a or b)

a. Treatment with ruxolitinib for ≥ 3 months b. 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. A female of childbearing potential (FCBP) must:

1. Have two negative pregnancy tests as verified by the Investigator prior to
starting study therapy. She must agree to ongoing pregnancy testing during the
course of the study, and after end of study therapy. This applies even if the
subject practices true abstinence* from heterosexual contact.

2. Either commit to true abstinence* from heterosexual contact (which must be
reviewed on a monthly basis and source documented) or agree to use, and be able
to comply with acceptable effective contraception** without interruption, -14
days prior to starting investigational product, during the study therapy
(including dose interruptions), and for 30 days after discontinuation of study
therapy.

Note: A female of childbearing potential (FCBP) is a female who: 1) has achieved
menarche at some point, 2) has not undergone a hysterectomy or bilateral
oophorectomy, or 3) has not been naturally postmenopausal (amenorrhea following
cancer therapy does not rule out childbearing potential) for at least 24
consecutive months (ie has had menses at any time in the preceding 24 consecutive
months).

11. Male subjects must:

1. Practice true abstinence* (which must be reviewed on a monthly basis) or agree to
use a condom during sexual contact with a pregnant female or a female of
childbearing potential while participating in the study, during dose
interruptions and for at least 30 days following investigational product
discontinuation, or longer if required for each compound and/or by local
regulations, even if he has undergone a successful vasectomy.

- True abstinence is acceptable when this is in line with the preferred and
usual lifestyle of the subject. [Periodic abstinence (eg, calendar,
ovulation, symptothermal, post-ovulation methods) and withdrawal are not
acceptable methods of contraception].

- Agreement to use highly effective methods of contraception that alone
or in combination result in a failure rate of a Pearl index of less
than 1% per year when used consistently and correctly throughout the
course of the study. Such methods include: Combined (estrogen and
progestogen containing) hormonal contraception: Oral; Intravaginal;
Transdermal; Progestogen-only hormonal contraception associated with
inhibition of ovulation: Oral; Injectable hormonal contraception;
Implantable hormonal contraception; Placement of an intrauterine
device; Placement of an intrauterine hormone-releasing system;
Bilateral tubal occlusion; Vasectomized partner.

Exclusion Criteria:

1. Any of the following laboratory abnormalities:

1. Platelets < 50,000/μL 2. Absolute neutrophil count (ANC) < 1.0 x 109/L 3. Myeloblasts ≥ 5 % in peripheral blood 4. Serum creatinine clearance 1.5 x ULN (upper limit of normal)

6. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) > 3 x ULN

7. Total bilirubin > 1.5 x ULN, subject's total bilirubin between 1.5 - 3.0 x ULN
are eligible if the direct bilirubin fraction is
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) 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