Diagnosis and Inclusion Criteria

   1. Primary MF, post-polycythemia vera MF, or post-essential thrombocythemia MF (as
   defined by Tefferi and Vardiman 2008

   2. Platelet count of <50,000/μL at Screening (Day -35 to Day -3)

   3. Dynamic International Prognostic Scoring System Intermediate-1, Intermediate-2, or
   High-Risk (Passamonti et al 2010

   4. Palpable splenomegaly ≥5 cm below the lower costal margin (LCM) in the midclavicular
   line as assessed by physical examination

   5. TSS of ≥10 on the MPN-SAF TSS 2.0 or a single symptom score of ≥5 or two symptoms of
   ≥3, including only the symptoms of left upper quadrant pain, bone pain, itching, or
   night sweats.The TSS criteria need only to be met on a single day.

   6. Age ≥18 years

   7. Eastern Cooperative Oncology Group performance status 0 to 2

   8. Peripheral blast count of <10% throughout the Screening period prior to randomization

   9. Absolute neutrophil count of ≥500/µL

10. Left ventricular cardiac ejection fraction of ≥50% by echocardiogram or multigated
   acquisition scan

11. Adequate liver and renal function, defined by liver transaminases (aspartate
   aminotransferase [AST]/serum glutamic-oxaloacetic transaminase [SGOT] and alanine
   aminotransferase [ALT]/serum glutamic pyruvic transaminase [SGPT]) ≤3 × the upper
   limit of normal (ULN) (AST/ALT ≤5 × ULN if transaminase elevation is related to MF),
   total bilirubin ≤4 x ULN (in cases where total bilirubin is elevated, direct bilirubin
   ≤4 × ULN, is required) and creatinine ≤2.5 mg/dL

12. Adequate coagulation defined by prothrombin time/international normalized ratio and
   partial thromboplastin time ≤1.5 × ULN

13. If fertile, willing to use effective birth control methods during the study

14. Willing to undergo and able to tolerate frequent MRI or CT scan assessments during the
   study

15. Able to understand and willing to complete symptom assessments using a
   patient-reported outcome instrument

16. Provision of signed informed consent

Exclusion Criteria

   1. Life expectancy <6 months

   2. Completed allogeneic stem cell transplant (allo-SCT) or are eligible for and willing
   to complete other approved available therapy including allogeneic stem cell

   3. History of splenectomy or planning to undergo splenectomy

   4. Splenic irradiation within the last 6 months

   5. Previously treated with pacritinib

   6. Treatment with any MF-directed therapy within 14 days prior to treatment Day 1

   7. Prior treatment with more than one JAK2 inhibitor

   8. Prior treatment with with ruxolitinib, if BOTH of the following conditions are met:

   i. exposure to higher-dose ruxolitinib (>10 mg daily) within 120 days prior to
   treatment Day 1 AND ii. total duration of treatment with higher-dose ruxolitinib (>10
   mg daily) was >90 days, from first to last exposure (i.e., this 90-day period starts
   on the date of first administration of ruxolitinib at a total daily dose of >10 mg and
   continues for 90 calendar days, regardless of whether higher-dose ruxolitinib is
   administered continuously or intermittently).

   9. Prior treatment with any JAK2 inhibitor other than ruxolitinib, irrespective of dose,
   with a duration of >90 days. The 90-day period starts on the date of first
   administration of JAK2 inhibitor therapy and continues for 90 calendar days,
   regardless of whether therapy is administered continuously or intermittently.

10. Treatment with an experimental therapy within 28 days prior to treatment Day 1

11. Systemic treatment with a strong cytochrome P450 3A4 inhibitor or a strong cytochrome
   P450 inducer within 14 days prior to treatment Day 1. Shorter washout periods may be
   permitted with approval of the Medical Monitor, provided that the washout period is at
   least five half-lives of the drug prior to treatment Day 1

12. Significant recent bleeding history defined as National Cancer Institute Common
   Terminology Criteria for Adverse Events (CTCAE) grade ≥2 within 3 months prior to
   treatment Day 1, unless precipitated by an inciting event (eg, surgery, trauma, or
   injury)

13. Systemic treatment with medications that increase the risk of bleeding, including
   anticoagulants, antiplatelet agents (except for aspirin dosages of ≤100 mg per day),
   anti-vascular endothelial growth factor (anti-VEGF) agents, and daily use of
   cyclooxygenase-1 (COX-1) inhibiting non-steroidal anti-inflammatory drugs (NSAIDs)
   within 14 days prior to treatment Day 1

14. Systemic treatment with medications that can prolong the QT interval within 14 days
   prior to treatment Day 1. Shorter washout periods may be permitted with approval of
   the Medical Monitor, provided that the washout period is at least five half-lives of
   the drug prior to treatment Day 1

15. Any history of CTCAE grade ≥2 non-dysrhythmia cardiac conditions within 6 months prior
   to treatment Day 1. Patients with asymptomatic grade 2 non-dysrhythmia cardiovascular
   conditions may be considered for inclusion, with the approval of the Medical Monitor,
   if stable and unlikely to affect patient safety.

16. Any history of CTCAE grade ≥2 cardiac dysrhythmias within 6 months prior to treatment
   Day 1. Patients with non-corrected QT interval CTCAE grade 2 cardiac dysrhythmias may
   be considered for inclusion, with the approval of the Medical Monitor, if the
   dysrhythmias are stable, asymptomatic, and unlikely to affect patient safety.

17. QT corrected by the Fridericia method (QTcF) prolongation >450 ms or other factors
   that increase the risk for QT interval prolongation (eg, hypokalemia [defined as serum
   potassium <3.0 mEq/L that is persistent and refractory to correction], or history of
   long QT interval syndrome).

18. New York Heart Association Class II, III, or IV congestive heart failure

19. Any active gastrointestinal or metabolic condition that could interfere with
   absorption of oral medication

20. Active or uncontrolled inflammatory or chronic functional bowel disorder such as
   Crohn's Disease, inflammatory bowel disease, chronic diarrhea, or chronic constipation

21. Other malignancy within 3 years prior to treatment Day 1. The following patients may
   be eligible despite having had a malignancy within the prior 3 years: patients with
   curatively treated squamous or basal cell carcinoma of the skin; patients with
   curatively treated non-invasive cancers; patients with organ-confined prostate cancer
   with prostate specific antigen (PSA) <20 ng/mL and National Comprehensive Cancer
   Network risk of Very Low, Low, or Favorable Intermediate; and patients with curatively
   treated non-metastatic prostate cancer with negative PSA.

22. Uncontrolled intercurrent illness, including, but not limited to, ongoing active
   infection, psychiatric illness, or social situation that, in the judgment of the
   treating physician, would limit compliance with study requirements

23. Known seropositivity for human immunodeficiency (HIV) virus. For patients in France,
   Czech Republic, and Italy only: testing for HIV is required during Screening.

24. Known active hepatitis A, B, or C virus infection. For patients in France, Czech
   Republic and Italy only: testing for hepatitis B and C is required during Screening.

25. Women who are pregnant or lactating

26. Concurrent enrollment in another interventional trial

27. Severe thrombocytopenia due to vitamin B12 deficiency, folate deficiency, or viral
   infection in the opinion of the investigator

28. Known hypersensitivity to pacritinib or any of the following inactive ingredients:
   microcrystalline cellulose, polyethylene glycol, and magnesium stearate; any
   contraindication to the "physician's choice" medicinal product selected by the
   investigator to be used as the comparator or to loperamide or equivalent antidiarrheal
   medication

29. Persons deprived of their liberty by a judicial or administrative decision

30. Persons subject to legal protection measures or unable to express their consent

31. Temporarily incapacitated persons