PACIFICA: This phase III trial is comparing the effectiveness of targeted therapy (pacritinib) with the physician's choice of treatment (either corticosteroids, hydroxyurea, danazol, or low-dose ruxolitinib) in people with Primary Myelofibrosis, Post-Polycythemia Vera Myelofibrosis, or Post-essential Thrombocythemia MyelofibrosisA Randomized, Controlled Phase 3 Study of Pacritinib Versus Physician's Choice in Patients With Primary Myelofibrosis, Post Polycythemia Vera Myelofibrosis, or Post-Essential Thrombocythemia Myelofibrosis With Severe Thrombocytopenia (Platelet Count <50,000>

Exclusion

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 allo-SCT
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. Any prior treatment with more than one JAK2 inhibitor
8. Treatment with an experimental therapy within 28 days prior to treatment Day 1
9. Systemic treatment with a strong CYP3A4 inhibitor or a strong cytochrome P450 (CYP450) 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
10. 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 (e.g., surgery, trauma, or injury)
11. 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-vascular endothelial growth factor [anti-VEGF]) agents, and daily use of cyclooxygenase-1 (COX-1) inhibiting nonsteroidal anti- inflammatory drugs (NSAIDs) within 14 days prior to treatment Day 1
12. 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
13. 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.
14. Any history of CTCAE grade ≥2 cardiac dysrhythmias within 6 months prior to treatment Day 1. Patients with non-QTc 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.
15. QT corrected by the Fridericia method (QTcF) prolongation >450 ms or other factors that increase the risk for QT interval prolongation (e.g., hypokalemia [defined as serum potassium <3.0 mEq/L that is persistent and refractory to correction], or history of long QT interval syndrome
16. New York Heart Association Class II, III, or IV congestive heart failure
17. Any active gastrointestinal or metabolic condition that could interfere with absorption of oral medication
18. Active or uncontrolled inflammatory or chronic functional bowel disorder such as Crohn's Disease, inflammatory bowel disease, chronic diarrhea, or chronic constipation
19. 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.
20. 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
21. Known seropositivity for human immunodeficiency virus
22. Known active hepatitis A, B, or C virus infection
23. Women who are pregnant or lactating
24. Concurrent enrollment in another interventional trial
25. Severe thrombocytopenia due to vitamin B12 deficiency, folate deficiency, or viral infection in the opinion of the investigator
26. 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.