Inclusion Criteria:

   - Intermediate -1 or -2 or high-risk Myelofibrosis (per Passamonti et al 2010)

   - Thrombocytopenia (platelet count ≤ 100,000/µL) at any time after signing informed
   consent

   - Palpable splenomegaly ≥ 5 cm on physical examination

   - Total Symptom Score ≥ 13 on the MPN-SAF TSS 2.0, not including the inactivity question

   - Patients who are platelet or red blood cell transfusion-dependent are eligible

   - Adequate white blood cell counts (with low blast counts), liver function, and renal
   function

   - At least 6 months from prior splenic irradiation

   - At least 1-4 weeks since prior myelofibrosis therapy, including any erythropoietic or
   thrombopoietic agent

   - Not pregnant, not lactating, and agree to use effective birth control

   - Able and willing to undergo frequent MRI or CT assessments and complete symptom
   assessments using a patient-reported outcome instrument

Exclusion Criteria:

   - Prior treatment with more than 2 JAK2 inhibitors or with pacritinib

   - There is no maximum cumulative prior JAK2 inhibitor treatment

   - History of (or plans to undergo) spleen removal surgery or allogeneic stem cell
   transplant

   - Ongoing gastrointestinal medical condition such as Crohn's disease, Inflammatory bowel
   disease, chronic diarrhea, or constipation

   - Active bleeding that requires hospitalization during the screening period

   - Cardiovascular disease, including recent history or currently clinically symptomatic
   and uncontrolled: congestive heart failure, arrhythmia, angina, QTc prolongation or
   other QTc risk factors, myocardial infarction

   - Other malignancy within last 3 years other than certain limited skin, cervical,
   prostate, breast, or bladder cancers

   - Other ongoing, uncontrolled illnesses (including HIV infection and active hepatitis A,
   B, or C), psychiatric disorder, or social situation that would prevent good care on
   this study

   - Life expectancy < 6 months