Key Inclusion Criteria:

   - Palpable splenomegaly at least 5 cm below the left costal margin

   - Confirmed diagnosis of PMF or post-PV/ET MF

   - Requires myelofibrosis therapy, in the opinion of the investigator

   - Classified as high risk OR intermediate-2 risk as defined by the International
   Prognostic Scoring System (IPSS) for PMF, or intermediate-1 risk (IPSS) associated
   with symptomatic splenomegaly, hepatomegaly, anemia (hemoglobin < 10.0 g/dL), and/or
   unresponsive to available therapy

   - Acceptable laboratory assessment obtained within 14 days prior to the first dose of
   study drug:

      - Absolute neutrophil count (ANC) ≥ 0.75 x 10^9/L in the absence of growth factor
      in the prior 7 days

      - Platelet Count ≥ 50 x 10^9/L (≥ 100 x 10^9/L if aspartate aminotransferase [AST]
      or alanine aminotransferase [ALT] is ≥ 2 x the upper limit of the normal range
      [ULN]) in the absence of platelet transfusion(s) or thrombopoietin mimetics in
      the prior 7 days

      - Peripheral blood blast count < 10%

      - AST and ALT ≤ 3 x ULN (≤ 5 x ULN if liver is involved by extramedullary
      hematopoiesis as judged by the investigator or if related to iron chelator
      therapy that was started within the prior 60 days)

      - Calculated creatinine clearance (CrCL) of ≥ 45 mL/min

      - Direct bilirubin ≤ 2.0 x ULN

   - Life expectancy of > 24 weeks

   - Males and females of childbearing potential must agree to use protocol-specified
   method(s) of contraception

   - Females who are nursing must agree to discontinue nursing before the first dose of
   study drug

   - Able to understand and willing to sign the informed consent form

Key Exclusion Criteria:

   - Prior splenectomy

   - Splenic irradiation within 3 months prior to the first dose of study drug

   - Eligible for allogeneic bone marrow or stem cell transplantation

   - Uncontrolled inter-current illness, per protocol.

   - Known positive status for human immunodeficiency virus (HIV)

   - Chronic active or acute viral hepatitis A, B, or C infection, or a hepatitis B or C
   carrier

   - Prior use of a JAK1 or JAK2 inhibitor

   - Use of chemotherapy, immunomodulating therapy, biologic therapy, radiation therapy, or
   investigational therapy within 4 weeks of the first dose of study drug

   - Presence of peripheral neuropathy ≥ Common Terminology Criteria for Adverse Events
   (CTCAE) Grade 2

   - Unwilling or unable to undergo a magnetic resonance imaging (MRI) or computed
   tomography (CT) scan

Note: Other protocol defined Inclusion/Exclusion criteria may apply.