Key Inclusion Criteria:

   - Individuals must have been on a stable dose of ruxolitinib for at least 4 weeks prior
   to study entry

   - Individuals with PMF, post-PV MF, or post-ET MF classified as high risk or
   intermediate risk as defined by the Dynamic International Prognostic Scoring System
   (DIPSS) for PMF or DIPSS Plus, if cytogenetics are available

   - Individuals with PMF, post-PV MF, or post-ET MF who are receiving ruxolitinib and meet
   2013 Revised International Working Group for Myelofibrosis Research and Treatment
   (IWG-MRT) and European Leukemia Net (ELN) response criteria with progressive and
   relapsed disease, with modifications for progressive disease complete remission (CR),
   partial remission (PR), or clinical improvement (CI)

   - European Cooperative Oncology Group (ECOG) performance status of ≤ 2

   - Required screening laboratory values as described in the protocol

   - Willing and able to comply with scheduled visits, drug administration plan, imaging
   studies, laboratory tests, other study procedures, and study restrictions including
   mandatory prophylaxis for pneumocystis jiroveci pneumonia (PJP)

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

Key Exclusion Criteria:

   - Individuals on a stable ruxolitinib dose of 5 mg once daily

   - History of prior allogeneic bone marrow progenitor cell or solid organ transplantation

   - Ongoing drug-induced liver injury, alcoholic liver disease, non-alcoholic
   steatohepatitis, primary biliary cirrhosis, extrahepatic obstruction caused by
   cholelithiasis, cirrhosis of the liver

   - Ongoing drug-induced pneumonitis

   - Ongoing inflammatory bowel disease

   - Ongoing alcohol or drug addiction

   - Symptomatic congestive heart failure (New York Heart Association Classification >
   Class II), unstable angina, or unstable cardiac arrhythmia requiring medication

   - Known hypersensitivity to the study investigational medicinal product (IMP), the
   metabolites, or formulation excipients

   - Unwilling or unable to take oral medication

   - Unresolved non-hematologic toxicities from prior therapies that are > Common
   terminology Criteria for Adverse Events (CTCAE) Grade 1 (with the exception of
   alopecia [Grade 1 or 2 permitted])

   - Pregnant or lactating females

   - Cytomegalovirus (CMV): Ongoing infection, treatment, or prophylaxis within the past 28
   days

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