Inclusion Criteria:

   - Diagnosis of primary myelofibrosis (PMF) according to the revised WHO criteria; or
   post-essential thrombocythemia-myelofibrosis (PET-MF) or post-polycythemia
   vera-myelofibrosis (PPV-MF) according to the International Working
   Group-Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria.

   - Dynamic International Prognostic Scoring System (DIPSS) intermediate-2 or highrisk MF.

   - Measurable splenomegaly prior to study entry as demonstrated by palpable spleen
   measuring ≥ 5 cm below the left costal margin OR spleen volume of ≥ 450 cm^3 measured
   by magnetic resonance imaging (MRI).

   - Active symptoms of MF as demonstrated by a symptom score of at least 5 points (on a 0
   to 10 scale) on at least one of the symptoms or a score of 3 or greater on at least 2
   of the symptoms.

   - Documented progressive disease during or after Janus kinase (JAK) inhibitor therapy.

   - Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2.

Exclusion Criteria:

   - Peripheral blood blast count of ≥ 10% or bone marrow blast count of ≥ 10%.

   - Prior treatment with imetelstat.

   - Any chemotherapy or MF-directed therapy, investigational drug, hydroxyurea,
   immunomodulatory or immunosuppressive therapy, corticosteroids or JAK inhibitor
   therapy ≤14 days prior to randomization.

   - Major surgery within 4 weeks prior to randomization.

   - Active systemic hepatitis infection requiring treatment (carriers of hepatitis virus
   are permitted to enter the study), of any type or known acute or chronic liver disease
   including cirrhosis.

   - Prior history of hematopoietic stem cell transplant.