Imetelstat Sodium in Treating Participants With Primary or Secondary Myelofibrosis

Inclusion Criteria

• Diagnosis of one of the following:
• Primary myelofibrosis (PMF) per the revised World Health Organization (WHO) criteria.
• Post-polycythemia vera/essential thrombocythemia myelofibrosis (Post-ET/PV MF) per the International Working Group for Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria.
• High-risk or Intermediate-2 risk MF (as defined by the Dynamic International Prognostic Scoring System [DIPSS-plus]).
• Life expectancy of greater than or equal to (>=) 12 weeks.
• Able to provide informed consent and be willing to sign an informed consent form.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2.
• Serum glutamic oxaloacetic transaminase (SGOT) (aspartate aminotransferase [AST]) = =1000/microliter (mcL).
• Platelet count >=50,000/mcL.
• Absence of active treatment with systemic anticoagulation and a baseline prothrombin time (PT) and activated partial thromboplastin time (aPTT) that does not exceed 1.5 x ULN.
• Females of childbearing potential must have a negative pregnancy test = 30 U/mL).
• Females of childbearing potential must agree to take appropriate precautions to avoid pregnancy (with at least 99% certainty) from screening through end of study; permitted methods for preventing pregnancy must be communicated to study participants and their understanding confirmed.
• Males must agree to take appropriate precautions to avoid fathering a child (with at least 99% certainty) from screening through follow-up; permitted methods for preventing pregnancy should be communicated to the participants and their understanding confirmed.

Exclusion Criteria

• Females who are pregnant or are currently breastfeeding.
• Any chemotherapy (e.g., hydroxyurea), immunomodulatory drug therapy (e.g., thalidomide), immunosuppressive therapy, corticosteroids > 10 mg/day prednisone or equivalent, growth factor treatment (e.g., erythropoietin) or Janus kinase (JAK) inhibitor therapy =<14 days prior to registration.
• Participants with another active malignancy.
• Note: participants with early stage squamous cell carcinoma of the skin, basal cell carcinoma of the skin or cervical intraepithelial neoplasia are eligible for enrollment.
• Known positive status for human immunodeficiency virus (HIV).
• Any unresolved toxicity greater or equal to grade 2 from previous anticancer therapy, except for stable chronic toxicities not expected to resolve.
• Incomplete recovery from any prior surgical procedures or had surgery =<4 weeks prior to registration, excluding the placement of vascular access.
• Presence of acute active infection requiring antibiotics.
• Uncontrolled intercurrent illness or any concurrent condition that, in the Investigator's opinion, would jeopardize the safety of the participant or compliance with the protocol.