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 \[AST\]) =\<2.5 x upper limit of normal (ULN) (or =\<5 x ULN if in the investigator's opinion the elevation is due to extramedullary hematopoiesis).
* Serum glutamic pyruvate transaminase (SGPT) alanine aminotransferase (ALT) =\<2.5 x ULN (or =\<5 x ULN if in the investigator's opinion the elevation is due to extramedullary hematopoiesis).
* Total bilirubin =\<3.0 mg/dL (or direct bilirubin \< 1 mg/dL).
* Creatinine =\<3.0 mg/dL.
* Absolute neutrophil count \>=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 =\<7 days prior to registration, unless they are surgically sterile for at least 3 months (i.e., hysterectomy), OR postmenopausal for at least 12 months (follicle-stimulating hormone \[FSH\] \>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.