Study to Evaluate Activity of 2 Dose Levels of Imetelstat in Participants With Intermediate-2 or High-Risk Myelofibrosis (MF) Previously Treated With Janus Kinase (JAK) Inhibitor

The purpose of this study is to evaluate the efficacy and safety of 2 dose regimens of imetelstat in participants with intermediate-2 or high-risk myelofibrosis (MF) whose disease is relapsed after or is refractory to Janus Kinase (JAK) inhibitor treatment. Key secondary endpoint includes overall survival.

This is a randomized (study medication assigned to participants by chance), multicenter (more than one hospital, medical school team or medical clinic work on a medical research study) study of 2 dosing regimens (treatment arms) of single-agent imetelstat in participants with intermediate-2 or high risk myelofibrosis (MF) whose disease is relapsed after or refractory to Janus Kinase (JAK) inhibitor treatment. The main study consists of 3 parts: Screening Phase (21 days before randomization); Treatment Phase (from randomization until study drug discontinuation); and Follow up Phase (until death, lost to follow-up, withdrawal of consent or study end, whichever occurs first). Participants received imetelstat 9.4 milligram (mg)/kilogram (kg) intravenously (IV) for every 3 weeks until disease progression, unacceptable toxicity, or study end OR imetelstat 4.7 mg/kg IV for every 3 weeks until disease progression, unacceptable toxicity, or study end. Initially, all participants were blinded to the treatment. After the first interim analysis, treatment for all participants was unblinded and participants assigned to the imetelstat 4.7 mg/kg arm could continue with their same imetelstat dose or have it increased to 9.4 mg/kg at the investigator's discretion. The percentage of spleen response and symptom response were evaluated as co-primary endpoints. Following completion of the primary analysis, participants benefiting from study treatment could continue to receive imetelstat in Extension phase for up to 2 years or until loss of benefit or unacceptable toxicity. Participants who had already stopped study treatment could enter the Extension phase to continue follow up for safety via serious adverse event collection and for survival status.

Eligibility Criteria:
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.