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.
