This is a multicenter study with 2 arms, and will include 3 phases: a) screening phase of up to 28 days before randomization during which participants will complete a 14-day washout period from all prior therapies including JAK-inhibitor treatment, and the participant's eligibility will be reviewed; b) treatment phase, from randomization until study treatment (imetelstat or BAT) discontinuation; and c) post treatment follow-up phase, that begins when the participant discontinues treatment, and will continue until death, lost to follow-up, withdrawal of consent, or study end, whichever occurs first. Participants will be randomized (2:1) into 2 Arms (Arm A will receive imetelstat and Arm B will receive BAT).~Participants who meet progressive disease criteria and discontinue BAT, may crossover to receive imetelstat treatment after sponsor's approval.
The purpose of the study is to evaluate the overall survival of participants treated with imetelstat compared to best available therapy with intermediate-2 or high-risk Myelofibrosis (MF) who are relapsed/refractory to Janus Kinase (JAK)-Inhibitor treatment.