This is a Phase 2/3, multicenter study of imetelstat that consists of 2 parts. Part 1 is an open‐label, single-arm design to assess the efficacy and safety of imetelstat. Approximately 55 participants will be enrolled in Part 1, including the expansion cohort, and be followed-up for safety, hematologic improvement and reduction in transfusion requirement. Part 2 of the study will be initiated if data from Part 1 are supportive of a satisfactory benefit/risk profile. Part 2 is a double‐blind, randomized design to compare the efficacy of imetelstat with placebo. Approximately 170 eligible participants will be randomized in a 2:1 ratio to receive either imetelstat or placebo, respectively. Each part of the study will consist of 3 phases: a Screening phase (up to 28 days); a treatment phase; and a post-treatment follow-up phase which will continue until death, lost to follow-up, withdrawal of consent, or the End of the Study (whichever occurs first). The End of the Study is defined as 2 years after the study entry of the last participant or anytime the sponsor terminates the study, whichever comes first.