IMAGINE is a two-part trial to evaluate the safety and preliminary efficacy of imetelstat in combination with azacitidine with or without venetoclax in patients with relapsed or refractory AML. The trial will consist of a safety run-in phase (Part A) employing a 3+3 design to monitor dose-limiting toxicities of imetelstat when administered in combination with a fixed dose of azacitidine. Part B will consist of a phase 1b trial employing a BOIN12 design to determine the optimal biological dose of imetelstat, starting at a lower dose level, in combination with azacitidine and venetoclax. Total of up to 36 participants will be accrued over 54 months at Mount Sinai Hospital. Estimated duration of trial is 114 months including recruitment, screening, treatment, and follow-up.