A Study to Evaluate Safety and Efficacy of Bomedemstat (MK-3543-017)

The primary purpose of the study is to transition participants into an extension study to collect long-term safety and efficacy data. The study will include participants who are safely tolerating bomedemstat, receiving clinical benefit from its use in estimation of the investigator, and have shown the following criteria: * Participants from the IMG-7289-202/MK-3543-005 (NCT05223920) study must have received at least 6 months of treatment with bomedemstat; * Essential thrombocythemia (ET) and polycythemia vera (PV) participants from studies other than IMG-7289-202/MK-3543-005 must have achieved confirmed hematologic remission. No hypothesis testing will be conducted in this study.

Inclusion Criteria: * Is from a bomedemstat study sponsored by Imago BioSciences, Inc. (a subsidiary of Merck \& Co., Inc.) or MSD, and established by the Sponsor as MK-3543-017 ready * Has received at least 6 months of treatment with bomedemstat in the IMG-7289-202/MK-3543-005 study, while safely tolerating bomedemstat, and receiving clinical benefit from its use in the estimation of the investigator * ET and PV participants from established feeder studies other than IMG-7289- 202/MK-3543-005 must have achieved confirmed hematologic remission, must be safely tolerating bomedemstat, and must be receiving clinical benefit from its use in the estimation of the investigator * Is not currently on a dose hold * Participant must be able to swallow oral medication and follow instructions for at-home dosing of bomedemstat Exclusion Criteria: * Has received prohibited concomitant medications * Ongoing or planned participation in another investigational study * Has noncompliance in prior bomedemstat study receiving \<90% of assigned doses excluding suspensions or holds as assigned by the investigator