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.