A Study of Bomedemstat (IMG-7289/MK-3543) Compared to Best Available Therapy (BAT) in Participants With Essential Thrombocythemia and an Inadequate Response or Intolerance of Hydroxyurea (MK-3543-006)

Inclusion Criteria: Has a diagnosis of ET per WHO 2016 diagnostic criteria for myeloproliferative neoplasms Has a bone marrow fibrosis score of Grade 0 or Grade 1, as per a modified version of the European Consensus Criteria for Grading Myelofibrosis Has a history of inadequate response to or intolerance of hydroxyurea per at least 1 of the following criteria, based on modified European LeukemiaNet (ELN) criteria for hydroxyurea resistance or intolerance: hydroxyurea resistance (or inadequate response) or hydroxyurea Intolerance Has an inadequate or loss of response to their most recent prior ET therapy, requiring a change of cytoreductive therapy Has a platelet count > 450 × 109/L (450k /μL) assessed up to 72 hours before first dose of study intervention Has an absolute neutrophil count (ANC) ≥0.75 × 109/L assessed up to 72 hours before first dose of study intervention Has a life expectancy of >52 weeks If capable of producing sperm, the participant agrees to not donate sperm and to either abstain from heterosexual intercourse or use a highly effective method of contraception for the duration of the study period and for at least the time needed to eliminate each study intervention after the final dose Participants assigned female sex at birth are eligible to participate if not pregnant or breastfeeding, and at least one of the following conditions applies: is not a participant of childbearing potential (POCBP) or is POCBP and uses a highly effective method of contraception or is abstinent from penile-vaginal intercourse during the study period and for at least the time needed to eliminate each study intervention after the last dose Exclusion Criteria: Known immediate or delayed hypersensitivity reaction or idiosyncrasy to drugs chemically related to bomedemstat or lysine demethylase or monoamine oxidase inhibitor (LSDi or MAOi) that contraindicates participation History of any illness/impairment of GI function that might interfere with drug absorption (eg, chronic diarrhea or history of gastric bypass surgical procedure), confound the study results or pose an additional risk to the individual by participation in the study Evidence at the time of Screening of increased risk of bleeding History of a malignancy, unless potentially curative treatment has been completed with no evidence of malignancy for 2 years. Note: The time requirement does not apply to participants who underwent successful definitive resection of basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or carcinoma in situ, excluding carcinoma in situ of the bladder Human immunodeficiency virus (HIV)-infected participants with a history of Kaposi's sarcoma and/or Multicentric Castleman's Disease Use of prohibited medication within 14 days of first dose of study intervention (eg, all hematopoietic growth factors, MAOIs, strong inhibitors and inducers of CYP3A4 or CYP2D6, drugs such as chloroquine whose metabolites are known to inhibit CYP3A4 or CYP2D6, Class 1c antiarrhythmics such as propafenone that are known to cause thrombocytopenias, etc.) or expected to require any of these medications during study treatment Has received prior treatment for their ET within 1 week (4 weeks for interferon) before first dose of study intervention Has received prior treatment with bomedemstat Has received an investigational agent or has used an investigational device within 4 weeks prior to study intervention administration Has an active infection requiring systemic therapy Has had major surgical procedure ≤4 weeks before first dose of study intervention or has not recovered from side effects of major surgical procedure >4 weeks before first dose