Bomedemstat vs Hydroxyurea for Essential Thrombocythemia (MK-3543-007)

Inclusion Criteria:

• Diagnosis of Essential Thrombocythemia (ET) based on World Health OrganizationCriteria for myeloproliferative neoplasms, and an indication for cytoreductivetherapy regardless of age or risk status

• Has a centrally assessed bone marrow fibrosis score of Grade 0 or Grade 1, as per amodified version of the European Consensus Criteria for Grading Myelofibrosis

• Has received no prior cytoreductive treatment for their ET

• Human Immunodeficiency Virus (HIV)-infected participants have well controlled HIV onantiretroviral therapy

• Participants who are Hepatitis B surface antigen (HBsAg) positive are eligible ifthey have received Hepatitis B Virus (HBV) antiviral therapy for at least 4 weeksand have undetectable HBV viral load

• Participants with history of Hepatitis C Virus (HCV) infection are eligible if HCVviral load is undetectable

Exclusion Criteria:

• History of any illness/impairment of gastrointestinal function that might interferewith drug absorption

• History of a malignancy, unless potentially curative treatment has been completedwith no evidence of malignancy for 2 years

• HIV-infected participants with a history of Kaposi's sarcoma and/or MulticentricCastleman's Disease

• Has an active infection requiring systemic therapy

• Has had a major surgery <4 weeks prior to first dose of study intervention or hasnot recovered from side effects of major surgery >4 weeks prior to first dose