Study of Bomedemstat in Participants With Essential Thrombocythemia (IMG-7289-CTP-201/MK-3543-003)

Inclusion Criteria: * Diagnosis of Essential Thrombocythemia per World Health Organization (WHO) diagnostic criteria for myeloproliferative neoplasms. * Requires treatment in order to lower platelet count based on patient age over 60 or history of thrombosis. * Have failed at least one standard therapy * Must have discontinued ET therapy at least 1 week (4 weeks for interferon) prior to study drug initiation. Exclusion Criteria: * Has undergone major surgery ≤4 weeks prior to starting study drug or has not recovered from side effects of such surgery. * Unresolved treatment related toxicities from prior therapies (unless resolved to ≤ Grade 1). * Uncontrolled active infection. * Current use of prohibited medications * Known HIV infection or active Hepatitis B or Hepatitis C virus infection * Other hematologic/biochemistry requirements, as per protocol * Use of investigational agent within last 14 days * Pregnant or lactating females