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 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

   - Participants may have received up to 3 prior lines of therapy including hydroxyurea

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) or the chosen best available therapy (including anagrelide, interferon
   alfa/pegylated interferon, ruxolitinib, or busulfan) 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