MK-3543 vs Hydroxyurea in Cytoreductive Therapy Naïve Essential Thrombocythemia

If you decide to participate in this study, you will have a screening visit to see if you are eligible for this study. If you are eligible to join this study, you will get a random assignment (like a coin flip) to 1 of 2 groups:

• Group 1: you will take bomedemstat and placebo (inactive substance or "sugar pill") for hydroxyurea
• Group 2: will take hydroxyurea and placebo for bomedemstat

Bomedemstat and hydroxyurea are capsules. They are both taken daily by mouth. This is a "blind" study, so you will not know what group assignment you get. After you get your group assignment, you will proceed to the study drug period. This period has 2 phases: Initial Study Drug Phase and Extended Study Drug Phase. The Initial Study Drug Phase lasts for about 1 year. You will visit our clinic every other week for the first 12 weeks, and then about once per month. When you finish the Initial Study Drug Phase, you will continue to use the study drug for up to 2 more years. This is called the Extended Study Drug Phase. During this phase, you will continue to get the same drug you got during the Initial Study Drug Phase. You will visit our clinic about once per month during this phase. During the study you will undergo tests and procedures including:

• Full physical examination
• 12-lead ECG
• Blood for safety assessments and urine or serum for pregnancy
• Bone marrow aspirate and biopsy
• Blood for pharmacokinetics and biomarker assessments