A Study to Evaluate Safety and Efficacy of Bomedemstat (MK-3543-017)

Participants will receive oral capsules of bomedemstat once daily for up to 10 years, with the starting dose as the same dose that the participant was on at the time of transition from the feeder study.

Drug: Bomedemstat

10, 15, 20, and 50 mg oral capsules

Plazo: Up to ~10 years

An AE is any undesirable physical, psychological or behavioral effect experienced by a patient during participation in an investigational study, in conjunction with the use of the drug or biologic, whether or not product-related. This includes any untoward signs or symptoms experienced by the patient from the time of first dose with bomedemstat under this protocol until completion of the study. The percentage of participants with AEs will be presented.

Plazo: Up to ~10 years

An AE is any undesirable physical, psychological or behavioral effect experienced by a patient during participation in an investigational study, in conjunction with the use of the drug or biologic, whether or not product-related. This includes any untoward signs or symptoms experienced by the patient from the time of first dose with bomedemstat under this protocol until completion of the study. The percentage of participants who discontinued study treatment due to an AE will be presented.

Plazo: Up to ~10 years

For participants who demonstrate durable clinicohematologic response (DCHR) in their feeder study, duration of durable clinicohematologic response (DODCHR) is defined as the time from the first documented evidence of confirmed reduction of platelet and white blood cell (WBC) counts until confirmed increase of platelet and WBC counts to above acceptable threshold, thrombotic or major hemorrhagic events, or disease progression to myelofibrosis (MF), myelodysplastic syndrome (MDS), or acute myeloid leukemia (AML). DODCHR will be reported.

Plazo: Up to ~10 years

For participants who showed clinicohematologic response in their feeder study, duration of clinicohematologic response is defined as the time from the first documented evidence of confirmed reduction of platelet and WBC count until confirmed increase of platelet and WBC counts to above acceptable threshold, thrombotic or major hemorrhagic events or disease progression to MF, MDS, or AML, or phlebotomy (PV participants only). The duration of clinicohematologic response will be reported.

Plazo: Up to ~10 years

For participants who showed confirmed hematologic response in their feeder study, duration of hematologic response is defined as the time from the first documented evidence of confirmed reduction of platelet and WBC count until confirmed increase of platelet and WBC counts to above acceptable threshold, or phlebotomy (PV participants only). The duration of hematologic response will be reported.

Plazo: Up to ~10 years

Disease Progression is defined as the transformation to post-ET myelofibrosis (ET participants only), post-PV myelofibrosis (PV participants only), spleen volume increase ≥25% (MF participants only), myelodysplastic syndrome, or acute myeloid leukemia as assessed by the investigator. The percentage of participants with transformation to MF or MDS/AML will be reported.

Plazo: Up to ~10 years

Spleen volume will be assessed by magnetic resonance imaging (MRI) (or computed tomography \[CT\] where applicable) at pre-specified timepoints. The percentage of participants with worsening of splenomegaly or transformation to MDS/AML will be reported.

Plazo: Up to ~10 years

Thrombotic events are defined as: new or recurrent acute myocardial infarction; unstable angina; stroke; transient ischemic attack (TIA); deep venous thrombosis (DVT); pulmonary embolism (PE); thrombotic digital ischemia; other thrombotic events such as peripheral limb ischemia or Budd-Chiari syndrome that are assessed to be due to underlying PV; other vascular occlusive events such as symptoms of cardiac, abdominal or peripheral limb ischemia supported by objective evidence of vessel disease and/or ischemia. The percentage of participants with thrombotic events will be presented.

Plazo: Up to ~10 years

Hemorrhagic events are defined as: Major Bleeding (MB) Events such as fatal bleeding, and/or symptomatic bleeding in a critical area or organ such as intracranial, intraspinal, intraocular, retroperitoneal, intra-articular or pericardial, or intramuscular with compartment syndrome, and/or bleeding causing a fall in hemoglobin level of 2 g/dL or more, or leading to transfusion of 2 or more units of whole blood or red cells; Clinically Relevant Non-Major Bleeding (CRNMB) Events Leading to hospitalization or increased level of care or clinically important, prompting a face-to-face medical evaluation. The percentage of participants with major hemorrhagic events will be presented.

A Multicenter, Open-Label, Extension Study Evaluating the Safety and Efficacy of Bomedemstat for the Treatment of Participants Enrolled in a Prior Bomedemstat Clinical Study