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

For participants with ET or PV: Duration of clinicohematologic response

For participants who showed durable clinicohematologic response (DCHR) in their feeder studies, duration of clinicohematologic response is defined as the time from the first documented evidence of confirmed reduction of platelet and white blood cell (WBC) count 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). The duration of clinicohematologic response will be reported.

Time frame: Up to ~10 years

For participants with ET or PV: Duration of hematologic remission

For participants who showed confirmed hematologic remission in their feeder studies, duration of hematologic remission 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. The duration of hematologic remission will be reported.

Time frame: Up to ~10 years

For participants with ET or PV: Percentage of participants with transformation to MF or MDS/AML

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.

Time frame: Up to ~10 years

For participants with MF: Percentage of participants with worsening of splenomegaly or transformation to MDS/AML

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.

Time frame: Up to ~10 years

For participants with MF, ET, or PV: Percentage of participants with thrombotic events

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.

Time frame: Up to ~10 years

For participants with MF, ET, or PV: Percentage of participants with major hemorrhagic events

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.

Time frame: Up to ~10 years