DCHR rate is the percentage of participants with DCHR, defined as a confirmed reduction of platelet count to ≤400 × 10\^9/L, absence of white blood cell (WBC) count elevation to \>10 × 10\^9/L locally assessed to be due to ET, starting by Week 24 and maintained for at least 24 weeks, and the absence of any thrombotic or major hemorrhagic events or disease progression to myelofibrosis (MF), myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) by Week 52.

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.

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.

An AE was defined as any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. The number of participants with one or more AEs are reported.

An AE was defined as any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. The number of participants who discontinued study intervention due to an AE are reported.

Hematocrit (Hct) was analyzed by taking blood samples from participants at designated time points during the study. Participants were considered responders if they achieved a sustained reduction of Hct to \<45% for 12 weeks (84 calendar days) by Week 36 AND there was no concomitant phlebotomy performed during the sustained reduction. Baseline data was defined as the data most recently collected prior to the first dose. A Clopper-Pearson (exact binomial) two-sided 95% confidence interval for the percentage of responders was presented. The percentage of participants who achieved a sustained 12-week reduction of Hct to \<45% without concomitant phlebotomy at Week 36 are reported.

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

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

Mean Spleen volume reduction (mL) in participants with MF as measured by central laboratory imaging analysis of MRI (or CT where applicable) approximately every 48 weeks. Per protocol only participants with MF were analyzed for this outcome measure. The change in spleen volume from baseline is presented.

Blood samples were taken at designated time points to determine platelet count. Percentage of participants with ET who achieve a reduction of platelet counts to \<= 400 K/uL (400 x 10\^9/L) in the absence of new thromboembolic events is presented.

An AE is any undesirable physical, psychological or behavioral effect experienced by a participant during the 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 participant from the time of first dose with study treatment until completion of the study. The number of participants who experienced an AE is reported.

An AE is any undesirable physical, psychological or behavioral effect experienced by a patient during participation in the 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 study treatment until completion of the study. The number of participants who discontinued study treatment due to an AE is reported.

Blood samples were collected at pre-specified timepoints to determine platelet counts. The percentage of participants who achieved reduction in platelet count to ≤400k/μL in the absence of new thrombolytic events is reported.

Blood samples collected to determine the AUC0-inf of bomedemstat.

Blood samples collected to determine the Cmax of bomedemstat.

Blood samples collected to determine the AUC0-inf of bomedemstat.

Blood samples collected to determine the Cmax of bomedemstat.

AUC0-inf for bomedemstat in plasma will be determined

Toxicity is defined as including dose limiting toxicities (DLTs), serious adverse events (SAEs) and adverse events (AEs) in study participants. Toxicity will be assessed using the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0, per physician discretion.

The RP2D of combination VenBom therapy will be determined as the maximum tolerated dose of study treatment as assessed by treating physician using the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0.

DLT was defined as any one of the following adverse events (AEs) that occured through Day 7 of the Initial Treatment Period (ITP) and was considered by the Investigator to be possibly, probably or definitely related to bomedemstat: * Thrombocytopenia leading to clinically significant sequelae (i.e., a clinically significant bleeding event or the need for prophylactic transfusions) * A clinically significant bleeding event in a participant with a platelet count \>50 x 10\^9/L (50 k/μL) * Any Grade 4 or 5 non-haematologic adverse event * Any Grade 3 non-haematologic adverse event with failure to recover to Grade 2 within 7 days of drug cessation, with the following exceptions: ≥ Grade 3 nausea, vomiting or diarrhea that responds to standard medical care; ≥ Grade 3 aesthenia lasting less than 14 days; any Grade 3 electrolyte abnormality unrelated to the underlying malignancy and persisting greater than 24 hours. The number of participants with a DLT were reported.

An AE was any undesirable physical, psychological or behavioral effect experienced by a participant, in conjunction with the use of the drug or biologic, whether or not product-related. This included any untoward signs or symptoms experienced by the participant from the time of first dose with bomedemstat until completion of the study. Serious AEs (SAEs) were any AE that resulted in death, life-threatening experience, required or prolonged inpatient hospitalization, persistent or significant disability/incapacity, congenital anomaly, or important medical events. The number of participants with at least one treatment-emergent (TE) SAE was reported for each arm.

An AE was any undesirable physical, psychological or behavioral effect experienced by a participant, in conjunction with the use of the drug or biologic, whether or not product-related. This included any untoward signs or symptoms experienced by the participant from the time of first dose with bomedemstat until completion of the study. The number of participants with at least one TE AE was reported for each arm.

An AE was any undesirable physical, psychological or behavioral effect experienced by a participant, in conjunction with the use of the drug or biologic, whether or not product-related. This included any untoward signs or symptoms experienced by the participant from the time of first dose with bomedemstat until completion of the study. The number of participants that discontinued study treatment with bomedemstat due to a TE AE was reported for each arm.

Cmax was defined as the maximum observed concentration after administration obtained directly from the concentration time profile. Blood and plasma samples were collected at pre-specified timepoints to calculate Cmax in participants of the Phase 1/2a portion of the study. As pre-specified by the protocol and Pharmacokinetic Analysis Plan (PAP), Phase 2b participants were excluded from this analysis.

Tmax was defined as the time to maximum concentration after administration obtained by inspection. Blood and plasma samples were collected at pre-specified timepoints to calculate Tmax in participants of the Phase 1/2a portion of the study. As pre-specified by the protocol and Pharmacokinetic Analysis Plan (PAP), Phase 2b participants were excluded from this analysis.

AUC0-24 was defined as the area under the concentration versus time curve calculated using the linear trapezoidal rule from the zero time-point to the 24-hour time-point concentration. Blood and plasma samples were collected at pre-specified timepoints to calculate AUC0-24 in participants of the Phase 1/2a portion of the study. As pre-specified by the protocol and Pharmacokinetic Analysis Plan (PAP), Phase 2b participants were excluded from this analysis.

CL/F was defined as the apparent total clearance of drug after oral administration. Blood and plasma samples were collected at pre-specified timepoints to calculate CL/F in participants of the Phase 1/2a portion of the study. As pre-specified by the protocol and Pharmacokinetic Analysis Plan (PAP), Phase 2b participants were excluded from this analysis.

Change in spleen volume was assessed based on calculated spleen volume (ml) measured by magnetic resonance imaging (MRI), or computerized tomography (CT) scan (where locally permitted) if the participant was not a candidate for MRI from Day 0. Percentage change from baseline in spleen volume was reported at Initial Treatment Period (ITP) Day 84, ITP Day 168, Additional Treatment Period 1 (ATP1) Day 84 (Study Day 253), and ATP1 Day 168 (Study Day 337).

Change in spleen size was assessed based on spleen palpation (in cm) at each visit. Percentage change from baseline in spleen size was reported at ITP Day 84, ITP Day 168, ATP1 Day 84 (Study Day 253), ATP1 Day 168 (Study Day 337), ATP2 Day 84 (Study Day 422), ATP2 Day 168 (Study Day 506), and ATP3 Day 84 (Study Day 591). As prespecified by the Statistical Analysis Plan, assessments for the Phase 1/2 groups were summarized using visit windowing after the Day 84 visit of the ITP to allow for comparison with the Phase 2b groups at ITP Day 168.

An SAE was defined as any adverse event, whether or not related to the study drug, that resulted in any of the following outcomes: * Death * Life-threatening experience * Required or prolonged inpatient hospitalization * Persistent or significant disability/incapacity * Congenital anomaly * Important medical events that, based upon appropriate medical judgment, may have jeopardized the participant and may have required medical or surgical intervention to prevent one of the outcomes listed above. The number of participants who experienced an SAE is presented.

An AE was defined as any undesirable physical, psychological, or behavioral effect experienced by a participant during participation in an investigational study, in conjunction with the use of the drug or biologic, whether or not related to the study drug. This included any untoward signs or symptoms experienced by the participant from the time of first dose until completion of the study. The number of participants who discontinued study treatment due to an AE is presented.

DLTs were assessed using the National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE) version 4.03. A DLT was defined as any of the following events that occurs during the DLT evaluation period and is considered by the Investigator possibly, probably, or definitely related to bomedemstat: * A clinically significant bleeding event * Any Grade 4 or 5 non-haematologic adverse event * Any Grade 3 non-haematologic adverse event with failure to recover to Grade 1 within 7 days of drug cessation, with the following exceptions: ≥ Grade 3 nausea, vomiting or diarrhoea that responds to standard medical care and ≥ Grade 3 aesthenia lasting less than 14 days * Any Grade 3 electrolyte abnormality unrelated to the underlying malignancy and persisting greater than 24 hours. Per protocol, DLTs were measured during the first 28-day treatment period. The number of participants who experienced a DLT is presented.

AEs were graded for severity on a scale of 1 to 5 using CTCAE criteria. Grade 1=mild; asymptomatic or mild symptoms; clinical or diagnostic observations only; or intervention not indicated. Grade 2=moderate, minimal, local, or noninvasive intervention indicated; or limiting age-appropriate instrumental activities of daily living. Grade 3=severe or medically significant but not immediately life-threatening; hospitalization or prolongation of hospitalization indicated; disabling; or limiting self-care. Grade 4=life-threatening consequences with urgent intervention indicated. Grade 5 =death related to AE. The number of participants who experienced any Grade 3 to 5 AE is reported.

Medical Events of Interest (MEOI) were adverse events that did not meet any criteria for a serious adverse event but were of particular interest in the context of the study. MEOIs included bleeding due to low platelets; electrocardiogram (ECG) QT corrected interval prolonged 481-500 milliseconds; and infection where, at a minimum, oral antibiotic, antifungal, or antiviral intervention is indicated. The number of participants with MEOIs is presented.

Cmax was defined as the maximum concentration of bomedemstat observed after administration of bomedemstat alone or administered with tretinoin. Plasma samples were collected at multiple time points to estimate the CMax of bomedemstat using non-compartmental methods of analysis following oral dosing of participants in Cycle 1 (Cycle 1 = 7 days of treatment and 7 days of rest for a total of 14 days). Unbound plasma concentrations of bomedemstat were estimated by assuming that protein binding was 60.57%. For the pharmacokinetic analysis, plasma concentrations below the assay limit of quantitation (LOQ) that occurred from predose to the first concentration ≥LOQ were treated as 0 and those that occured thereafter were treated as missing. The LOQ of the assay was 1 ng/mL. Per protocol, the Cmax was determined for participants in Cohort 1. The LOQ of the assay was 1 ng/mL. The Cmax of bomedemstat in unbound plasma is presented.

Tmax was the time required to reach the maximum concentration of bomedemstat observed after administration of bomedemstat alone or administered with tretinoin. Plasma samples were collected at multiple time points to estimate the TMax of bomedemstat using non-compartmental methods of analysis following oral dosing of participants in Cycle 1 (Cycle 1 = 7 days of treatment and 7 days of rest for a total of 14 days). For the pharmacokinetic analysis, plasma concentrations below the assay limit of quantitation (LOQ) that occurred from predose to the first concentration ≥LOQ were treated as 0 and those that occured thereafter were treated as missing. The LOQ of the assay was 1 ng/mL. Per protocol, the Tmax was determined for participants in Cohort 1. The Tmax of bomedemstat in plasma is presented.

AUC0-24hr was defined as the total exposure of bomedemstat over 24 hours after administration of bomedemstat alone or administered with tretinoin. Plasma samples were collected at multiple time points to estimate the AUC 0-24hr of bomedemstat using non-compartmental methods of analysis following oral dosing of participants in Cycle 1 (Cycle 1 = 7 days of treatment and 7 days of rest for a total of 14 days). Unbound plasma concentrations of bomedemstat were estimated by assuming that protein binding was 60.57%. For the pharmacokinetic analysis, plasma concentrations below the assay limit of quantitation (LOQ) that occurred from predose to the first concentration ≥LOQ were treated as 0 and those that occured thereafter were treated as missing. The LOQ of the assay was 1 ng/mL. Per protocol, the AUC 0-24hr was determined for participants in Cohort 1. The AUC 0-24hr of bomedemstat in unbound plasma is presented.

t1/2 was the time required to divide the bomedemstat concentration by two after reaching pseudo-equilibrium after administration of bomedemstat alone or administered with tretinoin. Plasma samples were collected at multiple time points to estimate the t1/2 of bomedemstat using non-compartmental methods of analysis following oral dosing of participants in Cycle 1 (Cycle 1 = 7 days of treatment and 7 days of rest for a total of 14 days). For the pharmacokinetic analysis, plasma concentrations below the assay limit of quantitation (LOQ) that occurred from predose to the first concentration ≥LOQ were treated as 0 and those that occured thereafter were treated as missing. The LOQ of the assay was 1 ng/mL. Per protocol, the t1/2 was determined for participants in Cohort 1. The t1/2 of bomedemstat in plasma is presented.

CL/F was the volume of plasma from which bomedemstat was eliminated per unit of time after administration of bomedemstat alone or administered with tretinoin. Plasma samples were collected at multiple time points to estimate the CL/F of bomedemstat using non-compartmental methods of analysis following oral dosing of participants in Cycle 1 (Cycle 1 = 7 days of treatment and 7 days of rest for a total of 14 days). For the pharmacokinetic analysis, plasma concentrations below the assay limit of quantitation (LOQ) that occurred from predose to the first concentration ≥LOQ were treated as 0 and those that occured thereafter were treated as missing. The LOQ of the assay was 1 ng/mL. Per protocol, the CL/F was determined for participants in Cohort 1. The CL/F of bomedemstat in plasma is presented.

Vz/F was the volume of distribution of bomedemstat during the terminal phase after administration of bomedemstat alone or administered with tretinoin. Plasma samples were collected at multiple time points to estimate the Vz/F of bomedemstat using non-compartmental methods of analysis following oral dosing of participants in Cycle 1 (Cycle 1 = 7 days of treatment and 7 days of rest for a total of 14 days). For the pharmacokinetic analysis, plasma concentrations below the assay limit of quantitation (LOQ) that occurred from predose to the first concentration ≥LOQ were treated as 0 and those that occured thereafter were treated as missing. The LOQ of the assay was 1 ng/mL. Per protocol, the Vz/F was determined for participants in Cohort 1. The Vz/F of bomedemstat in plasma is presented.

kel was the rate at which bomedemstat was removed from the body by excretion or metabolism after administration of bomedemstat alone or administered with tretinoin. Plasma samples were collected at multiple time points to estimate the kel of bomedemstat using non-compartmental methods of analysis following oral dosing of participants in Cycle 1 (Cycle 1 = 7 days of treatment and 7 days of rest for a total of 14 days). kel was obtained from the slope of the line, fitted by linear least squares regression, through the terminal points of the log (base e) concentration-time profiles. For the pharmacokinetic analysis, plasma concentrations below the assay limit of quantitation (LOQ) that occurred from predose to the first concentration ≥LOQ were treated as 0 and those that occured thereafter were treated as missing. The LOQ of the assay was 1 ng/mL. Per protocol, the kel was determined for participants in Cohort 1. The kel of bomedemstat in plasma is presented.

EFS was defined as the time from the first dose of study treatment to the date of a documented EFS event of resistance, relapse, progression, or death due to any cause as assessed by the investigator. Participants without documented events were censored at the date of the last disease assessment. The EFS, calculated using the Kaplan-Meier method, is presented.

EFS was defined as the time from the first dose of study treatment to the date of a documented EFS event of resistance, relapse, progression, or death due to any cause as assessed by the investigator. Participants without documented events were censored at the date of the last disease assessment. The EFS, calculated using the Kaplan-Meier method, is presented.

OS was defined as the time from the first dose of study treatment until death due to any cause. Participants without documented death at the time of the analysis were censored at the date of the last follow-up. The OS, calculated using the Kaplan-Meier method, is presented.

OS was defined as the time from the first dose of study treatment until death due to any cause. Participants without documented death at the time of the analysis were censored at the date of the last follow-up. The OS, calculated using the Kaplan-Meier method, is presented.

ORR was the percentage of participants with a response of complete remission (CR), CR with incomplete recovery (Cri), morphologic leukaemia-free state (MLFS), partial remission (PR), cytogenetic CR (CRc), molecular CR (CRm), or stable disease (SD): * CR: \<5% bone marrow (BM) blasts or blasts with Auer rods; no extramedullary disease (EMD); absolute neutrophil count ≥1000/μL; platelets ≥100,000/μL; independent of transfusion * Cri: CR with residual neutropenia (\<1,000/μL) or thrombocytopenia (\<100,000/μL); no EMD; does not require transfusion independence (TI) * MLFS: \<5% blasts in BM with marrow spicules; no EMD; does not require TI * PR: ≥ 50% decrease in BM blasts to a range of 5%-25%; Neutrophils ≥ 1,000/μL; Platelets ≥ 100,000/μL; independent of transfusion. ≤ 5% blasts is PR if Auer rods present * CRc: CR+reversion to a normal karyotype if initially abnormal * CRm: CR+no evidence of residual disease by molecular testing * SD: No evidence of CR, PR, progression, new dysplastic changes

BOR was the percentage of participants with a best overall response of CR, Cri, MLFS, PR, CRc, CRm, or SD: * CR: \<5% bone marrow (BM) blasts or blasts with Auer rods; no extramedullary disease (EMD); absolute neutrophil count ≥1000/μL; platelets ≥100,000/μL; independent of transfusion * Cri: CR with residual neutropenia (\<1,000/μL) or thrombocytopenia (\<100,000/μL); no EMD; does not require transfusion independence (TI) * MLFS: \<5% blasts in BM with marrow spicules; no EMD; does not require TI * PR: ≥ 50% decrease in BM blasts to a range of 5%-25%; Neutrophils ≥ 1,000/μL; Platelets ≥ 100,000/μL; independent of transfusion. ≤ 5% blasts is PR if Auer rods present * CRc: CR+reversion to a normal karyotype if initially abnormal * CRm: CR+no evidence of residual disease by molecular testing * SD: No evidence of CR, PR, progression, new dysplastic changes

ORR was the percentage of participants with a response of complete remission (CR), partial remission (PR), marrow CR (CRm), cytogenetic response (CyR), or stable disease (SD) lasting for ≥4 weeks: * CR: ≤5% myeloblasts in the bone marrow (BM) with normal maturation of all cell lines; persistent dysplasia will be noted; Hgb ≥11 g/dL, Platelets ≥100 X 10\^9/L, Neutrophils ≥1.0 X 10\^9/L, and 0% Blasts in the peripheral blood * PR: All CR criteria if abnormal before treatment except: BM blasts decreased by ≥50% over pretreatment but still \>5%. Cellularity and morphology not relevant * CRm: ≤ 5% myeloblasts in BM and decrease by ≥50% over pretreatment. Hematological improvement responses in the peripheral blood will be noted in addition to bone marrow CR. * CyR: Complete=Disappearance of the chromosomal abnormality without appearance of new ones. Partial= ≥50% reduction of the chromosomal abnormality * SD: Failure to achieve at least PR, but no evidence of progression for \>8 weeks

BOR was the percentage of participants with a best overall response of CR, PR, CRm, CyR, or SD lasting for ≥4 weeks: * CR: ≤5% myeloblasts in the bone marrow (BM) with normal maturation of all cell lines; persistent dysplasia will be noted; Hgb ≥11 g/dL, Platelets ≥100 X 10\^9/L, Neutrophils ≥1.0 X 10\^9/L, and 0% Blasts in the peripheral blood * PR: All CR criteria if abnormal before treatment except: BM blasts decreased by ≥50% over pretreatment but still \>5%. Cellularity and morphology not relevant * CRm: ≤ 5% myeloblasts in BM and decrease by ≥50% over pretreatment. Hematological improvement responses in the peripheral blood will be noted in addition to bone marrow CR. * CyR: Complete=Disappearance of the chromosomal abnormality without appearance of new ones. Partial= ≥50% reduction of the chromosomal abnormality * SD: Failure to achieve at least PR, but no evidence of progression for \>8 weeks