A Study of H3B-8800 (RVT-2001) in Participants With Lower Risk Myelodysplastic Syndromes

Inclusion Criteria:

1. Confirmed diagnosis of MDS, CMML, or AML.

   For the MDS Expansion cohort, participants must be lower-risk MDS, defined as low or intermediate-1 risk categorization per International Prognostic Scoring System (IPSS) criteria that carries a missense SF3B1 mutation.

   For the Dose Optimization cohort, participants must be transfusion-dependent, lower-risk MDS, defined as very-low to intermediate risk categorization per IPSS-R criteria that carries a missense SF3B1 mutation.
2. Participants must meet the following criteria relevant to their specific diagnosis:

   A. Participants with higher-risk MDS/CMML must be intolerant of hypomethylating agents (HMAs) or not have responded to 4 treatment cycles of decitabine or 6 treatment cycles of azacitidine, or must have progressed at any point after initiation of an HMA.

   B. For the Dose Escalation portion, participants with lower-risk MDS/CMML must be transfusion-dependent for red blood cells or platelets.

   For the MDS expansion cohort, lower risk MDS participants must be RBC transfusion dependent according to IWG 2006 criteria and must also have failed erythropoiesis stimulating agents (ESA) or have serum erythropoietin (EPO) levels greater than (\>) 500 units per liter (U/L).

   C. For the Dose Optimization cohort, lower-risk MDS participants must be RBC transfusion-dependent at baseline defined as ≥3 RBC units (concentrates) in 16-weeks in at least 2 transfusion episodes prior to the first dose of H3B-8800 (RVT-2001) and must also have failed ESA or have serum EPO levels \> 500 U/L. Any ESA use should be discontinued ≥6 weeks prior to enrollment.

   D. Participants with AML must either refuse or not be considered candidates for intensive induction chemotherapy using consensus criteria for defining such participants.

   E. Participants with CMML must have been treated with at least one prior therapy (hydroxyurea or a hypomethylating agent \[HMA\]).
3. Eastern Cooperative Oncology Group (ECOG) performance score of 0-2.
4. For MDS expansion and Dose optimization cohorts - absolute neutrophil count (ANC) greater than or equal to (\>=) 500/ microliter (mcL) (0.5\*10\^9/L).
5. For expansion and Dose optimization cohorts- platelet count \>50,000/mcL (50\*10\^9/L).
6. For Dose-optimization cohort: No prior HMA or lenalidomide in participants with lower-risk MDS.
7. Adequate baseline organ function.

Exclusion Criteria:

1. Diagnosis of a core binding factor leukemia (t(8;21), t(16;16) or inv(16)). Diagnosis of acute promyelocytic leukemia (t(15;17))
2. Participants are deemed candidate for hematopoietic stem cell transplants at the time of enrollment (for AML participants only).
3. Known prior or current retinal or optic nerve disease (example, Retinitis Pigmentosa, diabetic retinopathy, optic neuritis) not stable for at least 6 months.
4. History of clinically significant, uncorrected vitamin B12 or folate deficiency.