Inclusion Criteria:

Phase 2:

1. Has Confirmed MDS, CMML, MDS/MPN or AML diagnosis who are candidates to receive and benefit from single agent azacitidine and as applicable according to local country approvals and/or local institution standard practice.

Phase 3:

1. Has confirmed MDS or CMML and is a candidate to receive and benefit from single agent azacitidine as applicable according to local country approvals and/or local institution standard practice:

   a) French-American-British myelodysplastic syndrome subtypes: refractory anemia (RA) or refractory anemia with ringed sideroblasts (if accompanied by neutropenia or thrombocytopenia or requiring transfusions), refractory anemia with excess blasts (RAEB), refractory anemia with excess blasts in transformation (RAEB-T), and CMML or MDS with intermediate-2 or high risk MDS according to the International Prognostic Scoring System (IPSS).

2. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1.
3. Participants with adequate organ function.
4. For participants with prior allogeneic stem cell transplant, no evidence of graft-versus-host disease (GVHD).
5. Participants with no major surgery within 3 weeks before first study treatment.
6. Participants with no cytotoxic chemotherapy (excluding hydroxyurea) within 4 weeks before first study treatment.
7. Participants with projected life expectancy of at least 12 weeks.

Exclusion Criteria:

Phase 2 and 3:

1. Has an active uncontrolled gastric or duodenal ulcer.
2. Has poor medical risk because of other conditions.
3. Has known human immunodeficiency virus (HIV) infection.
4. Is known to be positive for Hepatitis B or C infection.
5. Has a life-threatening illness.
6. Has a history of other malignancies prior to study entry, with the exception of adequately treated in situ carcinoma of the breast or cervix uteri; localized basal cell carcinoma or squamous cell carcinoma of the skin; previous malignancy confined and surgically resected or adequately treated and controlled with other modalities; and any early stage malignancy for which no definitive therapy is required.
7. Participants with MDS/MPN including CMML who have clinical extramedullary disease including clinically palpable hepatomegaly or splenomegaly.
8. Has previous treatment with more than 1 cycle of decitabine, azacitidine, or guadecitabine (Phases 2 and 3 only).
9. Has been treated with any investigational drug or therapy within 2 weeks, or 5 half-lives, whichever is longer, before the protocol-defined first dose of study treatment, or ongoing clinically significant adverse events from previous treatment with investigational drug or therapy.
10. Has a known or suspected hypersensitivity to cedazuridine or azacitidine or any of their excipients.