This phase I study is evaluating how safe, tolerable and effective a new targeted therapy (CC-95251) is alone, and in combination with another drug (azacitidine) in people with relapsed or refractory acute myeloid leukaemia, and people with myelodysplastic syndromesA Phase 1, Open-label, Dose Finding Study of CC-95251 Alone and in Combination With Antineoplastic Agents in Subjects With Acute Myeloid Leukemia and Myelodysplastic Syndromes

Exclusion

• Acute promyelocytic leukemia
• Immediately life-threatening, severe complications of leukemia such as disseminated/uncontrolled infection, uncontrolled bleeding, and/or uncontrolled disseminated intravascular coagulation
• Participants who have received prior treatment with a CD47 or SIRPα targeting agent
• Participant is on chronic systemic immunosuppressive therapy or corticosteroids
• Prior systemic cancer-directed treatments or investigational modalities ≤ 5 half-lives or 4 weeks prior to starting study treatment, whichever is shorter (relapsed or refractory participants only).
• Any condition including, active or uncontrolled infection, or the presence of laboratory abnormalities, which places the participant at unacceptable risk if he/she were to participate in the study
• Pregnant or nursing participants.

Other protocol-defined inclusion/exclusion criteria apply