A Study to Assess the Effect of CC-95251 in Participants With Acute Myeloid Leukemia and Myelodysplastic Syndromes

Inclusion Criteria:

• Eastern Cooperative Oncology Group Performance Status of 0 to 2

For Parts A & B:

Relapsed or refractory (R/R) acute myeloid leukemia (AML) as defined by the 2016 WHO Classification
R/R myelodysplastic syndromes (MDS) as defined by the 2016 WHO Classification with intermediate, high or very high risk by Revised International Prognostic Scoring System (IPSS-R)

For Part C:

• Treatment-naïve (ie, previously untreated) MDS as defined by the 2016 WHO Classification with intermediate, high or very high risk by IPSS-R

Exclusion Criteria:

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