The main purpose of this study is to determine the safe and recommended dose of APR-246 in combination with azacitidine as well as to see if this combination of therapy improves overall survival.

Participants will be treated for a total of 6 cycles. For participants responding or who have stable disease following cycle 6, treatment may continue until one of the following criteria applies:

• Inter-current illness that prevents further administration of treatment,
• Unacceptable adverse event(s),
• Participant decides to withdraw from the study, or
• General or specific changes in the participant's condition render the participant unacceptable for further treatment in the judgment of the investigator.
• Evidence of disease progression by the International Working Group (IWG) 2006 criteria.

Participants who wish not to continue treatment at time of disease assessment at end of cycle 6 will complete their end of treatment visit upon completion of cycle 6.

Inclusion Criteria:

• Has signed the Informed Consent (ICF) and is able to comply with protocol requirements.
• Has adequate organ function according to study protocol guidelines.
• Age ≥18 years at the time of signing the informed consent form.
• Documented diagnosis of myelodysplastic syndrome (MDS), MDS/ myeloproliferative neoplasm (MPN), chronic myelomonocytic leukemia (CMML) or oligoblastic AML (20-30% myeloblasts) by World Health Organization (WHO) criteria.
• Documentation of a TP53 gene mutation by NGS based on central or local evaluation.
• For TP53 mutant patients with lower risk MDS (i.e., low or intermediate-1 risk by the International Prognostic Scoring System (IPSS)) and isolated deletion of 5q (del(5q)), failure of prior treatment with at least 4 full cycles of lenalidomide defined as no response to treatment, loss of response at any time point, progressive disease, or intolerance to therapy.
• An Eastern Cooperative Oncology Group (ECOG) performance status score of 0, 1, or 2 is required.
• If of childbearing potential, negative pre-treatment urine or serum pregnancy test.
• If of childbearing potential, willing to use an effective form of contraception such as hormonal birth control, intrauterine device or double barrier method during chemotherapy treatment and for at least six months thereafter.

Exclusion Criteria:

• Known history of HIV or active hepatitis B or active hepatitis C infection (testing not mandatory).
• Has any of the following cardiac abnormalities (as determined by treating MD): a. Symptomatic congestive heart failure; b. Myocardial infarction less than or equal to 6 months prior to enrollment; c. Unstable angina pectoris; d. Serious uncontrolled cardiac arrhythmia; e. QTc ≥ 470 msec
• Concomitant malignancies or previous malignancies with less than a 1-year disease free interval at the time of signing consent. Potential participants with adequately resected basal or squamous cell carcinoma of the skin, or adequately resected carcinoma in situ (e.g., cervix) may enroll irrespective of the time of diagnosis.
• Use of cytotoxic chemotherapeutic agents, or experimental agents (agents that are not commercially available) for the treatment of MDS, MDS/MPN, CMML or AML within 14 days of the first day of study drug treatment.
• No concurrent use of erythroid stimulating agents, G-CSF, GM-CSF is allowed during study except in cases of febrile neutropenia where G-CSF can be used for short term. Growth factors must be stopped 14 days prior to study.
• Women who are pregnant or breastfeeding.