Inclusion Criteria:

          1. Age ≥ 16 at the time of signing the informed consent form

          2. Patients with a diagnosis of AML (CR1 or CR2) according to WHO classification or high
             risk MDS (as per IPSS-R) undergoing allo-SCT using MAC or RIC preparative regimens,
             and with either peripheral blood or bone marrow as the source of hematopoietic stem
             cells.

          3. At the time of allo-SCT

               -  No prior allo-SCT; and

               -  No more than 1 antigen mismatch at HLA-A, -B, -C, -DRB1 or -DQB1 locus for either
                  related or unrelated donor; and

               -  No haplotype or cord blood donor; and

               -  Bone marrow blast <5% for AML and <10% for MDS patients

          4. Able to commence therapy between 42 to 84 days following allo-SCT

          5. Post-transplant bone marrow

               1. AML patients - blast count ≤ 5% confirmed within 28 days prior to starting study
                  therapy

               2. MDS patients - confirmation of CR post-transplant with blast count ≤ 5% in bone
                  marrow

          6. Adequate neutrophil and platelet engraftment within 14 days prior to starting study
             therapy defined as:

               -  ANC ≥ 1.0 x 10^9/L on two consecutive testing without daily use of myeloid growth
                  factor; and

               -  Platelet ≥ 50 x 10^9/L on two consecutive testing without platelet transfusion
                  within 1 week

          7. Adequate organ function:

               -  Serum AST and ALT < 4 x upper limit of normal (ULN)

               -  Serum bilirubin < 2 x ULN. Higher levels are acceptable if these can be
                  attributed to active red blood cell (RBC) precursor destruction within the bone
                  marrow (i.e., ineffective erythropoiesis) or Gilbert's syndrome

               -  Serum creatinine < 2 x ULN

          8. Adequate coagulation (PT ≤ 15 seconds and PTT ≤ 40 seconds)

          9. Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2

         10. Patients with adequately controlled GVHD (defined as GVHD grade <II with concurrent
             use of corticosteroids equivalent of prednisone at a dose ≤ 0.5 mg/kg) can be included

         11. Females of childbearing potential (FCBP) may participate, providing they meet the
             following conditions:

               1. Agree to use at least two effective contraceptive methods (oral, injectable, or
                  implantable hormonal contraceptive; tubal ligation; intra-uterine device; barrier
                  contraceptive with spermicide; or vasectomised partner) or practice true
                  abstinence throughout the study, and for 3 months following the last dose of
                  study therapy and

               2. Have a negative serum pregnancy test (sensitivity of at least 25 mIU/mL) at
                  screening; and

               3. Have a negative serum or urine (investigator's discretion) pregnancy test
                  (sensitivity of at least 25 mIU/mL) within 72 hours prior to starting study
                  therapy. This applies even if the subject practices complete abstinence from
                  heterosexual contact.

         12. Male patients with a female partner of childbearing potential must agree to the use of
             at least two physician-approved contraceptive methods throughout the course of the
             study and should avoid fathering a child during the course of the study and for 3
             months following the last dose study therapy

         13. Understand and voluntarily sign an informed consent from prior to any study related
             assessments or procedures being conducted

         14. Able to adhere to the study visit schedule (i.e., clinic visits at the study sites are
             mandatory, unless noted otherwise for study visits) and other protocol requirements

        Exclusion Criteria:

          1. Use of any of the following after transplantation and prior to starting study therapy:

               -  Any agents (chemotherapy or targeted agents) used for adjuvant therapy (note that
                  prophylactic use of these agents is allowed in this study, e.g., methotrexate for
                  GVHD or rituximab for Epstein-Barr Virus (EBV) reactivation)

               -  Unlicensed investigational agents/therapies used within 28 days prior to starting
                  study therapy

               -  Azacitidine, decitabine or other hypomethylating agent (HMA)

               -  Lenalidomide, thalidomide and pomalidomide used within 28 days prior to starting
                  study therapy

          2. Subjects who have undergone a haploidentical or cord blood transplant

          3. Active GVHD grade II or higher (acute GVHD Clinical Staging and Grading)

          4. Concurrent use of corticosteroids equivalent of prednisone at a dose > 0.5 mg/kg

          5. Known active viral infection with Human Immunodeficiency Virus (HIV), Hepatitis B
             Virus (HBV) or Hepatitis C Virus (HCV)

          6. Active uncontrolled systemic fungal, bacterial, or viral infection (defined as ongoing
             signs/symptoms related to the infection without improvement despite appropriate
             antibiotics, antiviral therapy, and/or other treatment)

          7. History of inflammatory bowel disease (e.g., Crohn's disease, ulcerative colitis),
             celiac disease (i.e., sprue), prior gastrectomy or upper bowel removal, or any other
             GI disorder or defect that may interfere with the absorption, distribution, metabolism
             or excretion of the investigational medicinal products (IMPs) and/or predispose the
             subject to an increased risk of gastrointestinal toxicity prior to allo-SCT

          8. Idiopathic thrombocytopenic purpura (ITP), disseminated intravascular coagulation,
             haemolytic uremic syndrome, thrombotic thrombocytopenic purpura (TTP)

          9. History of prior malignancies, except lobular breast carcinoma in situ, fully resected
             basal cell or squamous cell carcinoma of skin or treated cervical carcinoma in situ,
             Incidental histologic finding of prostate cancer (T1a or T1b using the tumor, node,
             metastasis (TNM) clinical staging system), CMML or Secondary AML arising from MDS.
             Cancer treated with curative intent ≥ 5 years previously will be allowed. Cancer
             treated with curative intent < 5 years previously will not be allowed.

         10. Significant active cardiac disease within the previous 6 months, including:

               -  New York Heart Association (NYHA) class III or IV congestive heart failure

               -  Unstable angina or angina requiring surgical or medical intervention; and/or

               -  Myocardial infarction

         11. Known or suspected hypersensitivity to azacitidine or mannitol

         12. Pregnant or lactating females

         13. Any significant medical condition, laboratory abnormality, or psychiatric illness that
             would prevent the patient from participating in the study.

         14. Any condition including the presence of laboratory abnormalities, which places the
             patient at unacceptable risk if he/she were to participate in the study

         15. Any condition that confounds the ability to interpret data from the study