Inclusion Criteria:

          -  Male or female aged ≥ 18 years;

          -  Patients with cytologically confirmed and documented de novo, secondary or
             therapy-related AML, excluding acute promyelocytic leukaemia (APL, French-American
             British M3 classification):

               -  with relapsed or refractory disease without established alternative therapy or

               -  secondary to MDS treated at least by hypomethylating agent or

               -  > 65 years not previously treated for AML and who are not candidates for
                  intensive chemotherapy nor candidates for established alternative chemotherapy Or
                  Patients with cytologically confirmed and documented MDS), in relapse or
                  refractory after previous treatment line including at least one hypomethylating
                  agent and have ≥10% bone marrow blasts;

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2

          -  Circulating white blood cells < 10^9 /L (with or without use of hydroxycarbamide).

          -  Adequate renal function defined as:

             • Serum creatinine ≤ 1.5 x ULN (upper normal limit) or calculated creatinine clearance
             (determined by MDRD) > 50 mL/min/1.73m2.

          -  LDH < 2 x ULN

          -  Adequate hepatic function defined as:

               -  AST and ALT ≤ 1.5 x ULN

               -  Total bilirubin level ≤ 1.5 x ULN, except for patients with known Gilbert's
                  syndrome (confirmed by the UGT1A1 polymorphism analysis), who are excluded if
                  total bilirubin>3.0 x ULN or direct bilirubin > 1.5 x ULN

          -  Serum CK/CPK ≤2.5 x ULN.

        Exclusion Criteria:

          -  Unlikely to cooperate in the study.

          -  Participant already enrolled in the study who has received at least one S64315
             infusion.

          -  Pregnancy, breastfeeding or possibility of becoming pregnant during the study.

          -  Participation in another interventional study requiring investigational treatment
             intake within 2 weeks or at least 5 half-lives (whichever is longer) prior to first
             dose of S64315 (participation in non-interventional registries or epidemiological
             studies is allowed).

          -  Presence of ≥ CTCAE grade 2 toxicity (except alopecia of any grade) due to prior
             cancer therapy, according to the National Cancer Institute Common Terminology Criteria
             for Adverse Events (NCI-CTCAE, version 4.03)

          -  Unresolved ≥ CTCAE grade 2 diarrhoea or medical conditions associated with chronic
             diarrhoea (such as irritable bowel syndrome, inflammatory bowel disease)

          -  Known carriers of HIV antibodies

          -  Known history of significant liver disease

          -  Uncontrolled hepatitis B or C infection

          -  Known active or chronic pancreatitis

          -  History of myocardial infarction (MI), angina pectoris, coronary artery bypass graft
             (CABG) within 6 months prior to starting study treatment.