The purpose of this study is to test a new experimental drug called SAR443579 for the treatment of participants with relapsed or refractory acute myeloid leukemia (R/R AML), B-cell acute lymphoblastic leukemia (B-ALL), and high risk-myelodysplasia (HR-MDS).

Experimental means that it has not been approved by the US Food and Drug Administration (FDA) to be prescribed to patients.

Participants will be placed into one of two phases of this drug trial – either the dose escalation phase, or the dose expansion phase. Participants will be informed of which study phase they’re taking part in.

During the dose escalation phase, the aim is to establish the maximum tolerated dose, and the dosage of the drug is increased a little at a time, with a new group, or cohort, of participants receiving each increasingly higher doses until the best dose is determined.

During the dose expansion phase, additional participants will be brought into the study to test the SAR443579 dosage determined during the dose escalation phase to assess the anti-leukemic activity of the drug (how good the drug is at treating disease).

SAR44579 will be administered via an IV infusion at dose schedules determined by the study team based on which portion of the study participants are involved in.

Study participation will last for up to 2.5 years.