 Subjects considered an adult according to local regulations at the time of signing informed consent may participate in this study. Subjects will be randomized in a 2:1 ratio to receive ASP2215 or salvage chemotherapy. Subjects will enter the screening period up to 14 days prior to the start of treatment. Prior to randomization, a salvage chemotherapy regimen will be pre-selected for each subject; options will include low-dose cytarabine LoDAC, azacitidine, mitoxantrone, etoposide, and intermediate-dose cytarabine MEC, or fludarabine, cytarabine, and granulocyte colony-stimulating factor G-CSF with idarubicin FLAG-IDA. The randomization will be stratified by response to first-line therapy and pre-selected salvage chemotherapy. Subjects will be administered treatment over continuous 28-day cycles. 