This study examines a new oral chemotherapy drug called tosedostat, in combination with cytarabine or decitabine. Tosedostat is thought to work by decreasing the availability of amino acids (building blocks the cell needs to make proteins) in cells. It has been shown in early studies to have activity against a variety of cancers, including leukemias. Patients with acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome (MDS) with specific genetic mutations have a poorer response to chemotherapy and a higher risk of relapse after treatment. Researchers are looking to see if combinations of chemotherapy drugs may improve outcomes for patients that do not respond as well with the current chemotherapy regimens, without increasing the risks of treatment.