The purpose of this research study is to test the safety of PRT1419 at different dose levels to find out what effects, good and/or bad, PRT1419 has on you and your type of cancer. The information collected from you and from other patients in this study will be used to decide the most appropriate dose of PRT1419 that should be used for studies that may be completed in the future. This study will enroll patients with the cancer types below: Acute Myeloid Leukemia (AML), Chronic myelomonocytic leukemia (CMML), High-Risk Myelodysplastic Syndrome (MDS) or MDS/ Myeloproliferative Neoplasms (MPN) Overlap Syndrome. This research study involves receiving an investigational drug referred to as PRT1419. The investigational drug, PRT1419 is a MCL1 inhibitor meaning that it blocks the MCL1 (myeloid cell leukemia 1) protein, which has been found to be increased in cancer patients with AML, CMML, MDS, and MDS/MPN Overlap Syndrome. Investigational means that the study drug is currently being tested and has not been approved for use by any health authority such as the U.S. Food and Drug Administration (FDA) or any other health or regulatory authority in other countries.