Additional Study Information: The purpose of this study is to find the highest dose of Flotetuzumab that can be given safely to patients who have a certain type of leukemia called Acute Myeloid Leukemia (AML) that has relapsed (has come back after treatment) or is refractory (never responded to treatment). Flotetuzumab is an experimental treatment that works by targeting certain proteins on the outside of leukemia cells and using the patient's own immune system to kill the leukemia cells.