A Phase 1, First-in-Human, Dose Escalation Study of MGD024, a CD123 x CD3 Bispecific DART Molecule, in Patients With Select Relapsed or Refractory Hematologic Malignancies

Inclusion Criteria: * Adult patients at least 18 years of age, able to provide informed consent and willing to comply with all study procedures. * Participants with * primary or secondary acute myeloid leukemia (AML) except acute promyelocytic leukemia, * primary or secondary myelodysplastic syndrome (MDS) with prognostic score of \>3 and \<20% bone marrow blasts, * classical Hodgkin lymphoma (cHL), * chronic myelogenous leukemia (CML), * b-cell acute lymphocytic leukemia (B-ALL), * hariy cell leukemia (HCL), * advanced systemic mastocytosis (ASM), or * blastic plasmacytoid dendritic cell neoplasm (BPDCM) * Relapsed after or refractory to at least one prior line of therapy and with no available potentially curative treatment option. * Evidence of at least 20% of malignant cells with CD123 expression. * Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2. * Life expectancy of at least 12 weeks. * Acceptable laboratory values, and heart function. * Continuing side effects of prior treatment are mild * Women and men of childbearing potential must agree to use highly effective forms of contraception throughout the study through 4 months after the last dose of MGD024. Exclusion Criteria: * Prior treatment with an anti-CD123-directed agent (except patients with BPDCN, who are allowed to have received prior tagraxofusp). * Known involvement of central nervous system (CNS) by the disease under investigation. * History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the patient's participation for the full duration of the trial, or is not in the best interest of the patient. * Systemic anti-cancer therapy, investigational therapy, corticosteroids or other immune suppressive drugs within 14 days of first dose * Vaccination with any live virus vaccine within 4 weeks prior to first dose. Inactivated annual influenza and SARS-CoV-2 vaccination are allowed.