XmAb18968 (CD3-CD38) in Relapsed or Refractory Acute Leukemia and T Cell Lymphoblastic Leukemia

The primary objective of this portion of the study is to determine a recommended phase II dose (RP2D) for XmAb18968. The trial will use a variation of the 3 + 3 design where both escalation and de-escalation are possible. There will be separate cohorts; Group A (T cell acute lymphoblastic leukemia, T cell lymphoblastic lymphoma) and Group B (acute myeloid leukemia). A minimum of 24 and a maximum of 60 subjects will be needed for the study. The first dose on Cycle 1 Day 1 (C1D1) will be split into two doses to ensure the safety of subjects and to closely monitor for CRS. The dose will be split into C1D1 and Cycle 1 Day 2 (C1D2) with approximately 25% of the dose given on C1D1 and 75% of the dose given on C1D2. Thereafter, subjects will receive the full dose planned for that cohort. Prior to enrolling subjects at the next applicable dose level, the Data Safety Monitoring Committee (DSMC) will review the results. Although AEs may occur at any point during treatment, only AEs occurring during Cycle 1 of treatment will necessarily influence decisions regarding dose escalation, expansion of a dose level, or evaluation of intermediate dose levels. Subjects will be monitored through all cycles of therapy for treatment-related toxicities. Toxicity will be evaluated according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. If multiple AEs are seen, the presence of a DLT will be based on the most severe AE experienced. The DLT will be based on the tolerability observed during the first 28 days (or up to 42 days for hematological DLTs) of treatment/observation. DLT will be defined as any of the following events: * Any grade 4 or higher non-hematological adverse reaction. * Cytokine release syndrome (CRS) is a possible side effect that can occur as a result of administration of XmAb18968. For this protocol, any grade 3 or higher CRS adverse event (AE) (per revised CRS grading system will be considered a DLT except grade 3 CRS AE that resolves to grade 1 within seven days). * Any subject meeting the criteria for Hy's Law case (i.e., severe drug-induced liver injury (DILI)). A Hy's Law case is defined as: aspartate aminotransferase (AST) or alanine transaminase (ALT) values ≥ 3 × upper limit of normal (ULN) AND with serum total bilirubin (TBIL) level \> 2 × ULN or international normalized ratio (INR) \> 1.5 without signs of cholestasis. * Any non-Hy's Law grade 3 liver abnormality lasting more than 72 hours will be considered a DLT. * Grade 3 electrolyte abnormalities - sodium (Na), potassium (K), chloride (Cl), carbon dioxide (CO2), calcium (Ca), magnesium (Mg), phosphate - that do not return to grade 1 or lower within 72 hours. * Any grade 4 neurotoxicity will be considered a DLT. Grade 3 neurotoxicity that lasts more than 72 hours will be considered a DLT. * Any grade 3 nausea, vomiting, or diarrhea that requires hospitalization, tube feeding or total parenteral nutrition. * Any adverse reaction that leads to dose reduction or withdrawal. * Grade 3 transaminitis (AST/ALT) elevation that does not return to grade 1 or lower within 72 hours. * Any grade 3 infection lasting more than seven days in the absence of active leukemia. * Any grade 3 bleeding with thrombocytopenia in the absence of active leukemia. * Any grade 4 or higher neutropenia lasting past cycle day 42 in the absence of active leukemia.