A Multicenter Phase II Study of Subcutaneous Blinatumomab for Treatment of Adult Patients With CD19-Positive Mixed Phenotype Acute Leukemia (MPAL)

* Inclusion Criteria: * General Criteria for all three Cohorts * Subjects must have histologically or cytologically confirmed MPAL based on 2022 WHO criteria * Subjects who have undergone allo-HSCT are eligible if they are ≥ 4 weeks post stem cell infusion, have no evidence of GVHD \> Grade 2, and are at least ≥ 1 week off of immunosuppressive therapy. Per FDA recommendation, patients should be off of calcineurin inhibitors (CNIs) for at least 4 weeks before receiving blinatumomab * Subjects with a CNS leukemia must be clinically stable (i.e., asymptomatic with no focal neurological signs and symptoms, or signs and symptoms unchanged over 4 weeks with no \> grade 2 manifestations) with a flow cytometric clear CSF in the 2 weeks prior to day 1 of SC-blinatumomab administration. * Ability to understand and willingness to sign a written informed consent document * Agree to comply with the study requirements and agree to come to the clinic/hospital for required study visits * Subjects with hematologic malignancies are expected to have hematologic abnormalities at study entry * Specific Criteria for Cohort A o Subjects should be ineligible for available induction therapy either if they are 75 years of age or older or if they have at least one of the following coexisting conditions precluding intensive chemotherapy: a history of CHF for which treatment is warranted or a report of EF ≤50% in the last 12 months, a history of chronic stable angina, a report of DLCO of ≤65% or FEV1 ≤65% in the last 12 months, ECOG performance status 3 or 4, Charlson comorbidity index (CCI) ≥3. * Specific Criteria for Cohort B * CD19+ MPAL in CR/CRh/CRi after at least one line of treatment with MRD positivity at a level of ≥0.1% using an assay with a minimum sensitivity of 0.01%. * ECOG performance status ≤2 * Subjects must have organ function as below: * Direct bilirubin ≤ 2.5 mg/dL * AST/ALT/Alkaline phosphatase ≤ 5 X institutional upper limit of normal * Serum creatinine ≤ 3 mg/dL * Specific Criteria for Cohort C * Confirmed R/R CD19+ MPAL * Previous cytotoxic chemotherapy (except for hydroxyurea) must have been completed by 5 half-lives of the drug(s) prior to day 1 of SC-blinatumomab. Per FDA recommendation, patients should have recovered to no more than Grade 1 toxicities from prior chemotherapy. * ECOG performance status ≤2 * Subjects must have organ function as below: * Direct bilirubin ≤ 2.5 mg/dL * AST/ALT/Alkaline phosphatase ≤ 5 X institutional upper limit of normal * Serum creatinine ≤ 3 mg/dL * Exclusion Criteria: * Criteria for all three Cohorts * Subjects receiving any other investigational agents, or concurrent chemotherapy, radiation therapy, or immunotherapy for cancer treatment not including corticosteroids or hydroxyurea * Active, uncontrolled infection; subjects with infection under active treatment and controlled with antimicrobials are eligible