A Study of Subcutaneous Blinatumomab in Children With R/R and and MRD+ B-Cell Precursor Acute Lymphoblastic Leukemia

Inclusion Criteria: * Age ≥28 days to \<12 years at the time of informed consent/assent. * Lansky Performance Status (LPS) of ≥ 50%. * For Phase 1b and Phase 2 cohort in participants with R/R B-ALL: * Participants with B-ALL relapsed after or refractory to any line of treatment including allogeneic hematopoietic stem cell transplant (HSCT). * Greater than or equal to 5% blasts in the bone marrow (BM) is considered as relapse in the BM. * For Phase 2 cohort in participants with MRD+ B-ALL: * Participants with MRD+ B-ALL must have between ≥ 0.1% and \< 5% blasts in the BM. * Prior CD19-directed therapy will be allowed (with demonstrated continued CD19+ expression) if treatment ended \>4 weeks prior to start of protocol therapy and no prior central nervous system (CNS) complications. * Any Philadelphia chromosome-positive (Ph+) participant intolerant or refractory to prior tyrosine kinase inhibitors (TKIs) are eligible. Exclusion Criteria: * Active ALL in the CNS. * History or presence of clinically relevant CNS pathology or event such as epilepsy, childhood seizure, paresis, aphasia, stroke, severe brain injuries, cerebellar disease, organic brain syndrome, psychosis, or severe (≥ grade 3) CNS events including immune effector cell-associated neurologic syndrome (ICANS) from prior CAR-T or other T-cell engager therapies. * Isolated EM disease. * Current autoimmune disease or history of autoimmune disease with potential CNS involvement. * Patients with Down Syndrome ar…