∙ Age

• Participants must be \< 22 years of age.

∙ Diagnosis

• Participants must have B-ALL with persistent or rising MRD between 0.1 and 4.99% without extramedullary disease following at least two prior induction attempts, relapse or after hematopoietic stem cell transplant

• Leukemia blasts demonstrating surface expression of CD22

∙ Performance Level

• Karnofsky or Lansky performance score ≥ 50% (corresponding to ECOG Score of ≥ 2). The Lansky performance score should be used for participants \< 16 years and the Karnofsky performance score for participants ≥ 16 years.

∙ Prior Therapy

• Patients must have fully recovered from the acute toxic effects of all prior anticancer therapy, defined as resolution of all such toxicities to ≤ Grade 2 or lower per the inclusion/exclusion criteria prior to entering this study.

• At least 14 days must have elapsed since the completion of cytotoxic therapy, with the exception of standard maintenance therapy and steroids.

• At least 7 days must have elapsed since completion of therapy with a biologic agent. For agents that have known adverse events occurring beyond 7 days after administration, this period prior to enrollment must be extended beyond the time during which adverse events are known to occur.

• At least 3 half-lives must have elapsed since prior therapy that included a monoclonal antibody with the exception of blinatumomab. Patients must have been off blinatumomab infusion for at least 7 days and all drug related toxicity must have resolved to Grade 2 or lower as outlined in the inclusion/exclusion criteria.

• At least 42 days must have elapsed since CAR-T cell therapy.

• Participant has received ≤ 1 prior bone marrow transplant.

• At least 90 days have elapsed since bone marrow transplant and participant is off immune suppression for ≥ 2 weeks, if applicable with no evidence of active GVHD.

• At least 2 weeks must have elapsed since local XRT (small port); ≥ 3 months must have elapsed if prior cranial or craniospinal XRT was received, if ≥ 50% of the pelvis was irradiated, or if TBI was received; ≥ 6 weeks must have elapsed if other substantial bone marrow irradiation was given.

∙ Organ Function Requirements

• Adequate renal function defined as glomerular filtration rate ≥ 60 cc/min/1.73m2 or serum creatinine based on age as follows:

‣ Age: \<6 months; maximum serum creatinine (mg/dL): 0.4 (male, female); Age: 6 months to \<1 year; maximum serum creatinine (mg/dL): 0.5 (male, female); Age: 1 to \< 2 years; maximum serum creatinine (mg/dL): 0.6 (male, female); Age: 2 to \< 6 years; maximum serum creatinine (mg/dL): 0.8 (male, female); Age: 6 to \<10 years; maximum serum creatinine (mg/ dL): 1 (male, female); Age: 10 to \<13 years; maximum serum creatinine (mg/dL): 1.2 (male, female); Age: 13 to \<16 years; maximum serum creatinine (mg/dL): 1.5 (male), 1.4 (female); Age: ≥ 16 years; maximum serum creatinine (mg/dL): 1.7 (male), 1.4 (female)

• Adequate hepatic function defined as:

‣ Direct bilirubin ≤ 1.4 mg/dL (if total bilirubin \> 1.4 mg/dL) and

⁃ AST or ALT ≤ 3 x ULN for age.

• Adequate cardiac function defined as shortening fraction of ≥ 27% or ejection fraction ≥ 45%.