This prospective, randomized, multicenter, open-label Phase 2 study is designed to evaluate the superiority of InO monotherapy vs ALLR3 after 1 cycle of induction treatment in paediatric participants (between 1 and <18 years) with High Risk (HR) or very high risk (VHR) first bone marrow relapse CD22-positive BCP ALL, and to evaluate the safety and tolerability, PK and long-term efficacy. Treatment with study intervention will end after induction therapy; follow-up will continue for up to 5 years from randomization.

This prospective, randomized, multicenter, open-label, Phase 2 study is designed to evaluate the superiority of InO monotherapy vs ALLR3, after 1 cycle of induction treatment in paediatric participants (between 1 and <18 years) with HR or VHR first bone marrow relapse CD22-positive BCP ALL, and to evaluate the safety and tolerability, PK and long-term efficacy. Treatment with study intervention will end after induction therapy; follow-up for efficacy and safety will continue for up to 5 years from randomization.

End of Treatment is defined as occurring upon recovery from 1 cycle of study therapy (Day 28 ± 2 days), or one day before initiation of new anticancer therapy, whichever occurs first.

Approximately 100 participants will be randomized (2:1) to receive 1 cycle of either InO monotherapy or ALLR3 (block 1) therapy during induction. Two interim analyses (approximately 50% and 75% enrollment) fort the primary outcome measure followed by a final analysis are planned to assess efficacy and futility (non-binding).

After completion of induction therapy (ie, study therapy), it is anticipated that the majority of responding participants will proceed immediately to consolidation therapy. Non-responders are expected to proceed with salvage therapy at the investigator's discretion. Participants responding to induction therapy are expected to proceed to SOC consolidation therapy upon recovery of blood counts, but no sooner than 7 days after last dose of study intervention.

All participants (responders and non-responders) will proceed to long-term follow-up for this study. All subsequent anticancer therapy will be determined by the treating physician.

Inclusion Criteria:

1. Male or female participants between 1 and <18 years of age.

2. Morphologically confirmed diagnosis of first relapse HR or VHR BCP ALL; HR first relapse is defined as relapse occurring within 18 to 30 months of original diagnosis of ALL or within 6 months of completion of primary therapy, and lacking any identified very high-risk genetic abnormalities (Groeneveld-Krentz et al, 2019) (ie, KMT2A::AFF1 fusion [t(4;11)(q21;q23)], TCF3-HLF fusion [t(17;19)(q22;p13)], TCF3-PBX1 fusion [t(1;19)(q23;p13.3)], hypodiploidy [<40 chromosomes] or masked low hypodiploidy (Molina et al, 2021), TP53 alteration). VHR first relapse is defined as relapse within 18 months of original diagnosis of ALL and/or with any of the following genetic abnormalities at original diagnosis or at relapse (Groeneveld-Krentz et al, 2019) (ie, KMT2A::AFF1 fusion [t(4;11)(q21;q23)], TCF3-HLF fusion [t(17;19)(q22;p13)], TCF3-PBX1 fusion [t(1;19)(q23;p13.3)], hypodiploidy [<40 chromosomes] or masked low hypodiploidy (Molina et al, 2021), TP53 alteration).

   • CD22-positive ALL as defined by local institution;
   • Bone marrow involvement of ≥ 5% leukemic blasts (≥ M2 status).

3. Adequate serum chemistry parameters:

   • An eGFR in participants 1 to <2 years of age, or eCrCl in those 2 to <18 years of age, ≥30 mL/min using the recommended formula in Section 10.10.2.
   • AST and ALT ≤5 × institutional ULN at the time of randomization or pre-cytoreduction/general anesthesia; (Refer to Appendix 9 for France-specific requirement on the ALT/AST threshold);
   • Total bilirubin ≤1.5 × institutional ULN unless the participant has documented Gilbert's syndrome;

4. Prior history of thrombosis during corticosteroid use and/or asparaginase are eligible provided the patient receives anti-coagulant prophylaxis per institutional guidelines.

5. Cardiac shortening fraction ≥ 30% by echocardiogram or ejection fraction >50% by MUGA.

6 Participants with combined bone marrow and testicular relapse are eligible assuming orchiectomy is performed prior to randomization or is planned at the end of induction therapy.

5.2. Exclusion Criteria

1. Any history of prior or ongoing hepatic SOS or prior liver failure [defined as severe acute liver injury with encephalopathy and impaired synthetic function (INR of ≥1.5)].
2. Prior allo-HSCT or CAR T-cell therapy.
3. Isolated extramedullary leukemia.
4. Philadelphia-chromosome positive ALL, ie. BCR-ABL/t(9;22) present.
5. Prior therapy with a calicheamicin-conjugated antibody (eg, InO or gemtuzumab ozogamicin).
6. Participants with active, uncontrolled bacterial, fungal, or viral infection.
7. Hypersensitivity/allergy to both PEG-ASP and Erwinia-ASP

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.