The researchers are doing this study to find out whether PK-targeted fludarabine is an effective Lymphodepletion (LD) chemotherapy approach for people with relapsed/refractory B-cell acute lymphoblastic leukemia (B-ALL) who will receive tisagenlecleucel CAR T-cell therapy. The researchers will compare PK-targeted fludarabine dosing with standard fludarabine dosing to see which treatment approach is more effective. The researchers will also look at whether PK-targeted fludarabine dosing is feasible (practical), the side effects of the study treatment, and how the study treatment affects people's quality of life. The researchers will measure quality of life by having participants complete questionnaires.

Inclusion Criteria:

• Patients with B-ALL and eligible to receive commercial tisagenlecleucel.

• Patient's weight > 9 kg at time of lymphodepleting chemotherapy

• Adequate organ function at time of LD is required and is defined:

  • Hepatic: Serum bilirubin ≤ 2 mg/dL, unless benign congenital hyperbilirubinemia
  • Hepatic: AST and ALT < 5x the upper limit of normal for age, unless thought to be leukemic disease-related
  • Renal: Calculated glomerular filtration rate (GFR) ≥ 70 ml/min/1.73m^2. (based on Schwartz formula GFR (mL/min/1.73 m²) = (36.2 × Height in cm) / Creatinine in mg/dL
  • Cardiac: LVEF ≥ 50% by multi-gated acquisition scan (MUGA), resting echocardiogram, or cardiac magnetic resonance imaging (MRI) within 6 weeks of screening
  • Pulmonary: Oxygen saturation as recorded by pulse oximetry of ≥ 90% on room air

• Adequate performance status:

  • Age ≥ 16 years: ECOG ≤ 1 or Karnofsky > 60% at treatment
  • Age < 16 years: Lansky ≥ 60% at treatment

• Willing to participate as research subject and provide written informed consent from parents/legal representative, patient, and age-appropriate assent as appropriate before any study specific screening procedures are conducted, according to local, regional or national law and legislation.

Exclusion Criteria:

• Have a known immediate or delayed hypersensitivity reaction or idiosyncrasy to the study drugs, or drugs chemically related to study treatment or excipients that contraindicate their participation, including fludarabine, cyclophosphamide and tisagenlecleucel.
• Patients with tisagenlecleucel that is deemed out of specification (OOS) will be excluded from this protocol
• Clinically significant active and uncontrolled infection confirmed by clinical evidence, imaging, or positive laboratory tests (e.g., blood cultures, PCR for DNA/RNA etc.)
• Patient/parent/guardian unable to give informed consent or unable to comply with the treatment protocol.
• Pregnant or lactating women

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.