A Study of Fludarabine Dosing in Children and Young Adults With B-cell Acute Lymphoblastic Leukemia

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