Background: The inability to receive L-asparaginase (ASNase) therapy due to hypersensitivity is associated with inferior outcomes in patients with ALL or LBL. JZP458, a recombinant Erwinia-derived ASNase from a Pseudomonas fluorescens expression platform, is approved by the FDA for patients with ALL/LBL who have developed hypersensitivity to E. coli–derived ASNase. Here, we report the efficacy and safety of IM JZP458 from COG AALL1931, a phase 2/3, open-label, multicenter, pharmacokinetic (PK) study. Methods: Eligible patients with ALL/LBL had a grade ≥3 allergic reaction or silent inactivation to a pegylated E. coli–derived ASNase. Each remaining dose of pegylated E. coli–derived ASNase was replaced with 6 doses of IM JZP458 on Monday/Wednesday/Friday (M/W/F) over 2-weeks. Three dosing cohorts were enrolled: Cohort 1a, 25 mg/m² M/W/F; Cohort 1b, 37.5 mg/m² M/W/F; Cohort 1c, 25 mg/m² M/W and 50 mg/m² F. Efficacy was assessed by the proportion of patients who achieved the last 72-hour (primary endpoint) or 48-hour (key secondary endpoint) nadir serum asparaginase activity (NSAA) levels ≥0.1 IU/mL in the first treatment course. A population PK (PPK) model was developed based on SAA data from AALL1931 to characterize the PK of JZP458 and to inform dosing decisions. Results: 167 patients were enrolled for IM dosing (Cohort 1a, n = 33; Cohort 1b, n = 83; Cohort 1c, n = 51). The median (range) age was 10 (1, 25) years. The median (range) of JZP458 courses received was 5 (1, 14) for Cohort 1a, 5 (1, 15) for Cohort 1b, and 4 (1, 11) for Cohort 1c. Mean (95% CI) SAA levels (IU/mL) at 72-hour were 0.16 (0.12, 0.19) for Cohort 1a, 0.33 (0.27, 0.39) for Cohort 1b, and 0.47 (0.35, 0.59) for Cohort 1c; and 0.45 (0.37, 0.53), 0.88 (0.76, 1.01), and 0.66 (0.54, 0.77), respectively, at 48-hour. Simulated data from the PPK model matched the observed data well. For Cohort 1c, the proportions of patients (95% CI) achieving NSAA levels ≥0.1 IU/mL at the last 72- and 48-hour in Course 1 were 90% (81%, 98%) and 96% (90%, 100%), respectively, based on observed data; and were 92% (91%, 93%) and 94% (93%, 95%) based on modeled data. The Table shows the rates of treatment-related adverse events (TRAEs; all grades) of interest per cohort. Overall, TRAEs leading to discontinuation included pancreatitis (6%), drug hypersensitivity (4%), anaphylactic reaction (2%), increased alanine aminotransferase (1%), and hyperammonemia (1%). There were no TRAEs leading to death. Conclusions: The totality of the results from AALL1931 demonstrate the positive benefit-risk profile of the IM JZP458 dosing regimen of 25 mg/m² M/W and 50 mg/m² F with a safety profile consistent with other asparaginases. Clinical trial information: NCT04145531.