Cladribine/cyclophosphamide lymphodepletion efficacy and toxicity in CD19 CAR-T therapy for B-ALL.

6549 Background: During an international fludarabine (Flu) shortage, our center adopted cladribine (Cla) as a substitute for Flu with cyclophosphamide (Cy) for lymphodepletion (LD) prior to CAR-T therapy in patients with relapsed/refractory (R/R) B-cell acute lymphoblastic leukemia (B-ALL). We evaluated clinical outcomes and toxicity with Cla/Cy LD. Methods: We conducted a retrospective, single-center analysis of R/R B-ALL patients who received CD19-directed CAR-T therapy between January 2018 and April 2025. The Cla/Cy regimen substituted Flu 30 mg/m² with Cla 5 mg/m²/day on days −5 to −3. ALL-HT risk was calculated (Nair et al, 2025). CRS and ICANS were graded per ASTCT; cytopenias per CTCAE v5.0. Early (<30 days) and late (31–100 days) infections were assessed. Overall survival (OS), Event free survival (EFS, event=progression or death) and Minimal residual disease (MRD) status by clonoSEQ assay at Day 30 bone marrow biopsy were assessed. Results: We analyzed 53 patients (brexu-cel n=49; tisa-cel n=4). 28 received Flu/Cy and 25 received Cla/Cy. Baseline characteristics are outlined in Table 1. Three patients died before day 30 and two before day 100. Ten patients underwent allo-SCT within 6 months of CAR-T. Rates of grade ≥3 CRS (4% vs 12%, p=0.29) and ICANS (40% vs 24%, p=0.25) did not differ. MRD negativity at day 30 was comparable (Cla/Cy 81.8% vs Flu/Cy 75.0%, p=0.71). Early infection rates were similar; however, late infections were more frequent with Flu/Cy (48% vs 17.4%, p=0.034), with a persistent trend after ALL-HT adjustment (adjusted OR 3.54, p=0.073). Grade ≥3 thrombocytopenia (35.7% vs 8.0%, p=0.022) and neutropenia (25.0% vs 4.0%, p=0.05) at day 90 were higher with Flu/Cy, with similar trends after adjustment (adjusted OR 4.71, p = 0.082 and adjusted OR 7.88, p = 0.067, respectively). Neutrophil recovery (ANC >1000) time was comparable (median 17 vs 18 days, p=0.10). With a median follow-up of 14.6(Flu/Cy) and 13.1 months(Cla/Cy), EFS (6.8 vs 18.1 months, p=0.45) and OS (33.9 months vs not reached, p=0.25) were similar. Conclusions: Cla/Cy demonstrated comparable efficacy to Flu/Cy with lower toxicity, including fewer late infections and severe cytopenias. These findings suggest Cla/Cy may represent a safer alternative lymphodepletion strategy. Prospective studies are warranted to validate these observations. Baseline characteristics of study cohort. Characteristic Flu/Cy (n=28) Clad/Cy (n=25) p-value Age, median (IQR) 38.0 (30.2–58.5) 47.0 (26.0–64.0) 0.77 Male sex, n (%) 18 (64.3) 13 (52.0) 0.53 White race, n (%) 25 (89.3) 20 (80.0) 0.35 KPS ≥80, n (%) 24 (85.7) 21 (84.0) 1.00 Marrow blasts pre-LD, median (IQR) 2.0 (1.0–3.5) 1.5 (0.5–11.0) 0.67 Ph+ ALL, n (%) 11 (39.3) 12 (48.0) 0.78 Prior allo-HSCT, n (%) 6 (21.4) 7 (28.0) 0.56 Prior lines of therapy, median (IQR) 2 (2–3) 3 (2–3) 0.96 High-risk ALL-HT, n (%) 12 (42.9) 3 (12.0) 0.016