7548 Background: ZUMA-5 is a Phase 2 study of axi-cel anti-CD19 CAR T-cell therapy in pts with R/R iNHL (follicular lymphoma [FL]; marginal zone lymphoma [MZL]). In the primary analysis, 11 pts (9 FL; 2 MZL) were retreated with axi-cel, achieving an overall response rate (ORR) of 100% (91% complete response [CR] rate) at a median follow-up of 2.3 mo post-reTx, with no Grade ≥3 cytokine release syndrome (CRS) or neurologic events (NEs; Chavez et al. ASH 2020. #2036). Here, we report updated clinical and translational outcomes with longer follow-up in pts retreated with axi-cel in ZUMA-5. Methods: Eligible pts with FL or MZL had R/R disease after ≥2 lines of therapy. Pts were considered for reTx if they progressed after a response at mo 3, had no evidence of CD19-negative relapse in biopsy, had no axi-cel neutralizing antibodies, and had no Grade 4 CRS or NEs with 1st Tx. Retreatment was per investigator discretion. At both Txs, pts received axi-cel (2×106 CAR T cells/kg) after conditioning chemotherapy. Results: As of 9/14/2020, 13 pts with iNHL (11 FL; 2 MZL) received axi-cel reTx, with 2 pts retreated after the primary analysis. Before their 1st Tx, pts had median 4 prior lines of therapy; 85% had stage 3–4 disease; 82% had FLIPI of ≥3; 46% were POD24; 77% had refractory disease. Among the 13 retreated pts, 85% had a CR to 1st Tx. Median 1st duration of response (DOR) was 8.2 mo. Detectable CD19 was confirmed in all evaluable biopsies from retreated pts at relapse, and median time from 1st Tx to reTx was 10.6 mo. Following reTx, the ORR was 100% (77% CR rate). After a median follow-up of 11.4 mo, the median DOR had not yet been reached; 46% of retreated pts had ongoing responses at data cutoff. At 1st Tx, CRS occurred in 9 pts (5 Grade 1, 4 Grade 2); NEs occurred in 5 (3 Grade 1, 1 Grade 2, 1 Grade 3). At reTx, CRS occurred in 8 pts (6 Grade 1, 2 Grade 2); NEs occurred in 4 (3 Grade 1, 1 Grade 2). Median peak levels of biomarkers typically associated with severe CRS and NEs were similar at reTx and 1st Tx (IL-6, 7.7 vs 5.7 pg/mL; IL-2, 1.8 vs 0.9 pg/mL; IFN-γ, 62.9 vs 64.2 pg/mL). In the 11 retreated pts with FL, tumor burden (median sum of product diameters [SPD]) was lower before reTx vs 1st Tx (1416 vs 4770 mm2). Engraftment index (CAR T-cell expansion relative to SPD) is an indirect proxy for effector:target ratio and a key covariate of response to axi-cel (Locke et al. Blood Adv. 2020). Though median peak CAR T-cell levels appeared lower at reTx vs 1st Tx (5.2 vs 14.3 CAR+ cells/µL blood), engraftment index was similar (0.003 vs 0.005 cells/µL×mm2). Conclusions: Axi-cel reTx achieved deep and durable responses, with an acceptable safety profile. Tumor CD19 positivity was maintained at relapse, and engraftment index was similar at both Txs, comparing favorably to previous reports in aggressive lymphomas (Locke et al. ASCO 2020. #8012). These data suggest axi-cel reTx is a promising option for pts with R/R iNHL. Clinical trial information: NCT03105336.