Background: Immune checkpoint inhibitors (ICI) have become standard of care for treatment of metastatic melanoma. Most patients with advanced melanoma progress on ICI and treatment options are limited for these patients. Progression may be through primary resistance (lack of response) or secondary resistance (initial response then progression). Lifileucel is an adoptive cell therapy using TIL, that has shown efficacy in patients with advanced melanoma who progress on/after an anti-PD-1 (Sarnaik, 2020). We present the 28-month (mos) follow-up data and highlight the impact of prior anti-PD-1 response and duration of exposure on outcome with lifileucel. Methods: C-144-01 is a Phase 2, open-label, multicenter study of efficacy and safety of lifileucel in patients with advanced melanoma who have progressed on anti-PD-1 therapy and BRAFi ± MEKi, if BRAF V600⁺. We report long-term follow up on Cohort 2 (N = 66). Tumors were resected at local sites, processed in central GMP facilities for TIL production in a 22-day manufacturing process. Therapy consisted of nonmyeloablative lymphodepletion using 2 days of cyclophosphamide and 5 days of fludarabine, a single infusion of lifileucel, and up to six doses of IL-2. Objective response rate (ORR) was assessed by RECIST 1.1. Data cutoff was Dec. 14, 2020. Results: Baseline characteristics: 3.3 mean prior therapies (100% anti-PD-1; 80% anti-CTLA-4; 23% BRAFi/MEKi), high baseline tumor burden (106 mm mean target lesion SOD), 42% liver/brain lesions, 40.9% LDH > ULN. ORR by investigator was 36.4% (3 CR, one new CR developed at 24 mos; 21 PR). Median duration of response (mDOR) was not reached at median follow-up of 28 mos (DOR range: 2.2- 35.2 mos). In responders, the median cumulative duration and median prior lines of anti-PD-1 therapy was 4.4 mos (range: 1.4-22.5 mos), and 1.5 (range: 1-4). Data in Table demonstrates a meaningful increase in DOR to TIL with primary anti-PD-1 resistance and lower duration of time on prior anti-PD-1 therapy. No new safety risks have been identified for lifileucel during long-term follow-up. Conclusions: One-time lifileucel treatment results in a 36.4% ORR, and mDOR was not reached at 28 mos of median study follow up. One PR converted to a new CR at 24 months as responses continue to deepen. DOR is positively associated with primary resistance to prior anti-PD-1 therapy and with shorter cumulative prior duration of anti-PD-1 therapy. Lifileucel may offer a better clinical outcome when used earlier upon detection of progression on prior anti-PD-1 rather than retreatment with anti-PD-1 based regimens. Clinical trial information: NCT02360579