TPS9608 Background: Metastatic melanoma (MM) patients who have progressed following immune checkpoint inhibitors (ICI; anti–PD-1/CTLA-4 therapy) and targeted agents (BRAF/MEK inhibitors) have limited treatment options and dismal prognosis. An autologous tumor-infiltrating lymphocytes (TIL) therapy (LN-144, lifileucel) has demonstrated efficacy and durable responses in heavily pretreated advanced melanoma. In a phase II study of TIL therapy (lifileucel) with traditional lymphodepletion (LD), using fludarabine 25 mg/m2 for 5 days and cyclophosphamide (Cy) 60 mg/kg for 2 days, in 153 MM patients previously treated with ICI and BRAF/MEK inhibitors, an overall response rate (ORR) of 31% was noted, and the median duration of response (DOR) was not reached after 27.6 month follow up. We hypothesize that a reduced dose of Flu/Cy LD for TIL therapy will have similar TIL expansion and persistence post-infusion, resulting in similar efficacy with a reduced toxicity profile. Methods: Study NCT06151847 is a single-center, open-label phase II pilot trial evaluating the efficacy, in vivo persistence, and safety of TIL therapy after reduced dose Flu/Cy LD. Key inclusion criteria include unresectable or metastatic melanoma (stage IIIC-IV) with disease progression after one or more lines of therapy. Four of the 12 planned patients have been enrolled as of February 2, 2024. Central TIL manufacturing from at least a 1.5 cm tumor specimen involves ex vivo expansion through cell culture in the presence of the interleukin (IL)-2 and an anti-CD3 monoclonal antibody (1-150 x10 9 viable cells). Patients will receive an outpatient reduced-dose Flu/Cy LD regimen of Flu (30 mg/m 2 on days -4, -3, -2, -1) and Cy (750 mg/m 2 on days -4, -3, -2). Infusion of TIL will be performed on day 0 followed by high-dose IL-2 (600,000 IU/kg up to 6 doses). The primary objective will be to ascertain TIL persistence using T-cell receptor (TCR) sequencing at day 42 which will be compared to historical data already generated utilizing traditional Flu/Cy LD. The secondary objectives will be to evaluate the efficacy parameters including ORR, DOR, progression-free survival (PFS), and overall survival (OS), and the safety profile of TIL in combination with a reduced dose Flu/Cy LD regimen. Exploratory objectives will involvecorrelative analyses to characterize the immunome and microenvironment. If TIL persistence is roughly equivocal between the reduced and high-dose Flu/Cy LD regimens, the design of a larger-scale trial using reduced-dose Flu/Cy LD for TIL therapy may be considered. Study follow-up will continue for one year. Clinical trial information: NCT06151847 .