e16264 Publication Only A pilot trial of personalized neoantigen pulsed autologous dendritic cell vaccine as adjuvant treatment in hepatocellular carcinoma. Qiuji Wu, Qiu Li; Cancer Center, West China Hospital, Sichuan University, Chengdu, China Background: The primary treatment for hepatocellular carcinoma (HCC) is radical surgery. However, HCC has a high propensity to recur postoperatively and no standard adjuvant ther- apies have been approved. This Phase I trial aims to evaluate the safety and efficacy of an adjuvant personalized neoantigen pulsed autologous dendritic cell vaccine (Neo-DCVac-02) inpatients with HCC. Methods: Eligible patients had histologically confirmed high-risk HCC after radical surgery. After obtaining the necessary baseline biopsy specimens and PBMCs of ap-propriate quality for DNA and RNA sequencing, neoantigen prediction, screening and synthesiswere performed. Subsequently, leukapheresis was performed and DCs were isolated and cul-tured. DCs were then pulsed with neoantigen peptides to produce Neo-DCVac-02. On day 0,patients were treated with cyclophosphamide at a dose of 250 mg/m2. The prepared Neo- DCVac-02 vaccine was administered subcutaneously to the axillary and inguinal regions bi- laterally on day 1, followed by daily administration of GM-CSF at a dose of 0.075 mg for 5 days(days 2-6). The primary objectives of this study were to assess feasibility and safety/tolerability.Secondary objectives included evaluation of immune responses (via vaccine response: IFN g ELISpot), relapse-free survival (RFS) and overall survival (OS). Results: A total of 32 patients provided informed consent to initiate the process of personalized neoantigen discovery, ofwhich 26 (81%) met the requirements for successful product selection for clinical manufactur-ing. Feasibility was demonstrated with 13 patients receiving Neo-DCVac-02. 13 patients did not receive the neoantigen vaccine (7 due to progressive disease, 6 due to withdrawal of informed consent because of COVID-19). The treatment was well tolerated with no grade 2+ treatment-related adverse events (TRAEs). The most common Grade 1 TRAEs were injection site reactions(75%), rash (16.7%), fatigue (8.3%), neutropenia (8.3%), and headache (8.3%). Five of 13evaluable patients showed increased antigen-specific T-cell activity and more than 50% of theneoantigen peptides in Neo-DCVac-02, and sustained cellular responses were observed up toone year. Increases in T-cell activation/co-stimulation seen after treatment with Neo-DCVac-02, as demonstrated by flow cytometric analysis, suggest immune priming. At a median follow- up of 29.7 months, median RFS was not reached, with 1- and 2-year RFS of 84.6% and 60%, respectively, and all patients are alive. Patients with a strong immune response had a longermedian relapse-free survival (not reached) compared to patients with a weak immune response(17.6 months, P = 0.003). Conclusions: These data demonstrate that Neo-DCVac-02 is safe, well tolerated and capable of inducing durable antigen-specific lymphocyte immune responses thatmay correlate with delayed HCC recurrence. Clinical trial information: NCT04147078.