386. SAFETY AND FEASIBILITY OF PERSONALIZED NEOANTIGEN DC VACCINES IN POSTOPERATIVE ESOPHAGUS SQUAMOUS CELL CARCINOMA: A PHASE 1B TRIAL

Abstract Background Immunotherapy, as a neoadjuvant treatment, currently stands at the forefront of esophageal squamous carcinoma research. Yet, the landscape of post-surgery adjuvant therapy remains largely unexplored, especially for patients who experience inadequate responses or develop resistance to initial immunotherapy. Addressing this gap, we have undertaken a single-center, single-arm, prospective, and exploratory clinical study. This study aims to evaluate the efficacy of a personalized neoantigen-loaded dendritic cell vaccine (Neo-DCVac) as a novel adjuvant treatment in ESCC patients who have previously undergone neoadjuvant immunochemotherapy (nICT). Methods Neo-DCVac was designed and administered to 12 ESCC patients undergoing radical mastectomy post-nICT. Tailored adjuvant therapy was based on tumor immune microenvironment (TIME) analysis, following Lam's Stratified Treatment Strategy. The immunization schedule included five doses at weeks 1, 2, 4, 6, and 8, with bi-monthly boosts during the consolidation phase, sustained over a year. Primary endpoints were safety and post-vaccination immune response. Secondary endpoints included 1- and 2-year overall survival (OS) and progression-free survival. T-cell response to neoantigen peptides was assessed using enzyme-linked immunospot (ELISPOT), flow cytometry, and enzyme-linked immunosorbent asssay (ELISA), alongside single-cell sequencing to explore the vaccine's efficacy mechanism. Results From April 2021 to October 2023, a total of 12 ESCC patients were enrolled. The occurrence of all-grade AEs was 38.5% (5/12), all of grade 1-2, with no dose-limiting toxicities. No treatment interruptions or treatment-related deaths were observed. Till October 27 2023, the median follow-up was 31.2 months. Survival analysis revealed 1-year and 2-year disease-free survival (DFS) and overall survival (OS) rates of 88.3%, 66.7%, and 100.0%, 91.7%, respectively. Patients with diver genes had a longer median DFS (not reached) compared with patients without diver genes (10.8 months, P = 0.001). Post-vaccine immunization, all patients showed a marked increase in peripheral blood T cell subtypes (CD3, CD4, CD8), with significant neoantigen peptide response in 75% (9/12) as evidenced by ELISPOT and ELISA tests. Single-cell sequencing coupled with CoNGA analysis revealed a distinct MAIT cell population, displaying 65 clonotypes and 146 cells with consistent GEX profiles and TCR sequences. GLIPH2 analysis on Neo-Vac immunotherapy patients identified shared peptide-MHC specificities, notably in patient ES0-01, and similar patterns in patients ES0-04 and ES0-07, suggesting T cell expansion and neoantigen-specific responses post-vaccination. Conclusions The neoantigen DC vaccine demonstrated favorable safety and exhibited notable efficacy in ESCC.