Exploration of the benefits and resistance mechanisms of immunotherapy based on PD-L1 expression and ctDNA profiles.

e16041 Background: Immunotherapy and chemotherapy combinations have become the standard treatment of HER2-negative advanced gastric cancer (AGC). However, there remains controversy regarding the use of PD-L1 expression as a predictive biomarker for the benefits of immunotherapy. Additionally, the underlying mechanisms of resistance to combined immunotherapy treatment are not yet fully understood. Methods: We retrospectively enrolled 245 AGC patients who received first-line treatment from January 2018 to December 2024 at our hospital. Concurrently, blood samples from 20 patients were prospectively collected at 3 time points: before the initiation of first-line immunotherapy (C1), after two cycles of treatment (C2), and during disease progression (P). These samples underwent next-generation sequencing (NGS) and T-cell receptor sequencing (TCR-seq). We aim to investigate the differences in clinical features across varied levels of PD-L1 expression, evaluate the efficacy of different first-line treatment regimens, and assess the value of circulating tumor DNA (ctDNA) in predicting long-term benefits from immunotherapy. Herein, we present partial outcomes. Results: A total of 245 patients were categorized into three groups based on PD-L1 expression: Low (N = 82, CPS < 1), Median (N = 63, CPS 1-4), and High (N = 100, CPS ≥ 5). Age and number of metastatic organs differed significantly among the groups. In the Low group, adding a PD-1 inhibitor or anti-angiogenic therapy to chemotherapy showed no advantage over chemotherapy alone. In the Median group, C+PD-1i+anti-angio treatment showed a numerically higher progression-free survival (PFS) compared to other treatments (10.3m vs 7.4m vs 7.5m, p = 0.677). In the High group, C+PD-1i improved median PFS over chemotherapy alone (6.9m vs 5.8m, P = 0.04), aligning with prior trials. Additionally, ctDNA analysis revealed that FAT2, TP53, PREX2, and NAV3 mutations were more common in the short-PFS group ( < 6 cycles), and MYC gene amplification was more frequent at the C2 time point in short-PFS group. At various time points, patients in the short PFS group exhibited a higher likelihood of harboring the same mutations (4 out of 5 compared to 3 out of 15), with significantly elevated maximum variant allele frequency (maxVAF) and circulating tumor DNA (ctDNA) levels. However, no significant difference observed in blood tumor mutational burden (TMB). Post-treatment, the ctDNA levels decreased in 50% of patients in the long-PFS group (7 out of 14), whereas only one patient in the short-PFS group (1 out of 4) experienced a reduction. Conclusions: The application of immunotherapy for AGC patients harboring PD-L1 expression CPS < 5 remains controversy. Several mutations in the blood may be associated with the benefits of immunotherapy. Lower maxVAF/ctDNA and easier clearance of ctDNA may be characteristics of patients with long-term benefits from immunotherapy.