Dynamic Changes in Circulating and Tissue Biomarkers and Therapeutic Response to HER2/PD-1 Dual Blockade in HER2-Positive Gastric Cancer

Background: HER2-positive gastric adenocarcinoma demonstrates heterogeneous responses to dual HER2/PD-1 blockade therapy. This study investigated integrated circulating tumor DNA, protein biomarkers, and immune cell profiling for predicting treatment outcomes and monitoring therapeutic resistance. Methods: This retrospective analysis included 72 patients with HER2-positive gastric or gastroesophageal junction adenocarcinoma receiving anti-HER2 therapy plus PD-1 inhibitors. Serial blood samples underwent circulating tumor DNA sequencing, protein biomarker measurement, and flow cytometric immune cell characterization. Multivariable analysis identified independent predictors and developed an integrated biomarker model for patient stratification. Results: Objective response rate was 40.3% with a median progression-free survival of 11.4 months. Sustained circulating tumor DNA suppression in 24 patients correlated with prolonged survival compared to early rebound patterns (p = 0.048). Co-occurring genomic alterations in EGFR, PIK3CA, MYC, and FGFR2 were detected in ctDNA and were associated with shorter progression-free survival. CD8+ T-cell increases and natural killer cell activity patterns differed between responders and non-responders. Multivariable analysis identified composite immune score (HR 0.58, p = 0.003), tumor stage (HR 2.31, p < 0.001), and performance status as independent predictors. An exploratory integrated biomarker model stratified patients into risk categories of differing outcome, although discrimination was modest (AUC 0.65) and requires prospective validation. Conclusions: Integrated circulating tumor DNA and immune biomarker monitoring may provide prognostic information for dual HER2/PD-1 blockade therapy. Serial molecular profiling enables early resistance detection and identifies evolutionary patterns that may inform therapeutic strategies, though prospective validation is required.