Early clues in the battle against advanced gastric cancer: How plasma ctDNA signals the effectiveness of PD-1 inhibitors with chemotherapy.

3042 Background: Combination therapy with Programmed Death 1 (PD-1) inhibitors and chemotherapy has shown efficacy in improving survival in advanced gastric cancer, but not all patients derive benefit. We aimed to explore whether early dynamic changes in circulating tumor DNA (ctDNA) can serve as predictors of treatment response and long-term survival in patients receiving first-line PD-1 inhibitors combined with chemotherapy. Methods: This is a prospective, single-center trial in which all patients are adults with advanced or metastatic GC/GEJC confirmed by pathology or imaging. 30 eligible patients will receive PD-1 mab (sintilimab/nivolumab (NIVO) Q3W) and chemotherapy (SOX/XELOX Q3W) regardless of programmed death ligand-1 status. Blood samples were collected before treatment and after two treatment cycles for ctDNA analysis. Plasma ctDNA was isolated for evaluating variant allele frequencies (VAF) of somatic mutations in 47 cancer-related genes using next-generation sequencing (NGS), ctDNA response was defined as clearance of maximum VAF (maxVAF) compared to baseline. CT scans were performed after every two cycles and will be categorized as responding (complete response plus partial response) or non-responding (progress disease plus stable disease) according to RECIST v1.1. Overall survival was calculated from the date of first PD-1 mab infusion to the time of death or censored at most recent follow-up; progression‐free survival was calculated from the date of first PD-1 mab infusion to the time of death or first documented progression, whichever came first, or censored at most recent follow-up. Results: In thirty advanced gastric cancer patients, the dynamic changes in maxVAF demonstrated a significant association with patients attaining either complete response (CR) or partial response (PR) to treatment (P=0.0021), but showed no correlation with stable disease (SD) or progress disease (PD). A substantial agreement was observed between ctDNA response and the best tumor shrinkage rate (Cohen's kappa=0.69). The objective response rate (ORR) among ctDNA responders was 85%, markedly higher compared to the 22% observed in non-responders (P=0.0073). Additionally, patients with ctDNA response had a significantly longer progression‐free survival (PFS) compared to patients without ctDNA response (median PFS 15.6 months vs 6.0 months; HR, 0.18; 95% CI, 0.06 to 0.61; p=0.003), overall survival (OS) was also significantly longer among ctDNA responders (median OS not reachNR vs 9.0 months; HR, 0.10; 95% CI, 0.01 to 0.88; p=0.011). Conclusions: ctDNA response serves as a potential biomarker for assessing treatment efficacy and long-term outcomes in advanced gastric cancer patients treated with first-line PD-1 inhibitors and chemotherapy. Clinical trial information: ChiCTR2200065366.