Clinical performance of Signatera Genome assay to predict treatment response and prognosticate outcomes in patients with locally advanced (LAR) resectable gastric or gastroesophageal junction (G/GEJ) adenocarcinoma: Results from the PLAGAST Study

Background: Circulating tumor DNA (ctDNA) is a clinically validated biomarker across multiple gastrointestinal cancers, used to predict recurrence risk, monitor response to neoadjuvant therapy (NAT), and detect molecular residual disease (MRD) post-definitive treatment. Signatera™ is a personalized, tumor-informed, multiplex PCR-NGS ctDNA assay that can be developed using either whole-exome sequencing (WES) or whole-genome sequencing (WGS). Initial clinical validation of Signatera Genome in select clinical contexts of a pancancer cohort showed robust performance. Here, we evaluated the performance of Signatera Genome in pts with LAR G/GEJ cancer enrolled in the PLAGAST prospective study (NCT02674373). Methods: A retrospective analysis was conducted using residual samples from pts with LAR G/GEJ cancer in the PLAGAST study (N=54). Signatera Genome assays were designed from matched tumor and normal WGS data and used to detect ctDNA in the corresponding pts' plasma samples collected at pre-NAT (or pre-surgery for NAT-naive), during-NAT, post-NAT, and post-surgery in the MRD window (within 2−12 weeks, before adjuvant treatment). ctDNA levels were quantified as mean tumor molecules per mL of plasma (MTM/mL). Longitudinal blood samples represented time points pre-/post-definitive treatment until recurrence or the end of follow-up. The correlation between ctDNA status and disease-free survival (DFS)/overall survival (OS) was assessed using Cox regression analysis. Results: The median pt age was 66 (34-86) years. The majority of pts were males (65%), had gastric cancer (57%), and received NAT (87%). Pts with post-NAT, pre-surgery ctDNA positivity had significantly worse outcomes (RFS: HR: 5.4, 95% CI: 1.68-17.37; p=0.005; OS: HR: 5.42, 95% CI: 1.5-28.73; p=0.0085). Postoperative survival analysis demonstrated that ctDNA positivity during the MRD window was also associated with worse RFS (HR: 9.78, 95% CI: 3.41-28.07, p<0.0001) and OS (HR: 12.98, 95% CI: 3.69-45.67; p<0.0001). Multivariate analyses showed ctDNA status post-NAT and within the MRD window to be the most significant independent risk factor for both RFS and OS compared to other clinicopathological features (post-NAT: RFS: HR: 5.53, p=0.005; OS: HR: 6.51, p=0.029; MRD window: RFS: HR: 4.97, p=0.014; OS: HR: 4.28, p=0.031). Conclusions: Persistent ctDNA positivity at post-NAT/pre-surgery timepoint and in the MRD window were strongly prognostic of inferior outcomes. These data indicate the robust clinical performance of Signatera Genome in pts with LAR G/GEJ adenocarcinoma and support its potential clinical utility in this disease setting. Prospective clinical trials are warranted to establish the clinical utility of the assay for guiding treatment decisions.