Clinical studies support the efficacy of programmed cell death 1 (PD-1) targeted therapy in a subset of patients with metastatic gastric cancer (mGC). With the goal of identifying determinants of response, we performed molecular characterization of tissues and circulating tumor DNA (ctDNA) from 61 patients with mGC who were treated with pembrolizumab as salvage treatment in a prospective phase 2 clinical trial. In patients with microsatellite instability-high and Epstein-Barr virus-positive tumors, which are mutually exclusive, dramatic responses to pembrolizumab were observed (overall response rate (ORR) 85.7% in microsatellite instability-high mGC and ORR 100% in Epstein-Barr virus-positive mGC). For the 55 patients for whom programmed death-ligand 1 (PD-L1) combined positive score positivity was available (combined positive score cut-off value ≥1%), ORR was significantly higher in PD-L1(+) gastric cancer when compared to PD-L1(-) tumors (50.0% versus 0.0%, P value <0.001). Changes in ctDNA levels at six weeks post-treatment predicted response and progression-free survival, and decreased ctDNA was associated with improved outcomes. Our findings provide insight into the molecular features associated with response to pembrolizumab in patients with mGC and provide biomarkers potentially relevant for the selection of patients who may derive greater benefit from PD-1 inhibition.

Gastroesophageal adenocarcinoma (GEA) is a lethal disease where targeted therapies, even when guided by genomic biomarkers, have had limited effi cacy. A potential reason for the failure of such therapies is that genomic profi ling results could commonly differ between the primary and metastatic tumors. To evaluate genomic heterogeneity, we sequenced paired primary GEA and synchronous metastatic lesions across multiple cohorts, fi nding extensive differences in genomic alterations, including discrepancies in potentially clinically relevant alterations. Multiregion sequencing showed signifi cant discrepancy within the primary tumor (PT) and between the PT and disseminated disease, with oncogene amplifi cation profi les commonly discordant. In addition, a pilot analysis of cell-free DNA (cfDNA) sequencing demonstrated the feasibility of detecting genomic amplifi cations not detected in PT sampling. Lastly, we profi led paired primary tumors, metastatic tumors, and cfDNA from patients enrolled in the personalized antibodies for GEA (PANGEA) trial of targeted therapies in GEA and found that genomic biomarkers were recurrently discrepant between the PT and untreated metastases. Divergent primary and metastatic tissue profi ling led to treatment reassignment in 32% (9/28) of patients. In discordant primary and metastatic lesions, we found 87.5% concordance for targetable alterations in metastatic tissue and cfDNA, suggesting the potential for cfDNA profi ling to enhance selection of therapy. SIGNIFICANCE:We demonstrate frequent baseline heterogeneity in targetable genomic alterations in GEA, indicating that current tissue sampling practices for biomarker testing do not effectively guide precision medicine in this disease and that routine profi ling of metastatic lesions and/or cfDNA should be systematically evaluated. Cancer Discov; 8(1);[37][38][39][40][41][42][43][44][45][46][47][48]

4001 Background: Adjuvant chemotherapy and/or chemoradiotherapy have been the standard of care in GC for years, supported by randomized trials. We compared the efficacy of different chemotherapy regimens and chemoradiotherapy in patients with D2-resected, stage II/III, node-positive GC. Methods: From Feb 2013 through Nov 2018, we randomly assigned, in a 1:1:1 ratio, patients with pathologically-staged II or III, node-positive, D2-resected GC, to receive adjuvant S-1 (40-60 mg twice daily 4-weeks-on/2-weeks-off) for one year, S-1 (2-weeks-on/1-week-off) plus oxaliplatin 130 mg/m2 (SOX) for six months, or SOX plus chemoradiotherapy 45 Gy (SOXRT). Randomization was stratified according to the type of surgery (total or subtotal gastrectomy), stage (II or III), and Lauren histologic classification (diffuse or intestinal). The primary endpoint was disease-free survival (DFS). A total of 900 patients had to be enrolled to demonstrate superiority of SOX or SOXRT to S-1 (hazard ratio [HR] 0.667), with 90% power at a two-sided significance level of 5%. Results: A total of 538 patients were included for this interim efficacy analysis. Median age was 58 years, men constituted 65%, and stage II and III were 31% and 69%, respectively. Baseline tumor and patient characteristics were balanced between treatment arms. Adverse events were as anticipated in each arm, generally well-tolerated and manageable. DFS in the control arm (S-1) were significantly shorter than in SOX and SOXRT arms (stratified HR for recurrence): S-1 vs. SOX, 0.617 (P = 0.016) and S-1 vs. SOXRT, 0.686 (P = 0.057). The DFS at 3-years was found to be 65%, 78% and 73% in S-1, SOX and SOXRT arms, respectively. No difference in DFS between SOX and SOXRT was found (HR 0.910, P = 0.667). Based on the results after the observation of 145 recurrence events at the cutoff date of Dec 27, 2018, the independent data monitoring committee considered the results sufficient to meet the endpoint of the trial and recommended early stopping of the trial. Conclusions: In patients with curatively D2-resected, stage II/III, node-positive GC, adjuvant SOX or SOXRT was effective in prolonging DFS, when compared to S-1 monotherapy. Clinical trial information: NCT0176146.