Phase II preoperative pembrolizumab for MSI high or MSS/PD-L1 gastric cancer followed by surgery and adjuvant therapy with pembrolizumab (NCT03257163): Results of a multicenter study.
Authors: Timothy Kennedy, Mihir Maheshkumar Shah, Rafi Kabarriti, Patrick M Boland, Haejin In, Chunxia Chen, Stephen M. Szabo, Howard S Hochster, Dirk F. Moore, Salma K. Jabbour
Journal: Journal of Clinical Oncology (2025)

441 Background: Data suggest that patients with tumors with microsatellite instability (MSI-H), EBV expression, or positive PD-L1 expression can benefit from immunotherapy. This trial evaluated PD-1 immunotherapy in this subset of patients with operable gastric cancer. Methods: We present results of a phase 2 multi-institutional clinical trial (NCT03257163). Patients with cT2-T4, N0-N3, M0 gastric adenocarcinoma with MSI-H, PDL1 CPS &gt; 1%, or EBV+ were included. Patients received 2 cycles preop pembrolizumab 200 mg IV q3 weeks followed by surgery. Pembrolizumab was given for 16 additional cycles postoperatively, concurrently with adjuvant chemoradiation (45 Gy) and 5 cycles of capecitabine (825 mg/m2 days 1-14 during cycles 3, 6 and 7; 625 mg/m2 BID days 1-14 during cycles 4 and 5 with radiation). Patients were defined as evaluable if they received &gt; one cycle of postop pembrolizumab. Primary endpoint is DFS, powered to detect 3-yr DFS of 70% with 89% power. Results: All enrolled patients received two cycles of pembrolizumab preoperatively (n=45). Eight patients did not undergo surgery (1 due to frailty, 2 with distant progression on preoperative imaging, 2 with peritoneal disease on surgical exploration and 3 with locally advanced, unresectable tumors). Six patients did not receive adjuvant therapy (2 due to surgical complications, 2 for failure to thrive, 1 refusal and 1 pending adjuvant therapy). 31 patients received adjuvant therapy and are included in the predetermined, evaluable analytic cohort. These 31 patients had a median age of 67 years (range 44 – 85) with 22.6% Hispanic, 29.0% Asian, 25.8% Black and 25.8% White. Median follow-up was 31.1 months (range 3.45-63.8). In these patients, 14 (45.2%) had MSI-H, 2 (6.5%) EBV+, and 29 (93.5%) had PDL1 expression &gt;1% tumors. Most tumors were advanced stage with 81% ≥ cT3 and 58% cN+. In the 31 evaluable patients, the 3-year DFS was 79.4%; 3-year OS was 78.9%. For MSI-H patients, 3-year DFS was 81.8%. For patients with MSS/PD-L1+ tumors, 3-year DFS was 77.8%. For the 45-patient cohort, 3-year DFS was 70%; 3-year OS was 65%. Three patients had pCR, two with MSI-H tumors and one with PD-L1 CPS of 4%. Downstaging occurred in 62.2% of patients, and 40% of surgical specimens had pT0/T1 tumors consistent with significant pathologic responses. Conclusions: A biomarker-driven preoperative treatment strategy for MSI-H, EBV+, and MSS/PDL1+ operable gastric adenocarcinoma patients is promising and warrants additional evaluation. This treatment strategy is novel and effective, and it incorporates immune checkpoint immunotherapy, low dose chemotherapy, and radiation thus avoiding more intensive combination chemotherapy regimens for patients with these biomarkers. Clinical trial information: NCT03257163 .