Pembrolizumab, Capecitabine, and Radiation Therapy in Treating Patients With Mismatch-Repair Deficient and Epstein-Barr Virus Positive Gastric Cancer

This phase II trial studies how well pembrolizumab works with capecitabine and radiation therapy in treating patients with mismatch repair deficient and Epstein-Barr virus positive gastric cancer. Monoclonal antibodies, such as pembrolizumab may interfere with the ability of tumor cells to grow and spread. Drugs used in chemotherapy, such as capecitabine, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Radiation therapy uses high energy x-rays to kill tumor cells and shrink tumors. Giving pembrolizumab, capecitabine and radiation therapy may work better at treating gastric cancer.

PRIMARY OBJECTIVES: I. To assess efficacy (disease-free survival) of operable gastric cancer treated with PD-1 blockade using pembrolizumab. SECONDARY OBJECTIVES: I. To characterize the safety and tolerability of pembrolizumab in the preoperative setting and postoperative setting with chemoradiation. II. To evaluate response rates, recurrence rates, and patterns of recurrence/metastasis. III. To characterize adverse events (AE) of pembrolizumab in combination with radiation therapy and capecitabine. IV. To evaluate overall survival rates. TERTIARY OBJECTIVES: I. To assess T cell responses and pathological responses in the tumor specimen. II. To correlate PD-L1 expression in tumor tissue and stroma with tumor tissue response. III. To evaluate ribonucleic acid (RNA) expression via Nanostring technology with tumor tissue response. OUTLINE: Patients receive pembrolizumab intravenously (IV) over 30 minutes on day 1. Treatment repeats every 21 days for up to 2 courses in the absence of disease progression or unacceptable toxicity. Within 2-6 weeks, patients undergo surgery. Beginning up to 56 days after surgery, patients receive pembrolizumab IV over 30 minutes on day 1 and capecitabine orally (PO) twice daily (BID) on days 1-14. Treatment repeats every 21 days for up to 5 courses in the absence of disease progression or unacceptable toxicity. Within 2-6 weeks of resting, patients continue to receive pembrolizumab IV over 30 minutes on day 1. Treatment repeats every 21 days for up to 11 courses in the absence of disease progression or unacceptable toxicity. Beginning course 4, patients undergo radiation therapy over 15-30 minutes on days 1-5 for up to 5 weeks. After completion of study treatment, patients are followed up every 12 weeks for 1 year, every 16 weeks for 2 years, every 4 months for year 2, and every 6 months for 2 years.