This study will evaluate the efficacy and safety of perioperative mFOLFOX plus Pembrolizumab combination regimen in participants with potentially resectable adenocarcinoma of the GEJ and stomach.~The study is a non-randomized, open-label, single-arm phase II study.~The enrolled participants will receive neoadjuvant combination of mFOLFOX every 2 weeks for 4 doses (on Days 1, 15, 29, 43) and Pembrolizumab every 3 weeks for 3 doses (on Days 1, 22, 43).~Serious adverse events (SAEs) will be assessed on an ongoing basis using CTCAE v4.0, and the Thall, Simon, and Estey's design to monitor the efficacy and toxicity continuously together.~Repeat PET-CT will be obtained approximately 2-3 weeks after completion of neoadjuvant combination therapy.~If no evidence of metastatic disease on PET-CT, participants will undergo potentially curative surgical resection 4-6 weeks after completion of neoadjuvant combination therapy. This will be followed by adjuvant combination therapy (to be started 6-8 weeks after surgery) consisting of mFOLFOX every 2 weeks for additional 4 doses (total 4 months of therapy) plus Pembrolizumab every 3 weeks for additional 12 doses (total 1 year of therapy). If there is evidence of metastatic disease on PET-CT, participant will come off the study (i.e., will not undergo surgical resection and adjuvant therapy).~Participants will continue to receive treatment until either one of the following occurs: completion of adjuvant therapy, development of radiographically confirmed progression, participant withdraws consent, intercurrent illness that prevents further administration of treatment, or sponsor-investigator decides to withdraw the participant.~Efficacy outcomes during the adjuvant chemotherapy phase will be determined by radiologic measurements by CT using RECIST v1.1. Assessment of response will be performed every 3 months for the first year and as per the standard institutional guidelines thereafter.
To evaluate the antitumor activity and safety/tolerability of the combination (mFOLFOX + Pembrolizumab) in patients with potentially resectable adenocarcinoma of the Gastroesophageal Junction (GEJ) and stomach.