Phase 2 Study of Telomelysin (OBP-301) in Combination With Pembrolizumab in Esophagogastric Adenocarcinoma

This is a phase II study of OBP-301 with pembrolizumab in advanced gastric and gastroesophageal junction adenocarcinoma that has progressed on at least 2 lines of prior therapy for advanced disease. Pembrolizumab has recently received FDA approval for PD-L1 positive gastric and GEJ adenocarcinoma based on the Keynote-059 study. The efficacy of pembrolizumab monotherapy is modest in PD-L1 positive patients (defined as a combined positive score, CPS, of \> 1), with only a \~15% overall response rate. This study will examine the addition of the oncolytic virus, OBP-301, administered prior to pembrolizumab in this patient population. Patients will be enrolled in a two-stage design, with 18 patients in the first stage. All patients will receive OBP-301 at 2x1012 viral particles (VP)/ tumor injection administered every two weeks x 4 injections as well as standard dose pembrolizumab 200 mg IV every 3 weeks. The tumor will be injected with OBP-301 four times (d1, d15, d29, d43). The preference is to inject the primary tumor endoscopically. Metastatic lesions may be injected on a case-by-case basis after discussion with the PI (Shah). All patients treated with OBP-301 will be eligible for the safety cohort. Primary Endpoints: * To examine the efficacy of OBP-301 with pembrolizumab in PD-L1 positive advanced gastric and gastroesophageal junction adenocarcinoma in the 3rd or 4th line setting, as assessed by the RECIST response rate. * To examine the safety of multiple OBP-301 intratumoral injections in combination with pembrolizumab in advanced gastroesophageal adenocarcinoma. Secondary Endpoint: • To examine other measures of efficacy of the combination of OBP-301 with pembrolizumab in advanced gastric and esophageal adenocarcinoma including the disease control rate, duration of response, overall survival and progression free survival.