A Study to Evaluate the Safety and the Activity of S095029 as Part of Combination Therapy in Advanced Gastroesophageal Junction/Gastric Cancers.

This study will investigate the safety, tolerability, and antitumor activity of S095029 (anti-NKG2A antibody) in combination with pembrolizumab in in microsatellite instability-high/Defective mismatch repair (MSI-H/dMMR) locally advanced unresectable or metastatic gastric /GEJ adenocarcinomas.

This Phase 1b/2 study will be conducted in two parts; a safety lead-in part (Phase 1b) to identify the RP2D of S095029 in combination with pembrolizumab and an expansion part (Phase 2) to evaluate anti-tumor activity and safety in participants with locally advanced unresectable or metastatic MSI-H/dMMR gastric /GEJ adenocarcinomas.

Inclusion Criteria: * Have a confirmed diagnosis of locally advanced and unresectable or metastatic gastric or gastro-esophageal junction adenocarcinoma * Participants' tumor must have an MSI-H/dMMR status according to institutional guidelines and/or according to the College of American Pathologists, determined at any time prior to enrolment. Exclusion Criteria: * Has received more than one previous line of treatment in the locally advanced and unresectable or metastatic setting. * Has received prior therapy with any checkpoint inhibitor (anti-PD-1, anti-programmed cell death ligand 1 (PDL1), anti-CTLA4). * Participants who have received prior systemic anti-cancer therapy including investigational agents within 4 weeks (shorter interval, at least 5 half-lives, for kinase inhibitors or other short half-life drugs) prior to first study treatment. * Prior radiotherapy if completed less than 2 weeks before first study treatment * Major surgery less than 4 weeks prior to the first study treatment or participants who have not recovered from the side effects of the surgery.