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.

Qualified scientific and medical researchers can request access to anonymized patient-level and study-level clinical trial data.

Access can be requested for all interventional clinical studies:

• used for Marketing Authorization (MA) of medicines and new indications approved after 1 January 2014 in the European Economic Area (EEA) or the United States (US).
• where Servier is the Marketing Authorization Holder (MAH). The date of the first MA of the new medicine (or the new indication) in one of the EEA Member States will be considered for this scope.

In addition, access can be requested for all interventional clinical studies in patients:

• sponsored by Servier
• with a first patient enrolled as of 1 January 2004 onwards
• for New Chemical Entity or New Biological Entity (new pharmaceutical form excluded) for which development has been terminated before any Marketing authorization (MA) approval.

After Marketing Authorization in EEA or US if the study is used for the approval.

Researchers should register on Servier Data Portal and fill in the research proposal form. This form in four parts should be fully documented. The Research Proposal Form will not be reviewed until all mandatory fields are completed.