This is a Phase 2, open-label, dose optimization/expansion study to assess the preliminary efficacy and safety of SAR445877 as a monotherapy for Chinese participants aged at least 18 years with advanced Gastric Cancer(GC)/Gastroesophageal Junction cancer (GEJ). Participants with advanced GC/GEJ who relapsed to at least 1 prior regimen which may or may not include an anti-PD1/PD-L1-based treatment depending on local standard of care, regardless combined positivity score (CPS) will be randomized in this study.

In this study, SAR445877 will be assessed as a monotherapy in approximately 30 participants with advanced unresectable or metastatic GC or Siewert Type 2 and 3 GEJ, and for whom receiving the standard of care (SOC) is not in his or her best interest, or where no SOC is established. Human epidermal growth factor receptor 2 (HER2) positive cases will not be eligible unless they have progressed on a HER2 targeted therapy. Those participants should have received at least 1 prior line of anti-cancer treatment which may or may not include an anti-PD1/PD-L1-based treatment depending on local standard of care. Metastatic microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) cases are not eligible.

Inclusion Criteria:

Age

- Participant must be at least 18 years of age inclusive (or country's legal age of majority if >18 years), at the time of signing the informed consent.

Cancer diagnosis:

• Histologically or cytologically confirmed diagnosis of advanced unresectable or metastatic GC or Siewert Type 2 & 3 GEJ.
• Participants with unknown HER2/neu status must have their HER2/neu status determined locally. Participants with HER2/neu negative are eligible. Participants with HER2/neu positive tumors must have documentation of disease progression on treatment containing an approved HER2 targeted therapy to be eligible.

Prior anticancer therapy:

- Participants should have failed or relapsed after at least 1 prior line of treatment which may or may not include an anti-PD1/PD-L1-based treatment or anti-Claudin 18.2 based treatment depending on local standard of care.

Measurable Disease:

- At least 1 measurable lesion per RECIST 1.1 criteria.

Exclusion Criteria:

Medical conditions

• Eastern Cooperative Oncology Group(ECOG)performance status of ≥2.
• Predicted life expectancy ≤3 months.
• Diagnosed of any other malignancies, either progressing or requiring active treatments, within 2 years prior to enrollment.
• Active brain metastases or leptomeningeal metastases.
• Known microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) tumor.
• History of treatment-related immune-mediated (or immune-related) AEs from immune- modulatory agents (including but not limited to anti-PD1/PD-L1 agents and anti cytotoxic T lymphocyte associated protein 4 monoclonal antibodies) that caused permanent discontinuation of the agent, or that were Grade 4 in severity, or have not resolved to Grade ≤1.
• Has any condition requiring ongoing/continuous corticosteroid therapy (>10 mg prednisone/day or an anti-inflammatory equivalent) within 1 week prior to the first dose of the study medicine.
• Any clinically significant cardiac (including valvular) or vascular (thromboembolic disorders) disease, within 6 months prior to the first IMP administration.
• Ongoing or recent (within 2 years) evidence of significant autoimmune disease that required treatment with systemic immunosuppressive treatments, which may suggest risk for immune-related AEs (irAEs).
• Has a known history or any evidence of interstitial lung disease or active, non-infectious pneumonitis within 3 years prior to the first dose of the study drug.
• Organ transplant requiring immunosuppressive treatment.
• Uncontrolled or active infection with human immunodeficiency virus (HIV ), hepatitis B, or hepatitis C infection, or has a diagnosis of immunodeficiency.

Note: Other Inclusion/Exclusion criteria may apply. The above information is not intended to contain all considerations relevant to a participant's potential participation in a clinical trial.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org