The goal of this clinical trial is to learn if givastomig in combination with standard therapy works to treat adults with cancer in the stomach and/or esophagus (GEA adenocarcinoma). It will also help the researchers to learn more about the safety of givastomig. The main questions it aims to answer are:

• Does the addition of givastomig to standard therapy increase the amount of time that participants survive without progression of their cancer?
• What toxicities do participants experience when taking givastomig?

Participants may be able to take part in the study if they have unresectable or metastatic GEA and if their cancer cells express certain proteins called Claudin 18.2 (CLDN18.2) and PD-L1. Participants whose cancer cells express a protein called HER2 cannot take part.

Up to 180 participants will be randomly assigned to received givastomig at one of two doses in combination with an immunotherapy medicine called nivolumab and chemotherapy OR to receive nivolumab and chemotherapy alone. These therapies will be given primarily via intravenous (into a vein) infusion every 2 or 3 weeks.

Participants will:

• Visit the study treatment center for infusions and/or check-ups and tests every 1-3 weeks
• Report any changes in their symptoms to their study doctors
• Have scans to check for any changes in their cancer every 8-12 weeks

This is a randomized, global, open-label, multicenter Phase 2 study evaluating the efficacy and safety of givastomig (TJ033721) in combination with nivolumab and chemotherapy compared with nivolumab and chemotherapy alone in participants with previously untreated, HER2-negative, CLDN18.2-positive, and PD-L1-positive locally advanced, unresectable, or metastatic gastroesophageal adenocarcinoma (GEA).

Approximately 180 participants will be randomized in a 1:1:1 ratio to one of three treatment arms. Two investigational arms will receive givastomig in combination with nivolumab and chemotherapy, and the control arm will receive nivolumab and chemotherapy alone. Chemotherapy will consist of either modified FOLFOX (mFOLFOX) or CAPOX, administered according to local standard of care. Participants enrolled in the United States, Japan, and South Korea will receive mFOLFOX only. Randomization will be stratified by chemotherapy regimen (mFOLFOX vs CAPOX) and by CLDN18.2 expression level (<75% vs ≥75% of tumor cells with membrane intensity score ≥2+).

Participants in the investigational arms will receive givastomig administered every 2 weeks or every 3 weeks, depending on the chemotherapy regimen. Tumor assessments will be performed at protocol-defined intervals and evaluated according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1.

Enrollment of participants with high CLDN18.2 expression (defined as membrane intensity score ≥1+ in ≥75% of tumor cells) will be capped at approximately 50% of the total study population. Enrollment of participants receiving CAPOX will be capped at approximately 30% of the total study population.

Study treatment will continue until disease progression, unacceptable toxicity, withdrawal of consent, death, or completion of study treatment, whichever occurs first. The duration of chemotherapy treatment will follow the respective product labeling or local standards of care.

Inclusion Criteria:

• Histologically confirmed unresectable, locally advanced, or metastatic gastric, gastroesophageal junction (GEJ), or esophageal adenocarcinoma (EAC).

• Treatment-naïve for advanced/metastatic disease (prior adjuvant/neoadjuvant therapy allowed if ≥6 months since last dose).

• CLDN18.2 positive (membrane intensity score ≥1+ on ≥1% of tumor cells).

• PD-L1 positive (CPS ≥1).

• At least 1 measurable lesion per RECIST v1.1.

• ECOG performance status 0 or 1.

• Adequate organ function, including:

  • Hematologic: WBC ≥2,000/μL; ANC ≥1,500/μL; platelets ≥100,000/μL; hemoglobin ≥9 g/dL
  • Hepatic: AST/ALT ≤3×ULN (≤5×ULN if liver metastases); bilirubin ≤1.5×ULN (≤3×ULN if Gilbert's)
  • Renal: Creatinine ≤1.5×ULN or eGFR ≥50 mL/min/1.73 m²

• Life expectancy ≥90 days.

• Women of childbearing potential (WOCBP) and men must use effective contraception during the study and for a defined period after treatment.

• Willing and able to provide informed consent and comply with study procedures

Exclusion Criteria:

• HER2-positive tumors.
• Second malignancy within 3 years, except certain skin or cervical cancers.
• Active or unstable gastrointestinal ulcer or bleeding within 6 weeks.
• Active autoimmune disease requiring systemic therapy within past 2 years or ongoing immunosuppressive therapy.
• Active pneumonitis or history requiring steroids/immunosuppressive therapy within 3 years.
• Participation in another therapeutic clinical trial.
• Major surgery or significant injury within 4 weeks prior to first dose, or planned major surgery within 6 months.
• Radiotherapy within protocol-specified timeframes without adequate recovery.
• Active CNS metastases or carcinomatous meningitis (previously treated brain metastases allowed if stable).
• Significant cardiovascular disease (NYHA Class 3-4 CHF, recent MI, unstable angina, TIA/stroke, or major cardiac procedures within 6 months).
• Active or uncontrolled HIV, hepatitis B, or hepatitis C infection, or immunodeficiency (controlled infection allowed).
• Receipt of live vaccine within 30 days or other vaccines within 7 days of first dose.
• Active infection requiring parenteral therapy.
• Known hypersensitivity to study drug components (e.g., DPD deficiency).
• Any other condition or laboratory abnormality that, in the investigator's judgment, increases risk or interferes with study participation.