The purpose of this clinical trial is to evaluate whether perioperative ivonescimab in combination with S-1 and oxaliplatin (SOX) is effective in treating locally advanced gastric or gastroesophageal junction (GEJ) adenocarcinoma. The study will also assess the safety profile of this treatment regimen.

Primary Objective:

To determine whether perioperative ivonescimab plus SOX improves the pathological complete response (pCR) rate compared with SOX alone in patients with locally advanced gastric or GEJ adenocarcinoma.

Study Design:

Participants will be randomly assigned to receive either ivonescimab plus SOX or SOX alone to evaluate the potential added benefit of ivonescimab in this setting.

Participation Details:

Participants will receive the assigned treatment (ivonescimab plus SOX or SOX alone) every 21 days for approximately 4 months.

They will visit the clinic once every 3 weeks for evaluations, laboratory tests, and monitoring.

Participants will be asked to keep a daily diary to record any symptoms or side effects experienced during the study.

To evaluate the efficacy and safety of ivonescimab in combination with S-1 and oxaliplatin (SOX) for the treatment of locally advanced gastric or gastroesophageal junction (GEJ) adenocarcinoma in the neoadjuvant (perioperative) setting, with the goal of improving the pathological complete response (pCR) rate.

Inclusion Criteria:

1. Age 18-75 years.
2. Histologically confirmed gastric or gastroesophageal junction (G/GEJ) adenocarcinoma.
3. Eastern Cooperative Oncology Group (ECOG) performance status 0-1.
4. Clinically staged as T3-4a N+ M0 by computed tomography (CT) or magnetic resonance imaging (MRI).
5. Considered eligible for curative resection.
6. No prior antitumor therapy for the current disease.
7. Adequate organ function, including hepatic, renal, and bone marrow function, as per prespecified laboratory criteria.
8. Expected survival of ≥6 months.

Exclusion Criteria:

1. Known mismatch repair-deficient (dMMR) or microsatellite instability-high (MSI-H) tumor.
2. Uncontrolled hypertension, defined as systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥ 90 mmHg despite optimized antihypertensive therapy, or hypertension complicated by acute events (e.g., hypertensive crisis, hypertensive encephalopathy) that cannot be stably controlled.
3. Tumor lesions with a bleeding tendency, including but not limited to: active ulcerative tumor lesions, hematemesis within 2 months prior to informed consent, high risk of major gastrointestinal bleeding as determined by the investigator.
4. History of thromboembolic or arterial/venous vascular events within 6 months prior to enrollment, such as cerebrovascular events (including transient ischemic attack), deep vein thrombosis, or pulmonary embolism.
5. Gastrointestinal perforation or gastrointestinal obstruction within 6 months prior to enrollment.

Individual Participant Data (IPD) may not be shared due to privacy and confidentiality concerns, legal or ethical restrictions, limitations in data sharing agreements or consent, intellectual property rights, commercial interests, or because of technical and resource constraints related to data anonymization and sharing.