Retlirafusp Alfa or Adebrelimab Plus SOX as Neoadjuvant Therapy for HER2-Negative Resectable Gastric or Gastroesophageal Junction Adenocarcinoma

This multicenter, randomized, open-label, exploratory phase II study aims to evaluate the efficacy and safety of perioperative Retlirafusp Alfa plus SOX versus adebrelimab plus SOX in adult patients with previously untreated, HER2-negative, resectable locally advanced gastric or gastroesophageal junction adenocarcinoma (clinical stage II-III, AJCC 8th edition). Approximately 98 participants will be randomized at a 1:1 ratio via stratified block randomization. Stratification factors include tumor location (gastric versus gastroesophageal junction) and PD-L1 combined positive score (CPS \

Inclusion Criteria 1. Voluntary signed written informed consent. 2. Age ≥ 18 years. 3. Histologically-confirmed gastric / gastroesophageal junction adenocarcinoma; Siewert II / III are eligible. 4. Resectable clinical stage II-III (cT1-2N+M0, cT3-4aNanyM0, AJCC 8th edition) confirmed by CT / MRI. 5. No prior anti-cancer therapy (surgery, radiotherapy, chemotherapy, targeted or immunotherapy). 6. Investigator-assessable for R0 resection; planned curative surgery after neoadjuvant therapy. 7. Able to swallow oral tablets. 8. ECOG-PS 0-1. 9. Available PD-L1 CPS result or 3-5 unstained tumor slides for testing (fresh or pre-randomization FFPE tissue acceptable). 10. Estimated life expectancy ≥ 12 months. 11. Adequate organ function (no transfusion / G-CSF in prior 14 d): * ANC ≥ 1.5 × 10⁹/L; platelets ≥ 80 × 10⁹/L; Hb ≥ 90 g/L * Total bilirubin \< 1.5 × ULN; ALT/AST ≤ 2.5 × ULN * Serum Cr ≤ 1.5 × ULN or CrCl \> 50 mL/min 12. Women of child-bearing potential: negative serum pregnancy test ≤ 72 h pre-randomization; effective contraception (longer of 3 months post-last retlirafusp alfa / adebrelimab or 6 months post-chemotherapy). Fertile-partner males: surgical sterility or same contraception window; no sperm donation during study. Exclusion Criteria 1. Known HER2-positive tumor. 2. Anticipated transthoracic surgical approach. 3. Unresectable disease, surgical contraindication or refusal of surgery. 4. Other malignancy within 5 years (exception: cured basal-cell skin, cervical / breast carcinoma-in-situ). 5. Laparoscopy shows peritoneal metastasis, positive peritoneal cytology (CY1P0) or AJCC 8th T4b disease. 6. Cardiac abnormalities: NYHA ≥ II heart failure or LVEF \< 50 %; unstable angina; MI within 1 year; QTc prolongation (male\>450 ms, female\>470 ms); significant ECG abnormality / high-grade AV-block / PR\>250 ms; risk factors for QTc prolongation. 7. Gastrointestinal obstruction signs/symptoms within prior 6 months (except symptoms resolved by definitive surgery). 8. Coagulopathy (INR\>2.0, PT\>16 s), bleeding diathesis or ongoing thrombolytic / anticoagulant therapy (prophylactic low-dose aspirin / LMWH allowed). 9. Clinically-significant bleeding tendency within 3 months pre-randomization. Persistently positive fecal occult blood requires gastroscopy. 10. GI perforation or fistula within prior 6 months. 11. Arterial / venous thromboembolic event within prior 6 months (superficial phlebothrombosis excluded). 12. Hereditary / acquired bleeding or thrombotic diathesis. 13. Active ulcer, unhealed wound or fracture. 14. Major vascular disease within prior 6 months. 15. CTCAE\>grade 2 severe infection within 4 weeks pre-randomization; active pulmonary inflammation; ongoing need for systemic antibiotics (≤48 h prophylactic IV antibiotics permitted). 16. Active hepatitis B (HBsAg+ plus HBV-DNA ≥ 2000 IU/mL) or active hepatitis C. 17. Congenital / acquired immunodeficiency including HIV infection. 18. Prior or planned organ / allogeneic bone-marrow transplantation. 19. Interstitial lung disease / pneumonitis (current or steroid-treated history); pulmonary fibrosis, pneumoconiosis, active tuberculosis or severe pulmonary impairment on screening CT. 20. Active autoimmune disease or high-risk relapse history (exceptions: hormone-replaced hypothyroidism; controlled type 1 diabetes; stable vitiligo, psoriasis, resolved childhood asthma; asthma requiring bronchodilators is excluded). 21. Systemic immunosuppressive corticosteroids (\>10 mg prednisone-equivalent daily) within 7 days pre-randomization. 22. Live-attenuated vaccine within 28 days pre-randomization or planned during-/within 60 days post-study-treatment. 23. Hypersensitivity to any study drug or excipients. 24. Investigational-drug clinical trial participation within prior 4 weeks. 25. Pregnancy, lactation or planned pregnancy during study treatment. 26. Investigator-judged other conditions interfering with study conduct or participant safety (substance abuse, severe psychiatric illness etc).