Adebrelimab Plus Apatinib Combined With SOX Regimen as Conversion Therapy for Gastric Cancer

Inclusion Criteria: * Age 18-75 years at the time of enrollment, with an estimated life expectancy of ≥ 3 months. * Histologically or cytologically confirmed gastric cancer or gastroesophageal junction cancer, predominantly adenocarcinoma. * Unresectable locally advanced gastric or gastroesophageal junction adenocarcinoma (Stage III or IV), as determined by the investigator based on CT, MRI, and/or PET-CT. * Disease with conversion (translational) therapeutic potential, as assessed by the investigator. * No prior anti-tumor treatment, including chemotherapy, radiotherapy, targeted therapy, immunotherapy, or other systemic anticancer therapies. * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2. * Adequate hematologic function within 14 days prior to enrollment: * White blood cell count ≥ 3.5 × 10⁹/L. * Absolute neutrophil count ≥ 1.5 × 10⁹/L. * Hemoglobin ≥ 90 g/L (9.0 g/dL). * Platelet count ≥ 100 × 10⁹/L. * Adequate hepatic function within 14 days prior to enrollment:Total bilirubin ≤ 1.5 × upper limit of normal (ULN);ALT and AST ≤ 2.5 × ULN in patients without liver metastases;ALT and AST ≤ 5 × ULN in patients with liver metastases;Adequate renal function:Serum creatinine ≤ 1.5 × ULN;Women of childbearing potential must have a negative serum or urine pregnancy test within 7 days prior to enrollment and agree to use effective contraception during the study and for at least 12 weeks after the last dose. * Male participants must be surgically sterile or agree to use effective contraception during the study and for at least 12 weeks after the last dose * Ability to understand and willingness to sign a written informed consent form * Expected to comply with study procedures and follow-up requirements Exclusion Criteria: * HER2-positive gastric or gastroesophageal junction adenocarcinoma. * Conditions that may significantly affect oral drug absorption, including inability to swallow, persistent nausea or vomiting, chronic diarrhea, or intestinal obstruction. * Known hypersensitivity or allergy to adebrelimab, apatinib, oxaliplatin, S-1 (tegafur/gimeracil/oteracil), or any of their excipients. * History of severe allergic reactions to monoclonal antibodies. * Active autoimmune disease or autoimmune disorders requiring systemic treatment. * Congenital or acquired immunodeficiency. * Use of systemic immunosuppressive therapy within 14 days prior to the first dose of study treatment. * Administration of live attenuated vaccines within 4 weeks prior to the first dose or planned during the study period. * Severe infection within 4 weeks prior to initiation of study treatment. * History of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation. * Evidence of interstitial lung disease, including pulmonary fibrosis, interstitial pneumonia, pneumoconiosis, radiation pneumonitis, drug-induced pneumonitis, or severely impaired pulmonary function. * Uncontrolled hypertension despite at least 3 months of antihypertensive treatment. * Uncontrolled clinically significant cardiovascular disease. * High risk of severe bleeding, as judged by the investigator. * Peripheral neuropathy of Grade \> 2 according to CTCAE. * Participation in another interventional clinical trial within 4 weeks prior to enrollment or within 5 half-lives of the investigational product, whichever is longer. * Any condition that, in the investigator's judgment, makes the participant unsuitable for study participation.