Inclusion Criteria:
* Voluntarily participate in the clinical study; fully understand and are informed about the study and sign the Informed Consent Form (ICF); willing to follow and able to complete all trial procedures.
* Age 18-80 years, gender is not limited.
* Patients with locally advanced unresectable, recurrent unresectable, or metastatic gastric cancer (GC) or gastroesophageal junction cancer (GEJC) confirmed by imaging and other examinations, and histopathologically confirmed as adenocarcinoma.
* Provide a report confirming HER2 overexpression or amplification negativity; defined as IHC 0/1+, or IHC 2+ with FISH/ISH negative.
* No prior systemic therapy for advanced or metastatic GC/GEJC (including anti-HER-2 therapy). Patients who have received adjuvant or neoadjuvant therapy (including chemotherapy, radiotherapy, or chemoradiotherapy) for GC/GEJC are eligible if the time to first recurrence or disease progression is greater than 6 months from the end of the last treatment. Prior use of anti-tumor Traditional Chinese Medicine preparations is allowed but must be discontinued at least 2 weeks before enrollment.
* ECOG performance status score of 0 or 1.
* Must have at least one measurable lesion according to RECIST v1.1 definitions.
* All acute toxicities caused by prior anti-tumor therapy or surgery must have resolved to Grade 0-1 (according to NCI CTCAE v5.0) or to the level specified in the inclusion/exclusion criteria. Alopecia, fatigue, and hearing loss, or other toxicities considered by the investigator not to pose a safety risk to the subject, are excluded.
* Adequate organ function (laboratory tests within 7 days prior to treatment):
* Hematology (No blood transfusion, G-CSF use, or drug correction within 14 days prior to screening):
* White blood cell count (WBC) ≥ 3,000/mm³ (3.0 × 10⁹/L);
* Absolute neutrophil count (ANC) ≥ 1,500/mm³ (1.5 × 10⁹/L);
* Platelet count (PLT) ≥ 100,000/mm³ (100 × 10⁹/L);
* Hemoglobin (Hb) ≥ 9.0 g/dL (90 g/L).
* Biochemistry (No albumin transfusion within 14 days prior to screening):
* Albumin ≥ 3.0 g/dL (30 g/L);
* Creatinine ≤ 1.5 × Upper Limit of Normal (ULN) or Creatinine Clearance ≥ 50 ml/min (calculated using the Cockcroft-Gault formula);
* Total Bilirubin (BIL) ≤ 1.5 × ULN;
* Aspartate Aminotransferase (AST) and Alanine Aminotransferase (ALT) levels ≤ 2.5 × ULN; for patients with liver metastases, ≤ 5 × ULN.
* Coagulation: International Normalized Ratio (INR) ≤ 1.5, Prothrombin Time (PT), and Activated Partial Thromboplastin Time (APTT) ≤ 1.5 × ULN.
* Urine: Urine protein \< 2+; if urine protein is ≥ 2+, 24-hour urine protein quantification must be ≤ 1g.
* Life expectancy ≥ 3 months.
* Women of childbearing potential must undergo a serum or urine pregnancy test within 7 days before starting treatment, with a negative result, and must not be lactating. All enrolled patients must use adequate barrier contraception throughout the treatment cycle and for 6 months after the end of treatment.
Exclusion Criteria:
* Known squamous cell carcinoma, undifferentiated carcinoma, other histological types of gastric cancer, or mixed gastric cancer containing other histological types.
* Active malignant tumors within the past 2 years, other than the tumor under study. Exceptions include subjects with locally curable cancers (that have been cured), such as basal or squamous cell skin cancer, superficial bladder cancer, and carcinoma in situ of the cervix or breast.
* Participation in a study of an investigational drug or receipt of investigational treatment or use of an investigational device within 4 weeks prior to the first dose.
* Enrollment in another clinical study, unless it is an observational, non-interventional clinical study or the follow-up period of an interventional study (defined as \>4 weeks since the last dose of the previous clinical study or \>5 half-lives of the study drug).
* Untreated Central Nervous System (CNS) metastases, or uncontrolled or symptomatic active CNS metastases. Patients with fully treated CNS metastases may be enrolled if neurological symptoms have returned to baseline levels at least 4 weeks prior to enrollment (excluding residual signs or symptoms related to CNS treatment). Additionally, subjects must have discontinued corticosteroids or be on a stable or tapering dose of prednisone ≤ 10 mg/d (or equivalent dose of other corticosteroids) for at least 4 weeks prior to enrollment.
* Pleural effusion or ascites that remains uncontrolled despite puncture and drainage within 14 days prior to enrollment; symptomatic or moderate-to-large pericardial effusion.
* Weight loss of more than 20% within 2 months prior to enrollment.
* Received the following treatments or medications prior to enrollment:
* Major surgery within 28 days prior to enrollment (tissue biopsy for diagnosis and PICC/port implantation are allowed).
* Use of immunosuppressive drugs within 14 days prior to enrollment, excluding nasal/inhaled corticosteroids or physiological doses of systemic steroids (i.e., ≤ 10 mg/d prednisone or equivalent).
* Vaccination with live attenuated vaccines within 28 days prior to enrollment or planned during the study period and within 60 days after the end of study drug treatment.
* Local anti-tumor therapy (e.g., radiotherapy or tumor embolization) within 28 days prior to enrollment.
* Diagnosed with any other malignant tumor within 5 years prior to entering the study, except for cured cutaneous basal cell carcinoma or squamous cell carcinoma, superficial bladder cancer, carcinoma in situ of the cervix, ductal carcinoma in situ of the breast, and papillary thyroid carcinoma amenable to local treatment.
* Presence of any active, known, or suspected autoimmune disease. Subjects in a stable state not requiring systemic immunosuppressive therapy are allowed, such as Type I diabetes, hypothyroidism requiring only hormone replacement therapy, and skin diseases not requiring systemic treatment (e.g., vitiligo, psoriasis, alopecia).
* Prior treatment with any anti-PD-1, anti-PD-L1, anti-CTLA-4 antibody, or any other antibody or drug targeting T-cell co-stimulation or checkpoint pathways.
* Significant clinical bleeding symptoms or definite bleeding tendency within 3 months prior to enrollment; gastrointestinal perforation and/or fistula within 6 months prior to enrollment; arterial/venous thromboembolic events within 6 months prior to enrollment, such as cerebrovascular accident (including TIA, cerebral infarction), deep vein thrombosis, and pulmonary embolism (except for those with gastric cancer bleeding/perforation where symptoms disappeared after surgical resection).
* Major vascular disease within 6 months before the start of study treatment (e.g., aortic aneurysm requiring surgical repair or recent peripheral arterial thrombosis).
* Severe, unhealed, or dehisced wounds, active ulcers, or untreated fractures.
* Presence of \> Grade 1 peripheral neuropathy.
* History of intestinal obstruction and/or clinical signs or symptoms of gastrointestinal obstruction within 6 months before starting study treatment, including incomplete obstruction related to the underlying disease or requiring routine parenteral hydration, parenteral nutrition, or tube feeding. Patients with incomplete obstruction/obstruction syndrome at initial diagnosis who have symptom relief after definitive (surgical) treatment may be enrolled.
* Interstitial lung disease, non-infectious pneumonitis, or uncontrolled systemic diseases (e.g., diabetes, hypertension, pulmonary fibrosis, acute pneumonia, etc.).
* Known history of active tuberculosis.
* Known allergy to the study drug or any of its excipients; or history of severe allergic reactions to other monoclonal antibodies.
* HIV infection or known Acquired Immunodeficiency Syndrome (AIDS); untreated active hepatitis (Hepatitis B defined as HBV-DNA ≥ 500 IU/ml; Hepatitis C defined as anti-HCV positive and HCV RNA above the lower limit of detection) or co-infection with Hepatitis B and C.
* Poorly controlled clinical cardiac symptoms or diseases, such as: (1) NYHA Class II or higher cardiac insufficiency or LVEF \< 50% on echocardiography; (2) Severe/unstable angina; (3) Myocardial infarction within 12 months prior to enrollment; (4) Clinically significant supraventricular or ventricular arrhythmias requiring treatment or intervention; (5) Symptomatic congestive heart failure; (6) QTc \> 480 ms.
* Systemic use of antibiotics ≥ 7 days within 4 weeks prior to enrollment, or unexplained fever \> 38.5°C during screening/before first dose (fever due to tumor causes is allowed per investigator judgment).
* Known history of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation.
* Participation in any other drug clinical study within 4 weeks or 5 half-lives prior to enrollment.
* Known history of psychotropic drug abuse or drug addiction.
* Presence of other severe physical or mental illnesses or laboratory abnormalities that may increase the risk of participation, interfere with study results, or render the patient unsuitable for the study in the opinion of the investigator.
