To evaluate efficacy and safety of Neoadjuvant of Sintilimab Combined Weekly Metronomic Chemotherapy (PLOF) in resectable locally advanced gastric cancer.

This is a single-arm clinical study to enroll 50 patients with gastric cancer (cTNM diagnosis of cT3-4aN1-3M0). Each enrolled patient will be assigned a case number. Both this case number and the patient's initials will be entered on each page of the case report form.

Enrolled patients receive a neoadjuvant regimen of POLF in combination with sindilizumab: preoperatively, they receive a POLF regimen (paclitaxel 60 mg/m2, oxaliplatin 50 mg/m2, and 5-fluorouracil 425 mg/m2) administered once weekly for a total of 6 doses, and in combination with sindilizumab 200 mg intravenously once every 3 weeks for a total of 2 doses. Upon completion of the evaluation, patients whose tumors were judged to be resectable underwent radical surgery and received six postoperative doses of the POLF regimen and two doses of Sindilizumab as adjuvant therapy.

Postoperative imaging evaluations will be performed every three months until disease recurrence. Survival follow-up was performed every three months after disease recurrence. Patients will receive neoadjuvant therapy for 6 weeks preoperatively and adjuvant therapy for 6 weeks postoperatively unless intolerable toxicity occurs, the patient refuses to continue treatment, or treatment is delayed beyond 3 weeks. Patients will be under study observation during treatment and 30 days after treatment termination, and will receive long-term follow-up for 5 years postoperatively. Ultimately, pCR and MPR will be the primary study endpoints, and ORR, DCR, 2-year PFS rate, 3-year OS rate and safety will be the secondary study endpoints to evaluate the efficacy and safety of the neoadjuvant regimen of POLF combined with sindilizumab, as well as to explore the immune activation effect and mechanism of the regimen using peripheral blood and tumor tissue samples.

Inclusion Criteria:

1. Signed written Informed Consent Form

2. Male or female, age ≥ 18 years old

3. Histologically confirmed gastric adenocarcinoma, diagnosed as locally progressive according to the AJCC 8th ed, cTNM diagnosis of cT3-4aN1-3M0 and resectable lesion as assessed by the investigator

4. No prior systemic therapy such as surgery, radiotherapy, or immunotherapy for the disease at hand

5. Consent to radical surgical treatment and no contraindications to surgery as determined by the surgeon

6. ECOG PS: 0-1 score

7. Expected survival > 6 months

8. Adequate organ function, must meet the following laboratory specifications:

   8.1 Absolute neutrophil count (ANC) ≥ 1.0x10^9/L; 8.2 Platelets ≥ 80x10^9/L; 8.3 Hemoglobin > 7g/dL; 8.4 Total bilirubin ≤ 1.5 x upper limit of normal (ULN) (Total bilirubin > 1.5 x ULN but direct bilirubin ≤ ULN are allowed to be enrolled); 8.5 AST, ALT ≤ 2.5×ULN; 8.6 Blood creatinine ≤ 1.5 x ULN or creatinine clearance ≥ 60 ml/min; 8.7 INR or PT ≤ 1.5 times ULN; 8.8 TSH within normal range (Enrollment allowed if baseline TSH is outside normal range but FT4 is within normal range); 8.9 Myocardial enzyme profile within normal range;

9. Negative pregnancy test in women of childbearing age

10. Need to use contraception with an annual failure rate of less than 1% if there is a risk of conception

Exclusion Criteria:

1. Endoscopically show signs of active bleeding from the lesion
2. Current participation in an interventional clinical study or treatment with another investigational drug or use of an investigational device within 4 weeks prior to the first dose of study drug
3. Prior therapy with anti-PD-1, anti-PD-L1, or anti-PD-L2 agents, or agents targeting CTLA-4, OX-40, CD137, etc.
4. Diagnosis of a malignant disease other than gastric cancer within 5 years prior to the first dose of therapy
5. Active autoimmune disease requiring systemic therapy within 2 years prior to the first dose of the drug
6. Live vaccination within 30 days prior to the first administration of the drug
7. Have received systemic systemic therapy with proprietary Chinese medicines with antitumor indications or immunomodulatory drugs within 2 weeks prior to the first administration of the drug
8. Have received systemic glucocorticoid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of treatment
9. Has not fully recovered from any intervention-induced toxicity and/or complications (excluding malaise or alopecia) prior to initiation of therapy
10. Known allogeneic organ transplantation (except corneal transplantation) or allogeneic hematopoietic stem cell transplantation
11. Known hypersensitivity to drugs used in this study
12. Known history of HIV infection
13. Untreated active hepatitis B
14. Active HCV infection
15. Pregnant or lactating women
16. The presence of any serious or uncontrolled systemic disease
17. Other factors that, in the judgment of the investigator, may affect the outcome of the study