Neoadjuvant SHR-A1811 and SHR-A1701 ± Pertuzumab for Potentially Immunotherapy-Sensitive GC or GEJ Adenocarcinoma

This is a prospective, single-center, randomized controlled phase II clinical trial.It plans to enroll a total of 24 eligible patients with locally advanced gastric/gastroesophageal junction adenocarcinoma who are HER2-positive and have potential benefit from immunotherapy (PD-L1 CPS≥1, EBV-positive, or dMMR/MSI-H). All patients were randomly assigned to the following two groups in a 1:1 ratio. Group A:Trastuzumab Rezatecan Dosage: 4.8 mg/kg, intravenous infusion Frequency: Once every 3 weeks (Q3W) Retlirafusp alfa (SHR-1701) Dosage: 30 mg/kg, intravenous infusion Frequency: Once every 3 weeks (Q3W) Group B: Trastuzumab Rezatecan Dosage: 4.8 mg/kg, intravenous infusion Frequency: Once every 3 weeks (Q3W) Retlirafusp alfa (SHR-1701) Dosage: 30 mg/kg, intravenous infusion Frequency: Once every 3 weeks (Q3W) Pertuzumab biosimilar (Betta Pharma): 840mg at first usage; subsequent maintenance: 420mg per dose,Q3W The neoadjuvant treatment phase will consist of 4 cycles, with each cycle lasting 21 days, for a total treatment duration of approximately 12 weeks. Imaging examinations will be performed after 2 cycles. Preoperative Assessment and Surgery Within 4 weeks after the completion of neoadjuvant therapy, patients will undergo imaging examinations for surgical feasibility assessment. Those eligible for surgery will undergo radical gastrectomy (D2 lymphadenectomy) 4-6 weeks after the last dose. The endpoints are efficacy and safety, specifically including: Primary Endpoint: pCR rate (the percentage of subjects achieving pathological complete response, defined as the absence of residual viable tumor cells in the primary tumor bed); Secondary Endpoints: MPR rate (the percentage of subjects achieving major pathological response, defined as ≤10% residual viable tumor cells in the tumor bed); EFS (Event-Free Survival, defined as the time from the initiation of treatment to the first occurrence of any of the following events: disease progression precluding surgical res