A Phase II, Open-Label, Multicenter Trial Comparing Disitamab Vedotin Plus Sintilimab and S-1 With Trastuzumab Plus Chemotherapy ± Sintilimab for First-Line Treatment of HER2-Positive Advanced Gastric or Gastroesophageal Junction Adenocarcinoma (RCTS2)

• Aged18-75 years, gender is not limited;

• Pathologically confirmed locally advanced gastric or gastroesophageal junction adenocarcinoma that is inoperable or has distant metastasis;

• HER2-Positive (IHC3+or IHC2+/FISH+) ;

• Has at least 1 measurable lesion as determined by RECIST 1.1;

• There is no systematic treatment in the past, or the patient has received neoadjuvant/adjuvant chemotherapy, but the disease progresses or relapses more than 6 months after the end of treatment;

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1;

• Adequate organ function;

• The life expectancy is at least 3 months;