This study was a single-arm, open, multi-center Phase Ib/II clinical trial to observe and evaluate the efficacy and safety of RC48-ADC plus S-1 in first-line treatment of advanced gastroesophageal adenocarcinoma with moderate expression of HER2.

Inclusion Criteria:

• At the same time, patients voluntarily participated in the study and signed informed consent;
• Either male or female, aged 18 or older;
• Patients diagnosed by pathological or cytological diagnosis of gastric cancer (GC), gastroesophageal junction carcinoma (GEJ) or esophageal adenocarcinoma had evidence of local advanced lesions or metastases that could not be surgically resected, and were mostly adenocarcinoma confirmed by histological examination;
• No previous systemic therapy; Or had received neoadjuvant/adjuvant chemotherapy but experienced disease progression or recurrence 6 months after the end of treatment;
• HER2 IHC 2+ and FISH-;
• ECOG scores 0-1;
• Estimated survival ≥3 months;
• Women of reproductive age had to undergo a pregnancy test (serum or urine) which was negative within 7 days of enrollment, and volunteer to use an appropriate method of contraception during the observation period and for 12 weeks after the last study drug was given. For men, surgical sterilization or consent to use appropriate methods of contraception during the observation period and for 12 weeks after the last administration of the study drug;
• Patients who comply are expected to be able to follow up on therapeutic outcomes and adverse reactions as required by the regimen.

Exclusion Criteria:

• Five years before first use of the study drug has been diagnosed as other malignant tumor, the effective treatment of basal cell carcinoma, squamous cell carcinoma of the skin and/or the effective removal of cervical cancer in situ and/or except breast cancer;
• Known hypersensitivity to RC48-ADC;
• HBV DNA>500 IU/ mL (or 2000 copies /ml), HCV RNA>103 copies /ml, HBsAg+ and anti-HCV antibody positive;
• History of HIV infection;
• History of receiving any anti-cancer drug/biologic treatment within 4 weeks prior to trial treatment;
• Assessed by the investigator to be unable or unwilling to comply with the requirements of the protocol.