The main purpose of this study is to evaluate safety, tolerability and the efficacy of Q-1802 plus SOC compared with SOC. .Pharmacokinetics (PK) ,Pharmacodynamics (PD) of Q-1802 and the immunogenicity profile of Q-1802 will be evaluated as well.

This study is a multicenter, open label, phase Ⅰb/Ⅱ clinical study conducted in unresectable patients with advanced or recurrent metastatic Claudin18.2 positive (medium and high expression) and HER-2 negative primary gastric adenocarcinoma or gastric esophageal junction adenocarcinoma. The Phase Ib dose escalation study included two dose groups each combined with the XELOX standard treatment regimen. Perform dose escalation to obtain MTD and/or RP2D doses for combined administration. The Phase II study adopted an open label parallel randomized controlled design. Further observe the efficacy and safety of Q-1802 combined with XELOX regimen in treating patients with moderate to high expression of Claudin 18.2, and compare and analyze the efficacy and safety of Q-1802 combined with XELOX regimen and XELOX regimen alone.

Inclusion criteria：

Patients with at least one measurable lesion per RECIST (v1.1);

Patients with medium or high expression of Claudin18.2 in tumor tissue samples tested by immunohistochemistry in central laboratory were included;

Patients with untreated, unresectable advanced or metastatic gastric adenocarcinoma or gastroesophageal junction adenocarcinoma with negative HER-2 immunohistochemistry or FISH test (HER-2 immunohistochemistry 0/1+ confirmed by a qualified local or central laboratory, or 2+ confirmed negative by FISH test) were included;

Eastern Cooperative Oncology Group (ECOG) performance status score 0 or 1 at screening and no deterioration occurs within two weeks before enrollment;

Life expectancy period ≥ 12 weeks;

Patients who have sufficient baseline organ function.

Exclusion Criteria:

Receive anti-tumor treatment within 4 weeks before the first administration or within 5 half-lives of the treatment drug, whichever is shorter;

Patients who have previously used Claudin 18.2 products for treatment;

With uncontrolled diseases;

Who are allergic to the study drug or any of its components;

Patients with a history of other primary malignant tumors at the time of screening, except for cured skin Basal-cell carcinoma or Cutaneous squamous-cell carcinoma or cervical Carcinoma in situ.