This study will assess the efficacy and safety of SHR-A1811 compared with treatment chosen by the investigator in participants with HER2-positive (defined as immunohistochemistry \[IHC\] 3+ or IHC 2+/in situ hybridization \[ISH\]+) gastric or GEJ adenocarcinoma (based on \[American Society of Clinical Oncology (ASCO) College of American Pathologists (CAP) guidelines who have progressed on or after a first-line anti-HER2 therapy-containing regimen.
Age 18-75 years old, male and female;
Histologically or cytologically confirmed gastric or gastroesophageal junction adenocarcinoma, and unresectable locally advanced or metastatic disease
Prior anti-HER-2 containing treatment
Progression on or after first-line standard treatment (Prior neoadjuvant or adjuvant therapy can be counted as a line of therapy if the subject progressed on or within 6 months of completing neoadjuvant or adjuvant therapy);
Receive the last dose of anti-cancer therapy(including chemotherapy, radiotherapy, biological therapy, targeted therapy or immunotherapy) within 4 weeks, prior to the first dose;
Known allergies to monoclonal antibodies and inactive ingredients of this product, and allergies to paclitaxel, docetaxel, and irinotecan concurrently;
The toxicity of prior anti-tumor therapy did not recover to the level specified by CTCAE v5.0 grade evaluation ≤ Grade 1 or inclusion/exclusion criteria;
Clinically active central nervous system metastases;