This is a Phase III, randomized, multicenter, open-label clinical trial designed to compare RC48-ADC to physician choice standard treatment in participants with human epidermal growth factor receptor 2 (HER2)-overexpression locally advanced or metastatic gastric cancer.

This study is a phase III multi-center, randomized, open-label, parallel control study to evaluate the efficacy and safety of recombinant humanized anti-HER2 monoclonal antibody-Monomethyl auristatin E (MMAE) conjugate for the treatment of HER2-overexpression locally advanced or metastatic gastric cancer. The HER2-overexpression is defined as: the HER2 IHC 3+ or 2+, regardless of FISH status.

Inclusion Criteria:

• Voluntary agreement to provide written informed consent.
• Male or female, Age ≥ 18 years.
• Predicted survival ≥ 12 weeks.
• Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.
• All female subjects will be considered to be of child-bearing potential unless they are postmenopausal, or have been sterilized surgically.Female subjects of child-bearing potential must agree to use two forms of highly effective contraception. Male subjects and their female partner who are of child-bearing potential must agree to use two forms of highly effective contraception.
• Willing to adhere to the study visit schedule and the prohibitions and restrictions specified in this protocol.
• Adequate organ function.
• All subjects must have inoperable, advanced or metastatic gastric or or gastroesophageal adenocarcinoma
• Have had progression or intolerance following receipt of at least two systemic chemotherapy for advanced or metastatic disease.
• The HER2 IHC test result is IHC 2+ or IHC 3+, the subject's previous test results (confirmed by the investigator) or the research center's test results are acceptable; the subject can provide the Specimen of primary or metastatic tumor for HER2 review/judgment.
• HER2 classic positive (definition: IHC3+ or IHC2+FISH+) and patients who have previously failed standard treatment
• According to the RECIST 1.1 standard, there is at least one measurable lesion.

Exclusion Criteria:

• Known hypersensitivity to Recombinant Humanized Anti-HER2 Monoclonal Antibody-MMAE Conjugate For Injection.
• History of receiving any anti-cancer drug/biologic treatment within 4 weeks prior to trial treatment.
• History of major surgery within 4 weeks of planned start of trial treatment.
• Has received a live virus vaccine within 4 weeks of planned start of trial treatment.
• Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the subject's participation for the full duration of the trial, or is not in the best interest of the subject to participate, in the opinion of the treating investigator.
• History of other malignancy within the previous 5 years, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, or cancers with a similar curative outcome as those mentioned above.
• Assessed by the investigator to be unable or unwilling to comply with the requirements of the protocol.