A Multicenter, Phase 3 Study of IBI343 Monotherapy Versus Treatment of Investigator's Choice in Subjects With Previously Treated, Claudin (CLDN) 18.2-positive, HER2-negative, Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria:

1. Able and willing to sign a written Informed Consent Form (ICF) and to comply withprotocol-specified visits and related procedures.

2. Has histopathologically confirmed unresectable locally advanced or metastaticadenocarcinoma of the gastric/gastroesophageal junction (G/GEJ AC).

3. Has received and progressed on at least 2 lines of systemic therapy (anti-PD-(L)1 incombination with platinum or fluoropyrimidines, paclitaxel/docetaxel, irinotecan). Aprior (neo)adjuvant systemic therapy that ended within 6 months prior to diseaserelapse is defined as the first line therapy. The subject has ≤ 4 prior lines ofsystemic therapy.

4. Has histopathologically confirmed CLDN18.2-positive disease.

5. Is a man or woman of 18 years of age or older at the time of signing the ICF.

6. Has an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.

Exclusion Criteria:

1. Has HER2-positive (defined as immunohistochemistry [IHC] 3+, or IHC 2+ and positiveby in situ hybridization) disease.

2. Is currently participating in another interventional clinical study, except when thesubject is during survival follow-up of an interventional clinical study.

3. Has a history of treatment with topoisomerase inhibitor-based antibody-drugconjugate(s).

4. Has received the last dose of an anti-cancer therapy (including traditional Chinesemedicine indicated for gastric cancer in the package insert, but excluding herbalprescriptions) within 4 weeks or 5 half-lives (whichever is shorter) prior to thefirst dose of study treatment.

5. Plans to receive other anti-cancer therapy during treatment with the study drug (palliative radiotherapy for symptomatic (e.g., pain) relief that does not affectresponse assessment is allowed).