A Phase Ib/II Study of ATG-022 Plus Pembrolizumab With/Without Chemotherapy in Participants With Claudin (CLDN) 18.2-positive, HER2-negative, Unresectable or Metastatic Gastric or Gastroesophageal Junction AdenocarcinomaUnresectable or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma

Known active central nervous system metastases and/or carcinomatous meningitis. Participants with previously treated brain metastases may participate provided they are radiologically stable (i.e., without evidence of progression) for at least 4 weeks as confirmed by repeat imaging performed during the study screening, are clinically stable and have not required steroid treatment for at least 14 days before the first dose of study intervention.

Prior exposure to CLDN 18.2 ADC, CLDN 18.2 chimeric antigen receptor T-cell immunotherapy or agents containing MMAE.

Prior systemic anticancer therapy including investigational agents within 4 weeks before the first dose of study treatment or within a period during which the investigational product or systemic anticancer treatment has not been cleared from the body (e.g., a period of 5 'halflives'), whichever is the most appropriate as judged by the investigator.

Received any prior immunotherapy and was discontinued from that treatment due to a Grade 3 or higher irAE (except endocrine disorders that can be treated with replacement therapy) or was discontinued from that treatment due to Grade 2 myocarditis or recurrent Grade 2 pneumonitis.