This study focuses on patients with previously untreated Advanced or Metastatic Gastric, Gastroesophageal Junction, or Esophageal Adenocarcinoma. This refers to a type of cancer found in the stomach, the area where the esophagus meets the stomach, or the lower part of the esophagus that has spread to other parts of the body. The purpose of the study is to compare the effectiveness and safety of different treatment combinations for this cancer.

Participants may receive a combination of the study drug pumitamig (also referred to as BNT327) and chemotherapy. The chemotherapy medications involved include oxaliplatin, fluorouracil, and capecitabine, sometimes used alongside calcium folinate. In the first part of the study, two different doses of pumitamig are tested to see which one works best. In the second part, the more effective dose is compared to a standard treatment consisting of nivolumab and chemotherapy.

The study is conducted in stages. Initially, the goal is to determine which dose of the new medicine is most effective at controlling tumor growth while minimizing side effects. Following this, the selected dose is evaluated to see if it can extend progression-free survival, which is the length of time a patient lives with the disease without it getting worse, and overall survival, which is the total length of time a patient remains alive.