A Study of Zolbetuximab (IMAB362) Plus CAPOX Compared With Placebo Plus CAPOX as First-line Treatment of Subjects With Claudin (CLDN) 18.2-positive, HER2-negative, Locally Advanced Unresectable or Metastatic Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma (GLOW).

Zolbetuximab is being studied in people with cancer in and around the stomach or where the food pipe (esophagus) joins the stomach, called GEJ cancer. Most people with this type of cancer have a protein called Claudin 18.2 in their tumor. Zolbetuximab is thought to work by attaching to the Claudin 18.2 protein in their tumor, which switches on the body's immune system to attack the tumor. There is an unmet medical need to treat people with advanced stomach cancer or GEJ cancer. This study will give more information about how well zolbetuximab works when given with chemotherapy in adults with advanced stomach cancer or GEJ cancer. In this study, adults with advanced stomach cancer or GEJ cancer will either be given zolbetuximab with chemotherapy or a placebo with chemotherapy. A placebo looks like zolbetuximab but doesn't have any medicine in it. Zolbetuximab with chemotherapy has already been approved to treat stomach cancer and GEJ cancer in some countries. This study is being done in countries where zolbetuximab has not yet been approved for use. If zolbetuximab becomes approved for use in those countries taking part in this study, the people taking part in those countries will leave this study and receive licensed zolbetuximab. The main aim(s) of the study is(are) to determine the efficacy of zolbetuximab combined with chemotherapy compared to a placebo combined with chemotherapy in treating adults with Claudin 18.2-positive, HER2-negative, locally advanced unresectable or metastatic gastric or GEJ adenocarcinoma. Adults with locally advanced unresectable or metastatic stomach cancer or GEJ cancer can take part. Locally advanced means the cancer has spread to nearby tissue. Unresectable means the cancer cannot be removed by surgery. Metastatic means the cancer has spread to other parts of the body. A tumor sample of their cancer will also have the Claudin 18.2 protein. They may have been previously treated with certain standard therapies but have not been treated with chemotherapy for their cancer. People cannot take part if they need to take medicines to suppress their immune system, have blockages or bleeding in their gut, have specific uncontrollable cancers such as symptomatic or untreated cancers in the nervous system, or have a specific heart condition, or infections. The study treatments are either zolbetuximab with chemotherapy or placebo with chemotherapy. People who take part will receive just one of the treatments by chance. Study treatment will be double-blinded. That means that the people in the study and the study doctors will not know who takes which of the study treatments. Study treatment will be given in cycles. The study treatment is given to people slowly through a tube into a vein. This is called an infusion. The chemotherapy is called CAPOX (capecitabine and oxaliplatin) and will be given as an infusion and also as tablets. People will have 1 infusion of either zolbetuximab or placebo together with oxaliplatin chemotherapy in 3-week (21-day) cycles. People will also take 1 tablet of capecitabine (chemotherapy) twice a day for the first 2 weeks (14 days) of each cycle. People may receive zolbetuximab or placebo until their cancer worsens, they cannot tolerate the treatment, or they need to start another cancer treatment. People will receive CAPOX for up to about 6 months (8 treatment cycles). After the 6 months, people may receive capecitabine chemotherapy only, until their cancer worsens, they cannot tolerate the study treatment, or they need to start another cancer treatment. People will visit the clinic on certain days during their treatment. The study doctors will check if people had any medical problems from zolbetuximab or the other study treatments. Also, people in the study will have health checks. On some visits, they will have scans to check for any changes in their cancer. People will have the option of giving a tumor sample after their study treatment has finished. People will visit the clinic within 7 days after they stop their study treatment. People will be asked about any medical problems and will have a health check. People who start treatment with licensed zolbetuximab will not need to attend the clinic for further visits and will receive standard of care health checks. People who continue study treatment will visit the clinic at 1 and 3 months after they stop their study treatment. They will continue to have scans every 9 or 12 weeks to check for any changes in their cancer. They will have telephone health checks every 3 months. The number of visits and checks done at each visit will depend on the health of each person and whether they completed their treatment or not.