Zolbetuximab is being studied in people with cancer in and around the stomach or where the food pipe (esophagus) joins the stomach, called gastroesophageal junction (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 gastric 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 study doctor will switch study treatment in those countries to the licensed zolbetuximab. If this happens, people taking part in those countries will leave this study and receive licensed zolbetuximab. The main aim of the study is to check if zolbetuximab and chemotherapy can prevent or delay the worsening of people's gastric cancer and GEJ cancer compared to placebo and chemotherapy. Adults with advanced 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 1 of the study 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. People will have 4 infusions in 6-week (42-day) cycles as follows: * Zolbetuximab or placebo - 2 infusions in a cycle. * Chemotherapy (called modified FOLFOX6 or mFOLFOX6) - 3 infusions in a cycle. The first infusion is combined with zolbetuximab or placebo on day 1 of each cycle. People may receive zolbetuximab or placebo until their cancer worsens, they cannot tolerate the study treatment, or they need to start another cancer treatment. People will receive mFOLFOX6 for up to 6 months (4 study treatment cycles). After the 6 months people may receive chemotherapy containing folinic acid and fluorouracil instead of mFOLFOX6. People may receive folinic acid and fluorouracil chemotherapy for more than 6 months, or 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 study treatment. The study doctors will check if people had any medical problems from taking 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 after they stop their study treatment. People who start treatment with licensed zolbetuximab or mFOLFOX6 outside of this study will not need to visit the clinic. People will be asked about any medical problems and will have a health check. People will visit the clinic at 1 month after they stop their study treatment. People will continue to have scans every 9 or 12 weeks to check for any changes in their cancer. People 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 study treatment or not.
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Progression Free Survival (PFS)
Timeframe: From date of randomization until the date of first documented radiological progression or date of death from any cause, whichever occurred first (up to 62 months and 18 days)