A Study to Evaluate the Safety and the Activity of S095029 as Part of Combination Therapy in Advanced Gastroesophageal Junction/Gastric Cancers.

To be considered for this study, you must have an official diagnosis of stomach cancer or gastroesophageal junction cancer that is advanced, meaning it cannot be removed by surgery or has spread. Your cancer must also have a specific genetic feature called MSI-H/dMMR, which your doctor will have tested for.

There are also some reasons why you might not be able to join. For example, if you've already had more than one line of treatment for your advanced cancer, or if you've previously received certain types of immunotherapy drugs, like those that target PD-1, PDL1, or CTLA4, you would not be eligible.

Also, if you've recently had other cancer treatments, such as chemotherapy or radiation therapy (within a few weeks before the study starts), or major surgery (in the last four weeks), you wouldn't be able to begin the study until enough time has passed for your body to recover.

If you join this study, you will receive two different medications: S095029 and pembrolizumab. The first part of the study will involve regular visits to the clinic so doctors can carefully monitor how you respond to the treatment and identify the best dose of S095029 to use. This will involve blood tests, scans, and other check-ups.

Once the ideal dose is found, the study will continue with more people. You will continue to have regular clinic visits, receive your study medications, and undergo various assessments to check how your body is handling the treatment and whether the cancer is responding. These assessments will include regular scans (like CT scans) to measure the size of your tumour(s). The total time you spend on the study can vary, but doctors will continue to monitor you even after treatment, to keep an eye on your progress and any side effects.