This study is testing a new medicine, called AZD7789, for people with advanced cancer. It is a Phase I/IIa study, meaning it's early in testing to see if the medicine is safe and works well. The study has two parts: Part A, where they adjust the dose, and Part B, where they use the best dose found. AZD7789 is given through a vein (IV). Participants must be at least 18 years old and have certain types of lung or stomach cancer that can't be treated with surgery or radiation. They also need to provide a tissue sample for testing. The study is open-label, which means both doctors and participants know what treatment is being given. There are some rules about who can join, such as having good organ health and not being pregnant. Participants cannot join if they have certain genetic mutations, unresolved side effects from past treatments, or serious health issues. This study is essential for people with advanced cancer to consider the potential benefits and risks of trying a new treatment.

• Study involves regular hospital visits and IV medication.
• Participants must provide tumor samples.
• Potential side effects and risks involved.