This study is for adults with advanced **small cell lung cancer** or **neuroendocrine tumors** that test positive for a marker called **DLL3**. Neuroendocrine tumors are a type of cancer affecting cells that release hormones into the blood. The study aims to find the safest dose of a new drug called **BI 764532**, which is a special molecule designed to help the immune system fight cancer. Participants will receive BI 764532 either weekly or every three weeks for up to three years, as long as it's helpful and tolerable. There are about 20 visits to the study site. Doctors will monitor for any side effects and overall health during the study. **Key Points:** - The study lasts up to 3 years with about 20 visits. - Participants receive BI 764532 weekly or every 3 weeks. - No prior clinical data on BI 764532 exists, so risks are unknown. Participants must have advanced cancer and no other treatments available. They should not have been treated with similar therapies before. Women who are pregnant or plan to become pregnant can't join.