This study is testing a new medicine called BL-M14D1 for people with a type of lung cancer called Small Cell Lung Cancer (SCLC) and other similar tumors called neuroendocrine tumors. Neuroendocrine tumors are rare growths that start in the hormone-producing cells. **Key Points:** - This is a Phase 1 open-label study, meaning everyone knows what treatment is being given and the study is mainly about checking safety and finding the right dose. - To join, you need to be at least 18 years old, have a certain type of cancer, and meet other health criteria. - The study will look at how safe the medicine is, how the body processes it (pharmacokinetics), and how well it works (efficacy). Participants must have tried other treatments first and need to be healthy enough to handle the study. They must agree to provide tissue samples and follow contraceptive guidelines if of childbearing potential. It's important to discuss with your doctor if this study is a good fit for you, especially considering the inclusion and exclusion criteria. Note: This study has specific health requirements and restrictions, including no recent chemotherapy or certain heart issues. Make sure to review these carefully.