This study tests the safety and effect of a drug called HER3-DXd in adults with a type of lung cancer called Non-Small Cell Lung Cancer (NSCLC) that has spread or cannot be removed with surgery. In the first part, called Dose Escalation, different doses of HER3-DXd are given to find the safest amount. In the second part, called Dose Expansion, the drug is given to more people to see how well it works against the cancer. The study lasts about 36 months, but the number of treatment cycles isn't fixed—participants can stop if they choose to, if their cancer worsens, or if side effects are too severe. To join, you should have NSCLC that has grown after previous treatments. The study will cover some costs, but you might face common risks from cancer treatments like side effects. Discuss with your doctor if you meet the criteria and if this study is a good fit.

• The study length is about 36 months, but treatment cycles vary.
• HER3-DXd is tested to find the safest dose and see if it shrinks tumors.
• Participants must have a type of lung cancer that has spread or can't be surgically removed.