This study is testing a new treatment for people with advanced breast cancer. The treatment combines two parts: **CLBR001**, which is a special type of cell therapy called a CAR-T cell (these are cells that are altered to better fight cancer), and **ABBV-461**, a molecule that helps control how the CAR-T cells work. Together, they are called a **switchable CAR-T cell therapy**.

Patients will receive one dose of the CLBR001 cells and then several doses of the ABBV-461. Doctors will check regularly to make sure the treatment is safe and see how well it is working against the cancer.

• The study involves regular visits to the doctor for safety checks and to see how the cancer is responding.
• Participants must be 18 or older and have tried all standard treatments before joining the study.
• People with recent serious infections or certain other health conditions cannot join the study.