This study tests if two medicines, **propranolol** and **pembrolizumab**, can help treat a tough type of breast cancer called **triple negative breast cancer** (TNBC). TNBC is a kind of cancer that does not respond to some treatments and can spread to other body parts. **Propranolol** is a medicine that affects the heart and blood flow. It might help make cancer treatments work better. **Pembrolizumab** is a type of **immunotherapy**, which helps the body's immune system fight cancer. This study aims to see if propranolol can make the immune system more responsive to pembrolizumab. Patients will take propranolol by mouth and receive pembrolizumab through an IV. They will also have CT scans, blood tests, and might have a piece of the tumor removed for study.
- Study involves taking medicines, having scans, and some tests.
- Participants must be okay with biopsies before and during the study.
- Participants must have already tried certain treatments without success.