This study is testing two investigational treatments, BNT323 and BNT327, to find the best dose for treating advanced breast cancer. The study has two parts. In Part 1, doctors will try different doses to find the right amount. In Part 2, they will test the best dose combination to see how well it works. Breast cancer types being studied include hormone receptor-positive, HER2-positive, HER2-low, HER2-ultralow, and triple-negative.

• This study is for people with advanced, difficult-to-treat breast cancer.
• Participants will be closely monitored to ensure their safety.
• Those taking part may receive either the combination therapy or one of the drugs alone.

To join, participants must have measurable disease (cancer that can be measured by doctors) and a heart function test showing a certain level of health. People with certain health issues or who have taken specific medications recently may not be eligible. The study is conducted across multiple sites, so participants will need to visit a study location for treatments and check-ups. Being part of this study could help in finding better treatments for breast cancer.