This study is testing a new drug called DB-1303/BNT323 against regular chemotherapy for people with a type of breast cancer. This cancer is called HER2-low, hormone receptor-positive metastatic breast cancer. Metastatic means the cancer has spread to other parts of the body. The study checks if the new drug helps people live longer without their cancer getting worse. To join, you must be at least 18 years old. You should have already tried at least two different treatments without success, and your cancer must have specific traits (HER2-low and hormone receptor-positive). You can't join if you've had certain heart or lung diseases or have been treated with some specific cancer drugs before.

• The study will take place in many locations worldwide and involves regular check-ups.
• You'll either get the new drug or a standard chemotherapy treatment.
• Before joining, make sure you can comply with the study rules and have a life expectancy of 12 weeks or more.

Participation could help find better treatments for this type of cancer, but it's important to understand the potential risks and benefits before joining. You need to follow rules on contraception and can't join if you have certain health issues.