Palazestrant and Ribociclib are medicines studied for treating advanced breast cancer that is hormone receptor-positive (ER+) and does not have too much of a protein called HER2 (HER2-). This study is comparing how well these medicines work when used together and how safe they are. It's a phase 3 trial, which means it's in the later stages of research to see if the treatment works better than current options. People in the study are split into two groups: one uses palazestrant with ribociclib and a placebo (a fake medicine), and the other uses letrozole, ribociclib, and a different placebo. The study is looking for 1,000 adults who have not yet had cancer treatment for their advanced breast cancer. Participants must have either a measurable disease, a type of bone disease, and a performance status of 0 or 1, meaning they are fully active or have some symptoms but can do light work.

• Study length and number of visits will be discussed with participants.
• Participants will receive either the new treatment or the current standard treatment.
• Potential side effects and risks will be monitored throughout the study.