This clinical trial is for people with a specific type of breast cancer called ER+/HER2- that has spread or is locally advanced. The study wants to see if a new drug pair, lasofoxifene and abemaciclib, is better than the current treatment, fulvestrant and abemaciclib. Participants will be either taking lasofoxifene and abemaciclib or fulvestrant and abemaciclib. Lasofoxifene is a pill taken daily, while fulvestrant is a shot given on specific days, and abemaciclib is a pill taken twice daily. This study is looking for pre- and postmenopausal women and men who have an ESR1 mutation and have tried treatments like palbociclib or ribociclib before.

• Study Length: The treatment requires regular doses and check-ups, which could take several months.
• Participation Visits: Regular visits are needed for treatment and monitoring.
• Eligibility: Must meet specific health criteria and cannot have certain conditions or previous treatments.