This study is testing a new treatment called elacestrant for patients with advanced breast cancer that has spread (metastatic). It is a two-part study: Phase 1b checks the safe dose of elacestrant with other medicines like alpelisib, everolimus, palbociclib, abemaciclib, and ribociclib. Phase 2 tests how well these combinations work. There are different groups of patients (treatment arms), each receiving elacestrant with different medicines. Up to 400 patients will participate.

To join, you must be 18 or older and have a specific type of breast cancer that is ER+/HER2- (certain proteins in cancer cells). You should also be in good health, without certain conditions like brain cancer or severe infections. Pregnant women and those unable to take oral medicines cannot join. The study will observe how safe and effective the treatment is.

• The study involves multiple visits to the hospital.
• Participants must not have had certain prior treatments.
• There may be risks and side effects from the treatment.