This study is for people with advanced breast cancer, which means the cancer has spread to other parts of the body. The study tests a new drug, palazestrant (OP-1250), against usual treatments like fulvestrant or aromatase inhibitors (anastrozole, letrozole, exemestane). Aromatase inhibitors are drugs that lower estrogen levels in the body, and fulvestrant blocks estrogen effects. The trial is open-label, so participants know which treatment they receive. To join, participants must have ER+ (estrogen receptor-positive) and HER2- (human epidermal growth factor receptor 2-negative) breast cancer, meaning their cancer grows with estrogen but not with HER2 proteins. Participants should have had prior treatment with CDK4/6 inhibitors, which are drugs that slow cancer cell growth. The study will enroll approximately 510 participants for different parts of the trial.

• The study lasts until the required number of participants is reached.
• Participants must visit study centers for evaluations.
• Potential risks include usual side effects of cancer treatments.