OP-1250 (Palazestrant) vs. Standard of Care for the Treatment of ER+/HER2- Advanced Breast Cancer

Key inclusion criteria: * Adult female or male participants. * ER+, HER2- locally advanced or metastatic breast cancer that is not amenable to curative therapy. * Evaluable disease (measurable disease or bone-only disease). * Previously received a CDK4/6 inhibitor in combination with an endocrine therapy in the advanced setting. One additional line of ET as a monotherapy is allowed. Duration of the most recent prior ET must be at least 6 months. * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. * Adequate hematologic, hepatic, and renal functions. * Female participants can be pre-, peri- or postmenopausal. * Male and pre- or peri-menopausal female participants must be willing to take a GnRH (or LHRH) agonist. Key exclusion criteria: * Symptomatic visceral disease, imminent organ failure, or any other reason that makes the participant ineligible for endocrine monotherapy. * Previously received chemotherapy in the advanced/metastatic setting. * Previously received treatment with elacestrant or an investigational estrogen receptor-directed therapy. * History of allergic reactions to study treatment. * Any contraindications to the selected standard-of-care endocrine therapy in the local prescribing information. * Symptomatic central nervous system metastases, carcinomatous meningitis, leptomeningeal disease, or a spinal cord compression that require immediate treatment. * Clinically significant comorbidities such as significant cardiac or cerebrovascular disease, gastrointestinal disorders that could affect absorption of study treatment.