STUDY OF OP-1250 ADMINSTERED AS MONOTHERAPY AND COMPARED TO STANDARD OF CARE FOR THE TREATMENT OF ER+, HER2– ADVANCED OR METASTATIC BREAST CANCER

Prior treatment

• Symptomatic visceral disease, imminent organ failure, or any disease burden that makes the participant ineligible for endocrine therapy. • Have received prior chemotherapy (including an antibody-drug conjugate) in the advanced/metastatic setting. • 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, or gastrointestinal disorders that could aff+F120ect absorption of study treatment, and others

• 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 will be allowed. • 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 (LHRH) agonist.