A PHASE 3 RANDOMIZED, OPEN-LABEL STUDY OF OP-1250 MONOTHERAPY VS STANDARD OF CARE FOR THE TREATMENT OF ER+, HER2– ADVANCED OR METASTATIC BREAST CANCER FOLLOWING ENDOCRINE AND CDK 4/6 INHIBITOR THERAPY (OPERA-01)

The purpose of this study is to learn more about the investigational study drug called OP- 1250 (palazestrant) on participants with ER+, HER2- advanced or metastatic breast cancer. OP-1250 is considered endocrine therapy (treatment that adds, blocks, or removes hormones). Estrogen is a type of hormone naturally produced by the body that helps develop and maintain female sex characteristics and the growth of bones. Estrogen can also cause certain types of breast cancer to multiply, grow, and move to different parts of the body. Estrogen works in the body by binding to proteins called estrogen receptors (ERs) which are found inside the cells of female reproductive tissue, breast tissue, and other types of tissue, and some cancer cells. OP-1250 belongs to groups of drugs known as complete estrogen receptor antagonists (CERANs) and selective ER degraders (SERDs). CERANs are drugs that completely blocks estrogen receptors, preventing estrogen from attaching to the receptor and carrying out its usual functions. SERDs are drugs that bind to and break down estrogen receptors.

Are you Eligible? (Inclusion Criteria)

• Participant is ≥ 18 years old. - Cannot be currently pregnant, breast feeding or planning to become pregnant - Any toxicities from prior anti-cancer therapy must have resolved