Hormone-receptor positive (HR+) breast cancer accounts for between 60-80% of all breast cancer cases, with the HR+/HER2- subtype having been identified as the most predominant in Western countries.
Approximately 30% of individuals diagnosed with HR+ breast cancer will eventually experience relapse with metastatic disease. Endocrine therapy targeting the estrogen receptor has historically been a fundamental component of treatment in the metastatic setting. First-line treatment previously recommended for post-menopausal women with HR+/HER2- metastatic breast cancer included either aromatase inhibitors or selective estrogen receptor degraders. Resistance to endocrine therapy in women with HR+ metastatic disease frequently occurs, however, with a number of different mechanisms being implicated. This has prompted the need for newer treatment options.