According to the World Health Organization, breast cancer is the most common cancer in women and the global incidence is expected to increase in the coming years.

The major cause of death from breast cancer is metastases, most commonly to the bone, liver, lung and brain. Approximately 30% of early-stage patients develop metastatic disease, and of those patients 90% relapse during the course of their treatment. About 5% of patients have distant metastases at the time of diagnosis. Patients with metastatic breast cancer have a five-year survival rate of only 25%, compared with a greater than 90% five-year survival rate for patients with only local disease at diagnosis. Importantly, even patients without metastases at diagnosis are at risk for developing metastases over time.

Approximately 70% of breast cancers express the estrogen receptor (ER) and depend on estrogen signaling for growth and survival. The standard of care for ER+ advanced/metastatic breast cancer calls for endocrine therapy at all stages of treatment, with patients typically cycling through multiple anti-estrogen therapies, such as aromatase inhibitors (AIs), selective estrogen receptor modulators (SERMs), and selective estrogen receptor degraders (SERDs).