The purpose of this study is to visualize and quantify ER-binding sites during treatment with Elacestrant (RAD1901)

The purpose of this study is to visualize and quantify ER-binding sites during treatment with Elacestrant (RAD1901) using 16α-18F-Fluoro-17β-estradiol (FES) positron emission tomography (PET) imaging

Key Inclusion Criteria:

1. Patients with histologically-proven, ER-positive, human epidermal growth factor receptor 2 (HER2)-negative, locally advanced, inoperative, and/or mBC
2. Tumor progression after ≥ 6 months of at least 1 line of hormonal systemic treatment (SERM, SERD, or aromatase inhibitor) in the metastatic setting
3. Measurable disease according to Response Evaluation Criteria in Solid Tumours (RECIST) criteria v1.1 or clinically evaluable disease
4. Greater than or equal to 18 years of age
5. Patients must be post-menopausal
6. Life expectancy >3 months

Key Exclusion Criteria:

1. Greater than 3 lines of endocrine therapy for metastatic disease.

2. Prior anti-cancer treatment or investigational drug therapy within the following windows:

   1. Tamoxifen or fulvestrant therapy < 42 days before 1st 18FES-PET scan
   2. Any other anti-cancer endocrine therapy < 14 days before 1st dose of study drug
   3. Any chemotherapy < 28 days before 1st dose of study drug
   4. Any investigational drug therapy < 28 days or 3 half-lives (whichever is longer) prior to the 1st dose of study drug 2. Patients with untreated or symptomatic central nervous system (brain) metastases 3. Patients with known endometrial disorders, including evidence of endometrial hyperplasia, dysfunctional uterine bleeding, or cysts