Circulating Tumor DNA

This is a single-arm, phase II study examining elacestrant in the adjuvant treatment of patients with ER+ breast cancer who test positive for circulating tumor DNA (ctDNA) during the screening period of the trial. Our trial will proceed in three separate phases: screening, treatment, and follow-up.

Inclusion Criteria for Screening: 1. Adults aged 18 years and older. 2. Previous diagnosis of anatomic stage IIB or anatomic stage III histopathologically or cytologically confirmed ER+, HER2-, breast cancer per local laboratory as per ASCO/CAP guidelines. In the context of this trial, ER status will be considered positive if \>10% of tumor cells demonstrate positive nuclear staining by immunohistochemistry, with or without progesterone receptor positivity. Patients with PR positive but ER-negative are not eligible. 3. Participants must have been diagnosed with ER+HER2- breast cancer at least five years ago and no more than 20 years ago and must have completed adjuvant endocrine therapy. 4. Participants must be off endocrine therapy for at least four weeks prior to screening. Exclusion Criteria for Screening: 1. Known current metastatic disease. 2. Known contraindication to receiving elacestrant as per FDA package insert. 3. Current treatment with endocrine therapy. 4. Prior treatment with elacestrant or other investigational SERDs. 5. Current or past invasive cancer other than breast cancer, except: 1. Adequately treated basal or squamous cell carcinoma of the skin. 2. Cancer survivors of previously diagnosed invasive cancer who were treated with curative intent and have no evidence of disease recurrence for five years or more and are considered low risk for future recurrence by the treating physician. 6. Patients in the screening phase, or in the randomized trial (treatment phase), cannot start receiving therapy on another therapeutic clinical trial. 7. Current use of strong and moderate CYP3A4 inducers/inhibitors or other prohibited concomitant medication unless an acceptable substitute is available, and the prohibited medication is discontinued at least five half-lives prior to initiation of elacestrant. 8. Participants who are pregnant. Inclusion Criteria for Treatment: 1. ctDNA positivity by NEXT Personal assay. 2. No evidence of metastatic disease on staging scans. a. If imaging, after review with a radiologist, is low probability for metastatic disease, patients may proceed with enrollment. Patients with suspicious but inconclusive imaging results should undergo a diagnostic biopsy; if biopsy is negative patients are eligible for enrollment. Patients with positive imaging that is conclusive of metastatic disease, or biopsy proven metastatic disease, are not eligible. 3. At the time of informed consent signature for treatment, participants may be either postmenopausal, premenopausal, or perimenopausal. a. Postmenopausal status is defined by: i. Age ≥60. ii. Age \1.0 x 109/L. 2. Platelets \>100 x 109/L. 3. Hemoglobin \> 8.0 g/dL. 4. Potassium, sodium, calcium, and magnesium CTCAE v5.0 grade \50 mL/min. 6. ALT and AST \