A Trial Using ARV-471 or Anastrozole in Post-Menopausal Women With Breast Cancer Prior to Surgery

Inclusion Criteria:

* Post-menopausal females ≥ 18 years.
* Histologically or cytologically confirmed ER+ and HER2- breast cancer (per local assessment). ER and HER2 status must be documented:

  * ER+ disease, with ER staining of ≥ 10% of tumor cell nuclei by immunohistochemistry (IHC) per American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines.
  * HER2- disease by either IHC or in situ hybridization per ASCO/CAP guidelines.
  * Ki-67 score ≥ 5%, analyzed locally.
* Clinical T1c-T4c, N0-N2, M0 breast cancer amenable to definitive surgical resection, without bilateral breast ductal carcinoma in situ or invasive breast cancer.
* The primary tumor must be at least 1.5 cm by imaging.
* Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
* Willingness to undergo a screening biopsy, an on-treatment biopsy and surgical resection.

Exclusion Criteria:

* Any other active malignancy within 3 years prior to enrollment, except for adequately treated basal cell or squamous cell skin cancer, or cervical carcinoma in situ.
* Any of the following in the previous 6 months: Myocardial infarction; Severe unstable angina; Coronary/peripheral artery bypass graft; Symptomatic congestive heart failure (New York Heart Association class III or IV); Cerebrovascular accident; Transient ischemic attack; Symptomatic pulmonary embolism or other clinically significant episode of thromboembolism.
* Any of the following in the previous 6 months: Congenital long QT syndrome; Torsade de Pointes; Sustained ventricular tachyarrhythmia and ventricular fibrillation; Left anterior hemiblock (bifascicular block); Ongoing cardiac dysrhythmias of NCI CTCAE ≥ Grade 2; Atrial fibrillation of any grade (≥ Grade 2 in the case of asymptomatic lone atrial fibrillation).
* corrected QT (Fridericia method) (QTcF) \> 470 msec.
* Active, uncontrolled bacterial, fungal or viral infection, including (but not limited to) hepatitis B virus (HBV), hepatitis C virus (HCV), and known human immunodeficiency virus (HIV) or acquired immunodeficiency syndrome (AIDS)-related illness.
* Active inflammatory gastrointestinal disease, chronic diarrhea, known uncontrolled diverticular disease, or previous gastric resection or lap band surgery.
* Cirrhosis meeting criteria for Child Pugh B and C.
* Prior treatment for breast cancer including systemic therapy (eg, chemotherapy, hormonal therapy), radiation, surgery, or any investigational agents.
* Any live vaccines within 14 days of planned start of first dose of study drug.
* Major surgery (as defined by the Investigator) within four weeks of first dose of study drug.