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: Co…