Tumor biopsy Ki-67 expression (% of tumor cells that are positive for Ki-67) at baseline and Cycle 1 Day 15 (C1D15) was collected. Ki-67 expression was assessed by immunohistochemical staining in a central laboratory. The log-transformed Ki-67 after approximately 2 weeks of treatment as a percentage of the baseline value, ie, the ratio between the Ki-67 measurements obtained from C1D15 visit and baseline was modelled using a generalized linear model (GLM) with both stratification factors (ie, baseline Ki-67 score and the tumor size) and treatment as co-variates. The treatment effects were back transformed into geometric means and their Confidence Intervals. The percent change, in other words, relative reduction, of Ki-67 after 2 weeks of treatment is reported as the complement of the ratio between the Ki-67 measurement from C1D15 and baseline, that is 100% × (1 - geometric mean ratio between Ki-67 at C1D15 and Ki-67 at baseline).

Ki-67 Evaluable Set included all enrolled participants who were randomized and received at least one dose of study treatment and had evaluable central Ki-67 measurements other than '0' or '< 1' from baseline and evaluable Ki-67 measurements from C1D15 visits.

An AE is any untoward medical occurrence in a participant or clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of study intervention. A serious adverse event (SAE) was an AE that resulted in any of the following outcomes: death; life threatening; persistent/significant disability/incapacity; initial or prolonged inpatient hospitalization; congenital anomaly/birth defect or was otherwise considered medically important. A TEAE is an AE that emerges or worsens on/after the first dose of ARV-471/Anastrozole to 30 days after the last administration of the study intervention (ie, study drug treatment or surgical resection, whichever occurs last).

Safety Analysis Set which consisted of all enrolled participants who received at least 1 dose of study intervention.

Local pathological assessment of the tissue from surgical resection (performed after approximately 5.5 months of treatment), at minimum, included pathologic stage (ypT and ypN stage) as described in the Laboratory Manual. Participants were analysed based on the current American Joint Committee on Cancer (AJCC) staging system as follows:

• Pathologic Tumor - post-neoadjuvant therapy pathologic tumor categorization (ypT): (ypTx, ypT0, ypTis, ypT1mi, ypT1a, ypT1b, ypT1c, ypT2, ypT3, ypT4a, ypT4b, ypT4c).
• Pathological Lymph Nodes - post-neoadjuvant therapy pathologic node categorization (ypN): (ypNX, ypN0, ypN0(i+), ypN0(mol+), ypN1, ypN1mi, ypN1a, ypN1b, ypN1c, ypN2, ypN2a, ypN2b, ypN3, ypN3a, ypN3b, ypN3c).
• ypT0 / ypN0 indicates no evidence of disease and the progressive grades indicate increasing size of tumor and increasing area of lymph node involvement respectively and ypTx/ypNX indicates non-measurable disease.

Full Analysis Set included all the enrolled participants who were randomized.

pCR is defined as no invasive cancer in the breast and sampled axillary lymph nodes following completion of neoadjuvant systemic therapy (ie, Pathologic Tumor - ypT = ypT0 or ypTis, and Pathologic Lymph Nodes - ypN = ypN0 in the current American Joint Committee on Cancer (AJCC) staging system). pCR rate is the percentage of participants with pCR.

Full Analysis set included all the enrolled participants who were randomized.

Modified Pre-operative Endocrine Prognostic Index (mPEPI) score is an investigational prognostic tool used to predict the risk of breast cancer recurrence. It will be derived from factors assigned a numerical score following Neoadjuvant endocrine treatment (NET). The factors include pathologic tumor size, and lymph node status and Ki67 expression in the surgical specimen. Total mPEPI score (mPEPI_T) per participant is the sum of mPEPI score of each factor.

mPEPI score of 0 indicates Pathological tumor size T1-T2, no lymph nodes and Ki67 level of 0%-2.7%, 1 indicates: Ki67 level >2.7%-7.3%, 2 indicates Ki67 level >19.7%-53.1% and 3 indicates: tumor sizeT3-T4, presence of lymph nodes and Ki67 level >53.1%.

Full Analysis Set included all the enrolled participants who were randomized.

Breast conserving surgery (BCS) Rate is the percentage of participants received breast conserving surgery.

Full Analysis Set included all the enrolled participants who were randomized.

The number of participants with Complete Response (CR), Partial Response (PR), Stable Disease (SD), Progressive Disease (PD), Not Evaluable (NE) per mRECIST calculated. CR = disappearance of all target lesions, PR is >=30% decrease in sum of diameters of target lesions, progressive disease (PD) is >=20% increase in sum of diameters of target lesions, stable disease (SD) is <30% decrease or <20% increase in sum of diameters of target lesions.

Full Analysis Set included all the enrolled participants who were randomized.

The percentage change from the baseline of the primary breast tumor size in physical exam calculated in caliper measurement. Caliper-based response is the maximum percentage decrease or minimum percentage increase if there is no decrease per participant.

Full Analysis Set included all the enrolled participants who were randomized. Here "Overall Number of Participants Analyzed" signifies those participants who were evaluable for this outcome measure.