PFS assessed by BICR was defined as the time from the date of randomization to the date of the first documentation of objective progression of disease (PD) per RECIST v1.1, or death due to any cause, whichever occurred first. PD as per RECIST v1.1 was defined as at least a 20% increase in the sum of diameters of target measurable lesions above nadir (smallest sum observed considering baseline and all assessments prior to the timepoint under evaluation), with a minimum absolute increase of 5 millimeters (mm) relative to nadir or unequivocal progression of existing non-target lesions or the presence of new lesions. PFS was censored on the date of last adequate disease assessment for those who did not have a PFS event, discontinued the study treatment due to withdrawal of consent prior to an event, started a new anticancer therapy prior to an event, had an event after a gap of 2 or more missing disease assessments, or lost to follow-up. Kaplan-Meier method was used.

Full analysis set (FAS) included all enrolled participants who were randomized.

PFS assessed by BICR was defined as the time from the date of randomization to the date of the first documentation of objective PD per RECIST v1.1, or death due to any cause, whichever occurred first. PD as per RECIST v1.1 was defined as at least a 20% increase in the sum of diameters of target measurable lesions above nadir (smallest sum observed considering baseline and all assessments prior to the timepoint under evaluation), with a minimum absolute increase of 5 mm relative to nadir or unequivocal progression of existing non-target lesions or the presence of new lesions. PFS was censored on the date of last adequate disease assessment for those who did not have a PFS event, discontinued the study treatment due to withdrawal of consent prior to an event, started a new anticancer therapy prior to an event, had an event after a gap of 2 or more missing disease assessments, or lost to follow-up. Kaplan-Meier method was used.

FAS included all enrolled participants who were randomized. Here, "Overall Number of Participants Analyzed" signifies participants with known ESR1 mutation-positive breast cancer.

OS was defined as the time from date of randomization to date of death due to any cause. In case of no death, OS time would be censored on the date participant was last known to be alive.

OS was defined as the time from date of randomization to date of death due to any cause. In case of no death, OS time would be censored on the date participant was last known to be alive.

OR was defined as the best overall response of confirmed complete response (CR) or partial response (PR) by BICR assessment as per RECIST v1.1 criteria. As per RECIST v1.1, CR was defined as complete disappearance of all target lesions, with the exception of nodal disease, complete disappearance of all non-target lesions and no new lesions. All nodes decreased to normal (short axis <10 mm); all target lesions and disease sites were assessed. PR was defined as at least a 30% decrease from baseline in the sum of diameters of all target lesions, non-PD/not evaluated for the non-target lesions and no new lesions. The short diameter was used in the sum for nodal target lesions, while the longest diameter was used in the sum for non-nodal target lesions, all target lesions were assessed.

FAS included all enrolled participants who were randomized. Here, "Overall Number of Participants Analyzed" included the participants with measurable disease at baseline (last available assessments collected on or prior to randomization date).

CBR: percentage of participants with clinical benefit response. Clinical benefit response: confirmed CR or PR at any time, or stable disease (SD) >=24 weeks per RECIST v1.1. CR: complete disappearance of all target lesions, of all non-target lesions and no new lesions; except for nodal disease, all nodes decreased to normal (short axis <10 mm); all disease sites, all target lesions assessed. PR: >=30% decrease from baseline in sum of diameters of all target lesions, non-PD/not evaluated for non-target lesions and no new lesions; all target lesions assessed. SD: did not qualify for CR, PR, PD. All target lesions assessed. PD per RECIST v1.1: at least 20% increased sum of diameters of target measurable lesions above nadir (smallest sum observed considering baseline and all assessments prior to timepoint under evaluation), with minimum absolute increase of 5mm relative to nadir or unequivocal progression of existing non-target lesions, or new lesions.

FAS included all enrolled participants who were randomized. Here, "Overall Number of Participants Analyzed" signifies number of participants evaluable for this outcome measure.

DOR was defined as time from first documentation of objective tumor response (CR or PR) to first documentation of PD, or death due to any cause, whichever occurred first. CR: complete disappearance of all target lesions, of all non-target lesions and no new lesions; except for nodal disease, all nodes decreased to normal (short axis <10 mm); all disease sites, all target lesions assessed. PR: >=30% decrease from baseline in sum of diameters of all target lesions, non-PD/not evaluated for non-target lesions and no new lesions; all target lesions were assessed. The short diameter is used in the sum for nodal target lesions, while longest diameter is used in sum for non-nodal target lesions, all target lesions were assessed. DOR was analyzed in participants with an OR. PD: at least 20% increase in sum of diameters of target measurable lesions above nadir, with minimum absolute increase of 5 mm relative to nadir or unequivocal progression of existing non-target lesions, or new lesions.

FAS included all enrolled participants who were randomized. Here, "Overall Number of Participants Analyzed" included the participants evaluable for this outcome measure.

Adverse event (AE): any untoward medical occurrence in clinical study participant, temporally associated with use of study treatment, whether or not considered related to study treatment. AEs included both SAEs and all other (non-SAEs) AEs. SAE: any untoward medical occurrence, at any dose met one or more of following criteria: death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, persistent or significant disability/incapacity, congenital anomaly/birth defect or other important medical events. TEAEs: AEs that occur on or after first dose of study treatment up to 28 days after last dose of study treatment. Relatedness to study drug were judged by investigator. As per National Cancer Institute Common Terminology Criteria for AE (NCI CTCAE) severity of AEs were graded as following, Grade 1: mild, Grade 2: moderate, Grade 3: severe, Grade 4: life-threatening; urgent treatment indicated, Grade 5: death related to AE.

Hematological parameters including neutrophil count decreased, white blood cell decreased, anemia, platelet count decreased, hemoglobin increased and leukocytosis were assessed. Baseline was defined as the last assessment on or prior to the date of the first dose of study treatment. As per NCI CTCAE v5.0, severity was graded as, Grade 1: mild, Grade 2: moderate, Grade 3: severe and Grade 4: life-threatening; urgent treatment indicated. Grade 0: Non-missing laboratory value that fell outside the grading range for the corresponding laboratory parameter. Categories with at least 1 non-zero values showing any shift in Grade from baseline to post-baseline were reported.

SAS included all randomized participants who receive at least 1 dose of study treatment. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure.

Serum chemistry parameters including alanine aminotransferase increased, alkaline phosphatase increased, aspartate aminotransferase increased, blood bilirubin increased, cholesterol high, creatinine increased, hypercalcemia, hyperkalemia, hypermagnesemia, hypernatremia, hypertriglyceridemia, hypoalbuminemia, hypocalcemia, hypoglycemia, hypokalemia, hypomagnesemia, hyponatremia were assessed. Baseline was defined as the last assessment on or prior to the date of the first dose of study treatment. As per NCI CTCAE v5.0 severity was graded as, Grade 1: mild, Grade 2: moderate, Grade 3: severe and Grade 4: life-threatening; urgent treatment indicated. Grade 0: Non-missing laboratory value that falls outside the grading range for the corresponding laboratory parameter. Categories with at least 1 non-zero values showing any shift in Grade from baseline to post-baseline were reported.

SAS included all randomized participants who received at least 1 dose of study treatment. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure. Here, "Number Analyzed" signifies participants evaluable for the specified rows.

Twelve lead ECGs were collected using an automated ECG machine that calculated heart rate and measured corrected QT (QTc interval, QT interval, PR interval and QRS complex). Criteria for heart rate was as follows, <= 50 beats/minute (min), >=100 beats/min, increase from baseline >= 20 beats/min, decrease from baseline >= 20 beats/min. Criteria for PR interval was >= 220 millisecond (msec). Criteria for QRS interval was >= 120 msec. Criteria for QT interval was as follows, <=450 msec, > 450 to <= 480 msec, >480 to <=500 msec, >500 msec, increase from baseline <= 30 msec, increase from baseline > 30 to <= 60 msec. Criteria for QTCF interval was as follows, <=450 msec, > 450 to <= 480 msec, > 480 to <= 500 msec, > 500 msec, increase from baseline <= 30 msec, increase from baseline > 30 to <= 60 msec, increase from baseline > 60 msec. Baseline was defined as the last assessment on or prior to the date of the first dose of study treatment.

SAS included all randomized participants who received at least 1 dose of study treatment. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure. Here, "Number Analyzed" signifies participants evaluable for this outcome measure

Criteria for QTcF interval for single beat were as follows, <= 450 msec, > 450 to <= 480 msec, > 480 to <=500 msec, > 500 msec, increase from baseline <= 30 msec, increase from baseline > 30 to <= 60 msec, increase from baseline > 60 msec. Baseline was defined as the last assessment on or prior to the date of the first dose of study treatment.

QTc substudy analysis set= subset of SAS participants in vepdegestrant (ARV-471) arm, randomized at selected sites and had ECG measurements to evaluate effect of ARV-471 on QTcF via serial triplicate ECGs (centrally read), had baseline ECG measurement (Cycle 1 Day 1 pre-dose) and at least one ECG measurement on Day 1 of Cycle 2 or Cycle 3 following >= 7 consecutive days of 200 mg ARV-471. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure.

The EORTC QLQ-C30 contains 30 items and is composed of 5 multi-item functional scales (physical functioning, role functioning, emotional functioning, cognitive functioning and social functioning), 3 multi-item symptom scales (fatigue, pain and nausea/vomiting), 6 single item symptom scales (dyspnea, insomnia, appetite loss, constipation, diarrhea, and financial impact), and one global quality of life scale. This questionnaire contained 30 questions organized into 5 multi-item functional scales, 3 multi-item symptom scales, 6 single item symptom scales, and one global quality of life scale. All the scales and single-item measures range in score from 0 to 100. Higher scores on the functional scales represent higher levels of functioning. Higher scores on the global health status/quality of life scale represent higher health status/quality of life. Higher scores on symptom scales/items represent a greater presence of symptoms.

The EORTC QLQ-BR23 was a 23-item breast cancer-specific companion module to the EORTC-QLQ-C30 and consisted of four functional scales (body image, sexual functioning, sexual enjoyment, future perspective) and four symptom scales (systemic side-effects, breast symptoms, arm symptoms, upset by hair loss). Each item was rated by choosing 1 of 4 possible responses that record the level of intensity (1= not at all, 2= a little, 3= quite a bit, and 4= very much) within each scale. All scores are converted to a 0 to 100 scale. For functional scales, higher scores represent a better level of functioning. For symptom-oriented scales, higher scores represented greater symptom severity.

The EQ-5D-5L was a 5-item participant-completed questionnaire designed to assess health status in terms of a single index value or utility score. There were 2 components, a Health State Profile where participants rated their level of problems (1=none, 2=slight, 3=moderate, 4=severe, 5=extreme/unable) in 5 dimensions (mobility, self-care, usual activities, pain/discomfort, and anxiety/depression), and a visual analogue scale (VAS) in which participants rated their overall health status from 0 (worst imaginable) to 100 (best imaginable). Responses to 5 dimensions comprised health state/ single utility index value. E.g. if a participant responds "no problems" for each 5 dimensions, then health state was coded as "11111" with a predefined index value to it. Every health state (coded as combination of responses) had a unique predefined utility index value assigned to it per US value sets, Overall index scores ranged from 0 to 1, with lower scores representing a higher level of dysfunction.

The BPI-SF consisted of items to measure participant perceptions of pain severity (item 3), assess degree of interference of pain on daily functioning, body diagrams on which participants indicate location of pain, record pain medication usage, VAS assessed degree of pain relief in last 24 hours (item 9a). Items in pain severity scale evaluated pain "at its worst", "at its least", and "on average" over previous 24 hours, as well as "pain now" (at time of assessment). Participants responded on 10- point numerical rating scale, where 0 = "no pain" and 10 = "pain as bad as you can imagine". Pain interference scale asked participants to rate how their pain interferes with "enjoyment of life", "general activity", "walking ability", "mood", "sleep", "normal work" and "relations with other people." Responses for interference scale were also based on 10-points scale, where 0 = "does not interfere" and 10 = "interferes completely". Higher scores=high levels of pain, impact attributed to pain.

Plasma concentrations of ARV-471 and its epimer ARV-473 were reported in this outcome measure.

PK concentration set included all participants who were in the Safety Analysis Set and had at least 1 concentration of either ARV-471 or ARV-473. Here, "Overall Number of Participants Analyzed" signifies participants evaluable for this outcome measure and "Number analyzed" signifies participants evaluable for a specified timepoint.

Quantitative change in plasma ctDNA levels from baseline to each protocol specified time point (up to EOT), as assessed using a validated assay.