OS is defined as the time interval from the date of randomization to the date of documented death (due to any cause). In the absence of observation of death, survival time was censored to last date the participant is known to be alive or at the cut-off date, whichever comes first. Analysis was performed by Kaplan-Meier method.

Analysis was performed on the ITT population.

Objective response is defined as percentage of participants having a partial response (PR) or complete response (CR) according to the RECIST version 1.1 assessed by ICR. As per RECIST 1.1, CR was defined as disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to less than (<) 10 millimeters (mm). PR was defined as at least a 30% decrease in the sum of diameters of target lesions, taking as reference the baseline sum diameters.

Analysis was performed on the ITT population.

Disease control is defined as percentage of participants having a confirmed CR, PR, or stable disease (SD) or Non-CR/Non-PD as BOR determined by ICR as per RECIST 1.1 from the date of randomization to the date of end of treatment. As per RECIST 1.1, CR: disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to <10 mm. PR: at least a 30% decrease in sum of diameters of target lesions, taking as reference baseline sum diameters. SD: neither sufficient shrinkage to qualify for PR nor sufficient increase to qualify for progressive disease (PD), taking as reference the smallest sum diameters. Non-CR/Non-PD: persistence of one or more nontarget lesion(s) and/or maintenance of tumor marker level above the normal limits. PD: at least 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions.

Analysis was performed on the ITT population.

Clinical Benefit is defined as percentage of participants having a confirmed CR, PR, SD, or Non-CR/Non-PD for at least 24 weeks determined by ICR as per RECIST 1.1 from the date of randomization to the date of end of treatment. As per RECIST 1.1, CR: disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to <10 mm. PR: at least a 30% decrease in the sum of diameters of target lesions, taking as reference the baseline sum diameters. SD: neither sufficient shrinkage to qualify for PR nor sufficient increase to qualify for PD, taking as reference the smallest sum diameters. Non-CR/Non-PD: persistence of one or more nontarget lesion(s) and/or maintenance of tumor marker level above the normal limits. PD: at least 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions.

Analysis was performed on the ITT population.

DOR is defined as time (in months) from first documented evidence of CR or PR until progressive disease (PD) determined by ICR as per RECIST 1.1 or death from any cause, whichever occurs first. For participants with ongoing response at the time of the analysis, DOR was censored at the date of the last valid disease assessment not showing documented progression performed before the initiation of a new anticancer treatment (if any). As per RECIST 1.1, CR: disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to <10 mm. PR: at least a 30% decrease in the sum of diameters of target lesions, taking as reference the baseline sum diameters. PD: at least 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions.

Analysis was performed on a subset of participants who had objective response.

PFS defined as the time (in months) interval from the date of randomization to the date of first documented tumor progression as per RECIST 1.1 assessed by ICR or death (due to any cause), whichever comes first. Progression as per RECIST 1.1: at least a 20 percent (%) increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions. The mutation status (wild type, mutant) of twelve specific mutations of the ESR1 gene was determined by multiplex droplet digital polymerase chain reaction (ddPCR), including their mutant frequency and concentration. Here, PFS is reported based on the ESR1 mutation status of participants: wild type and mutants. ESR1 was the gene encoding estrogen receptor alpha. ESR1 mutant type breast cancer was a disease where the ESR1 gene had a mutation (i.e., a type of error). ESR1 wild type breast cancer was a disease where the ESR1 gene was normal without a mutation. Analysis was performed by Kaplan-Meier method.

Analysis was performed on the ITT population. Here, "overall number of participants analyzed" = participants with available data for this outcome measure and "number analyzed" = participants with available data for each specified category.

Amcenestrant plasma concentrations at specified time points are reported.

Analysis was performed on Pharmacokinetic (PK)-evaluable population: all participant who were assigned to study intervention, took at least 1 dose of study intervention, had at least 1 available plasma concentration post treatment with adequate documentation of date and time of dosing and date and time of sampling. Here, 'number analyzed' = participants with available data for each specified category. Data for this outcome measure was not planned to be collected and analyzed for PCEM group arm.

Within-participant Steady state Ctrough was defined as the median value of the Ctrough across study using plasma concentration of predose samples at Cycle 1 Day 15 and Day 1 of Cycle 2, 3, 4 and 6 for each individual participant. Average (mean) of all calculated Ctrough values for all participants across study (Cycle 1 Day 15 and Day 1 of Cycle 2, 3, 4 and 6 ) was derived and reported in this outcome measure.

Analysis was performed on Pharmacokinetic-evaluable population. Here, 'overall number of participants analyzed' = participants with available data for this outcome measure. Data for this outcome measure was not planned to be collected and analyzed for PCEM group arm.

EORTC-QLQ-C30: cancer-specific instrument with 30 questions for evaluation of new chemotherapy & assessment of participant reported outcome. These include 5 functional scales, 9 symptom scales, & Global Health Status/quality of life scale (GHS/QoL). All 14 items/domains were scored on scale of 1 (not at all) to 4 (very much) and GHS/QoL, scored on scale of 1 (very poor) to 7 (excellent). All scales are transformed from raw scores to linear scales ranging 0 to 100. Higher score for functional & GHS/QoL = higher level of functioning, & higher score for symptoms scales = higher symptom burden. Least Square (LS) mean and Standard Error (SE) are derived from MMRM model with change from Baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from baseline values of overall treatment (i.e., each cycle [Cycle 1 up to Cycle 30]) was reported in this outcome measure.

Analysis was performed on safety population evaluable which includes all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, completed the baseline and at least 1 post baseline on-treatment assessment. Here, 'number analyzed' = participants with available data for each specified category.

EQ-5D-5L is a standardized measure of health status, provides a simple, generic measure of health for clinical and economic appraisal, and consists of 2 sections: the EQ-5D-5L health state utility index (descriptive system) and the EQ-5D-5L VAS. The Visual Analogue Scale is designed to rate the participant's current health state on a scale from 0 to 100, where 0 represents the worst imaginable health state and 100 represents the best imaginable health state. LS mean and SE are derived from MMRM model with change from baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from Baseline values of overall treatment (i.e., each cycle [Cycle 1 up to Cycle 30]) was reported in this outcome measure.

Analysis was performed on Safety population evaluable. Here, "overall number of participants analyzed" signifies participants with available data for this outcome measure.

EQ-5D-5L: consists of 2 sections: EQ-5D-5L health state utility index (descriptive system) & VAS. The EQ-5D descriptive system consists of 5 dimensions: mobility, self-care, usual activities, pain/discomfort & anxiety/depression. Each dimension has 5 levels: no problems, slight problems, moderate problems, severe problems, & extreme problems. Response options are measured with 5-point Likert scale (for 5L version). The EQ-5D-5L responses are converted into single index utility score between 0 to 1, where higher score indicates better health state & lower score indicate worse health state. LS mean and SE are derived from MMRM model with change from Baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from baseline values overall treatment (i.e., each cycle [Cycle 1 up to Cycle 30]) was reported in this outcome measure.

Analysis was performed on Safety population evaluable. Here, 'overall number of participants analyzed' = participants with available data for this outcome measure.

QLQ-BR23: disease-specific Health-related QOL assesses impact of breast cancer & side effects of treatment. EORTC-QLQ-BR23 contains 23 items: multi-item scales & single-item measures. 4 functional scales (body image, sexual functioning, sexual enjoyment, future perspective) & 4 scales related to symptoms of disease or treatment (arm symptoms, breast symptoms, systemic therapy side effects, & upset by hair loss). All items scored 1 (not at all) to 4 (very much). Scores of all scales transformed from raw scores to linear scales ranging 0 to 100. Higher score for functional scales = better outcome; higher score for symptoms scales = higher symptom burden. LS mean and SE are derived from MMRM model with change from Baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from baseline values of overall treatment (i.e., each cycle [Cycle 1 up to Cycle 30]) was reported.

Analysis was performed on Safety population evaluable. Here, 'number analyzed' = participants with available data for each specified category.

OS is defined as the time interval from the date of randomization to the date of documented death (due to any cause). In the absence of observation of death, survival time was censored to last date the participant is known to be alive or at the cut-off date, whichever comes first. Analysis was performed by Kaplan-Meier method.

Analysis was performed on the ITT population.

Objective response is defined as percentage of participants having a PR or CR according to the RECIST version 1.1 assessed by ICR. As per RECIST 1.1, CR was defined as disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to <10 mm. PR was defined as at least a 30% decrease in the sum of diameters of target lesions, taking as reference the baseline sum diameters.

Analysis was performed on the ITT population. Only those participants with response are reported.

Disease control is defined as percentage of participants having a confirmed CR, PR, or SD or Non-CR/Non-PD as BOR determined by ICR as per RECIST 1.1 from the date of randomization to the date of end of treatment. As per RECIST 1.1, CR: disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to <10 mm. PR: at least a 30% decrease in sum of diameters of target lesions, taking as reference baseline sum diameters. SD: neither sufficient shrinkage to qualify for PR nor sufficient increase to qualify for PD, taking as reference the smallest sum diameters. Non-CR/Non-PD: persistence of one or more nontarget lesion(s) and/or maintenance of tumor marker level above the normal limits. PD: at least 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions.

Analysis was performed on the ITT population. Only those participants with response are reported.

Clinical benefit is defined as percentage of participants having a confirmed CR, PR, SD, or Non-CR/Non-PD for at least 24 weeks determined by ICR as per RECIST 1.1 from the date of randomization to the date of end of treatment. As per RECIST 1.1, CR: disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to <10 mm. PR: at least a 30% decrease in the sum of diameters of target lesions, taking as reference the baseline sum diameters. SD: neither sufficient shrinkage to qualify for PR nor sufficient increase to qualify for PD, taking as reference the smallest sum diameters. Non-CR/Non-PD: persistence of one or more nontarget lesion(s) and/or maintenance of tumor marker level above the normal limits. PD: at least 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions.

Analysis was performed on the ITT population. Only those participants with response are reported.

DOR is defined as time (in months) from first documented evidence of CR or PR until PD determined by ICR as per RECIST 1.1 or death from any cause, whichever occurs first. For participants with ongoing response at the time of the analysis, DOR was censored at the date of the last valid disease assessment not showing documented progression performed before the initiation of a new anticancer treatment (if any). As per RECIST 1.1, CR: disappearance of all target and non-target lesions and normalization of tumor marker level. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to <10 mm. PR: at least a 30% decrease in the sum of diameters of target lesions, taking as reference the baseline sum diameters. PD: at least 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions.

Analysis was performed on a subset of ITT population who had an objective response.

PFS defined as the time (in months) interval from the date of randomization to the date of first documented tumor progression as per RECIST 1.1 assessed by ICR or death (due to any cause), whichever comes first. Progression as per RECIST 1.1: at least a 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions. The mutation status (wild type, mutant) of twelve specific mutations of the ESR1 gene was determined by multiplex ddPCR, including their mutant frequency and concentration. Here, PFS is reported based on the ESR1 mutation status of participants: wild type and mutants. ESR1 was the gene encoding estrogen receptor alpha. ESR1 mutant type breast cancer was a disease where the ESR1 gene had a mutation (i.e., a type of error). ESR1 wild type breast cancer was a disease where the ESR1 gene was normal without a mutation. Analysis was performed by Kaplan-Meier method.

Analysis was performed on the ITT population. Here, "overall number of participants analyzed" = participants with available data for this outcome measure and "number analyzed" = participants with available data for each specified category.

Amcenestrant plasma concentrations at specified time points are reported.

Analysis was performed on PK-evaluable population: all participants who were assigned to study intervention, took at least 1 dose of study intervention, had at least 1 available plasma concentration post treatment with adequate documentation of date and time of dosing and date and time of sampling. Here, 'number analyzed' = participants with available data for each specified category. Data for this outcome measure was not planned to be collected and analyzed for PCEM group arm.

Within-participant Steady state Ctrough was defined as the median value of the Ctrough across study using plasma concentration of pre-dose samples at Cycle 1 Day 15 and Day 1 of Cycle 2, 3, 4 and 6 for each individual participant. Average (mean) of all calculated Ctrough values for all participants across study (Cycle 1 Day 15 and Day 1 of Cycle 2, 3, 4 and 6) was derived and reported in this outcome measure.

Analysis was performed on PK-evaluable population:all participants who were assigned to study intervention,took at least 1 dose of study intervention,had at least 1 available plasma concentration post treatment with adequate documentation of date and time of dosing and date and time of sampling. Here, 'overall number of participants analyzed' = participants with available data for this outcome measure. Data for this outcome measure was not planned to be collected and analyzed for PCEM group arm.

EORTC-QLQ-C30: cancer-specific instrument with 30 questions for evaluation of new chemotherapy & assessment of participant reported outcome. These include 5 functional scales, 9 symptom scales, & Global Health Status/quality of life scale (GHS/QoL). All 14 items/domains were scored on scale of 1 (not at all) to 4 (very much) and GHS/QoL, scored on scale of 1 (very poor) to 7 (excellent). All scales are transformed from raw scores to linear scales ranging 0 to 100. Higher score for functional & GHS/QoL = higher level of functioning, & higher score for symptoms scales = higher symptom burden. LS mean and SE are derived from MMRM model with change from baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from baseline values of overall treatment (i.e., 183 weeks) was reported in this outcome measure.

Analysis was performed on safety population evaluable which includes all participants randomly assigned to study intervention, who took at least 1 dose of study intervention, completed the baseline and at least 1 post baseline on-treatment assessment. Here, 'number analyzed' = participants with available data for each specified category.

EQ-5D-5L is a standardized measure of health status, provides a simple, generic measure of health for clinical and economic appraisal, and consists of 2 sections: the EQ-5D-5L health state utility index (descriptive system) and the EQ-5D-5L VAS. The Visual Analogue Scale is designed to rate the participant's current health state on a scale from 0 to 100, where 0 represents the worst imaginable health state and 100 represents the best imaginable health state. LS mean and SE are derived from MMRM model with change from baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from baseline values of overall treatment (i.e., 183 weeks) was reported in this outcome measure.

Analysis was performed on Safety population evaluable. Here, "overall number of participants analyzed" signifies participants with available data for this outcome measure.

EQ-5D-5L: consists of 2 sections: EQ-5D-5L health state utility index (descriptive system) & VAS. The EQ-5D descriptive system consists of 5 dimensions: mobility, self-care, usual activities, pain/discomfort & anxiety/depression. Each dimension has 5 levels: no problems, slight problems, moderate problems, severe problems, & extreme problems. Response options are measured with 5-point Likert scale (for 5L version). The EQ-5D-5L responses are converted into single index utility score between 0 to 1, where higher score indicates better health state & lower score indicate worse health state. LS mean and SE are derived from MMRM model with change from baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from baseline values of overall treatment (i.e., 183 weeks) was reported in this outcome measure.

Analysis was performed on Safety population evaluable. Here, 'overall number of participants analyzed' = participants with available data for this outcome measure.

QLQ-BR23: disease-specific Health-related QOL assesses impact of breast cancer & side effects of treatment. EORTC-QLQ-BR23 contains 23 items: multi-item scales & single-item measures. 4 functional scales (body image, sexual functioning, sexual enjoyment, future perspective) & 4 scales related to symptoms of disease or treatment (arm symptoms, breast symptoms, systemic therapy side effects, & upset by hair loss). All items scored 1 (not at all) to 4 (very much). Scores of all scales transformed from raw scores to linear scales ranging 0 to 100. Higher score for functional scales = better outcome; higher score for symptoms scales = higher symptom burden. LS mean and SE are derived from MMRM model with change from baseline values as response variable, treatment, time, treatment-by-time interaction, Baseline value and stratifications factors as fixed effect. Average of LS mean change from baseline values of overall treatment (i.e., 183 weeks) was reported in this outcome measure.

Analysis was performed on Safety population evaluable. Here, "overall number of participants analyzed" = participants with available data for this outcome measure. Only those items with at least 10 participants by treatment group were included in the analysis.

An AE was any untoward medical occurrence in a participant or clinical study participant, temporally associated with the use of study treatment, whether or not considered related to the study treatment. An SAE was defined as any untoward medical occurrence that, at any dose: resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent disability/incapacity, was a congenital anomaly/birth defect or was a medically important event. TEAEs were defined as AEs that developed, worsened (according to the Investigator's opinion), or became serious during the on-treatment period.

Analysis was performed on Safety population evaluable.

PFS is defined as the time in months interval from the date of randomization to the date of first documented tumor progression as per Response Evaluation Criteria in Solid Tumors (RECIST 1.1) assessed by independent central review (ICR) or death (due to any cause), whichever comes first. Progressive Disease (PD) as per RECIST 1.1: at least a 20 percent (%) increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions. Analysis was performed by Kaplan-Meier method.

Analysis was performed on the ITT population.

PFS is defined as the time in months interval from the date of randomization to the date of first documented tumor progression as per RECIST 1.1 assessed by ICR or death (due to any cause), whichever comes first. PD as per RECIST 1.1: at least a 20% increase in sum of diameters of target lesions, unequivocal progression of existing non-target lesions. Analysis was performed by Kaplan-Meier method.

Analysis was performed on the ITT population.