AEs were defined as the appearance of (or worsening of any pre-existing) undesirable sign(s), symptom(s), or medical condition(s).

SAEs were defined as meeting at least 1 of the following criteria: is fatal or life-threatening, Results in persistent or significant disability/incapacity, constitutes a congenital anomaly/birth defect, is medically significant, requires inpatient hospitalization or prolongation of existing hospitalization. A SAE which caused death of the participant was considered as fatal SAE.

AEs were assessed and graded according to the Common Terminology Criteria for Adverse Events (CTCAE) version 4.03. Grade 1 to 5 were used to characterize the severity of the Adverse Event. Grade 1: mild; Grade 2: moderate; Grade 3: severe; Grade 4: life-threatening and Grade 5: death related to AE.

A participant with multiple severity grades for an AE is only counted under the maximum grade.

Safety set in the Core phase: All participants who received at least one dose of study treatment defined as either ribociclib, or letrozole, or goserelin, or leuprolide in the Core phase.

Time to progression (TTP) is defined as time from date of start of treatment to the date of first documented progression or death due to underlying cancer. Participants with symptoms of rapidly progressing disease without radiologic evidence were classified as progression only when clear evidence of clinical deterioration was documented and/or patient discontinued due to 'Disease progression' or death due to study indication. When there was no documentation of radiologic evidence of progression, and the patient discontinued for 'Disease progression' due to documented clinical deterioration of disease, the date of discontinuation was used as date of progression.

TTP was estimated using the Kaplan-Meier method. 95% CI of median was calculated according to Brookmeyer and Crowley method.

Full Analysis set: All participants who received at least one dose of study treatment defined as either ribociclib, or letrozole, or goserelin, or leuprolide in the Core phase.

Overall response rate (ORR) is defined as the percentage of participants with best overall response of complete response (CR) or partial response (PR) according to RECIST 1.1 based on investigator's assessment.

CR: Disappearance of all non-nodal target lesions. In addition, any pathological lymph nodes assigned as target lesions must have a reduction in short axis to < 10 mm PR: At least a 30% decrease in the sum of diameter of all target lesions, taking as reference the baseline sum of diameters.

95% CI was calculated using the exact binomial method.

Full Analysis set: All participants who received at least one dose of study treatment defined as either ribociclib, or letrozole, or goserelin, or leuprolide in the Core phase.

Clinical benefit rate (CBR) is defined as the percentage of participants with a best overall response of complete response (CR), or partial response (PR) or an overall lesion response of stable disease (SD), lasting as per local review, for a duration of at least 24 weeks. CR, PR and SD are defined according to RECIST 1.1 based on investigator's assessment.

CR: Disappearance of all non-nodal target lesions. In addition, any pathological lymph nodes assigned as target lesions must have a reduction in short axis to < 10 mm PR: At least a 30% decrease in the sum of diameter of all target lesions, taking as reference the baseline sum of diameters.

SD: Neither sufficient shrinkage to qualify for PR or CR nor an increase in lesions which would qualify for progressive disease.

95% CI was calculated using the exact binomial method.

Full Analysis set: All participants who received at least one dose of study treatment defined as either ribociclib, or letrozole, or goserelin, or leuprolide in the Core phase.

Change from baseline in FACT-B scores was assessed. FACT-B is a self-report instrument that measures multidimensional quality of life (QOL) in patients with breast cancer. The FACT-B consists of 37 questions that address physical, social, emotional, and functional well-being, with specific questions relevant to women with breast cancer. Each item has a score range of 0 (Not at all) to 4 (Very much), with a total score ranging from 0-148. The higher the score, the better the QOL reported by the participant. A positive change from baseline indicates improvement in QoL.

Due to the nature of the questionnaire, only females were asked to complete this questionnaire.

Female participants in the Full Analysis set population for whom baseline and at least one post baseline patient-reported outcome measurements are available. Number analyzed indicates number of participants with available data for this outcome measure at specified timepoints.

AEs were defined as the appearance of (or worsening of any pre-existing) undesirable sign(s), symptom(s), or medical condition(s).

SAEs were defined as meeting at least 1 of the following criteria: is fatal or life-threatening, Results in persistent or significant disability/incapacity, constitutes a congenital anomaly/birth defect, is medically significant, requires inpatient hospitalization or prolongation of existing hospitalization.

AEs were assessed and graded according to the Common Terminology Criteria for Adverse Events (CTCAE) version 4.03. Grade 1 to 5 were used to characterize the severity of the Adverse Event. Grade 1: mild; Grade 2: moderate; Grade 3: severe; Grade 4: life-threatening and Grade 5: death related to AE.

A participant with multiple severity grades for an AE is only counted under the maximum grade.

Safety set in the Extension phase: all patients who received at least one dose of study medications defined as ribociclib or letrozole or goserelin/leuprolide (if applicable) in the Extension Phase.

Clinical benefit as assessed by the Investigator during Extension phase

Safety set in the Extension phase: all patients who received at least one dose of study medications defined as ribociclib or letrozole or goserelin/leuprolide (if applicable) in the Extension phase.

Exploratory analysis performed in archival tumor samples collected during screening in the main study. Protein expression levels of the ribociclib plus letrozole cohort that did not achieve clinical benefit (progression within 3 months of treatment) and the cohort sensitive to ribociclib and letrozole (cohort with a time to progression of 22 months or more) were determined using using Single-Pot, Solid-Phase-enhanced, Sample Preparation-Clinical Tissue Proteomics (SP3-CTP). For normalization purposes a pooled internal standard sample, comprised of aliquots of every sample included in the study, was included in each experimental batch. Protein abundances were calculated as the log2 transformed abundances relative to the pooled internal standard. Positive values represent higher protein expression levels compared to the pooled internal standard. Expression levels of proteins that showed association to predicting response to study treatment are presented.

Participants included in the sub-study (only available for Canadian participants) with evaluable tumor samples collected during screening in the main study and with time to progression within 3 months (cohort not achieving clinical benefit) or with time to progression of 22 months or more (cohort sensitive to treatment)

On-treatment- Core phase: from first treatment in the Core phase up to 30 days post-treatment (for participants who did not enter the Extension phase) or up to last treatment in the Core phase (for participants who entered the Extension phase). Extension phase: from first dose of treatment in the Extension phase up to 30 days after last dose of treatment.

Post-treatment survival follow-up- Core phase: from 31 days post-treatment in the core phase up to end of study; Extension phase: from 31 days post-treatment in the Extension phase up to end of study.

Core phase: All participants who received at least one dose of study treatment defined as either ribociclib, or letrozole, or goserelin, or leuprolide in the Core phase.

Extension phase: all patients who received at least one dose of study medications defined as ribociclib or letrozole or goserelin/leuprolide (if applicable) in the Extension phase.