Phase IIIb Study of Ribociclib + ET in Early Breast Cancer

Key Inclusion criteria: * Participant is an adult, male or female ≥ 18 years of age at the time of informed consent form signature (IC). * Participant has a histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor positive breast cancer (BC) based on the most recently analyzed tissue sample tested by a local laboratory prior to enrollment. * Participant has HER2- BC defined as a negative in situ hybridization test or an immunohistochemistry (IHC) status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (FISH, CISH, or SISH) test is required by local laboratory testing based on the most recently analyzed tissue sample. * Participants may have already received any standard neoadjuvant and/or adjuvant ET, including tamoxifen or toremifene at the time of informed consent signature, but enrollment should occur within 36 months of prior ET start date and participants should have at least 3 years remaining of endocrine adjuvant therapy. * For participants with prior ET treatment \> 12 months, restaging is highly recommended (unless contradictory to local regulations) to rule out disease recurrence prior to enrollment. * The number of participants with prior ET between 12 and 36 months will be capped at 30%. The cap will not apply to Black or African American participants. * Participant has no contraindication to receive adjuvant ET in the study. * Participant after surgical resection where tumor was removed completely, with the final surgical specimen microscopic margins free from tumor, and belongs to one of the following categories: * Anatomic Stage Group III, or * Anatomic Stage Group IIB, or * A subset of Anatomic Stage Group IIA. * Participant has an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1 or 2. * Participant has adequate bone marrow and organ function. * ECG values assessed by KardiaMobile-6L device, or standard 12-lead ECG per local investigator where KardiaMobile-6L cannot be used, as: * QTcF interval at Screening \< 450 msec (QT interval using Fridericia's correction). * Mean resting heart rate 50-99 beats per minute (determined from the ECG). Key Exclusion criteria: * Participant with distant metastases of BC beyond regional lymph nodes (Stage IV according to AJCC 8th edition) and/or evidence of recurrence after curative surgery. * Participant is concurrently using other antineoplastic therapy with the exception of adjuvant ET. * Participant has any other concurrent severe and/or uncontrolled medical condition that would, in the Investigator's judgment, cause unacceptable safety risks, contraindicate participant participation in the clinical study or compromise compliance with the protocol, or limit life expectancy to ≤5 years. * Clinically significant, uncontrolled heart disease and/or cardiac repolarization abnormality. * Pregnant or breast-feeding (lactating) women or women who plan to become pregnant or breast-feed during the trial. * Women of child-bearing potential (CBP), defined as all women physiologically capable of becoming pregnant, unless they are using highly effective methods of contraception while taking study treatment and for 21 days after stopping the treatment. Other inclusion/exclusion criteria may apply