Critères d'inclusion : - Women (regardless of menopausal status) or men ≥18 years of age (or per local regulations).
- The participant has confirmed HR+, HER2-, early stage resected invasive breast cancer without evidence of distant metastases.
- The participant must have undergone definitive surgical treatment for the current malignancy.
- The participant must have tumor tissue from breast (preferred) or lymph node for exploratory biomarker analysis available prior to randomization.
- The participant must have axillary lymph node involvement by tumor and have one of the following indicating a higher risk of relapse:
4 or more axillary lymph nodes involved with cancer
Tumor size of at least 5 centimeters
Grade 3 histology
Ki67 index by central analysis of ≥20% on untreated breast tissue
- The participant must be randomized within 16 months from the time of surgery.
- If the participant is currently receiving or initiating standard adjuvant endocrine therapy at time of study entry, she/he must not have received more than 12 weeks of adjuvant endocrine therapy following his/her last non-endocrine therapy (surgery, chemotherapy, or radiation).
- Participants must have recovered from the acute effects of chemotherapy and radiotherapy and from surgical side effects following definitive breast surgery.
- Women of reproductive potential must have a negative blood pregnancy test and agree to use highly effective contraceptive methods.
- The participant has a Eastern Cooperative Oncology Group (ECOG) performance status ≤1.
- The participant has adequate organ function.
- The participant is able to swallow oral medications.
Critères de non-inclusion : - Stage IV (M1), Stage IA, and lymph node negative breast cancer.
- Participants with a history of previous breast cancer are excluded, with the exception of lobular carcinoma in situ (LCIS) or ductal carcinoma in situ (DCIS) treated by locoregional therapy alone ≥5 years ago. Participants with a history of any other cancer (except non-melanoma skin cancer or carcinoma in situ of the cervix), unless in complete remission with no therapy for a minimum of 5 years are excluded.
- Females who are pregnant or lactating.
- The participant has previously received treatment with any CDK4 and CDK6 inhibitor.
- The participant is receiving concurrent exogenous reproductive hormone therapy (for example, birth control pills, hormone replacement therapy, or megestrol acetate).
- The participant has previously received endocrine therapy for breast cancer prevention (tamoxifen or raloxifene or aromatase inhibitors).
- The participant has serious preexisting medical condition(s) that, in the judgment of the investigator, would preclude participation in this study.
- The participant has a personal history of any of the following conditions: syncope of cardiovascular etiology, ventricular arrhythmia of pathological origin or sudden cardiac arrest. Any participant with a history of venous thromboembolism (VTE) (for example, deep vein thrombosis [DVT] of the leg or arm and/or pulmonary embolism) will be excluded.
- The participant has active bacterial infection, fungal infection, or detectable viral infection or viral load.
- The participant has received an experimental treatment in a clinical trial within the last 30 days or 5 half-lives, whichever is longer.