NATALEE: This phase III trial is evaluating the safety and effectiveness of a targeted (biological) therapy (Ribociclib) with an endocrine therapy as an adjuvant treatment for people with hormone receptor positive (HR+) and HER2- breast cancerA Phase III Multi-center, Randomized, Open-label Trial to Evaluate Efficacy and Safety of Ribociclib With Endocrine Therapy as an Adjuvant Treatment in Patients With Hormone Receptor-positive, HER2-negartive Early Breast Cancer (New Adjuvant TriAl With Ribociclib

Exclusion

• Patient has received any CDK4/6 inhibitor
• Patient has received prior treatment with tamoxifen, raloxifene or AIs for reduction in risk ("chemoprevention") of breast cancer and/or treatment for osteoporosis within the last 2 years prior to randomization. Patient is concurrently using hormone replacement therapy.
• Patient has received prior treatment with anthracyclines at cumulative doses of 450 mg/m² or more for doxorubicin, or 900 mg/m² or more for epirubicin.
• Patient with a known hypersensitivity to any of the excipients of ribociclib and/or ET
• Patient with distant metastases of breast cancer beyond regional lymph nodes (stage IV according to AJCC 8th edition) and/or evidence of recurrence after curative surgery.
• Patient is concurrently using other anti-neoplastic therapy with the exception of adjuvant ET
• Patient has had major surgery, chemotherapy or radiotherapy within 14 days prior to randomization
• Patient has not recovered from clinical and laboratory acute toxicities related to prior anti-cancer therapies
• Patient has a concurrent invasive malignancy or a prior invasive malignancy whose treatment was completed within 2 years before randomization
• Patient has known HIV infection, Hepatitis B or C infection
• Clinically significant, uncontrolled heart disease and/or cardiac repolarization abnormality
• Patient is currently receiving any of the following substances within 7 days before randomization - Concomitant medications, herbal supplements, and/or fruits that are known as strong inhibitors or inducers of CYP3A4/5 or Medications that have a narrow therapeutic window and are predominantly metabolized through CYP3A4/5
• is currently receiving or has received systemic corticosteroids ≤ 2 weeks prior to starting trial treatment
• Patient has impairment of GI function or GI disease that may significantly alter the absorption of the oral trial treatments
• Patient has any other concurrent severe and/or uncontrolled medical condition that would, in the Investigator's judgment, cause unacceptable safety risks, contraindicate patient participation in the clinical trial or compromise compliance with the protocol
• Participation in other studies involving investigational drug(s) within 30 days prior to randomization or within 5 half-lives of the investigational drug(s) (whichever is longer), or participation in any other type of medical research judged not to be scientifically or medically compatible with this trial.
• Pregnant or breast-feeding (lactating) women or women who plan to become pregnant or breast-feed during the trial