A Phase IV Study to Collect Data on the Efficacy and Safety of RIBociclib in Older Women With Breastcancer

Inclusion Criteria: * Patient is a female ≥ 70 years old at the time of informed consent. * Advanced breast cancer (defined as locoregionally recurrent or metastatic not amenable to curative therapy). * Histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor positive breast cancer by local laboratory (defined as ER and/or PgR ≥1% or Allred \>2). * HER2-negative breast cancer defined as a negative in situ hybridization test or an IHC status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (FISH, CISH, or SISH) test as determined by local laboratory testing according to ASCO-CAP guidelines is necessary. * Patient has adequate bone marrow and organ function as defined by the following laboratory values (as assessed by local laboratory), preferably tested a maximum of 14 days before enrolment: * Patient has signed informed consent obtained before any trial-related activities and according to local guidelines. * Subjects must be able to communicate with the investigator and comply with the requirements of the study procedures. Exclusion Criteria: * Patient has a known hypersensitivity to any of the excipients of ribociclib or letrozole. * Patients who received any CDK4/6 inhibitor previously. * Patient who received any prior systemic antihormonal therapy or chemotherapy for advanced breast cancer. * Patient is concurrently using other systemic anti-cancer therapy (except bone modifying agents). * Patient with central nervous system (CNS) metastases and/or documented meningeal carcinomatosis unless they meet ALL the following criteria: * At least 4 weeks from prior therapy for CNS disease completion (including radiation and/or surgery) to starting the study treatment; * Clinically stable CNS lesions at the time of study treatment initiation and not receiving steroids and/or enzyme-inducing anti-epileptic medications for the management of brain metastases for at least 2 weeks (radiological confirmation of brain disease status is not necessary). * 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 study or compromise compliance with the protocol: (e.g. chronic pancreatitis, chronic active hepatitis, active untreated or uncontrolled fungal, bacterial or viral infections, etc.). * Patients who already have or who are at significant risk of developing QTc prolongation are not eligible for the study. This includes patients: * with long QT syndrome; * with uncontrolled or significant cardiac disease, including recent myocardial infarction, congestive heart failure, unstable angina and bradyarrhythmias; * with electrolyte abnormalities (potassium, magnesium, sodium and calcium) that are NCI CTCAE 4.03 grade 2 or higher (for details, see table 10 ). Note: phosphate testing is not mandatory, but should the investigator consider measuring it before enrolment, the same rules may be applied