Dose Escalation Study of LEE011 in Combination With Buparlisib and Letrozole in HR+, HER2-negative Post-menopausal Women With Advanced Breast Cancer.

Inclusion Criteria: 1. Women with advanced (recurrent or metastatic) breast cancer who received no prior therapy for advanced disease. 2. Patient is postmenopausal. 3. Patient may have received ≤ 2 lines of chemotherapy for metastatic or recurrent breast cancer in the dose-escalation phase. 4. Patient has a histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor positive breast cancer by local laboratory. 5. Patient has 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 is required by local laboratory testing. 6. Patient must have either: * Measurable disease, i.e., at least one measurable lesion as per RECIST 1.1 criteria or at least one predominantly lytic bone lesion Exclusion Criteria: 1. Patient who received any CDK4/6 or PI3K inhibitor. 2. Patient has active cardiac disease or a history of cardiac dysfunction including any of the following: * History of angina pectoris, symptomatic pericarditis, or myocardial infarction within 12 months prior to study entry * History of documented congestive heart failure (New York Heart Association functional classification III-IV) * Documented cardiomyopathy * Patient has a Left Ventricular Ejection Fraction (LVEF) \< 50% as determined by Multiple Gated acquisition (MUGA) scan or echocardiogram (ECHO) * History of any cardiac arrhythmias, e.g., ventricular, supraventricular, nodal arrhythmias, or conduction abnormality in the previous 12 months. * On screening, any of the following cardiac parameters: bradycardia (heart rate \< 50 at rest), tachycardia (heart rate \> 90 at rest), PR interval \> 220 msec, QRS interval \>109 msec, or QTcF \>450 msec. Systolic blood pressure \>160 or \<90 mmHg 3. Patient is currently receiving any of the following medications: * That are known strong inducers or inhibitors of CYP3A4. * That have a known risk to prolong the QT interval or induce Torsades de Pointes. * That have a narrow therapeutic window and are predominantly metabolized through CYP3A4. 4. Certain scores on an anxiety and depression mood questionnaires