The purpose of the Phase Ib is to:

1. determine the recommended dose of LEE011 in combination with a standard dose of letrozole as well as to provide additional safety and anti-tumor activity data in Asian non-Japanese patients
2. determine the recommended dose of LEE011 in combination with a standard dose of letrozole as well as to provide additional safety and activity data in Japanese patients
3. evaluate the safety and anti-tumor activity of LEE011 at the RP2D established in the dose escalation part in combination with a standard dose of letrozole, fulvestrant or tamoxifen plus goserelin in Japanese patients.

Inclusion Criteria:

• Women with advanced (locoregionally recurrent or metastatic) breast cancer not amenable to curative therapy (surgery and/or radiotherapy).
• Patient has a histologically and/or cytologically confirmed diagnosis of estrogen receptor positive and/or progesterone receptor positive breast cancer
• Patient has HER2-negative breast cancer
• Patient has adequate bone marrow and organ function

Exclusion Criteria:

• Patient who received any CDK4/6 inhibitor.
• Patient has a known hypersensitivity to any of the excipients of LEE011 or letrozole
• Patients with inflammatory breast cancer.
• Patient who received any prior systemic anti-cancer therapy (including hormonal therapy and chemotherapy) for advanced breast cancer
• Patient is currently using other anti-cancer therapy
• Patient has had major surgery within 14 days prior to starting study drug or has not recovered from major side effects.
• Patient who has received radiotherapy ≤ 4 weeks
• Patient has a concurrent malignancy or malignancy within 3 years
• Patient has metastases to the central nervous system (CNS).
• Patient has a known history of HIV infection

Other protocol-defined inclusion/exclusion criteria may apply