Study for Women and Men With Hormone-receptor Positive Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria:

* Patient is an adult, ≥ 18 years old at the time of informed consent and has signed informed consent before any trial related activities and according to local guidelines
* Women and men with advanced (locoregionally recurrent or metastatic) breast cancer not amenable to curative therapy.
* Patient has a histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor positive and HER2-negative breast cancer by local laboratory. Local pathology is sufficient for assessment.
* Patient must have either:

  1. Measurable disease, i.e., at least one measurable lesion as per RECIST 1.1 criteria ).
  2. Bone lesions: lytic or mixed (lytic + sclerotic) in the absence of measurable disease
  3. Non-measurable disease
* Patient has an Eastern Cooperative Oncology Group (ECOG) performance status ≤2

Exclusion Criteria

* Patient who received any CDK4/6 inhibitor or any mTOR inhibitor.
* Patient has a known hypersensitivity to any of the excipients of ribociclib or letrozole
* Patients with current inflammatory breast cancer.
* Patient has received \> 1 chemotherapy for the treatment of advanced/metastatic breast cancer
* Patient has received \> 2 endocrine therapies for the treatment of advanced/metastatic breast cancer
* Patient has central nervous system (CNS) involvement. If patient is fulfilling the following 3 criteria she/he is eligible for the trial.

  1. completed prior therapy (including radiation and/or surgery) for CNS metastases ≥ 28 days prior to the start of study and
  2. CNS tumor is clinically stable at the time of screening and
  3. Patient is not receiving steroids and enzyme inducing anti-epileptic medications for brain metastases
* Patient has active cardiac disease or a history of cardiac dysfunction