Inclusion Criteria:

          -  Patient has a histologically and/or cytologically confirmed diagnosis of estrogen
             receptor (ER) positive and/or progesterone receptor positive breast cancer by local
             laboratory (based on most recently analyzed biopsy).

          -  Patient has HER2-negative breast cancer (based on most recently analyzed biopsy)
             defined as a negative in situ hybridization test or an Immunohistochemistry (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.

          -  Patient must have either:

               -  Measurable disease, i.e., at least 1 measurable lesion as per Response Evaluation
                  Criteria in Solid Tumors (RECIST) 1.1 criteria (a lesion at a previously
                  irradiated site may only be counted as a target lesion if there is a clear sign
                  of progression since the irradiation). OR

               -  If no measurable disease is present, then at least 1 predominantly lytic bone
                  lesion must be present (patients with no measurable disease and only 1
                  predominantly lytic bone lesion that has been previously irradiated are eligible
                  if there is documented evidence of disease progression of the bone lesion after
                  irradiation).

          -  Patient has ECOG performance status 0 or 1.

        For premenopausal cohort:

          -  Patient is an adult, female ≥ 18 years old and < 60 years old at the time of informed
             consent and has signed informed consent before any trial related activities are
             conducted and according to local guidelines.

          -  Confirmed negative serum pregnancy test before starting study treatment or patient has
             had a hysterectomy.

          -  Patient has advanced (loco regionally recurrent not amenable to curative therapy or
             metastatic) breast cancer not amenable to curative therapy (e.g.

        surgery and/or radiotherapy).

          -  Patients who received ≤ 14 days of a NSAI (letrozole or anastrozole) with or without
             goserelin or goserelin ≤ 28 days for advanced breast cancer prior to randomization are
             eligible. Patients must continue treatment with the same hormonal agent + goserelin
             during the study. No treatment interruption is required for these patients prior to
             randomization.

          -  Patients who have received up to 1 line of chemotherapy for advanced breast cancer and
             have been discontinued 28 days before randomization are eligible.

        For postmenopausal cohort:

          -  Patient is an adult, female ≥ 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 with advanced (locoregionally recurrent or metastatic) breast cancer not
             amenable to curative therapy.

        Exclusion Criteria:

          -  Patient who has received a prior CDK4/6 inhibitor.

          -  Patient with symptomatic visceral disease or any disease burden that makes the patient
             ineligible for endocrine therapy per the investigator's best judgment

          -  Patient with CNS metastases.

          -  Patient who has not had resolution of clinical and laboratory acute toxicities related
             to prior anti-cancer therapy to National Cancer Institute (NCI) Common Terminology
             Criteria for Adverse Events (CTCAE) version 4.03 Grade ≤1.

          -  Patient has a known history of Human immunodeficiency Virus (HIV) infection (testing
             not mandatory).

          -  Clinically significant, uncontrolled heart disease and/or cardiac repolarization
             abnormality

          -  Patient is currently receiving any of the substances as defined in the protocol that
             cannot be discontinued 7 days prior to the start of the treatment:

        For premenopausal cohort:

          -  Pregnant or nursing (lactating) women.

          -  Women of child-bearing potential, defined as all women physiologically capable of
             becoming pregnant, unless they are using highly effective methods of contraception
             during dosing of study treatment and for 21 days after stopping study medication.

        Note: Use of oral (estrogen and progesterone), transdermal, injected or implanted hormonal
        methods of contraception as well as hormonal replacement therapy is not allowed in this
        study.

        For postmenopausal cohort:

        - Patient who received any prior systemic anti-cancer therapy (including hormonal therapy
        and chemotherapy) for advanced breast cancer.

        Note: Patients who received neo (adjuvant) therapy for breast cancer are eligible. If the
        prior neo (adjuvant) therapy included letrozole or anastrozole, the disease free interval
        must be greater than 12 months from the completion of treatment until randomization.

        - Patients who received ≤ 14 days of letrozole or anastrozole for advanced disease prior to
        randomization are eligible.

        Other protocol-defined inclusion/exclusion may apply.