Key Inclusion criteria:

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

        Patient has a histologically and/or cytologically confirmed diagnosis of ER-positive and/or
        PgR-positive breast cancer based on the most recently analyzed tissue sample, and all
        tested by local laboratory.

        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 and based on the most recently
        analyzed tissue sample.

        Patient must have measurable disease, i.e., at least one measurable lesion according to
        RECIST version 1.1. (a lesion in a previously irradiated site may only be counted as a
        target lesion if there is clear evidence of progression since the irradiation).

        Patient has an Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1.

        Standard 12-lead ECG values defined as the mean of the triplicate ECGs and assessed by the
        central laboratory:

          -  QTcF interval at screening < 450 ms (QT interval using Fridericia's correction)

          -  Mean resting heart rate 50 to 90 bpm (determined from the ECG)

        Women of childbearing potential (CBP), defined as all women physiologically capable of
        becoming pregnant, must have confirmed negative serum pregnancy test (for β-hCG) within 14
        days prior to randomization.

        Women of CBP must be willing to use highly effective methods of contraception.

        Key Exclusion Criteria:

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

        Patient who received any prior systemic anti-cancer therapy(including endocrine therapy,
        chemotherapy, prior CDK4/6 inhibitors) for aBC. Patients who received neo-/adjuvant therapy
        for breast cancer are eligible.

        Patient is concurrently using other anti-cancer therapy.

        Patient has had major surgery within 14 days prior to starting study drug or has not
        recovered from major toxicities.

        Patient has received extended-field radiotherapy ≤ 4 weeks or limited field radiotherapy ≤
        2 weeks prior to randomization, and has not recovered to grade 1 or better from related
        side effects of such therapy (with the exception of alopecia or other toxicities not
        considered a safety risk for the patient at investigator's discretion). Patients in whom ≥
        25% of the bone marrow has been previously irradiated are also excluded.

        Patient has a concurrent malignancy or malignancy within 3 years of the randomization date,
        with the exception of adequately treated basal or squamous cell skin carcinoma, or
        curatively resected cervical carcinoma in situ.

        Patients with central nervous system (CNS) involvement unless they meet specific stability
        criteria.

        Patient has clinically significant, uncontrolled heart disease and/or cardiac
        repolarization abnormality.

        Patient is currently receiving or has received systemic corticosteroids ≤ 2 weeks prior to
        starting study drug, and has not fully recovered from side effects of such treatment.

        Other protocol-defined Inclusion/Exclusion may apply.