INCLUSION CRITERIA

          1. Patient is an adult female ≥ 18 years old and < 60 years old at the time of informed
             consent.

          2. Patient has a histologically and/or cytologically confirmed diagnosis of
             estrogen-receptor positive and/or progesterone receptor positive breast cancer based
             on the most recently analyzed tissue sample and all tested by local laboratory. ER
             should be more than 10% ER positive or Allred ≥5 by local laboratory testing.

          3. 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.

          4. Women with inoperable locally advanced or metastatic breast cancer not amenable to
             curative therapy. Patients must fulfill at least one of the following criteria to be
             considered that combination chemotherapy is needed according to PI's judgment.
             However, for patients who are eligible under inoperable locally advanced breast cancer
             or criteria 4c, the recruitment is stopped to enrich patient population with visceral
             metastases.

               -  Symptomatic visceral metastases

               -  Rapid progression of disease or impending visceral compromise.

               -  Markedly symptomatic non visceral disease if the treating physician opt to give
                  chemotherapy for rapid palliation of patients symptoms.

          5. Patient is premenopausal or perimenopausal at the time of study entry.

               1. Premenopausal status is defined as either:

                    -  Patient had last menstrual period within the last 12 months. OR

                    -  If on tamoxifen within the past 14 days, plasma estradiol and FSH are in the
                       premenopausal range, according to local laboratory definition.

                    -  In case of therapy induced amenorrhea, plasma estradiol and/or FSH are in
                       the premenopausal range according to local laboratory definition.

                    -  Patients who have undergone bilateral oophorectomy are not eligible.

               2. Perimenopausal status is defined as neither premenopausal nor postmenopausal

          6. Patients must have not received neither prior hormonal therapy nor chemotherapy for
             advanced breast cancer, except LHRH agonist. Patients who received ≤ 14 days of
             tamoxifen or a NSAI (letrozole or anastrozole) with or without LHRH agonist for
             advanced breast cancer prior to randomization are eligible. Patient must have
             measurable disease.

        EXCLUSION CRITERIA;

          1. Patient has received prior systemic anti-cancer therapy (including hormonal therapy
             and chemotherapy, or any CDK4/6 inhibitor for advanced breast cancer.

               -  Patients who received (neo) adjuvant therapy for breast cancer are eligible. If
                  the prior neo (adjuvant) therapy included aromatase inhibitors, the treatment
                  free interval must be greater than 12 months from the completion of aromatase
                  inhibitor treatment until randomization.

               -  If patients have disease recurrence during adjuvant tamoxifen treatment, disease
                  free interval (defined as duration between the date of patient received complete
                  tumor resection for primary breast cancer lesion to the date of disease
                  recurrence documented) must be greater than 12 months.

               -  Patients who are receiving ≤ 14 days of tamoxifen or NSAI or LHRH agonists ≤ 28
                  days for advanced breast cancer prior to randomization are eligible.

          2. Patient has received extended-field radiotherapy ≤ 2 weeks prior to randomization 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). Patient from whom ≥ 25% of the bone marrow has been
             previously irradiated are also excluded.

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

          4. Patients who have lung metastases with oxygen demand in resting status.

          5. Patients who have liver metastases with bilirubin > 1.5 ULN.

          6. Patients with CNS involvement unless they meet ALL of the following criteria:

               -  At least 4 weeks from prior therapy completion (including radiation and/or
                  surgery) to starting the study treatment.

               -  Clinically stable CNS tumor at the time of screening and not receiving steroids
                  and/or enzyme inducing anti-epileptic medications for brain metastases

               -  Leptomeningeal metastases is not allowed, even with stable clinical condition