Inclusion Criteria:

          1. Histologically confirmed metastatic breast cancer with measurable or evaluable
             disease: Patients who have progressed on distant metastatic sites after curative
             surgery or have stage IV breast cancer at diagnosis

          2. Age > 19 years

          3. ECOG performance status 0 - 2

          4. Patient has HER2-negative breast cancer with IHC and/or FISH (or SISH, CISH) Patient
             has ER positive and/or PgR positive breast cancer by local laboratory testing

          5. Patient is premenopausal. Premenopausal status is defined as either:

             A. Patient had last menstrual period within the last 12 months B. If within three
             months of tamoxifen (tamoxifen) taking, C. In case of chemotherapy induced amenorrhea,
             the serum FSH ≤40IU/l

          6. A. Patient who have stage IV breast cancer at diagnosis, allow disease that progressed
             after 1st line chemotherapy. B. Patient who have stage IV breast cancer at diagnosis,
             allow disease that progressed after tamoxifen or goserelin. C. In case of
             recur/metastatic breast cancer, allow disease that progressed after 12 month of
             completion of neo/adjuvant chemotherapy .

          7. Urine or serum HCG test must be negative.

          8. Adequate bone marrow function (≥ ANC 1,500/ul, ≥ platelet 100,000/ul, ≥ Hemoglobin 9.0
             g/dl)

          9. Adequate renal function (≤ serum creatinine 1.5 mg/dl or CCr ≥ 50 ml/min)

         10. Adequate liver function (≤ serum bilirubin 1.5 mg/dl, ≤ AST/ALT x 3 upper normal
             limit)

         11. Patients who were already established on bisphosphonate therapy may continue on
             bisphosphonates.

         12. Patients agreed to use effective contraception or not of childbearing potential

         13. Written informed consent

         14. Consent to biomarker analysis.

        Exclusion Criteria:

          1. Postmenopausal women

          2. Serious uncontrolled intercurrent infections

          3. Serious intercurrent medical or psychiatric illness, including active cardiac disease

          4. Pregnancy or breast feeding

          5. Second primary malignancy(except in situ carcinoma of the cervix or adequately treated
             nonmelanomatous carcinoma of the skin or resected thyroid papillary carcinoma or other
             malignancy treated at least 5 years previously with no evidence of recurrence)

          6. Patients has received previous endocrine treatments such as, aromatase inhibitor,
             exemestane in the metastatic setting

          7. Patients has received previous treatment with CDK 4/6 inhibitors, mTOR inhibitors,
             PIK3CA inhibitors or capecitabine

          8. No symptomatic visceral metastasis

          9. Known brain metastases unless treated and stable

         10. Clinically significant uncontrolled conditions including, known active hepatitis B or
             hepatitis C.

         11. QTc interval > 480 msec, family or personal history of long or short QT syndrome, or
             known history of QTc prolongation or Torsade de Pointes.

         12. Known positive testing for human immunodeficiency virus or acquired immune deficiency
             syndrome.

         13. Unable to swallow and retain oral medication.

         14. Treatment radiotherapy within 4 weeks of the study

         15. Use of any investigational drug within 4 weeks of the study

         16. Treatment with chemotherapy within 3 weeks or hormone therapy within 2 weeks of the
             study