• Male or Female \>18 years of age on the day of informed consent signing.

• Progression on a CDK4/6 inhibitor in combination with an AI immediately prior to the enrollment on this study

• Histologically confirmed ER-positive, HER2-negative metastatic breast cancer. ER-positive is defined as ≥1% immunohistochemical (IHC) staining of any intensity. HER2 test result is negative if a single test (or both tests) performed show:

‣ IHC 1+ or 0

⁃ In situ hybridization negative based on:

• Single-probe average HER2 copy number \<4.0 signals/cell

∙ Dual-probe HER2/CEP17 ratio \<2.0 with an average HER2 copy number \<4.0 signals/cell.

• Measurable disease according to the RECIST 1.1 or bone-only disease.

• Postmenopausal status or receiving ovarian ablation with a gonadotropin-releasing hormone (GnRH) agonist. Postmenopausal status is defined by any one of the following criteria:

‣ Prior bilateral oophorectomy

⁃ Age ≥55 years

⁃ Age \<55 years and amenorrheic for at least 12 months (spontaneous cessation of menses for 12 consecutive months or more in the absence of chemotherapy, tamoxifen, toremifene, or ovarian suppression) and follicle-stimulating hormone and estradiol levels in the postmenopausal range without an alternative cause If the patient does not meet criteria for postmenopausal status but is receiving ovarian ablation therapy with a GnRH agonist, the patient is eligible for this trial, provided that the GnRH agonist is started at least 2 weeks prior to the first dose of trial treatment.

• Eastern Cooperative Oncology Group performance status of 0 or 1.

• Life expectancy ≥6 months.

• Patients who received chemotherapy must have recovered (Common Terminology Criteria for Adverse Events \[CTCAE\] Grade ≤1) from the acute effects of chemotherapy except for residual alopecia or Grade 2 peripheral neuropathy prior to randomization. A washout period of at least 21 days is required between last chemotherapy dose and randomization (provided the patient did not receive radiotherapy). Patients who received radiotherapy must have completed and fully recovered from the acute effects of radiotherapy. A washout period of at least 21 days is required between end of radiotherapy and randomization.

• Adequate organ function:

‣ Absolute neutrophil count ≥1500/µL (without granulocyte colony stimulating factor support within 2 weeks of laboratory test used to determine eligibility)

⁃ Platelets ≥100,000/µL (without transfusion within 2 weeks of laboratory test used to determine eligibility)

⁃ Hemoglobin ≥9 g/dL (without blood transfusion)

⁃ White blood cell count \>2,500/µL and \<15,000/µL

⁃ Lymphocyte count ≥500/µL

⁃ Serum bilirubin ≤1.5x upper limit of normal (ULN; patients with known Gilbert's disease who have serum bilirubin level ≤3 x ULN may be enrolled)

⁃ Serum transaminases (aspartate transaminase \[AST\] or alanine transaminase \[ALT\]) activity ≤3.0 x ULN with normal alkaline phosphatase (\[ALP\]; patients with liver metastases ≤5 x ULN) OR AST and ALT ≤1.5 x ULN with ALP \>2.5 x ULN

⁃ International normalized ratio and activated partial thromboplastin time ≤1.5 x ULN

⁃ Serum creatinine at or below the institutional normal value.

⁃ Able to swallow oral medication.

⁃ Patients who are made postmenopausal through use of GNRH agonists must be willing to use an adequate method of contraception for the course of the trial through 1 year after the last dose of trial treatment.

⁃ Patients who are made postmenopausal through use of GNRH agonists should have a negative serum pregnancy (β-human chorionic gonadotropin) within 7 days prior to trial treatment administration.

⁃ Willing and able to provide written informed consent/assent for the trial.