Key Inclusion Criteria:
1. Postmenopausal women defined as NCCN guideline at the time of informed consent.
2. Histological or cytological confirmation of advanced or metastatic ER+/HER2- breast cancer women who failed standard therapy or for which no standard therapy exists.
3. Prior therapy:
1. No more than 1 line of chemotherapy for advanced breast cancer
2. Recurrence or progression on at least one line of endocrine therapy in the advanced or metastatic disease setting and derived a clinical benefit from the endocrine therapy: Recurred or progressed while being treated with adjuvant endocrine therapy for a duration of at least 24 months, or progressed under endocrine therapy for more than 6 months in the advanced or metastatic setting
4. ECOG score 0-1.
5. Minimum life expectancy of a least 3 months as determined by the Investigator.
6. Evaluable disease per RECIST 1.1; for patients consent to tissue biopsy, disease suitable for tumor biopsy.
7. Sufficient bone marrow reserve and organ function.
Key Exclusion Criteria:
1. Previous treatment with any SERDs.
2. Patient any central nervous system metastasis.
3. Prior antitumor therapies:
1. Received chemotherapies within 3 weeks before the first dose.
2. Received systemic radiotherapy within 3 weeks before the first dose, or local radiotherapy within 7 days before the first dose
3. Received other anti-tumor therapy such as endocrine therapy, immunotherapy, and target therapy within 3 weeks or 5 half-lives of the drug before the first dose of the study drug
4. For bone metastasis, bisphosphonates and local remission therapy are allowed (7 days washout for local radiation therapy).
4. Patient who has participated in any other clinical trials for drugs or treatments within 5 half-lives for a prior investigational drug or 2 weeks from use of an investigational device prior to the first dose of study drug.
5. Patient who had major surgery or significant trauma within 4 weeks prior to the first dose of study drug (excluding needle biopsy), or has scheduled surgery during the study period.
6. Patient with serous unhealable wounds/ulcers/fractures within 4 weeks prior to the first dose of study drug.
7. Patient with adverse reactions to previous anti-tumor treatments who have not yet recovered to grade ≤1 according to CTCAE v5.0. (except for toxicities without safety risks as judged by Investigator, such as alopecia, grade 2 peripheral neuropathy etc.)
8. Patient who has used strong inhibitors or strong inducers of CYP3A, or grapefruit or grapefruit juice within 4 weeks prior to the first dose of study drug.
9. Patient unable to be administered oral medications or any condition that seriously affect digestion in the gastrointestinal tract at the judgement of the Investigator.
10. Patient with active infection within 1 week prior to the first dose of study drug, and currently need systemic anti-infective treatment.
11. Patient has a known history of the following: HIV infection without effective antiretroviral therapy (ART) or acceptable immune function, or syphilis infection, or HBsAg positive HBV or needs prophylaxis therapy or suppressive antiviral therapy before dosing, or has an HCV infection that hasn't completed curative antiviral treatment or with unacceptable viral load.
12. Patient has active cardiac disease or a history cardiac dysfunction.
13. Patient with third spacing that cannot be controlled clinically and is not suitable for the study by the Investigator's judgment.
14. Patient with known history of drug abuse.
15. Patient with mental disorder that, in the opinion of the Investigator, could lead to poor compliance with required study procedures.
16. Patient that cannot tolerate venous blood sampling.
17. Known to have other malignancy within the past 5 years, and is progressing or requires active treatment (except skin basal cell carcinoma, squamous cell carcinoma, or cervical carcinoma in situ who have received potentially radical treatment)
