Key Inclusion Criteria:  Postmenopausal women defined as NCCN guideline at the time of informed consent. Histological or cytological confirmation of advanced or metastatic ER+/HER2- breast cancer women who failed standard therapy or for which no standard therapy exists. Prior therapy: no more than 2 lines of chemotherapy for advanced breast cancer and recurrence or progression while being treated with endocrine therapy in adjuvant setting or in the advanced setting. ECOG score 0-2. Minimum life expectancy of a least 3 months as determined by the Investigator. Sufficient bone marrow reserve and organ function.  Key Exclusion Criteria:  Previous treatment with oral SERDs. Patient with central nervous system metastasis or meningeal metastasis. Patient who has received anti-tumor therapies including chemotherapy, radiotherapy, endocrine therapy, and immunotherapy within 4 weeks prior to the first dose of study drug. Patient who has participated in any other clinical trials for drugs or treatments within 3 weeks prior to the first dose of study drug. 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. Patient with serous unhealable wounds/ulcers/fractures within 4 weeks prior to the first dose of study drug. Patient with adverse reactions to previous anti-tumor treatments who have not yet recovered to grade ≤1 according to CTCAE v5.0. 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. Patient unable to be administered oral medications or any condition that seriously affect digestion in the gastrointestinal tract at the judgement of the Investigator. Patient with active infection within 1 week prior to the first dose of study drug, and currently need systemic anti-infective treatment. Patient has active cardiac disease or a history cardiac dysfunction. Patient with third spacing that cannot be controlled clinically and is not suitable for the study by the Investigator's judgment. Patient with known history of drug abuse. Patient with mental disorder that, in the opinion of the Investigator, could lead to poor compliance with required study procedures. Patient that cannot tolerate venous blood sampling.