This is a multicenter, open-label clinical study to evaluate the safety and antitumor activity of FCN-437c in combination with Fulvestrant for the treatment of post-menopausal female patients with ER+ and HER2- advanced breast cancer, FCN-437c in combination with Letrozole + Goserelin for the treatment of pre-menopausal female patients with ER+ and HER2- advanced breast cancer, and to evaluate the PK characteristics of the FCN-437c combination therapies. This study is consist of two cohorts, Cohort 1: FCN-437c in combination with Fulvestrant (1st or 2nd line treatment for postmenopausal ER+, HER2-advanced breast cancer); Cohort 2: FCN-437c in combination with Letrozole + Goserelin (1st line treatment for premenopausal ER+, HER2- advanced breast cancer). Thirty patients will be enrolled in each cohort, for a total of 60 patients. Tumor Assessment: Tumor evaluation will be performed every 8 weeks (±7 days) according to RECIST version 1.1 until disease progression, withdrawal of informed consent, or death; for patients who discontinue the drug due to toxicity, imaging evaluation is required until disease progression. End of Treatment and End of Study: End of Study (EOS) is defined as 2 years after the last patient's first dose or the end of treatment (whichever is earlier). At the end of the study, the investigator will decide whether the patients whose disease has not progressed shall continue taking FCN-437c and other combination agents or not based on clinical benefit. Cohort 1: Post-menopausal patients diagnosed with ER+, HER2- advanced breast cancer, who have not received prior systemic therapy for advanced breast cancer, or who have disease progression determined by imaging assessment during their 1st line endocrine therapy; Cohort 2: Pre-menopausal patients diagnosed with ER+, HER2- advanced breast cancer, who have not received prior systemic therapy for advanced breast cancer;