This is a phase III clinical trial to evaluate the efficacy and safety of BPI-16350 in combination with Fulvestrant versus placebo combined with Fulvesrant in Patients who have HR positive and HER2 negative locally advanced，recurrent or metastatic breast cancer with disease progression following endocrine therapy.

Inclusion Criteria:

• Has the pathologically-confirmed diagnosis of locally advanced, recurrent or metastatic, HR positive, HER2 negative Breast Cancer
• One previous line of chemotherapy for advanced/metastatic disease is allowed
• Disease Progression following endocrine therapy
• Have postmenopausal status
• Have 1 of the following, as defined by the Response Evaluation Criteria in Solid Tumors (RECIST) v1.1: measurable disease or nonmeasurable bone-only disease
• ECOG: 0~1
• Adequate organ function

Exclusion Criteria:

• Previous treatment with cytotoxic drugs within 4 weeks before enrollment
• Previous treatment with endocrine or small molecule drug tyrosine kinase inhibitor TKI within 2 weeks before enrollment
• Patients who received prior treatment with any CDK4/6 inhibitor, everolimus or fulvestant
• HER2 positive
• Patients with major surgery within 4 weeks, severe or unstable systemic disease, unstable/symptomatic CNS metastasis, other malignant tumors, ILD, clinical significant cardiac disease, bleeding or embolic disease, active infectious disease, conditions affecting drug swallow and absorption, etc
• Pregnancy or lactation
• Other conditions considered not appropriate to participate in this trial by the investigators