Study of Dalpiciclib Combined With Letrozole in the Maintenance Treatment of HR Positive and HER2 Negative Metastatic Breast Cancer After First-line Chemotherapy

Inclusion Criteria: 1. Has the pathologically-confirmed diagnosis of locally recurrent or metastatic, hormone-receptor positive, HER2 negative Breast Cancer. 2. Age: 18 - 75 years old 3. Eastern Cooperative Oncology Group performance status (ECOG PS)=0\~1 4. Functions of liver and kidney is normal 5. Agreed to take contraceptive measures during treatment Exclusion Criteria: 1. Confirmed diagnosis of HER2 positive disease. 2. Central nervous system metastasis 3. Patients who received prior treatment with any CDK4/6 inhibitor. 4. Clinically significant cardiovascular and cerebrovascular diseases,including but not limited to severe acute myocardial infarction within 6 months before enrollment, unstable or severe angina, Congestive heart failure (New York heart association (NYHA) class \> 2), or ventricular arrhythmia which need medical intervention； 5. Researchers believe that is not suitable for the study