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

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