A Study of GB491 in Combination With Letrozole in Patients With HR Positive and HER2 Negative Advanced Breast Cancer

Inclusion Criteria: * 1\. Age: 18-75 years old; * Have a pathologically-confirmed diagnosis of hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) breast cancer; * No previous systematic antitumor therapy for locally advanced or metastatic breast cancer; * Eastern Cooperative Oncology Group \[ECOG\] 0-1; * Have adequate organ and marrow function; * Agree to sign the informed consent; Exclusion Criteria: * Systematic treatment with any other CDK4/6 inhibitor； * Subjects with known allergy to GB491 or any component of Letrozole； * Confirmed diagnosis of HER2 positive disease; * Known uncontrolled, or symptomatic central nervous system metastases; * Had major surgery (or is expected to require major surgery during the study period), chemotherapy, radiation, any investigational drug, or other antitumor therapy within 4 weeks prior to randomization; * Have previously received chemotherapy for locoregionally recurrent or metastatic breast cancer; * 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, or ventricular arrhythmia which need medical intervention； * Known history of HIV infection or HIV seropositivity (including HIV antibody positive at the time of screening)