This study is designed to evaluate the safety and efficacy of SPH4336 combined with letrozole in first-line treatment of locally advanced or metastatic breast cancer
Patients who voluntarily participate in the study, completely understand the study, and voluntarily sign the informed consent form (ICF).
Female, ≥ 18 and ≤ 75 years of age at the time of signing the ICF.
ECOG（Eastern Cooperative Oncology Group） performance status 0 or 1.
Life expectancy ≥ 3 months.
Prior treatment with any CDK4/6 （Cyclin dependent kinase）inhibitor.
Inflammatory breast cancer.
Patients unsuitable for endocrine therapy at the investigator's discretion.
History of other malignancies within 5 years prior to the start of study treatment.