The purpose of this study is to investigate the efficacy and safety of BGB-43395 in combination with letrozole compared with investigator's choice of cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) in combination with letrozole in patients with advanced or metastatic hormone receptor positive (HR+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer (BC) who have not received prior systemic treatment for advanced or metastatic disease.
Participants must be at least 18 years of age or the legal age of consent in the jurisdiction in which the study is taking place at the time of signing the informed consent.
Participants with histologically confirmed locally advanced or metastatic HR+ HER2- breast cancer.
Participants must have a stable Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤ 1.
Adequate organ function.
Participants who have received prior systemic treatment in the advanced or metastatic setting.
Participants who have received prior treatment with any selective cyclin-dependent kinase 4 (CDK4) or cyclin-dependent kinase 2 (CDK2) targeting agent, or any other investigational anticancer drug in any disease setting, except for prior investigational or approved SERDs in the adjuvant setting, provided that disease recurrence occurred more than 12 months after the last dose of endocrine-based therapy.
Participants with active leptomeningeal disease or uncontrolled, untreated brain metastasis.