BGB-43395 Plus Letrozole Versus CDK4/6i Plus Letrozole for Patients With Advanced or Metastatic HR+/HER2- Breast Cancer Who Have Not Received Prior Treatment for Advanced or Metastatic Disease

Inclusion Criteria: * 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. Exclusion Criteria: * 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. Note: Other protocol-defined inclusion/exclusion criteria may apply.