An Open-Label, Randomized, Multicenter Phase 3 Study Investigating the Efficacy and Safety of BGB-43395 Plus Letrozole Versus CDK4/6 Inhibitors (Abemaciclib, Palbociclib, Ribociclib) Plus Letrozole in Patients With Advanced or Metastatic HR+/HER2- Breast Cancer Who Have Not Received Prior Systemic Anticancer 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…