A Study of a Medicine Called Atirmociclib in Combination With Giredestrant in Adult Participants With ER-positive, HER2-negative Early-stage Breast Cancer

This Phase 3 study will evaluate whether atirmociclib plus giredestrant improves invasive breast cancer-free survival compared with giredestrant alone in adults with ER-positive, HER2-negative early breast cancer at medium or high risk of recurrence. This is a Phase 3, double blind, placebo controlled study of atirmociclib plus giredestrant vs. giredestrant monotherapy as adjuvant treatment in adult patients with ER+, HER2- early breast cancer. The study plans to enroll 3800 participants randomized 1:1 into one of two treatment arms (Arm A - investigational: atirmociclib plus giredestrant taken continuously in a 28-day cycle) or (Arm B - comparator: placebo plus giredestrant taken continuously in a 28-day cycle). Participants may be treated with atirmociclib/placebo up to 3 years and giredestrant up to 5 years followed by long term follow up for survival and evaluation of recurrence of disease.