Study to Assess the Efficacy and Safety of Alpelisib Plus Fulvestrant in Participants With HR-positive (HR+), HER2-negative, Advanced Breast Cancer After Treatment With a CDK4/6 Inhibitor and an Aromatase Inhibitor.

The purpose of this study is to complement Study CBYL719C2301 (SOLAR-1) and obtain more comprehensive data on the efficacy and safety of alpelisib (BYL719) in combination with fulvestrant compared with placebo plus fulvestrant in men or postmenopausal women with HR-positive, HER2-negative advanced breast cancer with a PIK3CA mutation who progressed or relapsed on or after treatment with an AI plus a CDK4/6 inhibitor.