This is a Phase II open-label, 2-Part, multi-center study in Japan. The study will be conducted in two parts: Part 1 (Cohort 1) includes participants regardless of prior CDK4/6 inhibitor use and is designed to determine the recommended dose (RD), evaluate the tolerability and safety of alpelisib in combination with fulvestrant. Part 2 consists of 2 cohorts (the CDK4/6 inhibitor naive participants are in Cohort 2 and the CDK4/6 inhibitor pre-treated participants are in Cohort 3) which are designed to assess the efficacy and safety of alpelisib in combination with fulvestrant, will start once the RD of alpelisib is established.
Participants will be treated until disease progression, unacceptable toxicity, death or discontinuation from the study treatment for any other reason and will be followed for survival regardless of treatment discontinuation reason (except if consent is withdrawn or participant is lost to follow up).
