This clinical trial is focused on studying a type of breast cancer known as locally advanced (inoperable) or metastatic HR-positive/HER2-negative breast cancer. This type of cancer is characterized by its spread beyond the original site and its resistance to certain treatments. The study is testing a combination of two treatments: Capivasertib, which is taken as a film-coated tablet, and Fulvestrant, which is given as a solution for injection. The trial will compare the effects of this combination against a placebo combined with Fulvestrant. The purpose of the study is to evaluate how well the combination of Capivasertib and Fulvestrant works in slowing down the progression of the cancer.

Participants in the study will receive either the combination of Capivasertib and Fulvestrant or a placebo with Fulvestrant. The study is designed to be double-blind, meaning neither the participants nor the researchers will know who is receiving the actual treatment or the placebo. This helps ensure that the results are not biased. The trial will monitor the participants over time to assess the progression of the disease and any side effects that may occur. The study will also look at specific genetic changes, such as PIK3CA mutations, to see if they affect how well the treatment works.

The trial aims to provide valuable information on the safety and effectiveness of Capivasertib and Fulvestrant in treating this type of breast cancer. It will also gather data on overall survival rates, response to treatment, and quality of life for participants. The study is expected to continue until 2025, allowing researchers to collect comprehensive data on the long-term effects of the treatment. Participants will be closely monitored throughout the study to ensure their safety and well-being.