This clinical trial is focused on studying a type of breast cancer known as hormone receptor-positive, HER2-negative advanced breast cancer. This type of cancer has specific characteristics that make it different from other breast cancers. The study is testing a treatment that combines two medications: alpelisib and fulvestrant. Alpelisib is a medication that targets a specific mutation in the cancer cells called the PIK3CA mutation, while fulvestrant is used to block the effects of estrogen, a hormone that can promote the growth of breast cancer cells.

The purpose of the study is to find out if the combination of alpelisib and fulvestrant can help patients live longer without their cancer getting worse, compared to using fulvestrant with a placebo. A placebo is a substance with no active medication, used to compare the effects of the actual treatment. Participants in the study will be randomly assigned to receive either the combination of alpelisib and fulvestrant or fulvestrant with a placebo. The study will be conducted in a way that neither the participants nor the researchers know who is receiving the actual treatment or the placebo, which is known as a double-blind study.

Throughout the study, participants will take the medications for a period of up to 60 days, and their health will be monitored regularly to assess the effects of the treatment. The study aims to provide valuable information on whether this combination of medications can offer a better treatment option for people with this specific type of breast cancer. Participants will be closely observed for any changes in their condition and any side effects they may experience during the study.