This study focuses on a type of Advanced Breast Cancer that is Hormone-Receptor-Positive and HER2-Negative. This means the cancer cells grow in response to certain hormones and do not have an excess of a specific protein called HER2. The condition is also Endocrine-Resistant, meaning the cancer has stopped responding to hormone therapy, and it involves a Chromosome 8p Loss, which is a change in the genetic material of the cells. Additionally, the cancer does not have a PIK3CA Mutation, which is a specific change in a gene that often drives cancer growth.

The purpose of the study is to evaluate the effectiveness and safety of a combination of three medications. One group will receive Inavolisib, Ribociclib, and Fulvestrant, while another group will receive a placebo along with Ribociclib and Fulvestrant. Inavolisib and Ribociclib are taken as pills, while Fulvestrant is given as an injection into the muscle.

Participants in the study will be assigned to one of the two treatment groups. Over the course of the study, the combination of drugs will be monitored to see how the cancer responds and to observe any side effects. The study will track how long the cancer stays stable and how long patients live while receiving these treatments.