This study looks at the effectiveness and safety of adding a new drug, inavolisib, to a treatment for advanced breast cancer. It will be combined with a CDK4/6 inhibitor (a type of medicine that stops cancer cells from growing) and letrozole (a medicine that lowers estrogen levels) and compared to a placebo (a fake treatment). Participants need to have a specific kind of breast cancer called PIK3CA-mutated, hormone receptor-positive, HER2-negative advanced breast cancer.

**Key Points:**

• The study requires participants to have regular visits to check progress and side effects.
• People with certain health conditions, like diabetes or specific types of breast cancer, cannot join.
• Participants must be willing to provide tissue samples and have proper organ function.

Participants must not be pregnant or planning to get pregnant, and should not have received certain previous cancer treatments. If you meet the criteria and are interested, this study could help find better ways to treat advanced breast cancer.