To Study the Effect of Inavolisib in Combination With Fulvestrant in Participants With Breast Cancer

The purpose of this study is to evaluate the efficacy and safety of inavolisib in combination with fulvestrant compared with inavolisib in combination with fulvestrant in participants with PIK3CA-mutated, HR-positive, HER2-negative locally advanced or metastatic breast cancer (LA/mBC) in the post-cyclin-dependent kinase inhibitor (CDKi) setting.

Inclusion Criteria: * Documented ER +/ HER2- tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines * Disease progression after or during treatment with a combination of CDK4/6i and endocrine therapy: \= 1 study-eligible PIK3CA mutation * Life expectancy of \> 6 months * Ability, in the investigator's judgment, and willingness to comply with all study -related procedures, including completion of patient-reported outcomes Exclusion Criteria: * Metaplastic breast cancer * Prior treatment with chemotherapy in the recurrent locally advanced/metastatic setting * Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes * Prior treatment with PI3K/Akt/mTOR inhibitors in the recurrent locally advanced/metastatic setting * Requirement for daily supplemental oxygen * Symptomatic active lung disease, including pneumonitis