A Study Evaluating the Efficacy and Safety of Inavolisib Plus CDK4/6 Inhibitor and Letrozole vs Placebo + CDK4/6i and Letrozole in Participants With Endocrine-Sensitive PIK3CA-Mutated, Hormone Receptor-Positive, HER2-Negative Advanced Breast Cancer

Eligibility Criteria

• * Women or men with histologically or cytologically confirmed carcinoma of the breast
• * Documented ER-positive and/or progesterone receptor-positive tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines
• * Documented HER2-negative tumor according to ASCO/CAP guidelines
• * De-novo HR+ , HER2- ABC, or, alternatively, relapsed HR+ , HER2- ABC after at least 2 years of standard neoadjuvant/adjuvant endocrine therapy without disease progression during that treatment and disease-free interval of at least 1 year since the completion of that treatment
• * Participants who have bilateral breast cancers which are both HR-positive and HER2-negative
• * Confirmation of biomarker eligibility
• * Consent to provide fresh or archival tumor tissue specimen
• * Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1)
• * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
• * Adequate hematologic and organ function within 14 days prior to initiation of study treatment
• * Pregnant or breastfeeding, or intention of becoming pregnant during the study or within the time frame in which contraception is required
• * Metaplastic breast cancer
• * Any prior systemic therapy for locally advanced unresectable or metastatic breast cancer
• * Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes
• * Any history of leptomeningeal disease or carcinomatous meningitis
• * Known and untreated, or active CNS metastases. Participants with a history of treated CNS metastases are eligible
• * Active inflammatory or infectious conditions in either eye or history of idiopathic or autoimmune-associated uveitis in either eye
• * Symptomatic active lung disease
• * History of or active inflammatory bowel disease
• * Any active bowel inflammation
• * Prior hematopoietic stem cell or bone marrow transplantation
• * Treatment with strong cytochrome P450 (CYP) 3A4 inhibitors or strong CYP3A4 inducers within 4 weeks or 5 drug-elimination half-lives, prior to initiation of study treatment