A study to evaluate the efficacy and safety of triplet combination of inavolisib plus ribociclib and fulvestrant versus placebo plus ribociclib and fulvestrant in the first-line setting in participants with endocrine-therapy-resistant hormone receptor (HR)-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer (ABC).

Inclusion Criteria:

• Women or men with histologically or cytologically confirmed carcinoma of the breast that is locally advanced or metastatic and is not amenable to surgical or radiation therapy with curative intent
• Documented estrogen receptor (ER)-positive and/or progesterone receptor (PR)-positive tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines, defined as >=1% of tumor cells stained positive based on the most recent tumor biopsy and assessed locally (Allison et al. 2020)
• Participants must not have received any prior systemic therapy for locally advanced unresectable or metastatic breast cancer (mBC) and must have progressed during adjuvant endocrine-based treatment or within 12 months after completing adjuvant endocrine-based therapy with an aromatase inhibitor or tamoxifen
• Confirmed biomarker eligibility as documented through central laboratory testing of a tumor tissue sample documenting both the lack of a phosphatidylinositol-4,5-biphosphate 3-kinase catalytic subunit alpha gene (PIK3CA) mutation and the presence of heterozygous loss of chromosome 8p (i.e., PIK3CAnmd and chr8p loss)
• Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1)

Exclusion Criteria:

• Metaplastic breast cancer
• Radiotherapy within 2 weeks before randomization
• Appropriate for treatment with cytotoxic chemotherapy at time of entry into the study, as per national or local treatment guidelines (e.g., participants with visceral crisis)
• Type 2 diabetes requiring ongoing systemic treatment at the time of study entry; or any history of Type 1 diabetes
• Known and untreated, or active Central nervous system (CNS) metastases (progressing or requiring anticonvulsants or corticosteroids for symptomatic control). Participants with a history of treated CNS metastases are eligible
• Any history of leptomeningeal disease or carcinomatous meningitis

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing