A Study Evaluating the Efficacy and Safety of Inavolisib + Palbociclib + Fulvestrant vs Placebo + Palbociclib + Fulvestrant in Patients With PIK3CA-Mutant, Hormone Receptor-Positive, Her2-Negative, Locally Advanced or Metastatic Breast Cancer

Inclusion Criteria

Confirmed diagnosis of HR+/HER2- breast cancer
Metastatic or locally advanced disease not amenable to curative therapy
Progression of disease during adjuvant endocrine treatment or within 12 months of completing adjuvant endocrine therapy with an aromatase inhibitor or tamoxifen
Receiving LHRH agonist therapy for at least 2 weeks prior to Day 1 of Cycle 1 if pre/peri-menopausal
Confirmation of biomarker eligibility (detection of specified mutation(s) of PIK3CA via specified test)
Consent to provide fresh or archival tumor tissue specimen
Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1); evaluable "bone-only" disease is not eligible; "bone-only" disease with at least one measurable, soft-tissue component, even if considered disease that is limited to bone but has lytic or mixed lytic/blastic lesions and at least one measurable soft-tissue component per RECIST v1.1 may be eligible
Eastern Cooperative Oncology Group Performance Status of 0 or 1
Life expectancy of > 6 months
Adequate hematologic and organ function within 14 days prior to initiation of study treatment

Exclusion Criteria

Metaplastic breast cancer
Any history of leptomeningeal disease or carcinomatous meningitis
Any prior systemic therapy for metastatic breast cancer
Prior treatment with fulvestrant or any selective estrogen-receptor degrader, with the exception of participants that have received fulvestrant or any selective estrogen-receptor degrader as part of neoadjuvant therapy only and with treatment duration of no longer than 6 months
Prior treatment with any PI3K, AKT, or mTOR inhibitor, or any agent whose mechanism of action is to inhibit the PI3K-AKT-mTOR pathway
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 CNS metastases. Patients with a history of treated CNS metastases may be eligible
Active inflammatory or infectious conditions in either eye, or any eye conditions expected to require surgery during the study treatment period
Symptomatic active lung disease, or requiring daily supplemental oxygen
History of inflammatory bowel disease or active bowel inflammation
Anti-cancer therapy within 2 weeks before study entry
Investigational drug(s) within 4 weeks before randomization
Prior radiotherapy to >= 25% of bone marrow, or hematopoietic stem cell or bone marrow transplantation
Chronic corticosteroid therapy or immunosuppressants
Pregnant, lactating, or breastfeeding, or intending to become pregnant during the study or within 60 days after the final dose of study treatment
Major surgical procedure, or significant traumatic injury, within 28 days prior to Day 1 of Cycle 1