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;
             "bone-only" disease, even if considered measurable, is not 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 are 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