Inclusion Criteria:

- Patients with ER-positive locally advanced breast cancer or Metastatic Breast Cancer
(MBC). Postmenopausal women with locally advanced breast cancer or Metastatic Breast
Cancer whose disease has progressed during or after treatment with an aromatase
inhibitor. Part II: Postmenopausal women with locally advanced PIK3CA-mutant breast
cancer or PIK3CA-mutant MBC that has progressed during or after treatment with an AI.

- Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1

- Patients must have measurable disease by RECIST v1.1 or bone-only disease

- Adequate hematologic and end-organ function

- Estrogen receptor-positive disease and HER2-negative disease

Exclusion Criteria:

- Prior treatment with fulvestrant, PI3K inhibitor, or mTOR inhibitor for advanced
breast cancer or MBC

- Prior treatment with > one cytotoxic chemotherapy regimens or experienced recurrent
or progressive disease on > two endocrine therapies for metastatic breast cancer

- History of malabsorption syndrome or other condition that would interfere with
enteral absorption

- History of clinically significant cardiac or pulmonary dysfunction

- Clinically significant history of liver disease

- Active uncontrolled autoimmune disease or active inflammatory disease

- Immunocompromised status

- Symptomatic hypercalcemia

- Need for current chronic corticosteroid therapy

- Pregnancy, lactation, or breastfeeding

- Known untreated or active central nervous system (CNS) metastases Other
protocol-defined inclusion/exclusion criteria may apply.