Study of Alpelisib (BYL719) in Combination With Trastuzumab and Pertuzumab as Maintenance Therapy in Patients With HER2-positive Advanced Breast Cancer With a PIK3CA Mutation

Inclusion Criteria:

1. Participant has histologically-confirmed HER2-positive breast cancer that is advanced (loco-regionally not amenable to surgery or is metastatic).
2. Participant has received pre-study induction therapy with up to and including a maximum of 8 cycles of a taxane (docetaxel, paclitaxel, or nab-paclitaxel), plus trastuzumab and pertuzumab. 4 or 5 cycles of induction therapy are permitted if discontinuation of taxane was due to taxane toxicity. Of note, participants enrolled in Part 1 of this study received 4-6 cycles of pre-study induction therapy.
3. Participant has an Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
4. Participant has adequate bone marrow and organ function
5. Applies only to Part 2: Participant has a PIK3CA mutation(s) present in tumor prior to enrollment, locally confirmed per test listed in protocol or as determined by a Novartis designated central laboratory.

Exclusion Criteria:

1. Participant with inflammatory breast cancer at screening.
2. Participant with evidence of disease progression during the pre-study induction therapy and prior to first dose of alpelisib (or alpelisib/alpelisib matching-placebo for Part 2)
3. Participant with an established diagnosis of diabetes mellitus type I or uncontrolled type II based on fasting plasma glucose (FPG) and HbA1c.
4. Participant has a known history of acute pancreatitis within 1 year of screening or past medical history of chronic pancreatitis
5. Participant has clinically significant, uncontrolled heart disease and/or recent cardiac events
6. Participant has a history of Steven-Johnson Syndrome (SJS), erythema multiforme (EM) or Toxic Epidermal Necrolysis (TEN).
7. Participant has currently documented pneumonitis/interstitial lung disease

Other protocol-defined Inclusion/Exclusion may apply.