Sein

• Participant has histologically-confirmed HER2-positive breast cancer that is advanced (loco-regionally not amenable to surgery or is metastatic).
• 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.
• Participant has an Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
• Participant has adequate bone marrow and organ function
• 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.

• Participant with inflammatory breast cancer at screening.
• 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)
• Participant with an established diagnosis of diabetes mellitus type I or uncontrolled type II based on fasting plasma glucose (FPG) and HbA1c.
• Participant has a known history of acute pancreatitis within 1 year of screening or past medical history of chronic pancreatitis
• Participant has clinically significant, uncontrolled heart disease and/or recent cardiac events
• Participant has a history of Steven-Johnson Syndrome (SJS), erythema multiforme (EM) or Toxic Epidermal Necrolysis (TEN).
• Participant has currently documented pneumonitis/interstitial lung disease

Other protocol-defined Inclusion/Exclusion may apply.