Study Assessing the Efficacy and Safety of Alpelisib Plus Fulvestrant or Letrozole, Based on Prior Endocrine Therapy, in Patients With PIK3CA Mutant, HR+, HER2- Advanced Breast Cancer Who Have Progressed on or After Prior Treatments

Key Inclusion Criteria: Subjects eligible for inclusion in the Core Phase of the study fulfilled the following key inclusion criteria: * Subject was an adult male or female ≥ 18 years of age. * Subjects with histologically and/or cytologically confirmed diagnosis of estrogen receptor (ER)-positive and/or progesterone receptor (PgR)-positive breast cancer by local laboratory. * Subjects with a confirmed human epidermal growth factor receptor-2 (HER2)-negative aBC. * Subjects with gene encoding the p110alpha catalytic subunit of PI3K (phosphatidylinositol-3-kinase) (PIK3CA) mutation. * In case of women, both premenopausal and postmenopausal subjects were allowed to be included in this study. * Subjects with documented evidence of tumor progression on or after: i. Cyclin-Dependent Kinase 4 and 6 inhibitor (CDK4/6i) treatment as last treatment regimen in Cohorts A and B. ii. aromatase inhibitor (AI) treatment (either in adjuvant or metastatic setting) and received systemic chemotherapy or ET (monotherapy or combination except CDK4/6i + AI) as last treatment regimen in Cohort C. Upon completion of enrollment of Cohort B, subjects who received CDK4/6i + fulvestrant as immediate prior therapy were eligible for Cohort C. iii. No more than 2 prior anticancer therapies for aBC. iv. Received no more than 1 prior regimen of chemotherapy for the treatment of advanced/metastatic disease was permitted. v. Recovered to grade 1 or better from any adverse events (AEs) related to previou…