The purpose of this study is to complement Study CBYL719C2301 (SOLAR-1) and obtain more comprehensive data on the efficacy and safety of alpelisib (BYL719) in combination with fulvestrant compared with placebo plus fulvestrant in men or postmenopausal women with HR-positive, HER2-negative advanced breast cancer with a PIK3CA mutation who progressed or relapsed on or after treatment with an AI plus a CDK4/6 inhibitor.

This is a Phase III, randomized, double-blind, placebo-controlled, international, multi-center trial. Men and postmenopausal women will be randomized to either alpelisib plus fulvestrant or alpelisib-matching placebo plus fulvestrant. Randomization will follow a 1:1 randomization ratio and be stratified by presence of lung and/or liver metastases (yes vs. no) and setting at last prior CDK4/6 inhibitor therapy (adjuvant vs metastatic).

Study treatment with alpelisib plus fulvestrant or alpelisib-matching placebo plus fulvestrant will be initiated on Cycle 1 Day 1, and will continue until disease progression per RECIST v1.1 as per BIRC assessment, start of new antineoplastic therapy, death, lost to follow-up, or withdrawal of consent. A cycle is defined as 28 days.

Key Inclusion Criteria:

• Participant is an adult ≥ 18 years old at the time of informed consent and has signed informed consent before any trial related activities and according to local guidelines.
• Participant has a histologically and/or cytologically confirmed diagnosis of ER+ and/or PgR+ breast cancer by local laboratory.
• Participant has HER2-negative breast cancer defined as a negative in situ hybridization test or an IHC status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (Fluorescent in situ hybridization (FISH), Chromogenic in situ hybridization (CISH), or Silver-enhanced in situ hybridization (SISH)) test is required by local laboratory testing.
• Participant has at least one measurable lesion as per RECIST v1.1 criteria as assessed by Investigator (a lesion at a previously irradiated site may only be counted as a target lesion if there is clear sign of progression since the irradiation).
• Participant has recurrence or progression of disease during or after combined AI (i.e. letrozole, anastrozole, exemestane) and CDK4/6 inhibitor therapy. The combined AI and CDK4/6 inhibitor therapy does not need to be the latest treatment regimen (including adjuvant setting).
• Participant has received ≤ 2 prior lines of systemic therapies overall in the metastatic setting, of which a maximum of 1 line of prior treatment with chemotherapy (except for neoadjuvant/ adjuvant chemotherapy) is permitted.
• The presence of PIK3CA mutation(s) determined in tumor tissue prior to enrollment either by a Novartis designated laboratory or in tumor tissue or plasma ctDNA by a local laboratory using a Food and Drug Administration (FDA)-approved PIK3CA Companion Diagnostics (CDx) test for alpelisib or the CE-IVD QIAGEN Therascreen® PIK3CA RGQ PCR test.
• If female, then the participant must be in postmenopausal status.

Key Exclusion Criteria:

• Participant with symptomatic visceral disease or any disease burden that makes the participant ineligible for endocrine therapy (ET) per the Investigator's best judgment.
• Participant who relapsed with documented evidence of progression more than 12 months from completion of (neo)adjuvant endocrine/endocrine-based therapy with no treatment for metastatic disease.
• Participant has received prior treatment with fulvestrant, any oral selective estrogen receptor degrader (SERD), any Phosphatidylinositol-3-Kinase (PI3K), mammalian Target of Rapamycin (mTOR) or Protein Kinase B (AKT) inhibitor.

Other Inclusion and Exclusion Criteria do apply

Novartis is committed to sharing with qualified external researchers, access to patient-level data and supporting clinical documents from eligible studies. These requests are reviewed and approved by an independent review panel on the basis of scientific merit. All data provided is anonymized to respect the privacy of patients who have participated in the trial in line with applicable laws and regulations.

This trial data availability is according to the criteria and process described on www.clinicalstudydatarequest.com