Study to Assess the Efficacy and Safety of Alpelisib Plus Fulvestrant in Participants With HR-postitive (HR+), HER2-negative, Advanced Breast Cancer After Treatment With a CDK4/6 Inhibitor and an Aromatase Inhibitor.

Inclusion Criteria:

1. 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.
2. If female, then the participant must be in postmenopausal status. Postmenopausal status is defined either by: prior bilateral oophorectomy, age ≥60 or age <60 and amenorrheic for 12 or more months in the absence of chemotherapy, tamoxifen, toremifene, or ovarian suppression and follicle-stimulating hormone (FSH) and estradiol in the postmenopausal range per local normal range.
3. Participant has a histologically and/or cytologically confirmed diagnosis of ER+ and/or PgR+ breast cancer by local laboratory.
4. 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.
5. 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).
6. 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).
7. 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).
8. Participant must show the presence of a PIK3CA mutation(s) determined in tumor tissue prior ro 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.

Exclusion Criteria:

1. Participant with symptomatic visceral disease that makes the participant ineligible for endocrine therapy (ET) per the investigator's best judgment.
2. 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
3. Participant has received prior treatment with fulvestrant, any oral selective estrogen receptor degrader (SERDs), any Phosphatidylinositol-3-Kinase (PI3K), mammalian Target of Rapamycin (mTOR) or Protein Kinase B (AKT) inhibitor

Other Inclusion and Exclusion Criteria do apply