Background: Alpelisib, a PI3Kα-selective inhibitor and degrader, plus fulvestrant showed efficacy in hormone receptor-positive, HER2-negative, PIK3CA-mutated advanced breast cancer in SOLAR-1; limited data are available in the post-cyclin-dependent kinase 4/6 inhibitor setting. BYLieve aimed to assess alpelisib plus endocrine therapy in this setting in three cohorts defined by immediate previous treatment; here, we report results from cohort A., Methods: This ongoing, phase 2, multicentre, open-label, non-comparative study enrolled patients with hormone receptor-positive, HER2-negative, advanced breast cancer with tumour PIK3CA mutation, following progression on or after previous therapy, including CDK4/6 inhibitors, from 114 study locations (cancer centres, medical centres, university hospitals, and hospitals) in 18 countries worldwide. Participants aged 18 years or older with an Eastern Cooperative Oncology Group performance status of 2 or less, with no more than two previous anticancer treatments and no more than one previous chemotherapy regimen, were enrolled in three cohorts. In cohort A, patients must have had progression on or after a CDK4/6 inhibitor plus an aromatase inhibitor as the immediate previous treatment. Patients received oral alpelisib 300 mg/day (continuously) plus fulvestrant 500 mg intramuscularly on day 1 of each 28-day cycle and on day 15 of cycle 1. The primary endpoint was the proportion of patients alive without disease progression at 6 months per local assessment using Response Evaluation Criteria in Solid Tumors, version 1.1, in patients with a centrally confirmed PIK3CA mutation. This trial is registered with ClinicalTrials.gov, NCT03056755., Findings: Between Aug 14, 2017, and Jul 29, 2022 (data cutoff), 127 patients with at least 18 months' follow-up were enrolled into cohort A. 119 patients had a centrally confirmed PIK3CA mutation. At data cutoff, median follow-up was 21·8 months (IQR 10·8-37·6). 64 (53·8%; 95% CI 44·4-63·0) of 119 patients were alive without disease progression at 6 months. The most frequent grade 3 or worse adverse events were hyperglycaemia (37 [29%] of 127 patients), rash (13 [10%]), and rash maculopapular (11 [9%]). Serious adverse events occurred in 37 (29%) of 127 patients. No treatment-related deaths were reported., Interpretation: BYLieve showed activity of alpelisib plus fulvestrant with manageable toxicity in patients with PIK3CA-mutated, hormone receptor-positive, HER2-negative advanced breast cancer, after progression on a CDK4/6 inhibitor plus an aromatase inhibitor., Funding: Novartis Pharmaceuticals., Competing Interests: Declaration of interests HSR reports research support to the University of California San Francisco from Pfizer, Merck, Novartis, Eli Lilly, Genentech, OBI, Odonate, Daiichi, Eisai, Seattle Genetics, MacroGenics, Sermonix, and Immunomedics; a limited consulting role with Samsung; honoraria from Puma and Mylan; and travel support from Daiichi, Mylan, Pfizer, Merck, AstraZeneca, Novartis, and MacroGenics. FL reports consulting for AstraZeneca, Eisai, Genomic Health, and Eli Lilly; and travel support from Eli Lilly, Novartis, Pfizer, Roche, and Laboratories Pierre Fabre. EC reports honoraria, and consulting and paid speakers’ bureaus for Novartis, Pfizer, Eli Lilly, and Roche; and travel support from Roche and Pfizer. PD reports research funding from Roche and Novartis. YHP reports grants from Novartis, during the conduct of the study; personal fees from AstraZeneca, Pfizer, Daiichi Sankyo, and Eli Lilly; grants and personal fees from Merck, AstraZeneca, Eisai, and Novartis; grants and non-financial support from Pfizer; and personal fees and non-financial support from Roche, outside the submitted work. AP reports immediate family member employee of Novartis; personal honoraria from Pfizer, Novartis, Roche, MSD Oncology, Eli Lilly, Daiichi Sankyo, and Amgen; consultant for Amgen, Roche, Novartis, Pfizer, Bristol-Myers Squibb, Boehringer, Puma, Oncolytics Biotech, Daiichi Sankyo, and AbbVie; institutional honorarium from Nanostring Technologies; institutional research funding from Roche, Novartis, Incyte, and Puma; patents on HER2 and chemoendocrine score predictors; travel support from Daiichi Sankyo; an advisory role with Oncolytics and consultancy for Peptomyc; and is the founder of Reveal Genomics. TB reports personal fees and non-financial support from Roche; and grants, personal fees, and non-financial support from Novartis, AstraZeneca, and Pfizer, outside the submitted work. DJ reports personal fees from Novartis, Genentech, Eisai, Ipsen, and EMD Serono, during the conduct of the study. NT reports advisory board honoraria from AstraZeneca, Bristol-Myers Squibb, Lilly, Merck Sharpe & Dohme, Novartis, Pfizer, Roche/Genentech, Bicycle Therapeutics, Taiho, Zeno Pharmaceuticals, Repare Therapeutics; and research funding from AstraZeneca, BioRad, Pfizer, Roche/Genentech, Clovis, Merck Sharpe & Dohme, and Guardant Health. NS reports employment at Novartis. JPZ reports employment at and stock ownership in Novartis. JPZ's wife is employed by Johnson & Johnson. CA and Y-MS report employment at Novartis. ST reports employment at and stock ownership in Novartis. HK and W-CH reports consulting agreements and research funding for the conduct of this study from Novartis. SC reports grants and personal fees from Novartis and Pfizer; and personal fees from Eli Lilly, during the conduct of the study; and grants and personal fees from Hoffmann LaRoche, AstraZeneca, and Genomic Health, outside the submitted work. MR-B and PN declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved.)