Background: Hyperglycaemia is an early and frequent adverse event during alpelisib treatment. METALLICA aimed to evaluate prophylactic metformin to prevent or reduce hyperglycaemia occurrence in patients with HR+/HER2-/ PIK3CA -mutated advanced breast cancer (ABC)., Methods: Between August 13th, 2020 and March 23rd, 2022, this 2-cohort, phase 2, multicentre, single-arm trial (NCT04300790) enrolled patients with HR+/HER2-/PIK3CA-mutated ABC: cohort A, normal glycaemia (fasting plasma glucose <100 mg/dL [<5.6 mmol/L] and HbA1c <5.7%), and cohort B, prediabetes (fasting plasma glucose 100-140 mg/dL [5.6-7.8 mmol/L] and/or haemoglobin A1C [HbA1c] 5.7-6.4%). Participants were at least 18 years old, with Eastern Cooperative Oncology Group performance status of 0-1, and up to two prior lines of endocrine therapy (ET) for ABC. Alpelisib plus ET were administered in 28-day cycles after initiation of prophylactic metformin plus ET. Primary endpoint was the incidence of grade 3-4 hyperglycaemia over the first 8 weeks. Secondary endpoints included safety, progression-free survival (PFS), objective response rate (ORR), and clinical benefit rate (CBR). The primary objective for cohort A and B is met with ≤7 (14.6%) and ≤4 (20%) patients with grade 3-4 hyperglycaemia over the first 8 weeks, respectively., Findings: 233 patients were screened, and 68 (20.2%) patients were enrolled in cohorts A (n = 48) and B (n = 20). Median follow-up was 7.8 months (IQR 1.4-19.6). Over the first 8 weeks, one (2.1%) of 48 patients in cohort A (95% CI: 0.5-11.1; P < 0.0001), and three (15.0%) of 20 patients in cohort B (95% CI: 5.6-37.8; P = 0.016) had grade 3-4 hyperglycaemia. Serious treatment-related adverse events occurred in seven patients (10.3%). The most common were rash (two [2.9%]), vomiting (two [2.9%]), and diarrhoea (two [2.9%]). Discontinuation of alpelisib caused by AEs was reported in nine patients (13.2%), none caused by hyperglycaemia. At data cutoff (15 June, 2022), no treatment-related deaths were observed. In the full analysis set, median PFS was 7.3 months (95% CI: 5.9-not reached), ORR was 20.6% (95% CI: 11.7-32.1%), and CBR was 52.9% (95% CI: 40.4-65.2)., Interpretation: In HR+/HER2-/ PIK3CA -mutated ABC, prophylactic metformin before alpelisib plus endocrine treatment has low incidence and severity of alpelicib-induced hyperglycaemia., Funding: Novartis Pharmaceuticals., Competing Interests: ALC reports research support from Roche, Agendia, Lilly, Pfizer, Novartis, Merck Sharp & Dohme, Gilead, and Daichii-Sanyo; a consulting or advisory role with Lilly, Roche, Pfizer, and Novartis. Speakers’ bureaus for Lilly, AstraZeneca, Merck Sharp & Dohme; travel support from Roche, Pfizer, AstraZeneca; and stock or other ownership in MEDSIR and Initia-Research. JMPG reports a consulting or advisory role with Lilly, Roche, Eisai, Daichii Sankyo, AstraZeneca, Seattle Genetics; and employment at MEDSIR. MRB reports research support and participating in an advisory board of Novartis. PT reports a consulting or advisory role with AstraZeneca, Daiichi-Sankyo, Novartis and Seagen. Speakers’ bureaus for Pfizer, Novartis, Lilly, AstraZeneca, Daiichi-Sankyo and Seagen. IB reports grants and research support to the Institution: AstraZeneca, Lilly, Pfizer and Roche; honoraria and advisor colaboration: AstraZeneca, Roche, Novartis, Eisai, Celgene, Pfizer, Lilly, Pierre-Fabre, Bristol-Myers Squibb, Daiichi Sankyo, Grünenthal, Seagen and Veracyte and support for attending meetings and/or travel: AstraZeneca, Roche, Novartis, Pfizer, Lilly, Pierre-Fabre, Bristol-Myers Squibb and Daiichi Sankyo. BR reports travel support from Roche and Lilly. AFO reports travel support from Novartis. CS reports research support, consulting or advisory role from Genentech, AstraZeneca, Aragon Pharmaceuticals, Bayer Pharma, Byondis, Boehringer Ingelheim, Bristol-Myers Squibb, Cytomx Therapeutics, Daiichi Sankyo, F. Hoffmann-La Roche, Eisai, Genomic Health, GlaxoSmithKline, Innoup Farma, Eli Lilly, Macrogenics, Menarini Ricerche, Merck Sharp & Dohme, Merus, Millennium Pharmaceuticals, Novartis, Pfizer, Pierre Fabre, PintPharma, Puma Biotechnology, Sanofi, Seattle Genetics, Zymeworks. JPL reports honoraria from Seagen, Novartis, Pfizer, AstraZeneca/Daiichi Sankyo, Eli Lilly, Roche; and a consulting or advisory role with Seagen, Novartis, AstraZeneca/Daiichi Sankyo, Roche. EL reports honoraria from Ipsen, MSD, Pfizer, AstraZeneca/Daiichi Sankyo and Roche. EG reports honoraria from Roche/Genentech, Pfizer, Novartis, AstraZeneca, Seagen; a consulting or advisory role with AstraZeneca, Seagen, PharmaMar, Gilead Sciences; and speakers’ bureaus for Pfizer, Novartis, Roche. SGS reports honoraria for speaking from Novartis and GSK; Advisory role from Lilly, Seagen and Astra Zeneca; and travel expenses from Clovis and Pfizer outside the submitted work. EC reports consultancy and speakers’ bureaus for Novartis, Pfizer, Roche, Lilly, Celgene; and personal fees from Novartis, Pfizer, Roche, Lilly, Celgene, during the conduct of the study. JFC reports honoraria from Roche, AstraZeneca, Teva, Celgene, Pfizer, GSK, Novartis; and a consulting or advisory role with Roche, AstraZeneca, Pfizer, GSK, Novartis. MSC reports advisory Board of MEDSIR, Optimapharm; Speaker's Bureau in MEDSIR, Optimapharm; and Funding from MEDSIR, Optimapharm, FGP has taken part in advisory panels for Sanofi and Novo Nordisk; has received research support from Sanofi, Novo Nordisk, Boehringer Ingelheim Pharmaceuticals and Lilly; and has acted as a speaker for Sanofi, Novo Nordisk, Boehringer Ingelheim Pharmaceuticals, AstraZeneca Pharmaceuticals LP, Bristol-Myers Squibb Co. and Lilly. JC reports research support from ARIAD, Astrazeneca, Baxalta GMBH/Servier Affaires, Bayer, Eisai, Guardanth health, Merck Sharp & Dohme, Pfizer, Puma, Queen Mary University of London, Roche, Piqur; honoraria from Novartis, Eisai, Celgene, Pfizer, Roche, Samsung, Eli Lilly, Merck Sharp & Dohme Daiichi Sankyo; a consulting or advisory role with Celgene, Cellestia Biotech, AstraZeneca, Roche, Seattle Genetics, Daiichi Sankyo, ERYTECH Pharma, Polyphor, Athenex, Lilly, SERVIER, Merck Sharp & Dohme, GlaxoSmithKline, Leuko, Clovis Oncology, Bioasis, Boehringer Ingelheim, Ellipses Pharma, HiberCell, Bioinven, GEMoaB, Gilead Sciences, Menarini, Zymeworks, Reveal Genomics; stock or other ownership in Leuko, MEDSIR, Nektar; and travel support from Roche, Pfizer, Eisai, Novartis, Daiichi Sankyo. AU, EMD, SB, BB, MGC, PCC, AL, JLAR report no competing interests. JMPG, GM, PG, DAL, and MSC are employed by MEDSIR., (© 2024 Published by Elsevier Ltd.)