Background: The PI3K-mTOR pathway is frequently dysregulated in breast cancer. Combining an inhibitor targeting all class I PI3K isoforms and mTOR complex 1 (mTORC1)-mTOR complex 2 (mTORC2) with endocrine therapy and a CDK4/6 inhibitor might provide more effective tumour control than standard-of-care therapy. To evaluate this hypothesis, gedatolisib, a pan-PI3K-mTOR inhibitor, was assessed in a phase 1b trial combined with palbociclib and endocrine therapy in patients with hormone receptor-positive, HER2-negative, advanced breast cancer. Results from the dose expansion portion of this trial are reported herein., Methods: This multicentre, open-label, phase 1b study recruited female patients aged at least 18 years from 17 sites across the USA with hormone-receptor-positive, HER2-negative, advanced breast cancer and an Eastern Cooperative Oncology Group performance status of 0-1. Four patient groups were studied in the dose expansion portion of the study: treatment-naive in the advanced setting (first line; group A), progression on 1-2 lines of endocrine therapy but CDK4/6 inhibitor-naive (group B); and one or more previous lines (second-line and higher) of therapy, including a CDK4/6 inhibitor (groups C and D). Gedatolisib 180 mg was administered intravenously weekly in 28-day treatment cycles for groups A-C, and on days 1, 8, and 15 for group D. Letrozole (group A), fulvestrant (groups B-D), and palbociclib (all groups) were administered at standard doses and schedules. The primary endpoint was investigator-assessed objective response rate per RECIST version 1.1 in the evaluable analysis set. This trial is completed and registered with ClinicalTrials.gov, NCT02684032., Findings: Between Dec 19, 2017, and June 19, 2019, 103 female participants were enrolled in the dose expansion groups A (n=31), B (n=13), C (n=32), and D (n=27). Median follow-up was 16·6 months (IQR 5·7-48·4) for group A, 11·0 months (7·6-16·9) for group B, 3·6 months (1·8-7·5) for group C, and 9·4 months (5·3-16·7) for group D for the primary endpoint. Gedatolisib, palbociclib, and endocrine therapy induced an objective response in 23 (85·2%; 90% CI 69·2-94·8) of 27 evaluable first-line participants (group A). In the second-line and higher setting, an objective response was observed in eight (61·5%; 90% CI 35·5-83·4) of 13 evaluable group B participants, seven (25·0%; 12·4-41·9) of 28 evaluable group C participants, and 15 (55·6%; 38·2-72·0) of 27 evaluable group D participants; this included participants with both wild-type and mutated PIK3CA tumours. The most common grade 3-4 treatment-related adverse events were neutropenia (65 [63%] of 103), stomatitis (28 [27%]), and rash (21 [20%]). Grade 3-4 hyperglycaemia was reported in six (6%) participants. 23 (22%) of 103 participants had a treatment-related serious adverse event, and there were no treatment-related deaths. Nine (9%) participants discontinued treatment because of a treatment-emergent adverse event., Interpretation: Gedatolisib plus palbociclib and endocrine therapy showed a promising objective response rate compared with the published results for standard-of-care therapies and had an acceptable safety profile., Funding: Pfizer and Celcuity., Competing Interests: Declaration of interests RML received medical writing support and institutional research funding from Celcuity and Pfizer; institutional research funding from Novartis, Eli Lilly, Puma, Zentalis, Arvinas, and Accutar Biotechnology; and consulting fees from or participation on advisory boards for Celcuity, Pfizer, Novartis, Eli Lilly, Gilead, and Biotheryx. HSH received institutional research funding from Abbvie, Arvinas, Celcuity, Department of Defense, G1 Therapeutics, GlaxoSmith Kline, Horizon Oncology Research, Marker Therapeutics, Mersana, Novartis, Pfizer, Phoenix Pharmaceuticals, Pyonr, QuantumLeap Health, and Seagen, Zymeworks; consulting fees from or participation on advisory boards for Daiichi Sankyo–Astra Zeneca, Immunomedics–Gilead, and Novartis; and Speaker's Bureau fees for Eli Lilly. HSR received institutional research funding from Astellas Pharma, AstraZeneca, Daiichi Sankyo, F Hoffmann-La Roche–Genentech, Gilead Sciences, GlaxoSmithKline, Lilly, Merck, Novartis Pharmaceuticals Corporation, OBI Pharma, Pfizer, Pionyr; consulting fees from or participation on advisory boards for Napo Pharmaceutical, Puma Biotechnology, Blueprint, Scorpion Therapeutics, and Daiichi Sankyo; and honoraria from Blueprint, Mylan, and Puma Biotechnology. EMS-R received research funding from Susan G Komen, V Foundation, NIH, and Breast Cancer Research Foundation; consulting fees from or participation on advisory boards for Lilly, SeaGen, Immunomedics, Tersera, AstraZeneca, Merck, Novartis, and Mylan; other payments or honoraria from the American Society of Clinical Oncology, the National Comprehensive Cancer Network, American Association for Cancer Research–Bristol Myers Squibb–Winn Clinical Trials Workshop. JMS received institutional research funding from Celcuity, Pfizer, Merck, Seagen, Seattle Genetics, Minerva, Myriad Pharmaceuticals, Novartis, Genentech, Cascadian Therapeutics, Abbvie, and Nektar; consulting fees from Volastra and GE Healthcare; royalties or licenses from UW Comotion as a donation to the UW Breast Program; honoraria from Binaytara Foundation, Washington State Medical Oncology Society, and Hong Kong International Oncology Symposium; meeting or travel support from ECOG ACRIN, A2 Biotherapeutics, IwCAR-T, the Society of Nuclear Medicine and Molecular Imaging, and Translational Breast Cancer Research Consortium; and is a member of data safety monitoring or advisory boards for GE Healthcare, Sensei Biotherapeutics, and GlaxoSmithKline. ECD received institutional research funding from Merck, Novartis, Meryx, H3 Biomedicine, and G1 Therapeutics and participated on the data safety monitoring board for Sanofi. PK received institutional research funding from AstraZeneca, Hibercell, Sanofi, Eli Lilly, Zymeworks, and Genentech. SS is a full-time employee of Celcuity receiving a salary and holding shares in Celcuity; and received consulting fees from Celcuity (before employment), Artiva Biotherapeutics, Elevation Oncology, Kartos Therapeutics, Tellios Pharma, and Direct Biologics. SCM, BFS, and IG are full-time employees of Celcuity receiving a salary and holding shares in Celcuity. RW received institutional research funding from Celcuity and Pfizer, is a member of advisory boards for Celcuity and Seagen, and is a member of the investigational steering committee for this study., (Copyright © 2024 Elsevier Ltd. All rights reserved.)