Purpose: The aim of this study was to evaluate the effectiveness of CDK 4/6 inhibitors (CDK 4-6i) according to HER2 status (low/zero), and endocrine resistance/sensitivity, as well as the efficacy of second-line treatments, in a large real-world cohort., Methods: The GIM14/BIOMETA study (NCT02284581) is a retrospective/prospective study of the Gruppo Italiano Mammella evaluating treatment patterns and survival outcomes in patients with metastatic breast cancer (MBC). We retrieved data on patients with hormone receptor-positive/HER2-negative MBC receiving first-line CDK 4/6i., Results: Among 3832 patients enrolled in the GIM14-BIOMETA study, 701 were eligible. At a median follow-up of 24.80 months, no significant differences were found between HER2-zero and HER2-low subgroups in terms of first-line time to treatment discontinuation (TTD) (26.16 months [IQR 12.84-NR] vs. 27.60 months [IQR 12.12-64.44], p = 0.972) or overall survival (OS) (mOS>60 months for both groups, p = 0.398). Median TTD was 33.24 months (IQR 16.32-NR) for the endocrine sensitive subgroup, 19.92 months (IQR 8.88-51.24) for the secondary endocrine resistant subgroup and 17.40 months (IQR 7.44-24.72) for the primary endocrine resistant subset, respectively (p < 0.001). Among 239 patients receiving second-line treatment, no significant difference (p = 0.188) was found in terms of second-line TTD between those treated with capecitabine (6.11 months, IQR 2.96-11.47), taxane-based chemotherapy (5.06 months, IQR 2.99-9.99), everolimus plus exemestane (5.39 months, IQR 2.53-9.03) or fulvestrant (6.44 months, IQR 3.38-NR)., Conclusions: Endocrine therapy plus CDK 4/6i represents an effective treatment, regardless of HER2 status (low/zero). Second-line agents did not differ significantly in terms of TTD. Endocrine resistant cancers exhibit poor response to CDK 4/6i., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Chiara Molinelli reports honoraria for consultancy and consulting fees from Daiichi Sankyo, travel support to attend conferences from Gilead, Menarini and fees for medical writing from Lilly and Seagen. Eva Blondeaux reports honoraria from Eli Lilly and research funding (to institution) from Gilead. Michelino De Laurentiis reports honoraria or consultation fees for speaker, consultancy or advisory roles from Eli Lilly, Seagen, Therapeutics, GSK, Roche, Novartis, Genetic, Pfizer, Pierre Fabre, MSD, Menarini Stemline, Sophos Biotech, Max Farma, Celltrion Healthcare, Gilead, Daiichi Sankyo; institutional financial interests, financial support for clinical trials or contracted research, from: Novartis, Pierre Fabre, Pfizer, Roche, Stemline Therapeutics. Alessandra Fabi reports consulting fees and honoraria from Roche, Novartis, Lilly, Pfizer, MSD, Gilead, Seagen, Astra Zeneca, Daiichi Sankyo. Support for attending meetings and/or travel from Novartis, Lilly, Pfizer, MSD. Grazia Arpino reports research and medical writing from AstraZeneca; advisory boards, travel grants, activities as a speaker, consultancy from AstraZeneca, Daiichi Sankyo, Eisai, Eli Lilly, Gilead, Exact Science, Novartis, Roche, Seagen, Viatris. Andrea Fontana reports advisory boards from Roche, Novartis, Eli Lilly, Pfizer, MSD, Pierre Fabre, Eisai, Gilead, Seagen, Astra Zeneca, Exact Sciences; consulting fees from Dompé Farmaceutici S.p.A. D.G. Research funding from Menarini, Seagen, Novartis, AstraZeneca; advisory boards, activities as a speaker, consultancy from Seagen, Novartis, Eli Lilly, Roche, Gilead, Pfizer, Eisai, Exact Sciences. Fabio Puglisi reports advisory role for and receiving speaker honoraria from Amgen, AstraZeneca, Daichii Sankyo, Celgene, Eisai, Eli Lilly, Exact Sciences, Gilead, Ipsen, Menarini, MSD, Novartis, Pierre Fabre, Pfizer, Roche, Seagen, Takeda, and Viatris. Travel grants from AstraZeneca, Daichii Sankyo, Novartis, Roche. Research grants (to his institution) from Astrazeneca, Eisai, Roche. Claudia Bighin reports personal fees from Roche, Novartis, Lilly, Gilead, and Daiichi. Matteo Lambertini reports advisory role for Roche, Lilly, Novartis, Astrazeneca, Pfizer, Seagen, Gilead, MSD, Exact Sciences, Pierre Fabre, Menarini; speaker honoraria from Roche, Lilly, Novartis, Pfizer, Sandoz, Libbs, Daiichi Sankyo, Takeda, Menarini, AstraZeneca; travel Grants from Gilead, Daiichi Sankyo, Roche; research funding (to the Institution) from Gilead all outside the submitted work. Lucia Del Mastro reports grants from Eli Lilly, Novartis, Roche, Daiichi Sankyo, Seagen, AstraZeneca, Gilead, and Pierre Fabre; receiving consulting fees from Eli Lilly, Gilead, and Daiichi Sankyo; receiving speaker honoraria from Roche, Novartis, Pfizer, Eli Lilly, AstraZeneca, MSD, Seagen, Gilead, Pierre Fabre, Eisai, Exact Sciences, Ipsen, GSK, and Agendia-Stemline; receiving travel grants from Roche, Pfizer, Eisai, Daiichi Sankyo, and AstraZeneca; and having an advisory role for Novartis, Roche, Eli Lilly, Pfizer, Daiichi Sakyo, Exact Sciences, Gilead, Pierre Fabre, Eisai, AstraZeneca, Agendia, GSK, and Seagen. All the other authors declare no conflict of interest., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)