Purpose: This phase II study evaluated the impact of adding ribociclib to maintenance endocrine therapy (ET) treatment of physicians' choice following the first palliative chemotherapy in pre- and post-menopausal women with hormone receptor positive (HR+)/human epidermal growth factor 2 negative (HER2-) metastatic breast cancer (mBC)., Patients and Methods: The initial randomized study design was later amended into a single-arm study, and all subsequent patients received ribociclib and ET. The primary end point was locally assessed progression-free survival (PFS). Secondary end points included overall survival (OS), clinical benefit rate (CBR), safety, compliance, and quality of life (QoL)., Results: A total of 43 patients received ribociclib + ET and 10 patients received ET only. Median PFS was 12.4 months [95% CI 8.7-24.4] for patients who received ribociclib + ET and 4.75 months [95% CI 1.0-10.3] for those who received ET only. Median OS was not reached for patients who received ribociclib + ET, and 28 (65.1%) patients experienced clinical benefit [95% CI 49.1-79.0]. For patients who received ribociclib + ET, grade 3-4 hematological adverse events (AEs) occurred in 25 (58.1%) patients, and grade 3-4 non-hematological AEs occurred in 17 (39.5%) patients. During the study, 15 patients died - 14 of whom due to tumor-related reasons, and one patient due to pneumonia, which was not treatment-related., Conclusion: The results of the AMICA study show a promising efficacy and safety of maintenance treatment with ribociclib added to ET after at least stable disease following the first metastatic chemotherapy in patients with HR+/HER2-mBC., Trial Registration: Anti-hormonal Therapy With Ribociclib in HR-positive/HER2- Negative Metastatic Breast Cancer (AMICA), NCT03555877, https://clinicaltrials.gov/ct2/show/NCT03555877., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. T. Decker reports ad boards from Lilly, Novartis, and IOMEDICO. K. Luedtke-Heckenkamp reports personal fees from AstraZeneca, Gilead, Novartis, and Pfizer. She is on the advisory board of Daiichi Sankyo, Lilly, and Roche. K. Lübbe is on the advisory board of Genomic Health, Roche, Daiichy Sankyo, Lilly, MSD, EISAI, Seagen, and Novartis, and has received lecture fees from Novartis, AstraZeneca, Genomic Health, Roche, and Lilly. M. Schmidt reports personal fees from AstraZeneca, personal fees from BioNTech, personal fees from Daiichi Sankyo, personal fees from Eisai, personal fees from Lilly, personal fees from MSD, personal fees from Novartis, personal fees from Pfizer, personal fees from Pierre-Fabre, personal fees from Roche, and personal fees from SeaGen outside the submitted work; in addition, M. Schmidt has a patent for EP 2390370 B1: A method for predicting the response of a tumor in a patient suffering from or at risk of developing recurrent gynecologic cancer towards a chemotherapeutic agent issued and a patent for EP 2951317 B1: A method for predicting the benefit from inclusion of a taxane in a chemotherapy regimen in patients with breast cancer issued. C. Denkert reports grants from European Commission H2020, grants from German Cancer Aid Translational Oncology, grants from German Breast Group; personal fees from Novartis, Roche, MSD Oncology, Daiichi Sankyo, AstraZeneca, Lilly, Molecular Health, Merck, grants from Myriad to his institution, other from Sividon diagnostics (cofounder and former shareholder until 2016); In addition, C. Denkert has a patent VMScope digital pathology software with royalties paid, a patent WO2020109570A1 - cancer immunotherapy pending, and a patent WO2015114146A1 and WO2010076322A1- therapy response issued. V. Müller received speaker honoraria from Amgen, Astra Zeneca, Daiichi-Sankyo, Eisai, GSK, Pfizer, MSD, Medac, Novartis, Roche, Teva, Seagen, Onkowissen, high5 Oncology, Medscape, Gilead, and Pierre Fabre; consultancy honoraria from Hexal, Roche, Pierre Fabre, Amgen, ClinSol, Novartis, MSD, Daiichi-Sankyo, Eisai, Lilly, Sanofi, Seagen, and Gilead; institutional research support from Novartis, Roche, Seagen, and Genentech; and travel grants from Roche, Pfizer, Daiichi Sankyo, and Gilead. M. Thill is on the advisory boards of Agendia, Amgen, AstraZeneca, Aurikamed, Becton/Dickinson, Biom‘Up, ClearCut, Clovis, Daiichi Sankyo, Eisai, Exact Sciences, Gilead Science, Grünenthal, GSK, Lilly, MSD, Norgine, Neodynamics, Novartis, Onkowissen, Organon, Pfizer, pfm Medical, Pierre-Fabre, Roche, RTI Surgical, Seagen, Sirius Pintuition, and Sysmex. He has received support from Amgen, AstraZeneca, Celgene, ClearCut, Clovis, Daiichi Sanyko, Hexal, Neodynamics, Novartis, Pfizer, pfm medical, Roche, Servier, and Sirius Medical. He reports travels expenses from Amgen, Art Tempi, AstraZeneca, Clearcut, Clovis, Connect Medica, Daiichi Sankyo, Eisai, Exact Sciences, Hexal, I-Med-Institute, Lilly, MCI, Medtronic, MSD, Neodynamics, Norgine, Novartis, Pfizer, pfm Medical, Roche, RTI Surgical, and Seagen. He has received honoraria and funding from Amgen, Art Tempi, AstraZeneca, Biom’Up, Celgene, Clearcut, Clovis, Connect Medica, Eisai, Endomag, Exact Sciences, Gedeon Richter, Gilead Science, GSK, Hexal, I-Med-Institute, Jörg Eickeler, Laborarztpraxis Walther et al., Lilly, MCI, Medscape, MSD, Medtronic, Neodynamics, Novartis, Onkowissen, Pfizer, pfm medical, Roche, RTI Surgical, Seagen, StreamedUp, Sysmex, Vifor, and Viatris. N. Hirmas and N. Filmann declare to be GBG Forschungs GmbH employees. GBG Forschungs GmbH received funding for research grants from Abbvie, AstraZeneca, BMS, Daiichi-Sankyo, Gilead, Novartis, Pfizer, and Roche (paid to the institution); other (non-financial/medical writing) from Daiichi-Sankyo, Gilead, Novartis, Pfizer, Roche and Seagen (paid to the institution). GBG Forschungs GmbH has following royalties/patents: EP14153692.0, EP21152186.9, EP15702464.7, EP19808852.8 and VM Scope GmbH. S. Loibl reports grants or contracts paid to institute from Abbvie, AstraZeneca, DSI, Celgene, Gilead, Novartis, Pfizer, Roche, Molecular H; in addition, S. Loibl has royalties or licenses for Digital Ki67 Evaluator from VM Scope GmbH that were paid to institute; in addition, S. Loibl receives honorary for lectures paid to the institute from AstraZeneca, DSI, Gilead, Novartis, Pfizer, and Roche; non-financial for Medical Writing from DSI, Gilead, Novartis, Pfizer, Roche, and Seagen; in addition, S. Loibl has patent for EP14153692.0, EP21152186.9, EP15702464.7 and EP19808852.8, all patents via institute; in addition, S. Loibl declares participation on a Data Safety Monitoring Board or Advisory Board from Abbvie, Amgen, AstraZeneca, BMS, Celgene, DSI, Eirgenix, Eisai Europe, GSK, Gilead, Lilly, Merck, Novartis, Pfizer, Pierre Fabre, Relay Therapeutics, Roche, Sanofi, and Seagen, all paid to institute., (Copyright © 2023. Published by Elsevier Ltd.)