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Background: This study evaluated efficacy and safety of palbociclib, a CDK4/6 inhibitor, in heavily-pretreated hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR + /HER2 - ) metastatic breast cancer (mBC) patients during the compassionate use program in Spain from February 2015 to November 2017., Patients and Methods: Patient data were collected retrospectively from 35 hospitals in Spain. Patients with HR + /HER2 - mBC who had progressed on ≥4 treatments for advanced disease were eligible., Results: A total of 219 patients received palbociclib in combination with aromatase inhibitors (110; 50.2%), fulvestrant (87; 39.7%), tamoxifen (8; 3.6%) or as single agent (10; 4.6%). Mean age of the patients was 58 years; 31 patients (16.1%) were premenopausal and 162 (83.9%) were postmenopausal at the beginning of treatment with palbociclib. Patients had received a median of 3 previous lines of endocrine therapy (ET) for advanced disease. Real-world tumor response (rwTR) and clinical benefit rate were 5.9% (n = 13) and 46.2% (n = 101), respectively. The median real world progression-free survival (rwPFS) was 6.0 months (95% CI 5.7-7.0) and the median overall survival was 19.0 months (95% CI 16.4-21.7). Subgroup analysis revealed a significant difference in median rwPFS in patients treated with palbociclib plus fulvestrant depending on the duration of prior treatment with fulvestrant monotherapy (>6 versus ≤6 months; HR 1.93, 95% CI 1.37-2.73, p < 0.001). The most frequently reported toxicities were neutropenia, asthenia, thrombopenia and anemia., Conclusions: Palbociclib can be an effective and safe treatment option in patients with heavily pretreated endocrine-sensitive mBC, especially in those with longer PFS to previous ET., Competing Interests: Declaration of competing interest L. Manso reports consulting or advisory roles from Roche, AstraZeneca, Novartis, Tesaro, and Pfizer; speaker’s bureau participation from Roche, AstraZeneca, Novartis, Tesaro, and Pfizer; research funding from Tesaro; travel expenses from Roche, Novartis, and Tesaro. M. Oliveira reports receiving speaking and advisory honoraria as from Roche and Seattle Genetics; speaking fees from Novartis; and advisory honoraria from GSK, PUMA Biotechnology and AstraZeneca; and financial support from AstraZeneca, Philips, Genentech, Roche, Seattle Genetics, Zenith Epigenetics, GSK, Immunomedics, Novartis, Boehringer-Ingelheim, and PUMA Biotechnology. M. Ruiz-Borrego reports speaker grants and advisory fees from Pfizer, Eli Lilly and Co., and Novartis. Iñaki Álvarez-Busto reports consultant or advisory roles from Kyowa Kirin and Pharma Mar; speaker honoraria from Angelini, Astra-Zeneca, Bayer, Boehringer Ingelheim, EISAI, Grunenthal pharma, Novartis, Pierre Fabre, Pfizer, and Roche; financial support for attending symposia from Roche; and financial support for educational programs from Bristol-Myers Squibb and Roche. E. Aguirre reports financial support for attending symposiafrom Roche and MSD; financial support for educational programs from Astra Zeneca and MSD; and participation in advisory board from Roche, Pfizer and AstraZeneca. J. E. Alés-Martínez reports speaking honoraria from Roche, MSD and BMS; consulting honoraria from Tesaro and Pfizer; and travel grants from MSD, BMS, and Roche. D. Malón reports participation in advisories, formative activities, and financial support for attending symposia from Pfizer, Roche, Novartis, BMS, MSD, Eisai, and Ipsen. I. Echevarría reports receiving travel grants from Pfizer, Novartis, and Lilly; and speaking honoraria from Novartis. S. Servitja reports receiving speaking honoraria from Roche and Pfizer; advisory board participation with Genomichealth, AstraZeneca, and MSD; and financial support for attending symposia from Roche, Pfizer, and MSD. F. Moreno reports receiving financial support for attending symposia from Pfizer, Roche, Novartis; support from Pfizer as project sponsor; and positions on advisory board or board of directors or other type of management relationships from Roche, Novartis, Pfizer, and MSD. Other authors declare no conflicts of interest., (Copyright © 2020 The Author(s). Published by Elsevier Ltd.. All rights reserved.)