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Background: Metastatic breast cancer (mBC) causes nearly all BC-related deaths. Next-generation sequencing (NGS) technologies allow for the application of personalized medicine using targeted therapies that could improve patients' outcomes. However, NGS is not routinely used in the clinical practice and its cost induces access-inequity among patients. We hypothesized that promoting active patient participation in the management of their disease offering access to NGS testing and to the subsequent medical interpretation and recommendations provided by a multidisciplinary molecular advisory board (MAB) could contribute to progressively overcome this challenge. We designed HOPE (SOLTI-1903) breast cancer trial, a study where patients voluntarily lead their inclusion through a digital tool (DT). The main objectives of HOPE study are to empower mBC patients, gather real-world data on the use of molecular information in the management of mBC and to generate evidence to assess the clinical utility for healthcare systems. Trial design: After self-registration through the DT, the study team validates eligibility criteria and assists patients with mBC in the subsequent steps. Patients get access to the information sheet and sign the informed consent form through an advanced digital signature. Afterwards, they provide the most recent (preferably) metastatic archival tumor sample for DNA-sequencing and a blood sample obtained at the time of disease progression for ctDNA analysis. Paired results are reviewed by the MAB, considering patient's medical history. The MAB provides a further interpretation of molecular results and potential treatment recommendations, including ongoing clinical trials and further (germline) genetic testing. Participants self-document their treatment and disease evolution for the next 2 years. Patients are encouraged to involve their physicians in the study. HOPE also includes a patient empowerment program with educational workshops and videos about mBC and precision medicine in oncology. The primary endpoint of the study was to describe the feasibility of a patient-centric precision oncology program in mBC patients when a comprehensive genomic profile is available to decide on a subsequent line of treatment. [ABSTRACT FROM AUTHOR]

Background Palbociclib has gained a central role in the treatment of hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2−) advanced breast cancer (ABC). Despite its manageable toxicity profile, venous thromboembolism (VTE) or interstitial lung disease (ILD)/pneumonitis may infrequently occur. Therefore, we provide a comprehensive summary of the safety and tolerability of the combination of endocrine therapy and palbociclib among patients included in the randomized phase 2 PARSIFAL study. Materials and Methods Patients with endocrine-sensitive HR+/HER2- ABC and no prior therapy in an advanced setting (n  = 486) were randomly assigned 1:1 to receive fulvestrant–palbociclib (FP) or letrozole–palbociclib (LP). Laboratory tests and the incidence of adverse events (AEs) were recorded at baseline and day 1 of each cycle. Progression-free survival (PFS) was estimated for patients with and without VTE. Results A total of 483 patients were analyzed. Neutropenia, leukopenia, anemia, asthenia, arthralgia, fatigue, and diarrhea were the most frequent AEs in both groups. Febrile neutropenia occurred in 3 (1.2%) patients of the FP group and in 1 (0.4%) patient in the LP group. Six (2.5%; 0.4% grade 3) patients in the FP group and 6 patients (2.5%; 0.4% grade 3) in the LP group experienced ILD/pneumonitis. Pulmonary embolism was reported in 12 (5.0%) patients in the FP group and 6 (2.5%) patients in the LP group. Advanced age at baseline was the only factor significantly associated with an increased risk of pulmonary embolism (P  < .01). Conclusion The PARSIFAL data confirmed the favorable safety profile of both palbociclib regimens. VTE and ILD/pneumonitis were occasionally reported, and their early detection allowed patients to continue treatment effectively without detriment to efficacy. ClinicalTrials.gov Identifier NCT02491983; https://clinicaltrials.gov/ct2/show/NCT02491983). [ABSTRACT FROM AUTHOR]

Improved selection of cancer patients who are most likely to respond to immune checkpoint inhibitors remains an unmet clinical need. Recently, a positive correlation between levels of PD1 mRNA and clinical outcome in response to PD1 blockade across diverse tumor histologies has been confirmed in several datasets. ACROPOLI is a parallel cohort, non-randomized, phase II study that aims to evaluate the efficacy of the anti-PD1 immune checkpoint inhibitor spartalizumab as monotherapy in metastatic patients with solid tumors that express high levels of PD1 (cohort 1; n = 111). An additional cohort of 30 patients with tumors expressing low levels of PD1, where PD1/PD-L1 antibodies in monotherapy are standard treatment, will also be included (cohort 2). Primary end point is overall response rate in cohort 1. Clinical Trial Registration:NCT04802876 (ClinicalTrials.gov) Considering previous data, we hypothesized that anti-PD1 monotherapy is effective across multiple cancer types in patients with PD1-high mRNA expressing tumors. Given these observations, ACROPOLI is a phase II study designed to study spartalizumab in this population. [ABSTRACT FROM AUTHOR]

The use of cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) has proven to be a successful strategy in the treatment of advanced hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-) breast cancer (BC), leading to a strong interest in their possible role in the treatment of early luminal BC. In this review we collect the most relevant and recent information on the use of CDK4/6i for the treatment of early BC in the neoadjuvant and adjuvant settings. Specifically, we evaluate the results of the large phase 3 adjuvant trials recently released, which have yielded apparently divergent results. We also examine the relevance of biomarkers as response predictive factors for CDI4/6i, the combination between radiotherapy and CDK4/6i, and provide a critical discussion on the evidence that we have so far and future directions of the role of these drugs in the treatment of early BC. • CDK4/6 inhibitors have been successful in the treatment of HR+ HER2- breast cancer (BC). • The use of CDK4/6 inhibitors in early BC have been investigated in recent trials. • These trials on the adjuvant setting in early BC have yielded divergent results. • Differences in patient selection, drug schedule, and treatment duration could partially explain the outcomes. [ABSTRACT FROM AUTHOR]

Drug–drug interactions are of significant concern in clinical practice in oncology, particularly in patients receiving Cyclin-dependent kinase (CDK) 4/6 inhibitors, which are typically exposed to long-term regimens. This article presents the highlights from the 'First Workshop on Pharmacology and Management of CDK4/6 Inhibitors: Consensus about Concomitant Medications'. The article is structured into two modules. The educational module includes background information regarding drug metabolism, corrected QT (QTc) interval abnormalities, management of psychotropic drugs and a comprehensive review of selected adverse effects of palbociclib and ribociclib. The collaborative module presents the conclusions of the five working groups, each of which comprised five experts from different fields. From these conclusions positive lists of drugs for treating common comorbid conditions that can be safely administered concomitantly with palbociclib and/or ribociclib were developed. [ABSTRACT FROM AUTHOR]

Immunotherapy has revolutionized the oncology field during the last years, mainly with the introduction of immune checkpoint inhibitors in the clinical routine. Despite the recent approval of these drugs for the treatment of triple-negative breast cancer, most breast cancer patients cannot benefit from immunotherapy as most tumors are not considered immunoreactive. Therefore, new strategies must be developed to bring immunotherapy closer to breast cancer patients. The introduction of oncolytic viruses in the immuno-oncology field has shown promising results in cancer treatment, including breast cancer. However, a better understanding of their mechanisms of action, increase evidence of safety and efficacy, and the implications of its use as a systemic therapy must be examined in more depth. This review provides a summary of oncolytic virotherapy in the context of breast cancer, both in the pre-clinical and clinical setting. [ABSTRACT FROM AUTHOR]

In the phase II CORALLEEN trial, patients with PAM50 luminal B early breast cancer (EBC) were randomised to neoadjuvant ribociclib plus letrozole (R + L) or chemotherapy based on anthracyclines and taxanes. Results from the primary efficacy analysis showed a similar proportion of patients with response at surgery in both groups. How health-related quality of life (HRQoL) outcomes with R + L compare with chemotherapy in EBC setting is still unknown. Here, we report the results of the HRQoL analysis from the CORALLEEN study. A total of 106 women were randomised 1:1 to receive neoadjuvant R + L (n = 52) or chemotherapy (n = 54). Patient-reported outcomes were assessed using two questionnaires: EORTC QLQ-C30 and EORTC QLQ-BR23. Change from baseline in the global health status, functional, and symptom scales was analysed using linear mixed-effect models, and between-treatment differences were estimated along with 95% confidence interval (95% CI). At baseline, the overall questionnaire available rate was 94.3%, and patient-reported outcomes were similar between treatment groups. At the end of the study treatment (24 weeks), patients receiving R + L showed better global health status scores with a between-treatment difference of 17.7 points (95% CI 9.2–26.2; p-value <0.001). The R + L group also presented numerically better outcomes in all functional and symptom scales. The larger between-treatment differences in symptom severity were found in fatigue (−28.9; 95% CI −38.5 to −19.3), appetite loss (−23; 95% CI −34.9 to −11.2) and systematic therapy side-effects (−11.4; 95% CI −18.3 to −4.6). Neoadjuvant R + L was associated with better HRQoL outcomes compared with chemotherapy in patients with luminal B EBC. ClinicalTrials.gov Identifier: NCT03248427. • CORALLEEN trial included patients with luminal B early breast cancer. • Patients received neoadjuvant ribociclib plus letrozole (R + L) or chemotherapy. • Health-related quality of life (HRQoL) was better in the R + L arm. • R + L was associated with better global health status in comparison with chemotherapy. • R + L also presents better results in symptom and fatigue scales. [ABSTRACT FROM AUTHOR]

Background: In animal models of breast cancer, resistance to continuous use of letrozole can be reversed by withdrawal and reintroduction of letrozole. We therefore hypothesised that extended intermittent use of adjuvant letrozole would improve breast cancer outcome compared with continuous use of letrozole in postmenopausal women.Methods: We did the multicentre, open-label, randomised, parallel, phase 3 SOLE trial in 240 centres (academic, primary, secondary, and tertiary care centres) in 22 countries. We enrolled postmenopausal women of any age with hormone receptor-positive, lymph node-positive, and operable breast cancer for which they had undergone local treatment (surgery with or without radiotherapy) and had completed 4-6 years of adjuvant endocrine therapy. They had to be clinically free of breast cancer at enrolment and without evidence of recurrent disease at any time before randomisation. We randomly assigned women (1:1) to treatment groups of either continuous use of letrozole (2·5 mg/day orally for 5 years) or intermittent use of letrozole (2·5 mg/day orally for 9 months followed by a 3-month break in years 1-4 and then 2·5 mg/day during all 12 months of year 5). Randomisation was done by principal investigators or designee at respective centres through the internet-based system of the International Breast Cancer Study Group, was stratified by type of previous endocrine therapy (aromatase inhibitors only vs selective oestrogen receptor modulators only vs both therapies), and used permuted block sizes of four and institutional balancing. No one was masked to treatment assignment. The primary endpoint was disease-free survival, analysed by the intention-to-treat principle using a stratified log-rank test. All patients in the intention-to-treat population who initiated protocol treatment during their period of trial participation were included in the safety analyses. This study is registered with ClinicalTrials.gov, number NCT00553410, and EudraCT, number 2007-001370-88; and long-term follow-up of patients is ongoing.Findings: Between Dec 5, 2007, and Oct 8, 2012, 4884 women were enrolled and randomised after exclusion of patients at a non-adherent centre, found to have inadequate documentation of informed consent, immediately withdrew consent, or randomly assigned to intervention groups in error. 4851 women comprised the intention-to-treat population that compared extended intermittent letrozole use (n=2425) with continuous letrozole use (n=2426). After a median follow-up of 60 months (IQR 53-72), disease-free survival was 85·8% (95% CI 84·2-87·2) in the intermittent letrozole group compared with 87·5% (86·0-88·8) in the continuous letrozole group (hazard ratio 1·08, 95% CI 0·93-1·26; p=0·31). Adverse events were reported as expected and were similar between the two groups. The most common grade 3-5 adverse events were hypertension (584 [24%] of 2417 in the intermittent letrozole group vs 517 [21%] of 2411 in the continuous letrozole group) and arthralgia (136 [6%] vs 151 [6%]). 54 patients (24 [1%] in the intermittent letrozole group and 30 [1%] in the continuous letrozole group) had grade 3-5 CNS cerebrovascular ischaemia, 16 (nine [<1%] vs seven [<1%]) had grade 3-5 CNS haemorrhage, and 40 (19 [1%] vs 21 [1%]) had grade 3-5 cardiac ischaemia. In total, 23 (<1%) of 4851 patients died while on trial treatment (13 [<1%] of 2417 patients in the intermittent letrozole group vs ten [<1%] of 2411 in the continuous letrozole group).Interpretation: In postmenopausal women with hormone receptor-positive breast cancer, extended use of intermittent letrozole did not improve disease-free survival compared with continuous use of letrozole. An alternative schedule of extended adjuvant endocrine therapy with letrozole, including intermittent administration, might be feasible and the results of the SOLE trial support the safety of temporary treatment breaks in selected patients who might require them.Funding: Novartis and the International Breast Cancer Study Group. [ABSTRACT FROM AUTHOR]

Purpose: Development of a computational biomarker to predict, prior to treatment, the response to CDK4/6 inhibition (CDK4/6i) in combination with endocrine therapy in patients with breast cancer., Experimental Design: A mechanistic mathematical model that accounts for protein signaling and drug mechanisms of action was developed and trained on extensive, publicly available data from breast cancer cell lines. The model was built to provide a patient-specific response score based on the expression of six genes (CCND1, CCNE1, ESR1, RB1, MYC, and CDKN1A). The model was validated in five independent cohorts of 148 patients in total with early-stage or advanced breast cancer treated with endocrine therapy and CDK4/6i. Response was measured either by evaluating Ki67 levels and PAM50 risk of relapse (ROR) after neoadjuvant treatment or by evaluating progression-free survival (PFS)., Results: The model showed significant association with patient's outcomes in all five cohorts. The model predicted high Ki67 [area under the curve; AUC (95% confidence interval, CI) of 0.80 (0.64-0.92), 0.81 (0.60-1.00) and 0.80 (0.65-0.93)] and high PAM50 ROR [AUC of 0.78 (0.64-0.89)]. This observation was not obtained in patients treated with chemotherapy. In the other cohorts, patient stratification based on the model prediction was significantly associated with PFS [hazard ratio (HR) = 2.92 (95% CI, 1.08-7.86), P = 0.034 and HR = 2.16 (1.02 4.55), P = 0.043]., Conclusions: A mathematical modeling approach accurately predicts patient outcome following CDK4/6i plus endocrine therapy that marks a step toward more personalized treatments in patients with Luminal B breast cancer., (©2024 American Association for Cancer Research.)

Background: Palbociclib has gained a central role in the treatment of hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer (ABC). Despite its manageable toxicity profile, venous thromboembolism (VTE) or interstitial lung disease (ILD)/pneumonitis may infrequently occur. Therefore, we provide a comprehensive summary of the safety and tolerability of the combination of endocrine therapy and palbociclib among patients included in the randomized phase 2 PARSIFAL study., Materials and Methods: Patients with endocrine-sensitive HR+/HER2- ABC and no prior therapy in an advanced setting (n = 486) were randomly assigned 1:1 to receive fulvestrant-palbociclib (FP) or letrozole-palbociclib (LP). Laboratory tests and the incidence of adverse events (AEs) were recorded at baseline and day 1 of each cycle. Progression-free survival (PFS) was estimated for patients with and without VTE., Results: A total of 483 patients were analyzed. Neutropenia, leukopenia, anemia, asthenia, arthralgia, fatigue, and diarrhea were the most frequent AEs in both groups. Febrile neutropenia occurred in 3 (1.2%) patients of the FP group and in 1 (0.4%) patient in the LP group. Six (2.5%; 0.4% grade 3) patients in the FP group and 6 patients (2.5%; 0.4% grade 3) in the LP group experienced ILD/pneumonitis. Pulmonary embolism was reported in 12 (5.0%) patients in the FP group and 6 (2.5%) patients in the LP group. Advanced age at baseline was the only factor significantly associated with an increased risk of pulmonary embolism (P < .01)., Conclusion: The PARSIFAL data confirmed the favorable safety profile of both palbociclib regimens. VTE and ILD/pneumonitis were occasionally reported, and their early detection allowed patients to continue treatment effectively without detriment to efficacy., Clinicaltrials.gov Identifier: NCT02491983; https://clinicaltrials.gov/ct2/show/NCT02491983)., (© The Author(s) 2022. Published by Oxford University Press.)

Background: Chemotherapy efficacy in early-stage hormone receptor-positive (HR+) breast cancer (BC) according to menopausal status needs a biological explanation., Methods: We compared early-stage HR+ BC biological features before and after (neo)adjuvant chemotherapy or endocrine therapy (ET), and assessed oestrogen receptor (ER) pathway activity in both pre- and post-menopausal patients. The nCounter platform was used to detect gene expression levels., Findings: In 106 post-menopausal patients with HR+/HER2-negative BC randomized to neoadjuvant chemotherapy or ET (letrozole+ribociclib), a total of 19 oestrogen-regulated genes, including progesterone receptor (PGR), were found downregulated in the ET-based arm-only. We confirmed this finding in an independent dataset of 20 letrozole-treated post-menopausal patients and found, conversely, an up-regulation of the same signature in HR+/HER2-negative MCF7 cell line treated with estradiol. PGR was found down-regulated by 2 weeks of ET+anti-HER2 therapy in pre-/post-menopausal patients with HR+/HER2-positive (HER2+) BC, while anti-HER2 therapy alone increased PGR expression in HR-negative/HER2+ BC. In 88 pre- and post-menopausal patients with newly diagnosed HR+/HER2-negative BC treated with chemotherapy, the 19 oestrogen-regulated genes were found significantly downregulated only in pre-menopausal patients. In progesterone receptor (PR)+/HER2-negative BC treated with neoadjuvant chemotherapy (n=40), tumours became PR-negative in 69.2% of pre-menopausal patients and 14.8% of post-menopausal patients (p=0.001). Finally, a mean decrease in PGR levels was only observed in pre-menopausal patients undergoing anti-HER2-based multi-agent chemotherapy., Interpretation: Chemotherapy reduces the expression of ER-regulated genes in pre-menopausal women suffering from hormone-dependent BC by supressing ovarian function. Further studies should test the value of chemotherapy in this patient population when ovarian function is suppressed by other methods., Funding: Instituto de Salud Carlos III, Breast Cancer Now, the Breast Cancer Research Foundation, the American Association for Cancer Research, Fundació La Marató TV3, the European Union's Horizon 2020 Research and Innovation Programme, Pas a Pas, Save the Mama, Fundación Científica Asociación Española Contra el Cáncer, PhD4MDgrant of "Departament de Salut", exp SLT008/18/00122, Fundación SEOM and ESMO. Any views, opinions, findings, conclusions, or recommendations expressed in this material are those solely of the author(s)., Competing Interests: Declaration of Competing Interest Dr. Prat reports grants and personal fees from Pfizer, grants and personal fees from Lilly, personal fees from Nanostring tecnologies, grants and personal fees from Amgen, grants from Roche, personal fees from Oncolytics Biotech, personal fees from Daiichi Sankyo, personal fees from PUMA, personal fees from BMS, from Daiichi Sankyo, outside the submitted work. Dr. Perou reports personal fees from Bioclassifier LLC, outside the submitted work; in addition, Dr. Perou has a U.S. Patent No. 12,995,459 with royalties paid. Dr. Saura reports personal fees from AstraZeneca, Daiichi Sankyo, Eisai, Exact Sciences, Exeter Pharma, F. Hoffmann - La Roche Ltd, MediTech, Merck Sharp & Dohme, Novartis, Pfizer, Philips, Piere Fabre, Puma, Roche Farma, Sanofi-Aventis, SeaGen, and Zymeworks outside the submitted work. Dr. Ciruelos reports personal fees from Roche, Pfizer, Lilly, Novartis, Astra-Zeneca/Daiichy Sankio and from MSD outside the submitted work. Dr. Gavilá reports grants from Novartis, Pfizer, Astra-Zeneca and Lilly, outside the submitted work. Dr. Soberino reports grants and personal fees from MSD, grants from Sanofi and personal fees from Roche outside the submitted work. Dr. Carey reports uncompensed relationship with Sanofi, G1 Therapeutics, Genentech/Roche, AstraZeneca/Daiichi Sankyo, Apitude Health and Exact Sciences; in addition Dr. Carey declared that Companies who have provided research funds to her institution in the past 1–2 years were Syndax, Immunomedics, Novartis, Nanostring technologies, Abbvie, Seattle Genetics and Veracyte, outside the submitted work. Dr Bellett declares advisory board participation for Lilly, Pfizer and Novartis, as well as travel expenses from Pfizer, outside the submitted work. Dr. Muñoz reports expert testimony for Eisai, Novartis and Roche, advisory board for Pierre Fabre and travel grants from Lilly, Pfizer and Roche, outside the submitted work. Dr. Oliveira reports grants from Pfizer, grants, personal fees and non-financial support from Roche, grants and personal fees from Genentech, grants, personal fees and non-financial support from Novartis, grants and personal fees from Seattle Genetics, grants and personal fees from Astra-Zeneca, grants and personal fees from PUMA Biotechnolgy, grants from Immunomedics, grants from Boehringer-Ingelheim, non-financial support from Eisai, Grunenthal, GP Pharma and Pierre Fabre, outside the submitted work. Dr. Pernas reports personal fees and non-financial support from Novartis, personal fees from AstraZeneca, Daiichi-Sankyo, Polyphor, Seattle Genetics, Eisai, Roche and Pierre-Fabre, outside the submitted work. Dr. Cortes reports personal fees and travel expenses from Pfizer, personal fees from Lilly, Servier, Athenex, personal fees and travel expenses and advisory/honoraria from Roche, personal fees from Polyphor, personal fees and travel expenses from Daiichi Sankyo, Novartis and Eisai, personal fees from MSD, GSK, Astra-Zeneca, Celgene, Cellestia, Biothera, Merus, Seattle Genetics, Erytech, Leuko, Bioasis, Clovis Oncology, Boerhinger-Ingelheim, Samsung Bioepis, and stocks from MedSIR, outside the submitted work. The other authors have nothing to declare., (Copyright © 2021 The Author(s). Published by Elsevier B.V. All rights reserved.)

Purpose: Women with hormone receptor-positive early breast cancers have a persistent risk of relapse and biomarkers for late recurrence are needed. We sought to identify tumor genomic aberrations associated with increased late-recurrence risk., Experimental Design: In a secondary analysis of Study of Letrozole Extension trial, a case-cohort-like sampling selected 598 primary breast cancers for targeted next-generation sequencing analysis of gene mutations and copy-number gains (CNGs). Correlations of genomic aberrations with clinicopathologic factors and breast and distant recurrence-free intervals (BCFIs and DRFIs) were analyzed using weighted Cox models., Results: Analysis of mutations and CNGs was successfully performed for 403 and 350 samples, including 148 and 134 patients with breast cancer recurrences (median follow-up time, 5.2 years), respectively. The most frequent alterations were PIK3CA mutations (42%) and CNGs of CCND1 (15%), ERBB2 (10%), FGFR1 (8%), and MYC (8%). PIK3CA mutations and MYC CNGs were associated with lower ( P = 0.03) and higher ( P = 0.004) tumor grade, respectively; a higher Ki-67 was seen in tumor with CCND1, ERBB2 , and MYC CNGs ( P = 0.01, P < 0.001, and P = 0.03, respectively). FGFR1 CNG was associated with an increased risk of late events in univariate analyses [17/29 patients; BCFI: HR, 3.2; 95% confidence interval (CI), 1.48-6.92; P = 0.003 and DRFI: HR, 3.5; 95% CI, 1.61-7.75; P = 0.002) and in multivariable models adjusted for clinicopathologic factors., Conclusions: Postmenopausal women with hormone receptor-positive early breast cancer harboring FGFR1 CNG had an increased risk of late recurrence despite extended therapy. FGFR1 CNG may represent a useful prognostic biomarker for late recurrence and a therapeutic target., (©2020 American Association for Cancer Research.)

Purpose: This study aimed to identify biomarkers of resistance to endocrine therapy in estrogen receptor-positive (ER + ) breast cancers treated with prolonged neoadjuvant letrozole. Experimental Design: We performed targeted DNA and RNA sequencing in 68 ER + breast cancers from patients treated with preoperative letrozole (median, 7 months). Results: Twenty-four tumors (35%) exhibited a PEPI score ≥4 and/or recurred after a median of 58 months and were considered endocrine resistant. Integration of the 47 most upregulated genes (log FC > 1, FDR < 0.03) in letrozole-resistant tumors with transcription-binding data showed significant overlap with 20 E2F4-regulated genes ( P = 2.56E-15). In patients treated with the CDK4/6 inhibitor palbociclib before surgery, treatment significantly decreased expression of 24 of the 47 most upregulated genes in letrozole-resistant tumors, including 18 of the 20 E2F4 target genes. In long-term estrogen-deprived ER + breast cancer cells, palbociclib also downregulated all 20 E2F4 target genes and P-RB levels, whereas the ER downregulator fulvestrant or paclitaxel only partially suppressed expression of this set of genes and had no effect on P-RB. Finally, an E2F4 activation signature was strongly associated with resistance to aromatase inhibitors in the ACOSOG Z1031B neoadjuvant trial and with an increased risk of relapse in adjuvant-treated ER + tumors in METABRIC. Conclusions: In tumors resistant to prolonged neoadjuvant letrozole, we identified a gene expression signature of E2F4 target activation. CDK4/6 inhibition suppressed E2F4 target gene expression in estrogen-deprived ER + breast cancer cells and in patients' ER + tumors, suggesting a potential benefit of adjuvant CDK4/6 inhibitors in patients with ER + breast cancer who fail to respond to preoperative estrogen deprivation. Clin Cancer Res; 24(11); 2517-29. ©2018 AACR ., (©2018 American Association for Cancer Research.)