Abemaciclib Rechallenge after Progression on Abemaciclib plus Endocrine Therapy in Patients with Hormone Receptor-Positive HER2-Negative Metastatic Breast Cancer: Results from the Phase Ⅱ AGAIN Study (WJOG14220B)

Abemaciclib Rechallenge after Progression on Abemaciclib plus Endocrine Therapy in Patients with Hormone Receptor-Positive HER2-Negative Metastatic Breast Cancer: Results from the Phase Ⅱ AGAIN Study (WJOG14220B) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Abemaciclib Rechallenge after Progression on Abemaciclib plus Endocrine Therapy in Patients with Hormone Receptor-Positive HER2-Negative Metastatic Breast Cancer: Results from the Phase Ⅱ AGAIN Study (WJOG14220B) Meiko Nishimura, Takahiro Kogawa, Keiji Sugiyama, Yukinori Ozaki, and 16 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9265904/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract Purpose: In the postMONARCH trial, abemaciclib plus fulvestrant (FUL) yielded a progression-free survival (PFS) of 6.0 months in patients with hormone receptor (HR)-positive HER2-negative metastatic breast cancer who progressed after receiving CDK4/6 inhibitor plus endocrine therapy (ET). However, only 8% of these patients previously received abemaciclib. Methods: This multicenter, single-arm, phase II study enrolled patients who developed disease progression after abemaciclib plus ET. Patients were switched from aromatase inhibitor (AI)/tamoxifen (TAM) to FUL or from FUL to AI while continuing abemaciclib. The primary endpoint was PFS. The secondary endpoints were overall response rate (ORR), clinical benefit rate (CBR), chemotherapy-free interval (CFI), overall survival (OS), safety, and biomarker analysis was perfomed. Results This study enrolled 65 patients from June 2021 to November 2023, among which 64 patients were evaluable. The median PFS was 4.2 months (90% CI, 2.8–4.4). The median PFS was 7.1 months (95% CI, 3.9–11.3) in patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL (n = 28, 43.8%), whereas it was 2.8 months (95% CI, 1.9–4.2) in patients switching from abemaciclib plus FUL to abemaciclib plus AI (n = 36, 56.3%). Cyclin D1 ( CCND1 ) amplification at progression was associated with shorter median PFS (HR, 0.19; 95% CI, 0.05–0.7; p = 0.01) and OS (HR, 0.16; 95% CI, 0.03–0.8; p = 0.02). Conclusions The primary endpoint was not met, switching from abemaciclib plus AI/TAM to abemaciclib plus FUL could be a clinically effective option. CCND1 amplification may be a resistance to abemaciclib. jRCTs031210129; date of registration JUNE 2, 2021. CDK 4/6 inhibitor abemaciclib hormone receptor-positive HER2 negative breast cancer Figures Figure 1 Figure 2 Figure 3 INTRODUCTION The standard treatment for hormone (HR)-positive HER2-negative breast cancer is cyclin-dependent kinase 4/6 (CDK4/6) inhibitors plus endocrine therapy (ET) 1 , but no treatment approach has been established in patients who are refractory to this regimen. Some patients who have been pretreated with CDK4/6 inhibitor and ET may still benefit from CDK4/6 inhibitor rechallenge, especially in cases of acquired resistance to ET. Several randomized phase II trials testing the efficacy of CDK4/6 inhibitor rechallenge have yielded mixed results depending on the specific CDK4/6 inhibitor (MAINTAIN, PACE and PALMIRA trials) 2-4 . In the phase III postMONARCH trial, abemaciclib plus fulvestrant (FUL) yielded a progression-free survival (PFS) of 6.0 months in patients with HR-positive HER2-negative metastatic breast cancer who exhibited disease progression on a CDK4/6 inhibitor plus ET 5 . Abemaciclib rechallenge is an important next-line option after CDK4/6 inhibitors plus ET, but only 8% of patients received prior abemaciclib in the postMONARCH trial. Therefore, the evidence for abemaciclib rechallenge after prior abemaciclib plus ET remains limited, and there are still no biomarkers for identifying patients who are most likely to benefit from rechallenge with a CDK4/6 inhibitor. Accordingly, this single-arm, phase Ⅱ trial aimed to evaluate the clinical benefits of abemaciclib rechallenge. Translational research evaluating gene alterations was also conducted as accompanying exploratory research to assess the predictive biomarkers of abemaciclib resistance or sensitivity. METHODS Study design and participants This multicenter, single-arm, phase II study in Japan enrolled patients with disease progression after receiving abemaciclib plus ET. The main eligibility criteria were as follows:1)histologically confirmed HR-positive HER2-negative invasive breast cancer; 2) locally advanced, metastatic, or recurrent disease; 3) previously received no more than three lines of ET and one line of chemotherapy; and 4) previously received abemaciclib plus ET achieved clinical benefits, complete response or partial response, or stable disease for at least 6 months during abemaciclib-based treatment. This study excluded patients who were previously treated with immune checkpoint inhibitors, everolimus, olaparib, or other CDK4/6 inhibitors expect abemaciclib for advanced or metastatic disease. Since ribociclib is not approved in Japan, none of the patients in this study had a history of ribociclib treatment. Treatment and procedures Patients were switched from aromatase inhibitor (AI)/tamoxifen (TAM) to FUL, or from FUL to AI, while continuing abemaciclib. Abemaciclib was administered orally at a dose of 150 mg twice daily on days 1–28 of a 28-day cycle. If dose reduction was required due to adverse events during previous treatment, the starting dose of abemaciclib was adjusted to the previous level (100 mg or 50 mg). This was combined with another endocrine agent that had not been used in prior therapy. As ET, patients received either FUL or an AI. FUL (500 mg) was administered via intramuscular injection on days 1, 15, and 29, followed by maintenance dosing every 4 weeks. Alternatively, for those receiving an AI, anastrozole (1 mg), letrozole (2.5 mg), or exemestane (25 mg) was administered orally once daily throughout each 28-day cycle. Endpoints The primary endpointwas PFS. The secondary endpoints were overall response rate (ORR), clinical benefit rate (CBR), chemotherapy-free interval (CFI), overall survival (OS), and safety. Subgroup analyses were performed based on gene alterations (e.g., ESR1 or PIK3CA mutations), disease site (visceral/bone only/other), previous lines of therapy for advanced or metastatic disease (first/second/third and more), ET (FUL/non-FUL) or chemotherapy, performance status (0/1), organs involved (1/2/≥ 3), and disease-free interval (<36/≥36 months) and menopausal status, to examine the interaction of treatment effects on PFS and OS. Tumor response was assessed via computed tomography at baseline, every 6 weeks for 12 months, and then every 12 weeks until disease progression, death, or study discontinuation as per RECIST version 1.1. Adverse events (AEs) were recorded using the Common Terminology Criteria for Adverse Events version 5.0. Biomarker analysis To analyze circulating biomarkers, blood samples for liquid biopsy were collected at three time points: 1) prior to the initiation of abemaciclib rechallenge, 2) at two months post-rechallenge, and 3) during disease progression. Plasma was separated from the peripheral blood cells by centrifugation at 2,000 g for 10 minutes at 4°C, then circulating tumor DNA (ctDNA) was extracted by centrifugation at 1,600 g for 10 minutes at H.U. Frontier® (Minato-ku, Tokyo, Japan). At The Cancer Institute Hospital of Japanese Foundation for Cancer Research, plasma was separated by centrifugation at 300 g for 20 minutes, then ctDNA was extracted via centrifugation at 5,000 g for 20 minutes at 20°C and subsequently analyzed using the Oncomine Pan-Cancer Cell-Free Assay (Thermo Fisher Scientific®, Waltham, Massachusetts, USA). Statistical analysis The threshold and expectation of median PFS after the abemacilcib rechallenge were 3.0 and 5.0 months, respectively. A total of 59 cases were needed over the 2-year recruitment period and 1-year follow-up period to achieve a two-sided significance level of 5% and 80% power. Estimating a dropout/ineligibility rate of 10%, the target number for recruitment was 65 cases. Median PFS was estimated using the Kaplan–Meier method, with confidence interval (CI) calculated using the Brookmeyer and Crowley method. The protocol treatment was considered effective if the lower limit of the 90% CI for the median PFS exceeded 3 months. Subgroup analyses of PFS and OS were performed, and survival curves were generated using the Kaplan–Meier method. Hazard ratio (HR) and 95% CI were calculated using Cox proportional hazards models. Data analysis was performed using SAS version 9.4. Biomarker analysis was performed using R statistical software version 4.2.1 6 . Ethics approval and consent to participate All patients provided written informed consent before enrolment. The protocol was approved by the West Japan Oncology Group (WJOG) and the institutional review board/independent ethics committee (CRB3180008) in accordance with the ethical principles of the Declaration of Helsinki. The trial was registered with the Japan Registry of Clinical Trials (jRCTs031210129). RESULTS Patients and treatment From June 2021 to November 2023, 64 out of 65 enrolled patients were evaluable after 1 withdrawal (Supplementary Figure S1). By the end of the study period, 54 out of 64 patients discontinued treatment due to disease progression. Table 1 shows the patient characteristics. The median age was 56 years (range, 33–80 years) and the median number of prior treatment lines was 1 (range, 1–4). A total of 45 patients (70.3%) had visceral metastasis, while 13 patients (20.3%) had bone-only metastasis. Regarding treatment, 28 patients (43.7%) were switched from abemaciclib plus AI or TAM to abemaciclib plus FUL, whereas 36 (56.2%) were switched from abemaciclib plus FUL to abemaciclib plus AI. The posttreatment regimens were as follows: chemotherapy (n = 28, 43.8%); endocrine monotherapy (n = 17, 26.6%); abemaciclib plus ET (n = 7, 10.9%); other (n = 2, 3.1%); and none (n = 4, 6.3%). Efficacy The median PFS was 4.2 months (90% CI, 2.8–4.4) (Figure 1A). Since the lower limit of the 95% CI for PFS did not reach 3 months, the primary endpoint was not met. The secondary outcomes were as follows: ORR of 8.7% (95% CI, 2.4–20.8), CBR of 23.9% (95% CI, 12.6–38.8), CFI of 6.7 months (95% CI, 4.9–11.0), and OS of 28.7 months (95% CI, 25.4 not estimable [NE]) (Supplementary Table S1, Figure S2, 3). The median PFS was 7.1 months (95% CI, 3.9–11.3) in the abemaciclib plus FUL arm, while it was 2.8 months (95% CI, 1.9–4.2) in the abemaciclib plus AI arm (Figure 1B, C). On subgroup analyses of PFS, the only significant difference seen was that in prior ET (see Supplementary Table S2). Safety Most adverse events were low-grade. The most common grade 3 or higher adverse event was neutropenia, which occurred in 17 patients (26.6%) (Table 2). Diarrhea of any grade occurred in 14 patients (21.9%). Grade 3 febrile neutropenia and lung infections were observed in 2 patients (3.1%). No cases of interstitial lung disease were reported. The overall incidence of treatment-related AEs remained low throughout the study period. Biomarkers Figure 2 shows the frequency of ctDNA alterations seen before abemaciclib rechallenge and during disease progression. Before abemaciclib rechallenge, fibroblast growth factor receptor 1 ( FGFR1 ) amplification was associated with a shorter median PFS (HR, 5.05; 95% CI, 1.73–14.76; p = 0.003) and OS (HR, 4.44; 95% CI, 1.41–13.98; p = 0.01), while TP53 mutation was associated with shorter median OS (HR, 2.78; 95% CI, 1.07–7.25; p = 0.04) (Supplementary Table S3A). Two months after rechallenge, CCND1 amplification was associated with a shorter median PFS (HR, 2.40; 95% CI, 1.11–5.19; p = 0.03) (Supplementary Table S3A). During disease progression, the most frequently acquired gene alterations were TP53 mutations (n = 7, 13.0%) and CCND1 amplification (n = 5, 9.3%). The presence of TP53 mutations at progression was associated with a shorter median OS (mutant: 18.8 months [95% CI, 10.8–NA]; vs. wild-type: 28.6 months [95% CI, 25.3–NA], HR, 4.17; 95% CI, 1.24–14.02; p = 0.02) (Figure 3A, Supplementary Table S3B, C). Since TP53 mutation was associated with shorter OS at all time points (pre-rechallenge, two months post-rechallenge, and during progression), it was suggested as a prognostic biomarker. Out of 27 patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL, the most frequently acquired alteration was CCND1 amplification (n = 4, 14.8%). This alteration was associated with shorter median PFS (HR, 5.13; 95% CI, 1.43–18.4; p = 0.01) and OS (HR, 6.28; 95% CI, 1.26–31.25; p = 0.01) (Figure 3B, C). No significant differences in PFS or OS were seen based on ESR1 or PIK3CA mutations. DISCUSSION The AGAIN study was the first abemaciclib rechallenge study to switch from only abemaciclib. Although the primary endpoint was not met, patients who received abemaciclib plus FUL after progression on abemaciclib plus AI/TAM had a clinically meaningful median PFS of 7.1 months. Several CDK4/6 inhibitor rechallenge trials have been conducted to date. The MAINTAIN trial, a randomized phase II trial of ribociclib plus ET after progression of CDK4/6 inhibitors (palbociclib: 86.5%; ribociclib: 11.7%), demonstrated a significant PFS benefit (5.79 vs. 2.76 months; hazard ratio [HR]: 0.57; p = 0.006) 2 . On the other hand, several randomized phase Ⅱ studies (PACE and PALMIRA trials) did not demonstrate an improvement in PFS after switching patients from palbociclib plus ET to palbociclib plus ET 3, 4 . The postMONARCH trial, the first randomized phase Ⅲ study of abemaciclib rechallenge, demonstrated an improved median PFS of 6.0 months 5 , but the previous CDK4/6 inhibitors were mostly palbociclib (59%) and ribociclib (33%), with abemaciclib representing a minority (8%). Accordingly, no study has fully explored abemaciclib rechallenge after previous abemaciclib therapy. In recent years, there have been advancements in the development of treatments after resistance to CDK4/6 inhibitor plus ET. The CAPItello-291 study demonstrated that treatment with capivasertib (AKT inhibitor) plus FUL conferred a median PFS of 7.3 months in patients with AKT pathway-altered tumors who previously received a CDK4/6 inhibitor plus AI 7 . In the phase Ⅱ BYLieve study, alpelisib (PI3K inhibitor) plus FUL similarly conferred a median PFS of 7.4 in patients with PIK3CA-mutated breast cancer who previously received a CDK4/6 inhibitor 8 . Our study revealed a median PFS of 7.1 months in patients who received abemaciclib plus FUL after progression on abemaciclib plus AI/TAM, which is comparable to those who received targeted therapy after a CDK4/6 inhibitor. These findings suggest that abemaciclib rechallenge may be considered as a treatment option in cases where gene alterations of AKT pathway are absent, AKT inhibitors or PIK3CA inhibitors are not applicable, or treatment continuation is difficult to AEs (e.g., grade 3 or higher rash). In HR-positive HER2-negative advanced breast cancer, one of the mechanisms of resistance to AI is acquired alterations in the ESR1 gene 9 . In the EMBER-3 trial, treatment with imulunestrant (i.e., oral selective estrogen receptor degrader) plus abemaciclib significantly improved the median PFS of patients with ESR1 mutations who progressed after previous treatment with AI with or without CDK4/6 inhibitor 10 . In our study, there was no difference in PFS among patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL with ESR1 mutations. Most adverse events were low-grade, because the enrolled patients had previously managed AEs through supportive care or dose reduction. In a phase Ⅲ study of palbociclib plus FUL (PALOMA-3 trial), high cyclin E1 ( CCNE1 )mRNA expression was associated with palbociclib resistance 11 . In contrast, our biomarker analysis in the present abemaciclib rechallenge study revealed that CCND1 amplification was the most frequently acquired alteration, and this was associated with shorter PFS and OS among patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL. In the MONNALESSA-7 trial, patients with CCND1 alterations at baseline had shorter median PFS in both the ribociclib plus ET arm and ET-alone arm, suggesting the potential use of CCND1 as a prognostic biomarker 12 . However, CCND1 amplification was not associated with palbociclib sensitivity in the PALOMA-1/TRIO-18 trial 13 . Preclinical models and cell line studies have reported that tumors with CCND1 amplification and D-type cyclin activating features may be highly sensitive to abemaciclib 14 . Nevertheless, no preclinical or clinical data support that CCND1 amplification represents an acquired resistance to abemaciclib. Our study suggests that CCND1 amplification may be a potential predictor of resistance to abemaciclib in patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL. In PALOMA-3, patients with TP53 mutations had significantly worse PFS in both the palbociclib plus FUL arm and FUL-only arm 15 . In the MONALEESA-7 trial, patients with TP53 mutations similarly had worse PFS in both the ribociclib plus ET arm and ET-only arm 12 . In our study, patients with TP53 mutation similarly had shorter OS at each time point (pre-rechallenge, two months post-rechallenge, and during progression), and thus TP53 mutation was considered as a prognostic biomarker rather than a predictive marker. This study has several limitations. First, it is a single-arm, nonrandomized trial with a small sample size, resulting in broader 95% CI, and thus further studies are required to validate these results. Second, the results of the biomarker analysis are considered exploratory due to the very small number of patients with mutations. Lastly, tumor activity prior to enrolment in the AGAIN study was not assessed using the RECIST criteria. In conclusion, although the primary endpoint was not met, switching to abemaciclib plus FUL after progression on abemaciclib plus AI/TAM could be a clinically effective option. CCND1 amplification may be a potential predictor of resistance to abemaciclib in patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL. Declarations ACKNOWLEDGMENTS We would like to thank all the participants in this study, including patients, caregivers, physicians, and medical workers. FUNDING This study was sponsored by Eli Lilly Japan K.K. and funded by Japan Society of Clinical Oncology. DISCLOSURE SK has received honoraria from Kyowa Kirin Co., Ltd, Daiichi Sankyo Co., Ltd, Taiho Pharmaceutical Co., Ltd, Eli Lilly and Company, MSD K.K., AstraZeneca K.K., Chugai Pharmaceutical, Ltd., Eisai Co., Ltd., Takeda Pharmaceutical Co., Ltd., and Pfizer Japan Inc. CF have received honoraria from Daiichi Sankyo, Eisai, Pfizer, Chugai, MSD, Eli Lilly and material from Daiichi Sankyo. YF has received honoraria from Eisai Co. Ltd., Daiichi-Sankyo Co. Ltd., Ono Pharmaceutical Co. Ltd., Merck Biopharma Co. Ltd., Taiho Pharmaceutical Co. Ltd., Chugai Ltd., MSD & Co., Inc., Chugai Pharmaceutical Co. Ltd., Astellas Pharma Inc. AY has received honoraria from Pfizer, AstraZeneca, Chugai Pharmaceutical Co., Ltd. SW has received honoraria from Daiichi Sankyo Co., Ltd., Chugai Pharmaceutical Co., Ltd., MSD K.K., Eli Lilly Japan K.K., Kyowa Kirin Co., Ltd., Eisai Co., Ltd., Pfizer K.K. TT has received honoraria from Daiichi-Sankyo, Eli Lilly, Gilead Sciences, and MSD. HS has received honoraria from Daiichi Sankyo, Eli Lilly, Gilead Sciences, AstraZeneca, and Pfizer. HS has received research funding (outside of submitting work to her institution): Eli Lilly. YO has received honoraria from Eli Lilly, Pfizer, Daiichi Sankyo, AstraZeneca. TK has received grants from Eli Lilly, AstraZeneca, Guardant Health, Daiichi Sankyo, Ono Pharma, Chugai Pharma, Be One, and Eisai. TK has received consulting fees from Daiichi Sankyo, Astellas Pharma, and received honoraria from Daiichi Sankyo, Ono Pharma, Gilead sciences, Inc, Astellas Pharma, Eisai, AstraZeneca, Taiho Pharma, Chugai Pharma and MSD. TK has received payment for expert testimony from Astellas Pharma. TK has received support for attending meetings and/or travel from Phyzer and Eisai. TK has participated on a Data Safety Monitoring Board or Advisory Board from Daiichi Sankyo, Ono Pharma, Gilead sciences,Inc, Oncotherapy sciences, Eisai, Oncotherapy sciences, Eisai, Astra Zeneca, Taiho Pharma and Be One. TK received of equipment, materials, drugs, medical writing, gifts or other services from Daiichi Sankyo. The other authors have no conflict of interest. REFERENCES Gradishar WJ, Moran MS, Abraham J et al. NCCN Guidelines® Breast Cancer, Version 5.2025: Featured Updates to the NCCN Guidelines. J Natl Compr Cancer Netw 2025; 23 (11): 426-436. Kalinsky K, Accordino MK, Chiuzan C et al. Randomized Phase II Trial of Endocrine Therapy With or Without Ribociclib After Progression on Cyclin-Dependent Kinase 4/6 Inhibition in Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Metastatic Breast Cancer: MAINTAIN Trial. J Clin Oncol 2023; 41 (24): 4004-4013. Mayer EL, Ren Y, Wagle N et al. PACE: A Randomized Phase II Study of Fulvestrant, Palbociclib, and Avelumab After Progression on Cyclin-Dependent Kinase 4/6 Inhibitor and Aromatase Inhibitor for Hormone Receptor-Positive/Human Epidermal Growth Factor Receptor-Negative Metastatic Breast Cancer. J Clin Oncol 2024; 42 (17): 2050-2060. Llombart-Cussac A, Harper-Wynne C, Perello A et al. Second-Line Endocrine Therapy With or Without Palbociclib Rechallenge in Patients With Hormone Receptor-Positive/Human Epidermal Growth Factor Receptor 2-Negative Advanced Breast Cancer: PALMIRA Trial. J Clin Oncol 2025; 43 (18): 2084-2093. Kalinsky K, Bianchini G, Hamilton E et al. Abemaciclib Plus Fulvestrant in Advanced Breast Cancer After Progression on CDK4/6 Inhibition: Results From the Phase III postMONARCH Trial. J Clin Oncol 2025; 43 (9): 1101-1112. Kanda Y. Investigation of the Freely Available Easy-to-Use Software 'EZR' for Medical Statistics. Bone Marrow Transplant 2013; 48 (3): 452-458. Turner NC, Oliveira M, Howell SJ et al. Capivasertib in Hormone Receptor-Positive Advanced Breast Cancer. N Engl J Med 2023; 388 (22): 2058-2070. De Laurentiis M, Ferreira AM, Gligorov J et al. Alpelisib Plus Fulvestrant for PIK3CA-Mutated, HR-Positive, HER2-Negative Advanced Breast Cancer After a CDK4/6 Inhibitor (EPIK-B5): Phase III, Randomized, Double-Blind, Placebo-Controlled, Multicenter Study. Clin Cancer Res 2026; 32 (4_Supplement): RF7-02. Brett JO, Spring LM, Bardia A et al. ESR1 Mutation as an Emerging Clinical Biomarker in Metastatic Hormone Receptor-Positive Breast Cancer. Breast Cancer Res 2021; 23 (1): 85. Jhaveri KL, Neven P, Casalnuovo ML et al. Imlunestrant with or without Abemaciclib in Advanced Breast Cancer. N Engl J Med 2025; 392 (12): 1189-1202. Turner NC, Liu Y, Zhu Z et al. Cyclin E1 Expression and Palbociclib Efficacy in Previously Treated Hormone Receptor-Positive Metastatic Breast Cancer. J Clin Oncol 2019; 37 (14): 1169-1178. Bardia A, Su F, Solovieff N et al. Genomic Profiling of Premenopausal HR+ and HER2- Metastatic Breast Cancer by Circulating Tumor DNA and Association of Genetic Alterations With Therapeutic Response to Endocrine Therapy and Ribociclib. JCO Precis Oncol 2021; 5: 1408-1420. Finn RS, Crown JP, Lang I et al. The Cyclin-Dependent Kinase 4/6 Inhibitor Palbociclib in Combination with Letrozole Versus Letrozole Alone as First-Line Treatment of Oestrogen Receptor-Positive, HER2-Negative, Advanced Breast Cancer (PALOMA-1/TRIO-18): A Randomised Phase 2 Study. Lancet Oncol 2015; 16 (1): 25-35. Gong X, Litchfield LM, Webster Y et al. Genomic Aberrations that Activate D-type Cyclins Are Associated with Enhanced Sensitivity to the CDK4 and CDK6 Inhibitor Abemaciclib. Cancer Cell 2017; 32 (6): 761-776.e6. O'Leary B, Cutts RJ, Huang X et al. Circulating Tumor DNA Markers for Early Progression on Fulvestrant With or Without Palbociclib in ER+ Advanced Breast Cancer. J Natl Cancer Inst 2021; 113 (3): 309-317. Additional Declarations Competing interest reported. SK has received honoraria from Kyowa Kirin Co., Ltd, Daiichi Sankyo Co., Ltd, Taiho Pharmaceutical Co., Ltd, Eli Lilly and Company, MSD K.K., AstraZeneca K.K., Chugai Pharmaceutical, Ltd., Eisai Co., Ltd., Takeda Pharmaceutical Co., Ltd., and Pfizer Japan Inc. CF have received honoraria from Daiichi Sankyo, Eisai, Pfizer, Chugai, MSD, Eli Lilly and material from Daiichi Sankyo. YF has received honoraria from Eisai Co. Ltd., Daiichi-Sankyo Co. Ltd., Ono Pharmaceutical Co. Ltd., Merck Biopharma Co. Ltd., Taiho Pharmaceutical Co. Ltd., Chugai Ltd., MSD & Co., Inc., Chugai Pharmaceutical Co. Ltd., Astellas Pharma Inc. AY has received honoraria from Pfizer, AstraZeneca, Chugai Pharmaceutical Co., Ltd. SW has received honoraria from Daiichi Sankyo Co., Ltd., Chugai Pharmaceutical Co., Ltd., MSD K.K., Eli Lilly Japan K.K., Kyowa Kirin Co., Ltd., Eisai Co., Ltd., Pfizer K.K. TT has received honoraria from Daiichi-Sankyo, Eli Lilly, Gilead Sciences, and MSD. HS has received honoraria from Daiichi Sankyo, Eli Lilly, Gilead Sciences, AstraZeneca, and Pfizer. HS has received research funding (outside of submitting work to her institution): Eli Lilly. YO has received honoraria from Eli Lilly, Pfizer, Daiichi Sankyo, AstraZeneca. TK has received grants from Eli Lilly, AstraZeneca, Guardant Health, Daiichi Sankyo, Ono Pharma, Chugai Pharma, Be One, and Eisai. TK has received consulting fees from Daiichi Sankyo, Astellas Pharma, and received honoraria from Daiichi Sankyo, Ono Pharma, Gilead sciences, Inc, Astellas Pharma, Eisai, AstraZeneca, Taiho Pharma, Chugai Pharma and MSD. TK has received payment for expert testimony from Astellas Pharma. TK has received support for attending meetings and/or travel from Phyzer and Eisai. TK has participated on a Data Safety Monitoring Board or Advisory Board from Daiichi Sankyo, Ono Pharma, Gilead sciences,Inc, Oncotherapy sciences, Eisai, Oncotherapy sciences, Eisai, Astra Zeneca, Taiho Pharma and Be One. TK received of equipment, materials, drugs, medical writing, gifts or other services from Daiichi Sankyo. The other authors have no conflict of interest. Supplementary Files Supplementarymaterials.docx TableS2.pptx TableS3B.pptx TableS3C.pptx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 18 May, 2026 Reviews received at journal 04 May, 2026 Reviewers agreed at journal 02 May, 2026 Reviewers invited by journal 29 Apr, 2026 Editor assigned by journal 01 Apr, 2026 Submission checks completed at journal 01 Apr, 2026 First submitted to journal 30 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Nishimura","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABC0lEQVRIie2QsWrDMBCGTwgyqbuGEj1BQSKQsX0ViQyeMnUxdKjDUWXpAwgS6EN07aDgwYsfwKWhpQQ8dfAj1IpLacGJM5aib9AdSB/3nwAikT8JwXAy4CQLzZig/7rITlQmxOohBWhXeFdMNtLHY10sEWUD23OxwrvHs6fLBC2tG0hfga76x0zLDRoHNZPbjX1h9Wy+sKOJg/IayNr3K5XBnEHOJDet4ul88XBPISxEXH/C6dt7pwi3V24TYtmAUhGcBQWqvZLrYaU0qJxsg7XK89oXKuwCutTs4C5FseNNml8Jl9TVh78RytIdNKkeqwM/1iF/9Co8bCMx5Y4YvxDfDT9ViUQikX/OJy8OX0Q+yeS0AAAAAElFTkSuQmCC","orcid":"","institution":"Cancer Institute Hospital, Japanese Foundation for Cancer Research","correspondingAuthor":true,"prefix":"","firstName":"Meiko","middleName":"","lastName":"Nishimura","suffix":""},{"id":634337462,"identity":"cc2bcbbf-3d0e-4958-aefd-1b1732560df7","order_by":1,"name":"Takahiro Kogawa","email":"","orcid":"","institution":"Cancer Institute Hospital, Japanese Foundation for Cancer Research","correspondingAuthor":false,"prefix":"","firstName":"Takahiro","middleName":"","lastName":"Kogawa","suffix":""},{"id":634337465,"identity":"52466c54-3954-487f-b433-8fba05650727","order_by":2,"name":"Keiji Sugiyama","email":"","orcid":"","institution":"NHO Nagoya Medical Center","correspondingAuthor":false,"prefix":"","firstName":"Keiji","middleName":"","lastName":"Sugiyama","suffix":""},{"id":634337466,"identity":"0af84a0b-c986-425c-a80a-86d1857c9c1c","order_by":3,"name":"Yukinori Ozaki","email":"","orcid":"","institution":"Cancer Institute Hospital, Japanese Foundation for Cancer Research","correspondingAuthor":false,"prefix":"","firstName":"Yukinori","middleName":"","lastName":"Ozaki","suffix":""},{"id":634337467,"identity":"f54a8fed-3737-48bb-80ee-c9f0342476bf","order_by":4,"name":"Yuko Tanabe","email":"","orcid":"","institution":"Toranomon Hospital","correspondingAuthor":false,"prefix":"","firstName":"Yuko","middleName":"","lastName":"Tanabe","suffix":""},{"id":634337470,"identity":"3c2fa9cf-b7ae-4a49-a5b7-fd194ca7d3b8","order_by":5,"name":"Chikako Funasaka","email":"","orcid":"","institution":"National Cancer Center Hospital East","correspondingAuthor":false,"prefix":"","firstName":"Chikako","middleName":"","lastName":"Funasaka","suffix":""},{"id":634337471,"identity":"fe3d392a-8ea6-46ce-acfc-5817262981e0","order_by":6,"name":"Sasagu Kurozumi","email":"","orcid":"","institution":"IUHW Narita Hospital","correspondingAuthor":false,"prefix":"","firstName":"Sasagu","middleName":"","lastName":"Kurozumi","suffix":""},{"id":634337474,"identity":"fb0d7a1f-7b4d-4535-903c-51ccc51453b3","order_by":7,"name":"Akiyo Yoshimura","email":"","orcid":"","institution":"Aichi Cancer Center","correspondingAuthor":false,"prefix":"","firstName":"Akiyo","middleName":"","lastName":"Yoshimura","suffix":""},{"id":634337475,"identity":"250ba254-a57b-4ca4-9388-bbbf3e5686bb","order_by":8,"name":"Tsutomu Iwasa","email":"","orcid":"","institution":"Kindai University Faculty of Medicine","correspondingAuthor":false,"prefix":"","firstName":"Tsutomu","middleName":"","lastName":"Iwasa","suffix":""},{"id":634337476,"identity":"c742b289-13db-43ca-8a3a-8599099944be","order_by":9,"name":"Hitomi Sakai","email":"","orcid":"","institution":"Showa Medical University","correspondingAuthor":false,"prefix":"","firstName":"Hitomi","middleName":"","lastName":"Sakai","suffix":""},{"id":634337478,"identity":"fd778ba7-2435-4407-b66c-a0d2a49626d1","order_by":10,"name":"Mitsunori Morita","email":"","orcid":"","institution":"Hyogo Cancer Center","correspondingAuthor":false,"prefix":"","firstName":"Mitsunori","middleName":"","lastName":"Morita","suffix":""},{"id":634337485,"identity":"1c0690af-db33-4518-8977-d443560ec54a","order_by":11,"name":"Satomi Watanabe","email":"","orcid":"","institution":"Kishiwada City Hospital","correspondingAuthor":false,"prefix":"","firstName":"Satomi","middleName":"","lastName":"Watanabe","suffix":""},{"id":634337492,"identity":"c7fc1538-abcc-4f5e-a1dd-1dda2999fdac","order_by":12,"name":"Takaaki Fujii","email":"","orcid":"","institution":"Graduate School of Medicine, Gunma University","correspondingAuthor":false,"prefix":"","firstName":"Takaaki","middleName":"","lastName":"Fujii","suffix":""},{"id":634337501,"identity":"4cf53461-3b34-4029-888d-e6ce3b48c1db","order_by":13,"name":"Mitsugu Yamamoto","email":"","orcid":"","institution":"National Hospital Organization, Hokkaido Cancer Center","correspondingAuthor":false,"prefix":"","firstName":"Mitsugu","middleName":"","lastName":"Yamamoto","suffix":""},{"id":634337503,"identity":"9721713b-2047-4d3b-a12b-647ed2fc6927","order_by":14,"name":"Yasushi Tsuji","email":"","orcid":"","institution":"Tonan Hospital","correspondingAuthor":false,"prefix":"","firstName":"Yasushi","middleName":"","lastName":"Tsuji","suffix":""},{"id":634337505,"identity":"73f5661a-d956-49c9-891f-9fad825aa903","order_by":15,"name":"Mitsuo Terada","email":"","orcid":"","institution":"Nagoya City University Graduate School of Medical Sciences","correspondingAuthor":false,"prefix":"","firstName":"Mitsuo","middleName":"","lastName":"Terada","suffix":""},{"id":634337506,"identity":"2478ad38-e685-4de4-8d8c-d3dfc4a2cbd3","order_by":16,"name":"Takayo Ota","email":"","orcid":"","institution":"Izumi City General Hospital","correspondingAuthor":false,"prefix":"","firstName":"Takayo","middleName":"","lastName":"Ota","suffix":""},{"id":634337507,"identity":"896ee201-1cc1-46c4-8199-f996da1be7a4","order_by":17,"name":"Shinya Tokunaga","email":"","orcid":"","institution":"Osaka City General Hospital","correspondingAuthor":false,"prefix":"","firstName":"Shinya","middleName":"","lastName":"Tokunaga","suffix":""},{"id":634337510,"identity":"2d43941f-8557-462a-9a2c-1c270eaaad8b","order_by":18,"name":"Mototsugu Shimokawa","email":"","orcid":"","institution":"Yamaguchi University Graduate School of Medicine","correspondingAuthor":false,"prefix":"","firstName":"Mototsugu","middleName":"","lastName":"Shimokawa","suffix":""},{"id":634337511,"identity":"5a0bc6a7-d491-4de8-9515-c531476352fc","order_by":19,"name":"Toshimi Takano","email":"","orcid":"","institution":"Cancer Institute Hospital, Japanese Foundation for Cancer Research","correspondingAuthor":false,"prefix":"","firstName":"Toshimi","middleName":"","lastName":"Takano","suffix":""}],"badges":[],"createdAt":"2026-03-30 11:22:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9265904/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9265904/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108968329,"identity":"0454973d-598c-4efa-afe7-c9ce12c37bf2","added_by":"auto","created_at":"2026-05-11 10:00:25","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":171329,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e\u0026nbsp;(A). PFS (all)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;(B). PFS (ABE+AI/TAM→ABE＋FUL)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;(C). PFS (ABE+FUL→ABE＋AI)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAbbreviations: PFS, progression-free survival, ABE, abemaciclib; AI, aromatase inhibitor; TAM, tamoxifen; FUL, fulvestrant.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/38b89ca0668386e88dd4ad84.png"},{"id":108977621,"identity":"66fc0b80-b550-4011-8c1d-6c721cfcef13","added_by":"auto","created_at":"2026-05-11 11:32:21","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":33163,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFrequency of ctDNA alterations at before abemaciclib rechallenge and disease progression\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/2bc40b71c9ddc1c5082119a4.png"},{"id":108968333,"identity":"4d55c482-ac7f-4882-8514-df11978be250","added_by":"auto","created_at":"2026-05-11 10:00:25","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":160133,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(A) OS by \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eTP53 \u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003emutation at disease progression\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(B) PFS by \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eCCND1 \u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003eamplification at disease progression in patients who switched from abemaciclib + AI/TAM to abemaciclib + FUL.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(C) OS by \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eCCND1 \u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003eamplification at disease progression in patients who switched from abemaciclib + AI/TAM to abemaciclib + FUL.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAbbreviations: OS, overall survival; PFS, progression-free survival; AI, aromatase inhibitor; TAM, tamoxifen; FUL, fulvestrant.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/5f9242373fe79f50a6c48719.png"},{"id":108979773,"identity":"3f16572f-df88-45f9-b189-f4e98543b5ad","added_by":"auto","created_at":"2026-05-11 12:01:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":493234,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/7bd5bf9f-516c-4856-87e2-7d28f23ecdba.pdf"},{"id":108968330,"identity":"25d3265d-3b16-4e9d-b9f3-2eded9dcb533","added_by":"auto","created_at":"2026-05-11 10:00:25","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":900972,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterials.docx","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/e311aa783c0fc8879112543e.docx"},{"id":108968331,"identity":"9c0564e6-0182-459d-80cd-0e96f6660cb9","added_by":"auto","created_at":"2026-05-11 10:00:25","extension":"pptx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":52964,"visible":true,"origin":"","legend":"","description":"","filename":"TableS2.pptx","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/8b240a47c74cc9bbbbc12701.pptx"},{"id":108968334,"identity":"9b5b376e-ed23-49ff-a4c7-84ddd1b96a12","added_by":"auto","created_at":"2026-05-11 10:00:25","extension":"pptx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":57999,"visible":true,"origin":"","legend":"","description":"","filename":"TableS3B.pptx","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/b04e5b01459c401bcb2d78c2.pptx"},{"id":108968335,"identity":"e9841b7a-8ae5-4fb8-a92e-42a2d02df570","added_by":"auto","created_at":"2026-05-11 10:00:25","extension":"pptx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":47703,"visible":true,"origin":"","legend":"","description":"","filename":"TableS3C.pptx","url":"https://assets-eu.researchsquare.com/files/rs-9265904/v1/b632bcdeaf0b562aa9c0174f.pptx"}],"financialInterests":"Competing interest reported. SK has received honoraria from Kyowa Kirin Co., Ltd, Daiichi Sankyo Co., Ltd, Taiho Pharmaceutical Co., Ltd, Eli Lilly and Company, MSD K.K., AstraZeneca K.K., Chugai Pharmaceutical, Ltd., Eisai Co., Ltd., Takeda Pharmaceutical Co., Ltd., and Pfizer Japan Inc. CF have received honoraria from Daiichi Sankyo, Eisai, Pfizer, Chugai, MSD, Eli Lilly and material from Daiichi Sankyo. YF has received honoraria from Eisai Co. Ltd., Daiichi-Sankyo Co. Ltd., Ono Pharmaceutical Co. Ltd., Merck Biopharma Co. Ltd., Taiho Pharmaceutical Co. Ltd., Chugai Ltd., MSD \u0026 Co., Inc., Chugai Pharmaceutical Co. Ltd., Astellas Pharma Inc. AY has received honoraria from Pfizer, AstraZeneca, Chugai Pharmaceutical Co., Ltd. SW has received honoraria from Daiichi Sankyo Co., Ltd., Chugai Pharmaceutical Co., Ltd., MSD K.K., Eli Lilly Japan K.K., Kyowa Kirin Co., Ltd., Eisai Co., Ltd., Pfizer K.K. TT has received honoraria from Daiichi-Sankyo, Eli Lilly, Gilead Sciences, and MSD. HS has received honoraria from Daiichi Sankyo, Eli Lilly, Gilead Sciences, AstraZeneca, and Pfizer. HS has received research funding (outside of submitting work to her institution): Eli Lilly. YO has received honoraria from Eli Lilly, Pfizer, Daiichi Sankyo, AstraZeneca. TK has received grants from Eli Lilly, AstraZeneca, Guardant Health, Daiichi Sankyo, Ono Pharma, Chugai Pharma, Be One, and Eisai. TK has received consulting fees from Daiichi Sankyo, Astellas Pharma, and received honoraria from Daiichi Sankyo, Ono Pharma, Gilead sciences, Inc, Astellas Pharma, Eisai, AstraZeneca, Taiho Pharma, Chugai Pharma and MSD. TK has received payment for expert testimony from Astellas Pharma. TK has received support for attending meetings and/or travel from Phyzer and Eisai. TK has participated on a Data Safety Monitoring Board or Advisory Board from Daiichi Sankyo, Ono Pharma, Gilead sciences,Inc, Oncotherapy sciences, Eisai, Oncotherapy sciences, Eisai, Astra Zeneca, Taiho Pharma and Be One. TK received of equipment, materials, drugs, medical writing, gifts or other services from Daiichi Sankyo. The other authors have no conflict of interest.","formattedTitle":"Abemaciclib Rechallenge after Progression on Abemaciclib plus Endocrine Therapy in Patients with Hormone Receptor-Positive HER2-Negative Metastatic Breast Cancer: Results from the Phase Ⅱ AGAIN Study (WJOG14220B)","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eThe standard treatment for hormone (HR)-positive HER2-negative breast cancer is cyclin-dependent kinase 4/6 (CDK4/6) inhibitors plus endocrine therapy (ET)\u003csup\u003e1\u003c/sup\u003e, but no treatment approach has been established in patients who are refractory to this regimen. Some patients who have been pretreated with CDK4/6 inhibitor and ET may still benefit from CDK4/6 inhibitor rechallenge, especially in cases of acquired resistance to ET. Several randomized phase II trials testing the efficacy of CDK4/6 inhibitor rechallenge have yielded mixed results depending on the specific CDK4/6 inhibitor (MAINTAIN, PACE and PALMIRA trials)\u003csup\u003e2-4\u003c/sup\u003e. In the phase III postMONARCH trial, abemaciclib plus fulvestrant (FUL) yielded a progression-free survival (PFS) of 6.0 months in patients with HR-positive HER2-negative metastatic breast cancer who exhibited disease progression on a CDK4/6 inhibitor plus ET\u003csup\u003e5\u003c/sup\u003e. Abemaciclib rechallenge is an important next-line option after CDK4/6 inhibitors plus ET, but only 8% of patients received prior abemaciclib in the postMONARCH trial. Therefore, the evidence for abemaciclib rechallenge after prior abemaciclib plus ET remains limited, and there are still no biomarkers for identifying patients who are most likely to benefit from rechallenge with a CDK4/6 inhibitor. Accordingly, this single-arm, phase Ⅱ trial aimed to evaluate the clinical benefits of abemaciclib rechallenge. Translational research evaluating gene alterations was also conducted as accompanying exploratory research to assess the predictive biomarkers of abemaciclib resistance or sensitivity.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cp\u003e\u003cstrong\u003e\u003cem\u003eStudy design and participants\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis multicenter, single-arm, phase II study in Japan enrolled patients with disease progression after receiving abemaciclib plus ET. The main eligibility criteria were as follows:1)histologically confirmed HR-positive HER2-negative invasive breast cancer; 2) locally advanced, metastatic, or recurrent disease; 3) previously received no more than three lines of ET and one line of chemotherapy; and 4) previously received abemaciclib plus ET achieved clinical benefits, complete response or partial response, or stable disease for at least 6 months\u0026nbsp;during abemaciclib-based treatment. This study excluded patients who were previously treated with immune checkpoint inhibitors, everolimus, olaparib, or other CDK4/6 inhibitors expect abemaciclib for advanced or metastatic disease. Since ribociclib is not approved in Japan, none of the patients in this study had a history of ribociclib treatment.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eTreatment and procedures\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePatients were switched from aromatase inhibitor (AI)/tamoxifen (TAM) to FUL, or from FUL to AI, while continuing abemaciclib. Abemaciclib was administered orally at a dose of 150 mg twice daily on days 1–28 of a 28-day cycle. If dose reduction was required due to adverse events during previous treatment, the starting dose of abemaciclib was adjusted to the previous level (100 mg or 50 mg). This was combined with another endocrine agent that had not been used in prior therapy.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAs ET, patients received either FUL or an AI. FUL (500 mg) was administered via intramuscular injection on days 1, 15, and 29, followed by maintenance dosing every 4 weeks. Alternatively, for those receiving an AI, anastrozole (1 mg), letrozole (2.5 mg), or exemestane (25 mg) was administered orally once daily throughout each 28-day cycle.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eEndpoints\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe primary endpointwas PFS. The secondary endpoints were overall response rate (ORR), clinical benefit rate (CBR), chemotherapy-free interval (CFI), overall survival (OS), and safety. Subgroup analyses were performed based on gene alterations (e.g., \u003cem\u003eESR1\u003c/em\u003e or \u003cem\u003ePIK3CA\u003c/em\u003e mutations), disease site (visceral/bone only/other), previous lines of therapy for advanced or metastatic disease (first/second/third and more), ET (FUL/non-FUL) or chemotherapy, performance status (0/1), organs involved (1/2/≥ 3), and disease-free interval (\u0026lt;36/≥36 months) and menopausal status, to examine the interaction of treatment effects on PFS and OS. Tumor response was assessed via computed tomography at baseline, every 6 weeks for 12 months, and then every 12 weeks until disease progression, death, or study discontinuation as per RECIST version 1.1. Adverse events (AEs)\u0026nbsp;were recorded using the\u0026nbsp;Common Terminology Criteria for Adverse Events version 5.0.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eBiomarker analysis\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo analyze circulating biomarkers, blood samples for liquid biopsy were collected at three time points: 1) prior to the initiation of abemaciclib rechallenge, 2) at two months post-rechallenge, and 3) during disease progression.\u0026nbsp;Plasma was separated from the peripheral blood cells by centrifugation at 2,000 g for 10 minutes at 4°C, then circulating tumor DNA (ctDNA) was extracted by centrifugation at 1,600 g for 10 minutes at H.U. Frontier® (Minato-ku, Tokyo, Japan). At The Cancer Institute Hospital of Japanese Foundation for Cancer Research, plasma was separated by centrifugation at 300 g for 20 minutes, then ctDNA was extracted via centrifugation at 5,000 g for 20 minutes at 20°C and subsequently analyzed using the Oncomine Pan-Cancer Cell-Free Assay (Thermo Fisher Scientific®,\u0026nbsp;Waltham, Massachusetts, USA).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eStatistical analysis\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe threshold and expectation of median PFS after the abemacilcib rechallenge were 3.0 and 5.0 months, respectively. A total of 59 cases were needed over the 2-year recruitment period and 1-year follow-up period to achieve a two-sided significance level of 5% and 80% power. Estimating a dropout/ineligibility rate of 10%, the target number for recruitment was 65 cases. Median PFS was estimated using the Kaplan–Meier method, with confidence interval (CI) calculated using the Brookmeyer and Crowley method. The protocol treatment was considered effective if the lower limit of the 90% CI for the median PFS exceeded 3 months. Subgroup analyses of PFS and OS were performed, and survival curves were generated using the Kaplan–Meier method. Hazard ratio (HR) and 95% CI were calculated using Cox proportional hazards models. Data analysis was performed using SAS version 9.4. Biomarker analysis was performed using R statistical software version 4.2.1\u003csup\u003e6\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eEthics approval and consent to participate\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll patients provided written informed consent before enrolment. The protocol was approved by the West Japan Oncology Group (WJOG) and the institutional review board/independent ethics committee (CRB3180008) in accordance with the ethical principles of the Declaration of Helsinki. The trial was registered with the Japan Registry of Clinical Trials (jRCTs031210129).\u0026nbsp;\u003c/p\u003e"},{"header":"RESULTS","content":"\u003cp\u003e\u003cstrong\u003e\u003cem\u003ePatients and treatment\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFrom June 2021 to November 2023, 64 out of 65 enrolled patients were evaluable after 1 withdrawal (Supplementary Figure S1). By the end of the study period, 54 out of 64 patients discontinued treatment due to disease progression.\u003c/p\u003e\n\u003cp\u003eTable 1 shows the patient characteristics. The median age was 56 years (range, 33–80 years) and the median number of prior treatment lines was 1 (range, 1–4). A total of 45 patients (70.3%) had visceral metastasis, while 13 patients (20.3%) had bone-only metastasis. Regarding treatment, 28 patients (43.7%) were switched from\u0026nbsp;abemaciclib plus AI or TAM to abemaciclib plus FUL, whereas 36 (56.2%) were switched from abemaciclib plus FUL to abemaciclib plus AI.\u0026nbsp;The posttreatment regimens were as follows: chemotherapy (n = 28, 43.8%); endocrine monotherapy (n = 17, 26.6%); abemaciclib plus ET (n = 7, 10.9%); other (n = 2, 3.1%); and none (n = 4, 6.3%).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eEfficacy\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe median PFS was 4.2 months (90% CI, 2.8–4.4) (Figure 1A). Since the lower limit of the 95% CI for PFS did not reach 3 months, the primary endpoint was not met. The secondary outcomes were as follows: ORR of 8.7% (95% CI, 2.4–20.8), CBR of 23.9% (95% CI, 12.6–38.8), CFI of 6.7 months (95% CI, 4.9–11.0), and OS of 28.7 months (95% CI, 25.4 not estimable [NE]) (Supplementary Table S1, Figure S2, 3). The median PFS was 7.1 months (95% CI, 3.9–11.3) in the abemaciclib plus FUL arm, while it was 2.8 months (95% CI, 1.9–4.2) in the abemaciclib plus AI arm (Figure 1B, C). On subgroup analyses of PFS, the only significant difference seen was that in prior ET (see Supplementary Table S2).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eSafety\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMost adverse events were low-grade.\u0026nbsp;The most common grade 3 or higher adverse event was neutropenia, which occurred in 17 patients (26.6%) (Table 2). Diarrhea of any grade occurred in 14 patients (21.9%). Grade 3 febrile neutropenia and lung infections were observed in 2 patients (3.1%). No cases of interstitial lung disease were reported. The overall incidence of treatment-related AEs remained low throughout the study period.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eBiomarkers\u003c/em\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure 2 shows the frequency of ctDNA alterations seen before abemaciclib rechallenge and during disease progression. Before abemaciclib rechallenge, fibroblast growth factor receptor 1 (\u003cem\u003eFGFR1\u003c/em\u003e) amplification was associated with a shorter median PFS (HR, 5.05; 95% CI, 1.73–14.76; \u003cem\u003ep\u003c/em\u003e = 0.003) and OS (HR, 4.44; 95% CI, 1.41–13.98; \u003cem\u003ep\u003c/em\u003e = 0.01), while \u003cem\u003eTP53\u003c/em\u003e mutation was associated with shorter median OS (HR, 2.78; 95% CI, 1.07–7.25; \u003cem\u003ep\u003c/em\u003e = 0.04) (Supplementary Table S3A). Two months after rechallenge,\u0026nbsp;\u003cem\u003eCCND1\u003c/em\u003e amplification was associated with a shorter median PFS (HR, 2.40; 95% CI, 1.11–5.19;\u003cem\u003e\u0026nbsp;p\u003c/em\u003e = 0.03) (Supplementary Table S3A). During disease progression,\u0026nbsp;the most frequently acquired gene alterations were \u003cem\u003eTP53\u003c/em\u003emutations (n = 7, 13.0%) and \u003cem\u003eCCND1\u003c/em\u003e amplification (n = 5, 9.3%). The presence of \u003cem\u003eTP53\u0026nbsp;\u003c/em\u003emutations at progression was associated with a shorter median OS (mutant: 18.8 months [95% CI, 10.8–NA]; vs. wild-type: 28.6 months [95% CI, 25.3–NA], HR, 4.17; 95% CI, 1.24–14.02; \u003cem\u003ep\u003c/em\u003e = 0.02) (Figure 3A, Supplementary Table S3B, C).\u003c/p\u003e\n\u003cp\u003eSince \u003cem\u003eTP53\u003c/em\u003e mutation was associated with shorter OS at all time points (pre-rechallenge, two months post-rechallenge, and during progression), it was suggested as a prognostic biomarker. Out of 27 patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL, the most frequently acquired alteration was \u003cem\u003eCCND1\u0026nbsp;\u003c/em\u003eamplification (n = 4, 14.8%). This alteration was associated with shorter median PFS (HR, 5.13; 95% CI, 1.43–18.4; \u003cem\u003ep\u003c/em\u003e = 0.01) and OS (HR, 6.28; 95% CI, 1.26–31.25;\u003cem\u003e\u0026nbsp;p\u0026nbsp;\u003c/em\u003e= 0.01) (Figure 3B, C).\u0026nbsp;No significant differences in PFS or OS were seen based on \u003cem\u003eESR1\u0026nbsp;\u003c/em\u003eor \u003cem\u003ePIK3CA\u003c/em\u003e mutations.\u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eThe AGAIN study was the first abemaciclib rechallenge study to switch from only abemaciclib. Although the primary endpoint was not met, patients who received abemaciclib plus FUL after progression on abemaciclib plus AI/TAM had a clinically meaningful median PFS of 7.1 months.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Several CDK4/6 inhibitor rechallenge trials have been conducted to date. The MAINTAIN trial, a randomized phase II trial of ribociclib plus ET after progression of CDK4/6 inhibitors (palbociclib: 86.5%; ribociclib: 11.7%), demonstrated a significant PFS benefit (5.79 vs. 2.76 months; hazard ratio [HR]: 0.57; p = 0.006)\u003csup\u003e2\u003c/sup\u003e. On the other hand, several randomized phase Ⅱ studies (PACE and PALMIRA trials) did not demonstrate an improvement in PFS after switching patients from palbociclib plus ET to palbociclib plus ET\u003csup\u003e3, 4\u003c/sup\u003e. The postMONARCH trial, the first randomized phase Ⅲ study of abemaciclib rechallenge, demonstrated an improved median PFS of 6.0 months\u003csup\u003e5\u003c/sup\u003e, but the previous CDK4/6 inhibitors were mostly palbociclib (59%) and ribociclib (33%), with abemaciclib representing a minority (8%).\u0026nbsp;Accordingly, no study has fully explored abemaciclib rechallenge after previous abemaciclib therapy.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;In recent years, there have been advancements in the development of treatments after resistance to CDK4/6 inhibitor plus ET. The CAPItello-291 study demonstrated that treatment with capivasertib (AKT inhibitor) plus FUL conferred a median PFS of 7.3 months in patients with AKT pathway-altered tumors who previously received a CDK4/6 inhibitor plus AI\u003csup\u003e7\u003c/sup\u003e. In the phase Ⅱ BYLieve study, alpelisib (PI3K inhibitor) plus FUL similarly conferred a median PFS of 7.4 in patients with PIK3CA-mutated breast cancer who previously received a CDK4/6 inhibitor\u003csup\u003e8\u003c/sup\u003e. Our study revealed a median PFS of 7.1 months in patients who received abemaciclib plus FUL after progression on abemaciclib plus AI/TAM, which is comparable to those who received targeted therapy after a CDK4/6 inhibitor. These findings suggest that abemaciclib rechallenge may be considered as a treatment option in cases where gene alterations of AKT pathway are absent, AKT inhibitors or PIK3CA inhibitors are not applicable, or treatment continuation is difficult to AEs (e.g., grade 3 or higher rash).\u003c/p\u003e\n\u003cp\u003e In HR-positive HER2-negative advanced breast cancer, one of the mechanisms of resistance to AI is acquired alterations in the \u003cem\u003eESR1\u003c/em\u003e gene\u003csup\u003e9\u003c/sup\u003e. In the EMBER-3 trial, treatment with imulunestrant (i.e., oral selective estrogen receptor degrader) plus abemaciclib significantly improved the median PFS of patients with \u003cem\u003eESR1\u003c/em\u003e mutations who progressed after previous treatment with AI with or without CDK4/6 inhibitor \u003csup\u003e10\u003c/sup\u003e. In our study, there was no difference in PFS among patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL with \u003cem\u003eESR1\u0026nbsp;\u003c/em\u003emutations.\u003c/p\u003e\n\u003cp\u003eMost adverse events were low-grade, because\u0026nbsp;the enrolled patients had previously managed AEs through supportive care or dose reduction.\u003c/p\u003e\n\u003cp\u003eIn a phase Ⅲ study of palbociclib plus FUL (PALOMA-3 trial), high cyclin E1 (\u003cem\u003eCCNE1\u003c/em\u003e)mRNA expression was associated with palbociclib resistance\u003csup\u003e11\u003c/sup\u003e. In contrast, our biomarker analysis in the present abemaciclib rechallenge study revealed that \u003cem\u003eCCND1\u003c/em\u003e amplification was the most frequently acquired alteration, and this was associated with shorter PFS and OS among patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL. In the MONNALESSA-7 trial, patients with \u003cem\u003eCCND1\u003c/em\u003e alterations at baseline had shorter median PFS in both the ribociclib plus ET arm and ET-alone arm, suggesting the potential use of \u003cem\u003eCCND1\u0026nbsp;\u003c/em\u003eas a prognostic biomarker\u003csup\u003e12\u003c/sup\u003e. However, \u003cem\u003eCCND1\u003c/em\u003e amplification was not associated with palbociclib sensitivity in the PALOMA-1/TRIO-18 trial\u003csup\u003e13\u003c/sup\u003e. Preclinical models and cell line studies have reported that tumors with\u003cem\u003e\u0026nbsp;CCND1\u003c/em\u003e amplification and D-type cyclin activating features may be highly sensitive to abemaciclib\u003csup\u003e14\u003c/sup\u003e. Nevertheless, no preclinical or clinical data support that \u003cem\u003eCCND1\u003c/em\u003e amplification represents an acquired resistance to abemaciclib. Our study suggests that \u003cem\u003eCCND1\u0026nbsp;\u003c/em\u003eamplification may be a potential predictor of\u0026nbsp;resistance to abemaciclib in patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL.\u003c/p\u003e\n\u003cp\u003eIn PALOMA-3, patients with \u003cem\u003eTP53\u0026nbsp;\u003c/em\u003emutations had significantly worse PFS in both the palbociclib plus FUL arm and FUL-only arm\u003csup\u003e15\u003c/sup\u003e. In the MONALEESA-7 trial, patients with \u003cem\u003eTP53\u003c/em\u003e mutations similarly had worse PFS in both the ribociclib plus ET arm and ET-only arm\u003csup\u003e12\u003c/sup\u003e. In our study, patients with\u0026nbsp;\u003cem\u003eTP53\u003c/em\u003e mutation similarly had shorter OS at each time point (pre-rechallenge, two months post-rechallenge, and during progression), and thus \u003cem\u003eTP53\u003c/em\u003e mutation was considered as a prognostic biomarker rather than a predictive marker.\u003c/p\u003e\n\u003cp\u003eThis study has several limitations. First, it is a single-arm, nonrandomized trial with a small sample size, resulting in broader 95% CI, and thus further studies are required to validate these results. Second, the results of the biomarker analysis are considered exploratory due to the very small number of patients with mutations. Lastly, tumor activity prior to enrolment in the AGAIN study was not assessed using the RECIST criteria.\u003c/p\u003e\n\u003cp\u003eIn conclusion, although the primary endpoint was not met, switching to abemaciclib plus FUL after progression on abemaciclib plus AI/TAM could be a clinically effective option. \u003cem\u003eCCND1\u0026nbsp;\u003c/em\u003eamplification may be a potential predictor of resistance to abemaciclib in patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL.\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe would like to thank all the participants in this study, including patients, caregivers, physicians, and medical workers.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFUNDING\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was sponsored by Eli Lilly Japan K.K. and funded by Japan Society of Clinical Oncology.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDISCLOSURE\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSK has received honoraria from Kyowa Kirin Co., Ltd, Daiichi Sankyo Co., Ltd, Taiho Pharmaceutical Co., Ltd, Eli Lilly and Company, MSD K.K., AstraZeneca K.K., Chugai Pharmaceutical, Ltd., Eisai Co., Ltd., Takeda Pharmaceutical Co., Ltd., and Pfizer Japan Inc. CF have received honoraria from Daiichi Sankyo, Eisai, Pfizer, Chugai, MSD, Eli Lilly and material from Daiichi Sankyo. YF has received honoraria from Eisai Co. Ltd., Daiichi-Sankyo Co. Ltd., Ono Pharmaceutical Co. Ltd., Merck Biopharma Co. Ltd., Taiho Pharmaceutical Co. Ltd., Chugai Ltd., MSD \u0026amp; Co., Inc., Chugai Pharmaceutical Co. Ltd., Astellas Pharma Inc. AY has received honoraria from Pfizer,\u0026nbsp;AstraZeneca, Chugai Pharmaceutical Co., Ltd. SW has received honoraria from Daiichi Sankyo Co., Ltd.,\u0026nbsp;Chugai Pharmaceutical Co., Ltd.,\u0026nbsp;MSD K.K., Eli Lilly Japan K.K., Kyowa Kirin Co., Ltd., Eisai Co., Ltd., Pfizer K.K. TT has received honoraria from Daiichi-Sankyo, Eli Lilly, Gilead Sciences, and MSD. HS has received honoraria from Daiichi Sankyo, Eli Lilly, Gilead Sciences, AstraZeneca, and Pfizer. HS has received research funding (outside of submitting work to her institution): Eli Lilly. YO has received honoraria from Eli Lilly, Pfizer, Daiichi Sankyo, AstraZeneca. TK has received grants from Eli Lilly, AstraZeneca,\u0026nbsp;Guardant Health,\u0026nbsp;Daiichi Sankyo, Ono Pharma, Chugai Pharma, Be One, and\u0026nbsp;Eisai. TK has received consulting fees from\u0026nbsp;Daiichi Sankyo, Astellas Pharma, and received\u0026nbsp;honoraria from\u0026nbsp;Daiichi Sankyo, Ono Pharma, Gilead sciences, Inc, Astellas Pharma, Eisai, AstraZeneca,\u0026nbsp;Taiho Pharma, Chugai Pharma and MSD. TK has received payment for expert testimony from\u0026nbsp;Astellas Pharma. TK has received support for attending meetings and/or travel from Phyzer and Eisai. TK has participated on a Data Safety Monitoring Board or Advisory Board from Daiichi Sankyo, Ono Pharma, Gilead sciences,Inc, Oncotherapy sciences, Eisai, Oncotherapy sciences, Eisai, Astra Zeneca, Taiho Pharma and\u0026nbsp;Be One. TK received of equipment, materials, drugs, medical writing, gifts or other services from Daiichi Sankyo.\u0026nbsp;The other authors have no conflict of interest.\u003c/p\u003e"},{"header":"REFERENCES ","content":"\u003col\u003e\n\u003cli\u003eGradishar WJ, Moran MS, Abraham J et al. NCCN Guidelines\u0026reg; Breast Cancer, Version 5.2025: Featured Updates to the NCCN Guidelines. J Natl Compr Cancer Netw 2025; 23 (11): 426-436.\u003c/li\u003e\n\u003cli\u003eKalinsky K, Accordino MK, Chiuzan C et al. Randomized Phase II Trial of Endocrine Therapy With or Without Ribociclib After Progression on Cyclin-Dependent Kinase 4/6 Inhibition in Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Metastatic Breast Cancer: MAINTAIN Trial. J Clin Oncol 2023; 41 (24): 4004-4013.\u003c/li\u003e\n\u003cli\u003eMayer EL, Ren Y, Wagle N et al. PACE: A Randomized Phase II Study of Fulvestrant, Palbociclib, and Avelumab After Progression on Cyclin-Dependent Kinase 4/6 Inhibitor and Aromatase Inhibitor for Hormone Receptor-Positive/Human Epidermal Growth Factor Receptor-Negative Metastatic Breast Cancer. J Clin Oncol 2024; 42 (17): 2050-2060.\u003c/li\u003e\n\u003cli\u003eLlombart-Cussac A, Harper-Wynne C, Perello A et al. Second-Line Endocrine Therapy With or Without Palbociclib Rechallenge in Patients With Hormone Receptor-Positive/Human Epidermal Growth Factor Receptor 2-Negative Advanced Breast Cancer: PALMIRA Trial. J Clin Oncol 2025; 43 (18): 2084-2093.\u003c/li\u003e\n\u003cli\u003eKalinsky K, Bianchini G, Hamilton E et al. Abemaciclib Plus Fulvestrant in Advanced Breast Cancer After Progression on CDK4/6 Inhibition: Results From the Phase III postMONARCH Trial. J Clin Oncol 2025; 43 (9): 1101-1112.\u003c/li\u003e\n\u003cli\u003eKanda Y. Investigation of the Freely Available Easy-to-Use Software \u0026apos;EZR\u0026apos; for Medical Statistics. Bone Marrow Transplant 2013; 48 (3): 452-458.\u003c/li\u003e\n\u003cli\u003eTurner NC, Oliveira M, Howell SJ et al. Capivasertib in Hormone Receptor-Positive Advanced Breast Cancer. N Engl J Med 2023; 388 (22): 2058-2070.\u003c/li\u003e\n\u003cli\u003eDe Laurentiis M, Ferreira AM, Gligorov J et al. Alpelisib Plus Fulvestrant for PIK3CA-Mutated, HR-Positive, HER2-Negative Advanced Breast Cancer After a CDK4/6 Inhibitor (EPIK-B5): Phase III, Randomized, Double-Blind, Placebo-Controlled, Multicenter Study. Clin Cancer Res 2026; 32 (4_Supplement): RF7-02.\u003c/li\u003e\n\u003cli\u003eBrett JO, Spring LM, Bardia A et al. ESR1 Mutation as an Emerging Clinical Biomarker in Metastatic Hormone Receptor-Positive Breast Cancer. Breast Cancer Res 2021; 23 (1): 85.\u003c/li\u003e\n\u003cli\u003eJhaveri KL, Neven P, Casalnuovo ML et al. Imlunestrant with or without Abemaciclib in Advanced Breast Cancer. N Engl J Med 2025; 392 (12): 1189-1202.\u003c/li\u003e\n\u003cli\u003eTurner NC, Liu Y, Zhu Z et al. Cyclin E1 Expression and Palbociclib Efficacy in Previously Treated Hormone Receptor-Positive Metastatic Breast Cancer. J Clin Oncol 2019; 37 (14): 1169-1178.\u003c/li\u003e\n\u003cli\u003eBardia A, Su F, Solovieff N et al. Genomic Profiling of Premenopausal HR+ and HER2- Metastatic Breast Cancer by Circulating Tumor DNA and Association of Genetic Alterations With Therapeutic Response to Endocrine Therapy and Ribociclib. JCO Precis Oncol 2021; 5: 1408-1420.\u003c/li\u003e\n\u003cli\u003eFinn RS, Crown JP, Lang I et al. The Cyclin-Dependent Kinase 4/6 Inhibitor Palbociclib in Combination with Letrozole Versus Letrozole Alone as First-Line Treatment of Oestrogen Receptor-Positive, HER2-Negative, Advanced Breast Cancer (PALOMA-1/TRIO-18): A Randomised Phase 2 Study. Lancet Oncol 2015; 16 (1): 25-35.\u003c/li\u003e\n\u003cli\u003eGong X, Litchfield LM, Webster Y et al. Genomic Aberrations that Activate D-type Cyclins Are Associated with Enhanced Sensitivity to the CDK4 and CDK6 Inhibitor Abemaciclib. Cancer Cell 2017; 32 (6): 761-776.e6.\u003c/li\u003e\n\u003cli\u003eO\u0026apos;Leary B, Cutts RJ, Huang X et al. Circulating Tumor DNA Markers for Early Progression on Fulvestrant With or Without Palbociclib in ER+ Advanced Breast Cancer. J Natl Cancer Inst 2021; 113 (3): 309-317.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"breast-cancer-research-and-treatment","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"brea","sideBox":"Learn more about [Breast Cancer Research and Treatment](https://www.springer.com/journal/10549)","snPcode":"10549","submissionUrl":"https://submission.nature.com/new-submission/10549/3","title":"Breast Cancer Research and Treatment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"CDK 4/6 inhibitor, abemaciclib, hormone receptor-positive HER2 negative breast cancer ","lastPublishedDoi":"10.21203/rs.3.rs-9265904/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9265904/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003ePurpose:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn the postMONARCH trial, abemaciclib plus fulvestrant (FUL) yielded a progression-free survival (PFS) of 6.0 months in patients with hormone receptor (HR)-positive HER2-negative metastatic breast cancer who progressed after receiving CDK4/6 inhibitor plus endocrine therapy (ET). However, only 8% of these patients previously received abemaciclib.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis multicenter, single-arm, phase II study enrolled patients who developed disease progression after abemaciclib plus ET. Patients were switched from aromatase inhibitor (AI)/tamoxifen (TAM) to FUL or from FUL to AI while continuing abemaciclib. The primary endpoint was PFS. The secondary endpoints were overall response rate (ORR), clinical benefit rate (CBR), chemotherapy-free interval (CFI), overall survival (OS), safety, and biomarker analysis was perfomed.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study enrolled 65 patients from June 2021 to November 2023, among which 64 patients were evaluable. The median PFS was 4.2 months (90% CI, 2.8–4.4). The median PFS was 7.1 months (95% CI, 3.9–11.3) in patients who switched from abemaciclib plus AI/TAM to abemaciclib plus FUL (n = 28, 43.8%), whereas it was 2.8 months (95% CI, 1.9–4.2) in patients switching from abemaciclib plus FUL to abemaciclib plus AI (n = 36, 56.3%). Cyclin D1 (\u003cem\u003eCCND1\u003c/em\u003e)\u003cem\u003e \u003c/em\u003eamplification at progression was associated with shorter median PFS (HR, 0.19; 95% CI, 0.05–0.7; \u003cem\u003ep\u003c/em\u003e = 0.01) and OS (HR, 0.16; 95% CI, 0.03–0.8; \u003cem\u003ep\u003c/em\u003e = 0.02).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe primary endpoint was not met, switching from abemaciclib plus AI/TAM to abemaciclib plus FUL could be a clinically effective option. \u003cem\u003eCCND1\u003c/em\u003e amplification may be a resistance to abemaciclib.\u003c/p\u003e\n\u003cp\u003ejRCTs031210129; date of registration JUNE 2, 2021.\u003c/p\u003e","manuscriptTitle":"Abemaciclib Rechallenge after Progression on Abemaciclib plus Endocrine Therapy in Patients with Hormone Receptor-Positive HER2-Negative Metastatic Breast Cancer: Results from the Phase Ⅱ AGAIN Study (WJOG14220B)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-11 10:00:20","doi":"10.21203/rs.3.rs-9265904/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"261403336159351629411057366424754271407","date":"2026-05-18T11:32:42+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-04T18:13:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"233870281588726720535457262076625300712","date":"2026-05-03T01:25:42+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-30T02:45:35+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-01T08:30:18+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-01T08:30:10+00:00","index":"","fulltext":""},{"type":"submitted","content":"Breast Cancer Research and Treatment","date":"2026-03-30T10:48:52+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"breast-cancer-research-and-treatment","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"brea","sideBox":"Learn more about [Breast Cancer Research and Treatment](https://www.springer.com/journal/10549)","snPcode":"10549","submissionUrl":"https://submission.nature.com/new-submission/10549/3","title":"Breast Cancer Research and Treatment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"6cd0f352-15f4-4ba2-b3d8-efee7260bbab","owner":[],"postedDate":"May 11th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"261403336159351629411057366424754271407","date":"2026-05-18T11:32:42+00:00","index":46,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-04T18:13:52+00:00","index":31,"fulltext":""},{"type":"reviewerAgreed","content":"233870281588726720535457262076625300712","date":"2026-05-03T01:25:42+00:00","index":29,"fulltext":""},{"type":"reviewersInvited","content":"29","date":"2026-04-30T02:45:35+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-11T10:00:20+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-11 10:00:20","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9265904","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9265904","identity":"rs-9265904","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}