Hormone Receptor-positive Metastatic Breast Cancer Research Articles

Abstract PO1-23-09: A bedside-to-bench translational analysis demonstrates that NF1 alterations promote CDK4/6 inhibitor (CDK4/6i) resistance in hormone receptor-positive (HR+) metastatic breast cancer (MBC)

Abstract Background: CDK4/6i are part of the standard management of HR+ MBC. The MAPK pathway has been implicated in mediating resistance to CDK4/6 blockade (Wander 2020). A variety of molecular alterations dysregulate MAPK signaling, including NF1 mutations (NF1-MUT) (Wallace 2012), though NF1 has not been implicated in CDK4/6i resistance. To assess this, we analyzed retrospective clinical data from multiple institutions, real-world data (RWD) from the GuardantINFORM database, and preclinical models of HR+ breast tumors. The primary objective of this study was to characterize clinical outcomes of CDK4/6i in patients (pts) with NF1-MUT MBC, and to validate those findings in laboratory models. Methods: Pts with pathogenic (p)NF1-MUT were identified via sequencing of circulating tumor DNA (ctDNA) or tumor tissue. Pathogenic was defined as nonsense, insertion/deletion, frameshift, or oncogenic missense mutations. Progression free survival (PFS) on CDK4/6i was analyzed, and intrinsic resistance was defined as progression < 6 months on 1st line (1L) CDK4/6i or < 3 mo on 2nd line and beyond (2L+). Acquired resistance was defined as PFS >6 mo (1L) or >3 mo (2L+) with a NF1 alteration detected post-progression. RWD from the GuardantINFORM dataset had Guardant360 (G360) ctDNA testing linked with claims data on >37,000 pts with advanced or MBC tested between June 2014 and March 2023. Pts on CDK4/6i within 90 days pre-G360 were stratified as NF1-MUT (n = 28) vs. NF1-non-mutant (NF1-nonMUT) (n = 1133) and analyzed for real-world time-to-next-treatment (rwTTNT) and real-world overall survival (rwOS). Propensity score weighting for age, sex, line of therapy, and year of G360 were used to create a balanced control group. To determine whether NF1 loss-of-function alterations were causal of CDK4/6i resistance, we used CRISPR/Cas9 to delete NF1 in MCF7 breast cancer cells. MCF7 wild-type (WT) and two NF1-knockout (KO) clones were treated with palbociclib ± estrogen deprivation, followed by cell viability assays. MAPK and PI3K/AKT pathway activation were measured by immunoblot analysis. Results: Across 4 institutions, 40 pts with HR+, HER2- MBC expressing pNF1-MUT and prior CDK4/6i were included. The mean age at MBC diagnosis was 56 years, and the majority of pts received no prior treatment in the metastatic setting before CDK4/6i (range 0 - 5 prior lines). Intrinsic or acquired resistance to CDK4/6i was seen in 29/40 (73%) tumors harboring pNF1-MUT. We analyzed pts with baseline pNF1-MUT who received 1L CDK4/6i plus endocrine therapy (n = 13) and found that the median PFS was 7.7 months (range 2 - 18 mo). We next examined GuardantINFORM RWD and discovered a significant difference in rwTTNT and rwOS on CDK4/6i for NF1-MUT (n = 28) vs. weighted NF1-nonMUT (n = 28) tumors. Mean age (62 years) and level of pretreatment (majority 2L+) were similar between the groups. Compared to NF1-nonMUT, NF1-MUT pts had a shorter median rwTTNT on CDK4/6i regimens (4.2 vs. 12.4 mo, weighted log-rank p< 0.0001) and worse rwOS (15.8 vs 45.2 mo, weighted log-rank p=0.016). In MCF7 cells, NF1-KO exhibited a palbociclib IC50 11- to 13-fold higher than NF1-WT controls. In estrogen-deprived media, treatment with 250 nM palbociclib for 6 days reduced MCF7 NF1-WT cell viability 75% but only 5-20% in NF1-KO cells. As expected, phosphorylation of ERK and AKT were strongly induced in NF1-KO cells relative to NF1-WT. Conclusions: Using multicenter clinical and real-world evidence, we demonstrate that NF1-MUT is associated with inferior duration and worse outcomes on CDK4/6i in MBC. Pts with baseline pNF1-MUT on 1L CDK4/6i and endocrine therapy had a median PFS below typical responses expected in clinical practice. A link between NF1 loss and CDK4/6i resistance was supported by in vitro experiments in HR+ breast cancer cells. NF1 deletion was accompanied by activation of ERK and AKT, therefore, future directions include investigating alternative or combination treatment strategies targeting the MAPK and/or PI3K/AKT pathways in these tumors. *MRL and RCP contributed equally. ABH and SAW are co-senior authors. Citation Format: Maxwell Lloyd, M Rosario Chica-Parrado, Todd Knepper, Chang-Ching Lin, Emily Podany, Christine Walko, Fabiana Napolitano, Caroline Weipert, Jiemin Liao, Arielle Medford, Lianne Ryan, Jennifer Keenan, Nicole Zhang, Shiyuan Liu, Gerburg Wulf, Katherine Clifton, Cynthia Ma, Hyo Han, Leif Ellisen, Aditya Bardia, Carlos Arteaga, Ariella Hanker, Seth Wander. A bedside-to-bench translational analysis demonstrates that NF1 alterations promote CDK4/6 inhibitor (CDK4/6i) resistance in hormone receptor-positive (HR+) metastatic breast cancer (MBC) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-23-09.

Abstract PO5-05-07: Efficacy and safety of first line CDK4/6i plus endocrine therapy for patients with HR+/HER2- metastatic breast cancer:initial real-world experience at Ho Chi Minh city oncology hospital, Viet Nam

Abstract Background The cyclin-dependent kinases (CDK)4/6 inhibitors (CDK4/6i) palbociclib and ribociclib have been available in Viet Nam since 2021 for the treatment of hormone receptor-positive metastatic breast cancer (MBC). Data regarding CDK4/6i benefit in Vietnamese MBC are sparse. This study evaluated the impact of CDK4/6i in first-line HR+/HER2− MBC using real-world experience at Ho Chi Minh City Oncology Hospital. Methods A single-institution study analyzed retrospective data from 108 patients treated with palbiciclib or ribociclib with endocrine therapy in first-line in HR+/HER2– MBC patients at Ho Chi Minh city oncology hospital. Patient characteristics, PFS, treatment response, and toxicity profiles were analyzed. Results Baseline patients’ characteristics are detailed in Table 1.The median follow-up was 12.5 months (95% CI: 10.9-13.3). Best responses by the line of therapy are in Table 2. The median PFS has still not been reached. The PFS rates at 6, 12 and 18 months were 94.4%, 93.5% and 91.5%, respectively. Patients with bone-only metastasis have a better PFS than visceral metastasis (p = 0.059). Patients with recurrent disease had shorter PFS (p = 0.083) than those presenting with de novo metastasis. The most common reasons for toxicity were neutropenia (92.6%), anemia (35.2%), thrombopenia (22.2%) and the incidence of diarrhea was only 2,8%. None were related to QTc-prolongation Conclusion The real-world effectiveness and safety with CDK4/6 inhibitors in our institution HR+/HER2– MBC patients mimics that observed in the clinical trials. This finding supports the use of CDK4/6i in combination with endocrine therapy as standard of care for HR+/HER2- MBC in Viet Nam. The study also suggests the need for more study of under-represented minority populations. Further studies are ongoing to validate these findings incorporating additional cancer centers. Keywords: advanced/metastatic breast cancer, Asia, CDK4/6 inhibitors, real world Table 1. Patient Demographics and Baseline Disease Characteristics (N=108) Characteristics N (%) Age, y: Median 52 Stage Recurrent from earlier stage, stages 0–III 67 (62) De novo, newly diagnosed stage IV at enrolment 41 (38) DFI < 12 months 5 (4.6) 12-24 months 12 (11.1) > 24 months 51 (47.2) No. of metastatic sites 1 49 (45.4) 2 37 (34.3) ≥ 3 22 (20.4) Metastasis pattern Bone only 30 (27.8) Other 78 (72.2) CDK4/6i Palbociclib 35 (32.4) Ribociclib 73 (67.6) Partner ET AIs 94 (87) Fulvestrant 14 (13) Table 2. Best responses of therapy (N=108) Best responses (N,%) CR 2 (1.9) PR 29 (26.9) SD 67 (62.1) PD 10 (9.3) Total 108 (100) ORR 31 (28.8) CBR 76 (70.3) Table. Citation Format: Hoang-Quy Nguyen, Hong-Duc Phan-Thi, Thi-Hong-Van Le. Efficacy and safety of first line CDK4/6i plus endocrine therapy for patients with HR+/HER2- metastatic breast cancer:initial real-world experience at Ho Chi Minh city oncology hospital, Viet Nam [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-05-07.

Abstract PS12-06: A phase 2 study of abemaciclib monotherapy for patients with retinoblastoma-positive (Rb+), triple-negative metastatic breast cancer

Abstract Background: Cyclin-dependent kinase (CDK) 4/6 inhibitors can significantly extend survival when given in combination with endocrine therapy as treatment for hormone receptor-positive metastatic breast cancer. Preclinical studies suggest CDK4/6 inhibitor monotherapy might also be effective in a subset of triple-negative breast cancers (TNBCs), including those that express a functional retinoblastoma (RB) protein and/or those of the Luminal Androgen Receptor (LAR) subtype. Currently, the clinical activity of CDK4/6 inhibitor therapy in TNBC has not been reported. Methods: We conducted a single-arm phase II study of abemaciclib monotherapy in patients with locally advanced or metastatic TNBC. Key eligibility criteria included: (i) Measurable disease by RECIST 1.1; (ii) Between 1-3 prior lines of systemic therapy for advanced TNBC; (iii) RB-positive tumor (defined as ≥ 50% of tumor cells staining positive for RB by immunohistochemistry [archival or fresh sample] on central testing); (iv) ECOG PS 0/1. Patients were treated with abemaciclib 200 mg orally (28-day cycles) twice daily until disease progression, unacceptable toxicity, withdrawal of consent, or death. Tumor biopsies were mandatory at baseline and C2D1 if tumor tissue was safely accessible. The primary outcome was objective response rate (ORR). Key secondary endpoints included progression-free survival (PFS), overall survival (OS), clinical benefit rate (CBR: CR + PR + SD ≥ 24 weeks), and safety and tolerability. The study had a two-stage design. Thirteen patients were enrolled in the first stage, with the plan to enrol a further 25 patients if at least one objective response was observed. Results: One unconfirmed partial response was observed in stage 1, and a total of 27 patients were enrolled before the trial was closed early due to slow accrual. The median age was 61 years and patients had received a median of 2 prior lines of systemic therapy for metastatic disease. Twelve patients had received prior immunotherapy. After a median follow-up of 28.5 months, the confirmed ORR was 0% and the CBR was 15%, with 4 of 27 patients experiencing stable disease for ≥ 24 weeks. The median PFS was 1.94 months (95% CI: 1.8 – 11.5 months), and the median OS was 8.44 months (95% CI: 4.6 – 15.6 months). There was no significant difference in PFS or OS between patients with PD-L1-positive versus PD-L1-negative disease, or Androgen Receptor (AR) positive versus negative tumors by immunohistochemistry. The most common adverse events of grade 2 or higher were diarrhea (41%), neutropenia (41%), anemia (30%), and nausea (30%). RNA-sequencing of baseline biopsies has been performed to identify biomarkers associated with clinical benefit, and results will be presented at the meeting. Conclusions: Abemaciclib monotherapy did not show clinical activity in patients with Rb+ metastatic TNBC. This finding suggests that future trials of CDK4/6 inhibition as monotherapy in TNBC are not warranted. Citation Format: Shom Goel, Bojana Jovanović, Xiangying Chu, Melissa Hughes, Ayesha Mohammed-Abreu, Julie Kasparian, Timothy Erick, Molly DiLullo, Eileen Wrabel, Rinath Jeselsohn, Nabihah Tayob, Stuart Schnitt, Nancy Lin, Sara Tolaney. A phase 2 study of abemaciclib monotherapy for patients with retinoblastoma-positive (Rb+), triple-negative metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS12-06.

Abstract PS12-08: MORPHEUS Hormone Receptor-Positive Breast Cancer: interim analysis of a Phase Ib/II, study of fulvestrant ± atezolizumab and abemaciclib triplet treatment in patients metastatic disease

Abstract BACKGROUND Endocrine therapy (ET) is the mainstay of treatment for metastatic hormone receptor-positive breast cancer (HR+ BC). ET resistance and disease progression are expected, thus novel therapies, like cancer immunotherapy, are needed. Prior data suggest that abemaciclib (abema), a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor, has immunomodulatory activity. In the MORPHEUS HR+ BC study (NCT03280563), atezolizumab (atezo; anti–programmed death-ligand 1 [PD-L1]) was tested in combination with fulvestrant (FUL), with and without abema, in patients (pts) with HR+ metastatic BC. We present 24-week interim analyses. METHODS Pts with measurable disease progression during first- or second-line therapy for metastatic or inoperable locally advanced HR+ BC and prior treatment with a CDK4/6 inhibitor were randomized to receive FUL (control) or FUL + atezo (1200 mg intravenous every 3 weeks) or FUL + atezo + abema (150 mg twice a day); prior FUL was not permitted. Pts were treated until loss of clinical benefit or unacceptable toxicity. Primary endpoints were objective response rate (ORR) per Response Evaluation Criteria in Solid Tumours (RECIST) v1.1 and safety. Progression-free survival (PFS) was a secondary endpoint. Baseline tumor samples were analyzed for PD-L1 expression (SP263), CD8 T cell infiltration, and gene expression by RNAseq. RESULTS As of Dec 2022, 40, 31, and 25 pts (38, 30, and 20 evaluable pts) were randomized to atezo + abema + FUL, atezo + FUL, and FUL, respectively. Pts were followed for ≥ 24 weeks. Demographics were similar among the groups, with most pts receiving prior palbociclib (palbo) as part of their only prior metastatic therapy. Details and best confirmed ORRs are shown in the table. Median PFS was 6.34 months (95% confidence interval [CI] 5.52, 16.03) in the atezo + abema + FUL arm, 3.15 months (95% CI 1.51, 7.79) in the atezo + FUL arm, and 1.95 months (95% CI 1.45, 4.93) in the FUL arm. The hazard ratio of atezo + abema + FUL vs FUL was 0.43 (95% CI 0.24, 0.78). Safety data are shown in the table. Mild/moderate (grade 1/2) interstitial lung disease (ILD)/pneumonitis (7.7%) was observed in the atezo + abema + FUL arm. At baseline, tumors exhibited low prevalence of PD-L1 (median immune cells: 0.5%, tumor cells: 0%) and CD8 infiltration (12% inflamed phenotype), which did not associate with response in any arm. RNAseq analysis indicated that response to atezo + abema + FUL was strongly associated with low baseline expression levels of proliferation and metabolism signatures and trended with high expression of some immune signatures. CONCLUSIONS The triplet therapy of atezo + abema + FUL showed improved ORR and PFS compared with FUL monotherapy in the second- or third-line setting post-CDK4/6 inhibitor. This combination of atezo + abema + FUL was tolerable, with no unexpected safety signals, including no high-grade ILD/pneumonitis. Efficacy and safety Data are number of patients (%), unless otherwise specified. * Patient was treated in the second line and incorrectly included in this group. Abema, abemaciclib; AE, adverse event; ALT, alanine aminotransferase; AST, aspartate aminotransferase; atezo, atezolizumab; CDK4/6, cyclin-dependent kinase 4/6; CI, confidence interval; ful, fulvestrant; irAE, immune-related adverse event; L, line; ORR, objective response rate; PFS, progression-free survival; TRAE, treatment-related adverse event. Citation Format: Kyung Hae Jung, Seock-Ah Im, Denise Yardley, Sara Hurvitz, Keun Seok Lee, Amir Sonnenblick, Shlomit Shachar, Antoinette Tan, Elizabeth Comen, Einav Gal-Yam, Adam Brufsky, Hope Rugo, Jing Zhu, Kelly DuPree, Vanessa Breton, Fiona Young, Richard Schwab, Edward Cha, Melinda Telli. MORPHEUS Hormone Receptor-Positive Breast Cancer: interim analysis of a Phase Ib/II, study of fulvestrant ± atezolizumab and abemaciclib triplet treatment in patients metastatic disease [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS12-08.

Abstract PO3-23-10: Continued treatment with CDK4/6 inhibitors slow tumor progression by extending G1-phase duration in drug resistant populations

Abstract CDK4/6 inhibitors (CDK4/6i) have significantly improved the prognosis of patients with metastatic hormone receptor-positive breast cancer. However, the development of drug resistance leads to the discontinuation of CDK4/6i treatment. The potential benefits of maintaining CDK4/6i treatment in the context of resistance and their underlying molecular mechanisms remain elusive. Here we show that despite tumor progression, continued treatment with CDK4/6i notably extends the G1 phase in drug-resistant cells, thereby slowing their growth. Our single-cell data reveal that the duration of the G1 phase in these resistant cells is approximately tripled under continuous CDK4/6i treatment compared to conditions with CDK4/6i withdrawal, while the durations of the S and G2 phases remain comparable. The maintenance of CDK4/6i induces an alternative inactivation of the retinoblastoma (Rb) protein in drug-resistant cells through its post-translational degradation. However, this alternative form of Rb inactivation is ineffective and thus induces slow activation kinetics of E2F transcription factors, significantly delaying G1 progression. The beneficial effect of maintaining CDK4/6i treatment was further validated in breast cancer xenograft models. Furthermore, our findings suggest that given the spectrum of various CDK4/6i, switching between different CDK4/6i types after the emergence of drug resistance may further delay tumor progression. This study provides new mechanistic insights into the maintenance of CDK4/6i treatment despite the development of drug resistance. Citation Format: Jessica Armand, Sungsoo Kim, Hee Won Yang. Continued treatment with CDK4/6 inhibitors slow tumor progression by extending G1-phase duration in drug resistant populations [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-23-10.

Abstract PO1-16-07: Patient Characteristics and Treatment Sequencing Among HER2-low Metastatic Breast Cancer Patients with Rapid vs. Delayed Progression on First-line Endocrine Therapy

Abstract Background Patients with HER2-low (IHC 1+ or 2+ and negative ISH) and hormone-receptor positive (HR+) metastatic breast cancer (mBC) who rapidly progress on first-line (1L) endocrine therapy may experience different treatment pathways than those with delayed progression. The goal of this multisite study is to describe the characteristics, treatment patterns and outcomes of these patients with rapid vs. delayed progression. Methods This was a retrospective cohort study of HER2-low, HR+, mBC patients who received 1L endocrine-based treatment regimen at the Huntsman Cancer Institute, Moffitt Cancer Center, and the Winthrop P. Rockefeller Cancer Institute between 2017 and 2021. With no conventional definition, our study defined rapid progression as disease progression within 12 months of 1L treatment initiation vs >12 months/no progression during study follow-up as delayed progression. Demographic and clinical characteristics, and 1L and second-line (2L) treatment patterns were assessed using descriptive statistics. Using Kaplan-Meier analysis, overall survival (OS) was estimated from treatment initiation to death while progression-free survival (PFS) was estimated from treatment initiation to death or disease progression according to physician notes in patients’ charts. Censoring was performed at the date of last follow-up or 31 Dec 2021, whichever occurred earlier. Results The study included 118 patients who received 1L endocrine therapy (see table), mostly in combination with a CDK4/6 inhibitor (n = 87, 73.7%). Rapid progression was observed for 25 (21.2%) patients with a median 1L PFS and OS of 6.4 months (95% CI: 5.4-8.2) and 20.5 months (95% CI: 14.4-30.5), respectively, compared to delayed progressors (median 1L PFS = 30.5 months, 95% CI: 21.5-54.2 and median OS = 62.1 months, 95% CI: 46.4 -upper bound not reached). Rapid progressors were more likely than delayed progressors to be current smokers (n = 6, 24.0% vs n = 7, 7.5%, p = 0.015) or had liver metastasis at diagnosis of advanced disease (n = 11, 44.4% vs n = 17, 18.3%, p = 0.007) but less likely to have bone as the only metastasis site (n = 6, 24.0% vs n = 46, 49.5%, p = 0.023). A smaller proportion of rapid progressors were on CDK4/6 inhibitors in 1L (n = 17, 68.0%) compared to delayed progressors (n = 70, 75.3%) but the difference was not statistically significant. A substantial proportion of rapid progressors (68%, n = 17) continued to receive endocrine therapy in 2L, including endocrine therapy alone (n = 6, 24.0%), endocrine therapy with targeted therapy (n = 6, 24.0%) and endocrine therapy with CDK4/6 inhibitors (n = 5, 20.0%). Among patients on 1L endocrine therapy who continued to a 2L treatment, 2L PFS and OS of rapid progressors (n = 24) were 5.7 months (95% CI: 3.2-10.6) and 14.1 months (95% CI: 6.8-24.2), respectively, compared to 9.2 months (95% CI: 3.1-13.7) and 29.2 months (95% CI:14.4- upper bound not reached) among delayed progressors (n = 39). Conclusion In this study of patients with HER2-low and HR+ mBC, many patients with rapid disease progression on 1L endocrine-based regimens continued to receive endocrine therapy in their next line of therapy and experienced poor clinical outcomes. This preliminary chart review study highlights the unmet need for more effective treatments for patients with rapid progression on 1L endocrine-based regimens. Further research with a larger sample size is warranted to confirm these findings. Patient characteristics and treatment patterns Demographic characteristics and treatment patterns of stage IV breast cancer patients with HER2-low disease and received 1L endocrine therapy Citation Format: Chia Jie Tan, Connor Willis, Trang Au, Matthew Schabath, Chenghui Li, Kristen Kelley, Xiaoqing Xu, Leah Park, Clara Lam, Sandhya Mehta, Jackie Kwong, Diana Brixner, David Stenehjem. Patient Characteristics and Treatment Sequencing Among HER2-low Metastatic Breast Cancer Patients with Rapid vs. Delayed Progression on First-line Endocrine Therapy [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-16-07.

Abstract PO5-05-05: Clinical outcomes of CDK4/6 inhibitors in patients with bone only metastatic breast cancer

Abstract Background: CDK4/6 inhibitors (CDK 4/6i) have significantly advanced the treatment of hormone receptor-positive (HR+) metastatic breast cancer in the last decade. Several clinical trials including PALOMA, MONALEESA, and MONARCH have shown improved progression-free survival (PFS) rates when CDK4/6i were combined with endocrine therapy (ET). These results led to recommending their use in combination with ET as the first line of therapy in the metastatic settings. Bone metastasis is the most common site of disease in patients with HR+ metastatic breast cancer, about 40% of whom have bone as the only site of metastasis (bone-only disease [BoD]). While previous studies have shown that BoD can have a unique clinical course and a more favorable prognosis compared to non-BoD metastasis, most of the aforementioned CDK 4/6i clinical trials have not shown significant improvements in PFS in BoD subset of patients. Here we aimed to investigate the survival outcomes of BoD patients at a single institution center and evaluate CDK 4/6i activity in preclinical models of bone metastasis in breast cancer. Patients and methods: Patients with HR+, HER2-negative metastatic breast cancer from a retrospective CDK4/6i study at Washington University School of Medicine were included in this analysis. We selected all the patients who received CDK4/6i in the first- or second-line settings from June 2014 to March 2020. Outcomes were collected through June 10, 2023. The Kaplan-Meier method with log rank test and multivariable cox model were used for survival analysis. Results: 114 female patients (n=75 first line, n=39 second line, n=20 African American, n=94 Caucasian) were included in this study. All patients received Palbociclib as the first exposure to CDK4/6 inhibitor in combination with ET for at least two months. Median age was 60 years (range, 28-87) and forty-seven patients (41%) were in the BoD group. Survival outcomes for BoD group were compared to the rest of patients who had either bone and visceral or visceral only metastatic lesions (BV group). Our results demonstrated a significantly reduced median PFS in BoD group compared to BV group (13.1 months [95%CI:8-18.2] vs 19.6 months [95%CI:6.8-32.4], p=0.01) respectively. There was no significant difference in overall survival between BoD and BV groups (36.6 months [95%CI:32.2-41] vs 41.7 months [95%CI 36.1-47.2] p=0.74) respectively. In multivariate analysis regarding the associated risk factors with PFS, after adjusting for age and race, BoD had a HR of 1.62 (95%CI:1.04-2.54) for PFS. Among the 75 patients who received CDK4/6i as their first line of therapy, BoD (n=32 patients [42.7%]) also showed an inferior (p=0.05) median PFS (20 months [95%CI 12.9-27]) compared to BV group (n=43 patients [57.3%], median PFS=28.4 months [95%CI: 16.3-40.4]). To investigate potential treatment resistance mechanisms in bone metastases, we used the triple negative 4T1 and the ER+ luminal B PyMT Bo1 breast tumor cell line derived mammary gland models in immunocompetent mice. Single agent CDK4/6i, ribociclib, decreased primary mammary gland tumor burden. This effect was abrogated when CD8+ T-cells were depleted. In the bone colonization model using these cell lines, ribociclib had little effect on bone tumor burden. Arg1+ immune suppressive macrophages were significantly decreased in the primary mammary tumor but not in the bone tumor after ribociclib administration, suggesting a relative treatment resistance and immune suppression in the bone tumor microenvironment compared to the primary mammary gland. Conclusion: CDK4/6i play a significant role in the treatment of patients with HR+ metastatic breast cancer. Our single institution, retrospective study and preclinical data suggests that CDK4/6i may not be as effective for the treatment of tumors residing in the bone microenvironment, warranting future studies to understand the mechanisms of CDK4/6 inhibitors action in bone versus visceral metastases. Citation Format: Mahta Mardani, Suleyman Noordeen, Katherine Clifton, Cynthia Ma, Jingqin Luo, Jing Xi, Nusayba Bagegni, Foluso Ademuyiwa, Rama Suresh, Ashley Frith, Andrew Davis, Ron Bose, Lindsay Peterson, Shana Thomas, Yu Tao, Takayuki Kobayashi, Jingyu Xiang, Yalin Xu, Xinming Su, Katherine Weilbaecher. Clinical outcomes of CDK4/6 inhibitors in patients with bone only metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-05-05.

The Prognostic Value of Low HER2 Expression on Treatment Outcomes in Metastatic Hormone Receptor-positive Breast Cancer Patients Treated with CDK4/6 Inhibitors and Endocrine Therapy

The Prognostic Value of Low HER2 Expression on Treatment Outcomes in Metastatic Hormone Receptor-positive Breast Cancer Patients Treated with CDK4/6 Inhibitors and Endocrine Therapy

Abstract 5827: Identification of potential novel drivers and biomarkers of CDK4/6 inhibitor resistance in metastatic breast cancer

Abstract Background: Inhibitors of the cyclin-dependent kinases 4 and 6 (CDK4/6i) have become part of the standard of care for patients with hormone receptor-positive metastatic breast cancer. Despite their initial benefit for most patients, the development of resistance to this treatment is almost inevitable. However, there is a lack of predictive or clinical markers that can be utilized to monitor resistance development, and their definition remains an unmet challenge. Methods: We aimed to identify novel drivers and potential biomarkers of CDK4/6i resistance by creating and characterizing two ribociclib-resistant breast cancer cell lines. Ribociclib-resistant derivatives of MCF7 cells and the CTC (circulating tumor cell)-derived cell line CTC-ITB-01 were created by long-term exposure to the inhibitor for a minimum of 6 months and potential novel resistance drivers were identified by transcriptome analysis using RNA-sequencing. In addition to mRNAs encoding proteins that may contribute to resistance, we also investigated microRNAs as they are poorly characterized in the context of CDK4/6i resistance. Additionally, changes in the kinome activity were assessed by using PamGene technology. Results: The RNA-sequencing analysis identified ≥ 2000 genes with differential expression in both resistant cell lines compared to their respective parental counterparts. Over Representation Analysis revealed that a high number of these genes encode proteins associated with the cell cycle, proliferation, epithelial-to-mesenchymal transition, various signaling pathways like Wnt, and the complement system. Evaluation of kinase activity using functional kinome profiling in the resistant versus parental CTC-ITB-01 cell line showed increased activity of the CDK and MAPK kinase subgroups, indicating a multifactorial resistance mechanism. Furthermore, the analysis of deregulated ncRNAs demonstrated increased expression of miR-146a-5p in both resistant cell lines and interestingly, a significant decrease of miR-205-5p levels in the resistant CTC-ITB-01 cell line. MiR-205-5p is described as a negative regulator of EMT and progression in breast cancer but has not been linked to CDK4/6i resistance yet. This result was validated by qPCR and miRNA in situ-hybridization. Conclusion: Through our comprehensive transcriptome analysis of two ribociclib-resistant cell lines, we were able to identify several novel potential resistance drivers. Their utility as predictive or monitoring biomarkers requires further investigation. Furthermore, discovering new resistance mechanisms could also help find new therapeutic strategies after CDK4/6i resistance development. Acknowledgment: This project received funding from the Deutsche Krebshilfe DETECT high project (Nr.: 70114705) and the Horizon 2020 program RNADiagon (Nr.: 824036). Citation Format: Leonie F. Ott, Malik Alawi, Malgorzata Stoupiec, Elena Laakmann, Malte Kriegs, Volkmar Müller, Klaus Pantel, Sabine Riethdorf. Identification of potential novel drivers and biomarkers of CDK4/6 inhibitor resistance in metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5827.

Application of fluorocarbon nanoparticles of 131I-fulvestrant as a targeted radiation drug for endocrine therapy on human breast cancer

BackgroundBreast cancer is the most prevalent malignant tumor among women, with hormone receptor-positive cases constituting 70%. Fulvestrant, an antagonist for these receptors, is utilized for advanced metastatic hormone receptor-positive breast cancer. Yet, its inhibitory effect on tumor cells is not strong, and it lacks direct cytotoxicity. Consequently, there's a significant challenge in preventing recurrence and metastasis once cancer cells develop resistance to fulvestrant.MethodTo address these challenges, we engineered tumor-targeting nanoparticles termed 131I-fulvestrant-ALA-PFP-FA-NPs. This involved labeling fulvestrant with 131I to create 131I-fulvestrant. Subsequently, we incorporated the 131I-fulvestrant and 5-aminolevulinic acid (ALA) into fluorocarbon nanoparticles with folate as the targeting agent. This design facilitates a tri-modal therapeutic approach—endocrine therapy, radiotherapy, and PDT for estrogen receptor-positive breast cancer.ResultsOur in vivo and in vitro tests showed that the drug-laden nanoparticles effectively zeroed in on tumors. This targeting efficiency was corroborated using SPECT-CT imaging, confocal microscopy, and small animal fluorescence imaging. The 131I-fulvestrant-ALA-PFP-FA-NPs maintained stability and showcased potent antitumor capabilities due to the synergism of endocrine therapy, radiotherapy, and CR-PDT. Throughout the treatment duration, we detected no notable irregularities in hematological, biochemical, or histological evaluations.ConclusionWe've pioneered a nanoparticle system loaded with radioactive isotope 131I, endocrine therapeutic agents, and a photosensitizer precursor. This system offers a combined modality of radiotherapy, endocrine treatment, and PDT for breast cancer.Graphical

Exceptional Clinical Response to Alpelisib in a Patient with Metastatic Breast Cancer with Hyperbilirubinemia and a PIK3CA Mutation

In the era of personalized and precision medicine, the importance of next-generation sequencing in metastatic breast cancer has been increasingly recognized. This case highlights one such example in which a patient with heavily pretreated, hormone receptor-positive metastatic breast cancer was discovered to have a PIK3CA mutation and started on a PIK3 inhibitor with an exceptional response. This patient had been facing a visceral crisis with diffuse liver metastasis and hyperbilirubinemia. Alpelisib was able to achieve rapid disease control within a matter of weeks with clinical and objective laboratory improvement. To date, there has been no published data regarding the safety of alpelisib in patients with elevated bilirubin and this case report lends support to safety and efficacy in this situation.

An Italian Real-World Study Highlights the Importance of Some Clinicopathological Characteristics Useful in Identifying Metastatic Breast Cancer Patients Resistant to CDK4/6 Inhibitors and Hormone Therapy.

Background: Cyclin-dependent kinase 4 and 6 (CDK4 and CDK6) inhibitors have changed the therapeutic management of hormone receptor-positive (HR+) metastatic breast cancer (mBC) by targeting the cell cycle machinery and overcoming endocrine resistance. However, a large number of patients present disease progression due to cancer cells resisting CDK4/6 inhibitors. Our research considers which clinicopathological characteristics could be useful in identifying patients who might respond to CDK4/6 inhibitors by analyzing a retrospective case series of patients with HR+ mBC who were treated with hormone therapy plus CDK4/6 inhibitors. Methods: Approximately 177 mBC patients were enrolled, of whom 66 were treated with CD4/6 inhibitors plus letrozole and 111 were treated with CDK4/6 inhibitors and fulvestrant. A multistate model was used. Results: A low body surface area and older age were associated with an increased risk of developing neutropenia. A high Ki67 index, the presence of visceral metastases, and not having previously undergone adjuvant chemotherapy were prognostic factors of disease progression/death. As expected, some of the neutropenic patients who had previously undergone multiple lines of treatment were at a higher risk of disease progression/death. Furthermore, neutropenia status was associated with a more than doubled risk of progression/death compared to patients without neutropenia (HR = 2.311; p = 0.025). Conclusions: Having identified certain factors that could be associated with the development of neutropenia and considering that neutropenia itself is associated with an increased risk of progression, we believe that the baseline characteristics should be taken into account to reduce cases of neutropenia and disease progression.

C-MET-positive circulating tumor cells and cell-free DNA as independent prognostic factors in hormone receptor-positive/HER2-negative metastatic breast cancer

BackgroundEndocrine therapy resistance in hormone receptor-positive/HER2-negative (HR+/HER2−) breast cancer (BC) is a significant clinical challenge that poses several unmet needs in the management of the disease. This study aimed to investigate the prognostic value of c-MET-positive circulating tumor cells (cMET+ CTCs), ESR1/PIK3CA mutations, and cell-free DNA (cfDNA) concentrations in patients with hormone receptor-positive (HR+) metastatic breast cancer (mBC).MethodsNinety-seven patients with HR+ mBC were prospectively enrolled during standard treatment at Samsung Medical Center. CTCs were isolated from blood using GenoCTC® and EpCAM or c-MET CTC isolation kits. PIK3CA and ESR1 hotspot mutations were analyzed using droplet digital PCR. CfDNA concentrations were calculated using internal control copies from the ESR1 mutation test. Immunocytochemistry was performed to compare c-MET overexpression between primary and metastatic sites.ResultsThe proportion of c-MET overexpression was significantly higher in metastatic sites than in primary sites (p = 0.00002). Survival analysis showed that c-MET+ CTC, cfDNA concentration, and ESR1 mutations were significantly associated with poor prognosis (p = 0.0026, 0.0021, and 0.0064, respectively) in HR+/HER2− mBC. By contrast, EpCAM-positive CTC (EpCAM+ CTC) and PIK3CA mutations were not associated with progression-free survival (PFS) in HR+/HER2− mBC. Multivariate analyses revealed that c-MET+ CTCs and cfDNA concentration were independent predictors of PFS in HR+/HER2− mBC.ConclusionsMonitoring c-MET+ CTC, rather than assessing c-MET expression in the primary BC site, could provide valuable information for predicting disease progression, as c-MET expression can change during treatment. The c-MET+ CTC count and cfDNA concentration could provide complementary information on disease progression in HR+ /HER2− mBC, highlighting the importance of integrated liquid biopsy.

A Phase I Study of Ruxolitinib in Combination with Abemaciclib for Patients with Primary or Post-Polycythemia Vera/Essential Thrombocythemia Myelofibrosis

Background and Significance: Patients (pts) with primary or secondary myelofibrosis (MF) experience variable degrees of constitutional symptoms, cytopenias and risk of progression to acute myeloid leukemia leading to a reduced life-expectancy compared to age-matched controls. Although several oral Janus Kinase (JAK) inhibitors such as ruxolitinib (RUX) have been approved by the US FDA based on improvements in symptom burden and spleen volume compared to physician's choice of best available therapy, the duration of response to these agents is generally time-limited and treatment with JAK inhibitors is not considered to have disease-modifying potential. Furthermore, the median overall survival after RUX discontinuation is only 11-14 months highlighting the need for novel mechanism-based therapies for pts with disease progression on RUX. JAK2 V617F mutations promote transition from the G1 to S-phase of the cell cycle via increased expression of CDC25A, which is also upregulated in primary MPN patient samples. Additional cell cycle regulators such as CDK6 and Cyclin D have also been implicated in MF pathogenesis and contribute to myeloproliferation. Finally, we and others have shown synergistic effects of the combination of CDK4/6 inhibitors and RUX in murine models of MF (Rampal et al., CCR 2021) including improvement in bone marrow fibrosis suggesting the potential for disease-modifying activity. Several CDK4/6 inhibitors have been approved by the US FDA for the treatment of hormone receptor-positive metastatic breast cancer establishing a robust record of safety data. These preclinical data and known safety profile of CDK4/6 and RUX as single agents, provide rationale for combining CDK4/6 and JAK inhibition in pts with MF. Methods: This multicenter, phase I dose-escalation trial (NCT05714072) evaluates the safety of RUX + the CDK4/6 inhibitor abemaciclib in primary or secondary MF pts with intermediate-1/2 or high-risk disease by DIPSS who require treatment and had an inadequate response to RUX. Inadequate response to RUX is defined by (I) palpable splenomegaly ≥5 cm below the left costal margin at study entry AND/OR (II) active MPN symptoms, as defined by the presence of one symptom score ≥5 or two symptom scores ≥3 using the screening MPN-SAF TSS. Pts must have been on a stable dose of 10 mg or 15 mg BID of RUX for at least 4 weeks prior to enrollment with adequate platelet (≥75 x 10 9/L) and neutrophil counts (≥1.5 x 10 9/L). Key exclusion criteria include prior treatment with a CDK4/6 inhibitor, accelerated- or blast-phase MPN, platelet count <50 x 10 9/L in the 4 weeks before screening or platelet transfusion(s) within 8 weeks before screening, and absolute neutrophil count <0.5 x 10 9/L in the 4 weeks before screening. Among the currently FDA-approved CDK4/6 inhibitors, abemaciclib was chosen for combination therapy due to less myelosuppressive effects compared with other CDK4/6 inhibitors. This trial uses a conventional “3+3” dose-escalation design ( Figure), pts will be treated with increasing doses of abemaciclib added to a stable dose of RUX (10 mg or 15 mg BID). The pre-study dose of RUX will be maintained to determine the maximum tolerated dose of the combination of RUX and abemaciclib. The primary endpoint is to determine the maximum tolerated dose of RUX + abemaciclib. Secondary endpoints include measures of efficacy such as overall response rate, duration of response and survival endpoints. Exploratory endpoints include changes in allele fraction of driver mutations, gene expression profiling, and clonal architecture to identify biomarkers of response, mechanisms of resistance, and to assess for any disease-modifying effects of the combination therapy. At this time, 2 pts have been enrolled at dose level 0 without any dose-limiting toxicities observed. In line with the known adverse event profile of abemaciclib, grade 1 diarrhea occurred in one patient. One patient developed grade 3 neutropenia which promptly resolved with holding abemaciclib. No unexpected safety signal was noted and enrollment continues. This trial is open to enrollment at Memorial Sloan Kettering Cancer Center and will commence at MD Anderson Cancer Center later this year.

Changes in cell-free DNA after short-term palbociclib and fulvestrant treatment for advanced or metastatic hormone receptor-positive and human epidermal growth factor 2-negative breast cancer.

Here, we investigated the potential predictive and elucidating efficacy of cell-free DNA (cfDNA) changes on clinical outcomes and biological effects, respectively, after short-term palbociclib and fulvestrant treatment for patients with hormone receptor (HR)-positive and human epidermal growth factor 2 (HER2)-negative advanced or metastatic breast cancer (ABC). In this secondary analysis of the Japan Breast Cancer Research Group-M07 (FUTURE) trial, blood cfDNA was obtained before palbociclib treatment and on day 15 of cycle one (28-day cycle). Target enrichment was performed using next-generation sequencing; progression-free survival (PFS) was compared based on cfDNA changes between baseline and day 15 of cycle one after combination therapy. Fifty-six patients (112 paired blood samples) were examined. The median follow-up time was 8.9months. PIK3CA (30.4%, 17/56), FOXA1 (30.4%, 17/56), and ESR1 (28.6%, 16/56) were most frequently mutated at baseline. The number of mutated genes was significantly decreased on day 15 compared with that at baseline (paired t test: P value = 0.025). No significant difference was observed in PFS (decrease group, 7.9m vs the others, 9.3m; log-rank P value = 0.75; hazard ratio, 1.13; 95% confidence interval, 0.53-2.41). Among patients without previous aromatase inhibitor treatment (n = 15), three (20%) had ESR1 mutations after progression to fulvestrant. No significant association was observed between changes in mutated genes after short-term palbociclib and fulvestrant treatment and disease progression; a significant reduction in cfDNA mutation level was observed on day 15 of cycle one. Clinical meanings of cfDNA should be investigated in the future trials.

Neratinib+ fulvestrant+ trastuzumab for HR-positive, HER2-negative, HER2-mutant metastatic breast cancer: outcomes and biomarker analysis from the SUMMIT trial.

HER2 mutations are targetable alterations in patients with hormone receptor-positive (HR+) metastatic breast cancer (MBC). In the SUMMIT basket study, patients with HER2-mutant MBC received neratinib monotherapy, neratinib+ fulvestrant, or neratinib+ fulvestrant+ trastuzumab (N+ F+ T). We report results from 71 patients with HR+, HER2-mutant MBC, including 21 (seven in each arm) from a randomized substudy of fulvestrant versus fulvestrant+ trastuzumab (F+ T) versus N+ F+ T. Patients with HR+ HER2-negative MBC with activating HER2 mutation(s) and prior cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) therapy received N+ F+ T (oral neratinib 240 mg/day with loperamide prophylaxis, intramuscular fulvestrant 500 mg on days 1, 15, and 29 of cycle 1 then q4w, intravenous trastuzumab 8mg/kg then 6 mg/kg q3w) or F+ T or fulvestrant alone. Those whose disease progressed on F+ T or fulvestrant could cross-over to N + F + T. Efficacy endpoints included investigator-assessed objective response rate (ORR), clinical benefit rate (RECIST v1.1), duration of response, and progression-free survival (PFS). Plasma and/or formalin-fixed paraffin-embedded tissue samples were collected at baseline; plasma was collected during and at end of treatment. Extracted DNA was analyzed by next-generation sequencing. ORR for 57 N+ F+ T-treated patients was 39% [95% confidence interval (CI) 26% to 52%); median PFS was 8.3months (95% CI 6.0-15.1 months). No responses occurred in fulvestrant- or F+ T-treated patients; responses in patients crossing over to N+ F+ T supported the requirement for neratinib in the triplet. Responses were observed in patients with ductal and lobular histology, 1 or ≥1 HER2 mutations, and co-occurring HER3 mutations. Longitudinal circulating tumor DNA sequencing revealed acquisition of additional HER2 alterations, and mutations in genes including PIK3CA, enabling further precision targeting and possible re-response. The benefit of N+ F+ T for HR+ HER2-mutant MBC after progression on CDK4/6is is clinically meaningful and, based on this study, N+ F+ T has been included in the National Comprehensive Cancer Network treatment guidelines. SUMMIT has improved our understanding of the translational implications of targeting HER2 mutations with neratinib-based therapy.

421P Changes in cell-free DNA after short-term palbociclib and fulvestrant treatment for advanced or metastatic hormone receptor-positive and human epidermal growth factor 2-negative breast cancer

421P Changes in cell-free DNA after short-term palbociclib and fulvestrant treatment for advanced or metastatic hormone receptor-positive and human epidermal growth factor 2-negative breast cancer

CDK4/6 Inhibitor Resistance in Hormone Receptor-Positive Metastatic Breast Cancer: Translational Research, Clinical Trials, and Future Directions.

The emergence of CDK4/6 inhibitors, such as palbociclib, ribociclib, and abemaciclib, has revolutionized the treatment landscape for hormone receptor-positive breast cancer. These agents have demonstrated significant clinical benefits in terms of both progression-free survival and overall survival. However, resistance to CDK4/6 inhibitors remains a challenge, limiting their long-term efficacy. Understanding the complex mechanisms driving resistance is crucial for the development of novel therapeutic strategies and the improvement of patient outcomes. Translational research efforts, such as preclinical models and biomarker studies, offer valuable insight into resistance mechanisms and may guide the identification of novel combination therapies. This review paper aims to outline the reported mechanisms underlying CDK4/6 inhibitor resistance, drawing insights from both clinical data and translational research in order to help direct the future of treatment for hormone receptor-positive metastatic breast cancer.

Prognostic value of [18F]-FDG PET/CT in patients with meta-static breast cancer treated with cyclin-dependent inhibitors.

The addition of cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) to endocrine therapy impressively improved the outcome of patients with hormone receptor-positive metastatic breast cancer. Despite their great efficacy, not all patients respond to treatment and many of them develop acquired resistance. The aim of this retrospective study was to assess the role of [18F]-FDG PET/CT in predicting PFS and OS in breast cancer patients treated with CDK4/6i. 114 patients who performed an [18F]-FDG PET/CT scan before (PET1) and 2-6 months (PET2) after starting treatment were retrospectively enrolled. Metabolic response was evaluated by EORTC, PERCIST and Deauville Score and correlated to PFS and OS. In patients who did not progress at PET2 (n = 90), PFS rates were not significantly different between classes of response by EORTC and PERCIST. Conversely, patients showing a Deauville score ≤3 had a longer PFS (median PFS 42 vs 21.0 months; p = 0.008). A higher total metabolic tumor volume at PET1 (TMTV1) was also associated with a shorter PFS (median 18 vs 42 months; p = 0.0026). TMTV1 and Deauville score were the only independent prognostic factors for PFS at multivariate analysis and their combination stratified the population in four definite classes of relapse risk. Conversely, the above parameters did not affect OS which was only influenced by a progressive metabolic disease at PET2 (3-years survival rate 29.8 vs 84.9%; p<0.0001). TMTV and metabolic response by Deauville score were significant prognostic factors for PFS in patients with breast cancer treated with CDK4/6i. Their determination could help physicians to select patients who may need a closer follow up.

Capecitabine Plus Aromatase Inhibitor as First Line Therapy for Hormone Receptor Positive, HER2 Negative Metastatic Breast Cancer.

(1) Background: recent evidence suggests that long low-dose capecitabine regimens have a synergistic effect with endocrine therapy as aromatase inhibitors (AIs), and might increase overall survival for hormone-receptor-positive, HER2-negative, metastatic breast cancer compared to both treatments. We performed a retrospective study to confirm the efficacy and expand the safety data for capecitabine plus AI (a combination henceforth named XELIA) for this indication. (2) We conducted a single-center retrospective cohort study of 163 hormone receptor-positive metastatic breast cancer patients who received either the XELIA regimen, capecitabine, or an aromatase inhibitor (AI) as single agents in first-line treatment. The primary endpoint was progression-free survival, and the secondary endpoints were overall survival, best objective response, and toxicity incidence. (3) Results: the median progression-free survival for patients receiving XELIA, AI, and capecitabine was 29.37 months (20.91 to 37.84; 95% CI), 20.04 months (7.29 to 32.80; 95% CI) and 10.48 (8.69 to 12.28; 95% CI), respectively. The overall response rate was higher in the XELIA group (29.5%) than in the AI (14.3%) and capecitabine (9.1%) groups. However, the differences in overall survival were not statistically significant. Apart from hand-foot syndrome, there were no statistically significant differences in adverse events between the groups. (4) Conclusions: this retrospective study suggests that progression-free survival and overall response rates improved with the XELIA regimen compared to use of aromatase inhibitors and capecitabine alone. Combined use demonstrated an adequate safety profile and might represent an advantageous treatment in places where CDK 4/6 is not available. Larger studies and randomized clinical trials are required to confirm the effects shown in our study.