Aromatase Inhibitor Resistance Research Articles

Abstract 3829: Identifying the genetic basis for resistance to aromatase inhibitors (AIs) in metastatic breast cancer (mBC) patients

Abstract Background: AIs, such as letrozole, anastrozole, and exemestane, are used to treat hormonal receptor positive (HR+) breast cancer (BC) patients. Many patients either respond poorly or relapse on AIs. A comprehensive view of the underlying genomic landscape of such patients may aid in a better understanding of the resistance mechanisms and the development of new targeted therapies. This study leveraged a real-world clinico-genomic dataset of BC patients to identify key biomarkers and the corresponding high frequency mutations in this relapse/refractory patient population. Methods: This retrospective study used ConcertAI’s Genome360TM BC dataset (N=3223). A cohort of HR+/HER2− mBC patients who were either refractory to or relapsed on AIs (N=863) were included. To identify key resistance genes/pathways, only genomic data (from NGS tests) from a biopsy performed after tumor progression on AIs was considered. Prevalence analysis was done only based on known pathogenic/likely pathogenic variants. Pathway analysis of identified biomarkers with the mechanism of action of AIs was performed to identify and validate key resistance pathways. Further, signatures of unique protein alteration were identified within the biomarkers of interest. Results: The median age of patients at the time of diagnosis was 54 years. Adenocarcinoma and Lobular/Ductal carcinoma were the two most prevalent histologies. 51% of patients in this cohort were treated with AIs in the first line. Table 1 lists the 20 most frequently altered genes observed in this cohort. Genes with a potential role in AI resistance based on pathway analysis are indicated. The most common mutations in selected biomarkers are also provided. These biomarkers were observed at a much lower frequency in patients who responded well to AIs. Conclusion: This study provides insights into the genetic basis of resistance to AIs in mBC patients, opening up possibilities for development of new treatments to overcome resistance. Table 1: Top 20 biomarkers and their variants found in AI relapse/refractory mBC Patients. Biomarker Prevalence (%) Potential Role in AI Resistance Key Variants ESR1 37.81 Yes D538G, Y537, Y537N, E380Q, Y537C PIK3CA 37.04 Yes H1047R, E545K, E542K, H1047L, E726K, N345K TP53 35.56 GATA3 14.25 Yes Frameshift Mutation (Mainly) CDH1 11.64 Yes Frameshift Mutation (Mainly) MAP3K1 9.35 KMT2C 9.02 PTEN 8.38 Yes Frameshift Mutation (Mainly) ARID1A 8.17 RB1 7.38 Yes Frameshift Mutation (Mainly) DNMT3A 5.31 ATM 5.30 TBX3 5.10 SPEN 4.90 NF1 4.82 Yes Frameshift Mutation (Mainly) AKT1 4.74 Yes E17K ERBB2 3.21 Yes L755S, V777L, D769Y KRAS 2.74 Yes G12V, G12D, G12A BRCA1/2 2.69 CDKN2A 1.78 Citation Format: Neeraj Singh, Smita Agrawal. Identifying the genetic basis for resistance to aromatase inhibitors (AIs) in metastatic breast cancer (mBC) patients [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 3829.

Acetyl-CoA carboxylase 1 controls a lipid droplet-peroxisome axis and is a vulnerability of endocrine-resistant ER+ breast cancer.

Targeting aromatase deprives ER+ breast cancers of estrogens and is an effective therapeutic approach for these tumors. However, drug resistance is an unmet clinical need. Lipidomic analysis of long-term estrogen-deprived (LTED) ER+ breast cancer cells, a model of aromatase inhibitor resistance, revealed enhanced intracellular lipid storage. Functional metabolic analysis showed that lipid droplets together with peroxisomes, which we showed to be enriched and active in the LTED cells, controlled redox homeostasis and conferred metabolic adaptability to the resistant tumors. This reprogramming was controlled by acetyl-CoA-carboxylase-1 (ACC1), whose targeting selectively impaired LTED survival. However, the addition of branched- and very long-chain fatty acids reverted ACC1 inhibition, a process that was mediated by peroxisome function and redox homeostasis. The therapeutic relevance of these findings was validated in aromatase inhibitor-treated patient-derived samples. Last, targeting ACC1 reduced tumor growth of resistant patient-derived xenografts, thus identifying a targetable hub to combat the acquisition of estrogen independence in ER+ breast cancers.

A novel scaffold long-acting selective estrogen receptor antagonist and degrader with superior preclinical profile against ER+ breast cancer

Breast cancer is one of the most common malignant tumors in women worldwide, with the majority of cases showing expression of estrogen receptors (ERs). Although drugs targeting ER have significantly improved survival rates in ER-positive patients, drug resistance remains an unmet clinical need. Fulvestrant, which overcomes selective estrogen receptor modulator (SERM) and AI (aromatase inhibitor) resistance, is currently the only long-acting selective estrogen receptor degrader (SERD) approved for both first and second-line settings. However, it fails to achieve satisfactory efficacy due to its poor solubility. Therefore, we designed and synthesized a series of novel scaffold (THC) derivatives, identifying their activities as ER antagonists and degraders. G-5b, the optimal compound, exhibited binding, antagonistic, degradation or anti-proliferative activities comparable to fulvestrant in ER+ wild type and mutants breast cancer cells. Notably, G-5b showed considerably improved stability and solubility. Research into the underlying mechanism indicated that G-5b engaged the proteasome pathway to degrade ER, subsequently inhibiting the ER signaling pathway and leading to the induction of apoptosis and cell cycle arrest events. Furthermore, G-5b displayed superior in vivo pharmacokinetics and pharmacodynamics properties, coupled with a favorable safety profile in the MCF-7 tamoxifen-resistant (MCF-7/TR) tumor xenograft model. Collectively, G-5b has emerged as a highly promising lead compound, offering potent antagonistic and degradation activities, positioning it as a novel long-acting SERD worthy of further refinement and optimization.

Clinical relevance of circulating ESR1 mutations during endocrine therapy for advanced hormone-dependent endometrial carcinoma

ObjectiveEndocrine therapy is frequently administered in patients with hormone dependent (HR+) metastatic endometrial cancer. ESR1 mutations have emerged as a key mechanism of aromatase inhibitor (AI) resistance in HR + metastatic breast cancer and can be monitored using circulating tumor DNA (ctDNA). The aim of this study was to explore the incidence and clinical relevance of circulating ESR1 mutations in patients treated by AI or megestrol acetate (M) for advanced endometrial carcinoma.MethodologyThis single-center retrospective study was performed at the Henri Becquerel Center (Rouen) and looked for circulating ESR1 gene mutations by droplet digital PCR (E380Q, L536R, Y537S, Y537N, Y537C, D538G, S463P) in patients with advanced HR + endometrial carcinoma treated between 2008 and 2020 for at least 30 days by AI or M. Analyses were performed before exposure and at progression/during endocrine therapy.ResultsTwenty-two patients were included: 13 were treated with AI, 12 of whom progressed; 9 patients were treated with M, 8 of whom progressed. 68.1% of the patients had low-grade endometrial carcinoma and 54.5% had received chemotherapy in the metastatic setting. The median duration of treatment was 152 days (min 47 – max 629) with AI and 155 days (min 91-max 1297) with M. Under AI, there was no ESR1 mutation at baseline, and one Y537C mutation at progression with a variant allele frequency (VAF) of 0.14%. Under M, one patient had a Y537C (VAF 0.2%) at baseline that disappeared during treatment. Another patient had a Y537S mutation emergence at progression after 91 days of treatment (VAF 1.83%). There was no significant difference between the circulating DNA concentration before and after hormone therapy (p = 0.16).ConclusionESR1 mutations do not seem to be involved in the mechanisms of resistance to AI or M in HR+ endometrial cancer. The clinical relevance of their detection is not demonstrated.

SERPINA3-ANKRD11-HDAC3 pathway induced aromatase inhibitor resistance in breast cancer can be reversed by HDAC3 inhibition

Endocrine resistance is a major challenge for breast cancer therapy. To identify the genes pivotal for endocrine-resistance progression, we screened five datasets and found 7 commonly dysregulated genes in endocrine-resistant breast cancer cells. Here we show that downregulation of serine protease inhibitor clade A member 3 (SERPINA3) which is a direct target gene of estrogen receptor α contributes to aromatase inhibitor resistance. Ankyrin repeat domain containing 11 (ANKRD11) works as a downstream effector of SERPINA3 in mediating endocrine-resistance. It induces aromatase inhibitor insensitivity by interacting with histone deacetylase 3 (HDAC3) and upregulating its activity. Our study suggests that aromatase inhibitor therapy downregulates SERPINA3 and leads to the ensuing upregulation of ANKRD11, which in turn promotes aromatase inhibitor resistance via binding to and activating HDAC3. HDAC3 inhibition may reverse the aromatase inhibitor resistance in ER-positive breast cancer with decreased SERPINA3 and increased ANKRD11 expression.

The genomic landscape associated with resistance to aromatase inhibitors in breast cancer.

Aromatase inhibitors (AI) are drugs that are widely used in treating estrogen receptor (ER)-positive breast cancer patients. Drug resistance is a major obstacle to aromatase inhibition therapy. There are diverse reasons behind acquired AI resistance. This study aims at identifying the plausible cause of acquired AI resistance in patients administered with non-steroidal AIs (anastrozole and letrozole). We used genomic, transcriptomic, epigenetic, and mutation data of breast invasive carcinoma from The Cancer Genomic Atlas database. The data was then separated into sensitive and resistant sets based on patients' responsiveness to the non-steroidal AIs. A sensitive set of 150 patients and a resistant set of 172 patients were included for the study. These data were collectively analyzed to probe into the factors that might be responsible for AI resistance. We identified 17 differentially regulated genes (DEGs) among the two groups. Then, methylation, mutation, miRNA, copy number variation, and pathway analyses were performed for these DEGs. The top mutated genes (FGFR3, CDKN2A, RNF208, MAPK4, MAPK15, HSD3B1, CRYBB2, CDC20B, TP53TG5, and MAPK8IP3) were predicted. We also identified a key miRNA - hsa-mir-1264 regulating the expression of CDC20B. Pathway analysis revealed HSD3B1 to be involved in estrogen biosynthesis. This study reveals the involvement of key genes that might be associated with the development of AI resistance in ER-positive breast cancers and hence may act as a potential prognostic and diagnostic biomarker for these patients.

Abstract 3863: Aminoflavone confers activity against aromatase inhibitor-resistant breast cancer cells in part via miR125b2-3p restoration

Abstract Aromatase inhibitor therapy is the most crucial adjuvant treatment option for postmenopausal women with hormone receptor-positive (HR+) non-metastatic breast cancer. However, up to 30% of these patients will experience relapse due in part to aromatase inhibitor resistance. We recently identified miR125b2-3p as a putative tumor suppressor in HR+ breast cancer. We also found that small molecule Aminoflavone (AF) induces miR125b2-3p expression in HR+ breast cancer cells. In the current study, we hypothesized that AF confers anticancer actions in breast cancer cells resistant to aromatase inhibitors via miR125b2-3p reactivation. Using the Alamar Blue and colony forming assays, we discovered that AF demonstrated anticancer activity in estrogen-deprived resistant (EDR) cells designed to mimic aromatase inhibitor resistance. AntagomiR125b2-3p enhanced, while miR125b2-3p mimics diminished colony formation of EDR cells. AF-mediated suppression of colony formation in EDR cells was attenuated during co-treatment with antagomiR125b2-3p and was enhanced during co-treatment with miR125b2-3p mimics. Both miR125b2-3p mimics and AF suppressed EDR cell migration. AntagomiR125b2-3p counteracted AF-mediated suppression of cell migration. Our in silico profiling studies identified the alpha6-integrin, a mediator of breast cancer stem cell activity and metastasis, as one miR-125b2-3p target. Immunohistochemistry of patient tumor samples revealed that alpha 6-integrin protein expression intensified following relapse on the aromatase inhibitor anastrozole. MCF-7 cells transfected to overexpress the alpha 6-integrin were less responsive to anastrozole than those transfected with empty vector. These data suggest that AF restores the tumor suppressor function of miR125b2-3p to inhibit the alpha 6-integrin and confer activity against aromatase inhibitor-resistant breast cancer. Citation Format: Eileen J. Brantley, Nicole Mavingire, Salma Khan, Gayathri Nagaraj, Shawnee Angeloni, Charles Wang, Ubaldo Soto. Aminoflavone confers activity against aromatase inhibitor-resistant breast cancer cells in part via miR125b2-3p restoration. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3863.

Abstract PD10-07: PD10-07 Low plasma estradiol, low expression of estrogen responsive genes and TP53 mutations are associated with poor anti-proliferative response to aromatase inhibitors

Abstract Background: Aromatase inhibitors (AIs) are highly effective at reducing recurrences and mortality in postmenopausal patients with estrogen receptor positive breast cancer (ER+ BC). Poor anti-proliferative (Ki67) response or ER+ BCs to AIs after 2 weeks is associated with worse long-term outcomes. Factors that relate to the degree of the response may identify markers and/or mechansims of resistance. Methods: The PeriOperative Endocrine Therapy for Individualizing Care (POETIC) trial randomized 4,480 with ER+ BC to 2 weeks’ AI before surgery or no presurgical treatment. All patients within the bottom 15% of Ki67 responders to AI (poor responders [PRs]; n=177 with RNA extracted) were selected from and matched to good responders (GRs) within the 50% showing the best response (n=190). Matching was based on baseline Ki67 levels as measured by immunohistochemistry (IHC). Response to AI was measured by the percentage change in Ki67 after 2 weeks’ treatment. PRs were further divided into groups expressing high ESR1 (PRs ESR1HIGH; n=119) and low ESR1 (PRs ESR1LOW; n=58) levels since there were very few GRs with low ESR1. RNAseq, targeted exome DNA sequencing of 87 BC/resistance related genes and measurement of plasma estradiol levels by mass spectrometry were performed to understand mechanisms of de novo resistance. Intrinsic subtypes were estimated from RNAseq data. Results: More than 90% of PRs ESR1LOW were non-luminal subtypes with low expression of estrogen-responsive genes. In contrast, 11% of PRs ESR1HIGH were non-luminal compared to 4% of GRs but only HER2-enriched subtypes were significantly higher in PR ESR1HIGH (p=0.05, Fisher exact). While AI treatment had limited impact on Ki67 IHC values in PRs ESR1HIGH, PGR expression was more than 2-fold lower after 2 weeks of AI. Gene-set enrichment analysis showed significantly lower expression of estrogen-response genes in PRs ESR1HIGH compared to GRs (FDR< 10-9) at baseline despite similar percentage of Luminal subtypes in PRs ESR1HIGH and GRs. Plasma estradiol levels were correlated with expression of estrogen-response genes (FDR=0.01) and levels were significantly lower in PRs ESR1HIGH compared to GRs (p=0.003, Mann Whitney). PRs ESR1HIGH had significantly more mutations in RB1, TP53, ARID1B and DNAH11 genes (p< 0.05, Fisher exact). TP53 mutations were significantly enriched in Luminal-A PRs ESR1HIGH compared to GRs (22% and 3% respectively; p=0.003, Fisher exact), but not in Luminal-B tumors (23% and 15% mutated respectively). Discussion and conclusions: In approximately 33% of PRs, de novo AI resistance was associated with and most likely due to low expression of ER/ESR1 and estrogen-responsive genes in non-luminal tumors. In the remaining tumors, AI treatment still impacted some estrogen responsive genes but had limited downstream impact on suppressing proliferation. This might be due to mutations including in TP53 that limit suppression of proliferation downstream of estrogen signaling. The proportion of Luminal tumors in GRs and PRs ESR1HIGH was similar, suggesting better outcome of Luminal-A tumors on AI is likely due to their better intrinsic prognosis rather than better response to endocrine therapy. Citation Format: Eugene F. Schuster, Elena López-Knowles, Anastasia Alataki, Lila Zabaglo, Elizabeth Folkerd, David Evans, Kally Sidhu, Holly Tovey, Perry Maxwell, Nicholas Turner, Stephen Johnston, Manuel Salto-Tellez, Maggie Chon U Cheang, John Robertson, Ian Smith, Judith Bliss, Mitch Dowsett. PD10-07 Low plasma estradiol, low expression of estrogen responsive genes and TP53 mutations are associated with poor anti-proliferative response to aromatase inhibitors [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD10-07.

Abstract PD10-08: PD10-08 Immune cell infiltration associated with poor anti-proliferative response to aromatase inhibitors in postmenopausal women with primary ER-positive HER2-negative breast cancer

Abstract Background: Aromatase inhibitors (AIs) are one of the main treatment strategies for the clinical management of estrogen receptor-positive (ER+) breast cancer (BC). Despite prolonged time to recurrence and initial clinical responses, >20% of patients eventually relapse, and previous studies have shown an association of poor anti-proliferative response to AIs and worse outcome. High immune activity in ER+ tumors may be associated with worse outcome, in contrast to ER-negative BC where immune infiltration is a feature associated with better outcome. Our work focused on understanding the correlations between immune cell infiltration and response to AI. Methods: All patients with ER+ HER2- tumors within the bottom 15% of Ki67 anti-proliferative responders to AIs (poor responders [PRs]; n=177) were selected from the PeriOperative Endocrine Therapy for Individualizing Care (POETIC) trial and matched on baseline Ki67 levels to good responders (GRs) within the 50% showing the best response (n=190). Response to AI was measured by the Ki67 percentage change after 2 weeks of treatment. PRs were further divided into groups expressing high ESR1 (PRs ESR1HIGH; n=119) and low ESR1 (PRs ESR1LOW; n=58) levels to represent PR subgroups that showed partial or no response to AIs. The percentage of stromal tumor-infiltrating lymphocytes (TILs) was assessed. Multiple immunofluorescence was performed for ER, CD3, CD20, CD68, FOXP3, and CD3/FOXP3 in 15 baseline samples from each of the GR, PR ESR1HIGH, and PR ESR1LOW populations and immune cell density in stromal or tumor compartments was estimated. Spearman correlations of TILs with Consensus tumor microenvironment (TME) deconvolution and Molecular Signatures Database hallmark gene sets were conducted. The relationship between the immune markers’ density and genes, hallmark gene sets and Consensus TME was assessed. Results: The percentage of TILs was significantly higher in the PR ESR1HIGH and PR ESR1LOW compared to the GRs (adjusted p< 0.05). As expected, TILs were highly correlated with T cells (particularly T-regulatory cells) and immune hallmark gene sets. There was a tendency for higher density of each of the immune markers in PRs compared to GRs, with significant differences being observed in stromal B-cell marker CD20 density (p< 0.05). Analysis showed a significant correlation between TILs and stromal FOXP3 marker density (FDR< 0.05), and stromal biomarker density was highly correlated to the gene expression of the encoding genes of the same tumors (CD3/CD3D, FOXP3/FOXP3, and CD20/MS4A1) (FDRs< 0.05). There was also a strong and significant correlation between the stromal expression of CD20, CD3, FOXP3, and CD3/FOXP3 with the immune hallmark gene sets (FDRs< 0.05). Finally, the immune phenotyping showed the expected correlations with TME deconvolution, with particularly strong correlations of CD20 and CD3 with B- and T-cell gene signatures, respectively (FDRs< 0.05). Conclusions: Different immune features indicated a broad involvement of several immune cell types in PRs to AIs, suggesting that the immune system might be associated with resistance of ER+ breast tumors to AI treatment. Spatial gene expression profiling is ongoing to characterize these tumors further and investigate potential mechanisms of AI resistance. Citation Format: Anastasia Alataki, Gene Schuster, Lila Zabaglo, Perry Maxwell, Elena López-Knowles, Elizabeth Folkerd, David Evans, Kally Sidhu, Holly Tovey, Nicholas Turner, Stephen Johnston, Maggie Chon U Cheang, John Robertson, Manuel Salto-Tellez, Ian Smith, Judith Bliss, Mitch Dowsett. PD10-08 Immune cell infiltration associated with poor anti-proliferative response to aromatase inhibitors in postmenopausal women with primary ER-positive HER2-negative breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD10-08.

Long noncoding RNA DIO3OS induces glycolytic-dominant metabolic reprogramming to promote aromatase inhibitor resistance in breast cancer

Aromatase inhibition is an efficient endocrine therapy to block ectopic estrogen production for postmenopausal estrogen receptor (ER)-positive breast cancer patients, but many develop resistance. Here, we show that aromatase inhibitor (AI)-resistant breast tumors display features of enhanced aerobic glycolysis with upregulation of long noncoding RNA (lncRNA) DIO3OS, which correlates with poor prognosis of breast cancer patients on AI therapies. Long-term estrogen deprivation induces DIO3OS expression in ER-positive breast tumor cells, which further enhances aerobic glycolysis and promotes estrogen-independent cell proliferation in vitro and in vivo. Mechanistically, DIO3OS interacts with polypyrimidine tract binding protein 1 (PTBP1) and stabilizes the mRNA of lactate dehydrogenase A (LDHA) by protecting the integrity of its 3’UTR, and subsequently upregulates LDHA expression and activates glycolytic metabolism in AI-resistant breast cancer cells. Our findings highlight the role of lncRNA in regulating the key enzyme of glycolytic metabolism in response to endocrine therapies and the potential of targeting DIO3OS to reverse AI resistance in ER-positive breast cancer.

Ruxolitinib and exemestane for estrogen receptor positive, aromatase inhibitor resistant advanced breast cancer

Circulating IL-6, an activator of JAK/STAT signaling, is associated with poor prognosis and aromatase inhibitor (AI) resistance in hormone-receptor positive (HR+) breast cancer. Here we report the results of a phase 2 single-arm Simon 2-stage trial combining Ruxolitinib, an oral selective inhibitor of JAK1/2, with exemestane, a steroidal AI, in patients with HR+ metastatic breast cancer (MBC) after progression on non-steroidal AI (NSAI). Safety and efficacy were primary objectives, and analysis of inflammatory markers as predictors of response was a key secondary objective. Twenty-five subjects enrolled. The combination of ruxolitinib and exemestane was safe, though anemia requiring transfusion in 5/15 (33%) at the 25 mg dose in stage 1 led to a reduction to 15 mg twice daily in stage 2 (with no additional transfusions). Clinical benefit rate (CBR) in the overall study population was 24% (95% CI 9.4–45.1); 6/25 patients demonstrated stable disease for ≥6 months. Median progression-free survival was 2.8 months (95% CI 2.6–3.9). Exploratory biomarkers revealed high levels of systemic inflammation and 60% harbored a high-risk IL-6 genotype. Pharmacodynamics demonstrated modest on-target inhibition of phosphorylated-STAT3 by ruxolitinib at a tolerable dose. Thus, ruxolitinib combined with exemestane at a tolerable dose was safe but minimally active in AI-resistant tumors of patients with high levels of systemic inflammation. These findings highlight the need for more potent and specific therapies targeting inflammation in MBC.

HER2-enriched subtype and novel molecular subgroups drive aromatase inhibitor resistance and an increased risk of relapse in early ER+/HER2+ breast cancer.

SummaryBackgroundOestrogen receptor positive/ human epidermal growth factor receptor positive (ER+/HER2+) breast cancers (BCs) are less responsive to endocrine therapy than ER+/HER2- tumours. Mechanisms underpinning the differential behaviour of ER+HER2+ tumours are poorly characterised. Our aim was to identify biomarkers of response to 2 weeks’ presurgical AI treatment in ER+/HER2+ BCs.MethodsAll available ER+/HER2+ BC baseline tumours (n=342) in the POETIC trial were gene expression profiled using BC360™ (NanoString) covering intrinsic subtypes and 46 key biological signatures. Early response to AI was assessed by changes in Ki67 expression and residual Ki67 at 2 weeks (Ki672wk). Time-To-Recurrence (TTR) was estimated using Kaplan-Meier methods and Cox models adjusted for standard clinicopathological variables. New molecular subgroups (MS) were identified using consensus clustering.FindingsHER2-enriched (HER2-E) subtype BCs (44.7% of the total) showed poorer Ki67 response and higher Ki672wk (p<0.0001) than non-HER2-E BCs. High expression of ERBB2 expression, homologous recombination deficiency (HRD) and TP53 mutational score were associated with poor response and immune-related signatures with High Ki672wk. Five new MS that were associated with differential response to AI were identified. HER2-E had significantly poorer TTR compared to Luminal BCs (HR 2.55, 95% CI 1.14–5.69; p=0.0222). The new MS were independent predictors of TTR, adding significant value beyond intrinsic subtypes.InterpretationOur results show HER2-E as a standardised biomarker associated with poor response to AI and worse outcome in ER+/HER2+. HRD, TP53 mutational score and immune-tumour tolerance are predictive biomarkers for poor response to AI. Lastly, novel MS identify additional non-HER2-E tumours not responding to AI with an increased risk of relapse.FundingCancer Research UK (CRUK/07/015).

Fulvestrant plus capivasertib versus placebo after relapse or progression on an aromatase inhibitor in metastatic, oestrogen receptor-positive, HER2-negative breast cancer (FAKTION): overall survival, updated progression-free survival, and expanded biomarker analysis from a randomised, phase 2 trial

Capivasertib, an AKT inhibitor, added to fulvestrant, was previously reported to improve progression-free survival in women with aromatase inhibitor-resistant oestrogen receptor (ER)-positive, HER2-negative advanced breast cancer. The benefit appeared to be independent of the phosphoinositide 3-kinase (PI3K)/AKT/phosphatase and tensin homologue (PTEN) pathway alteration status of tumours, as ascertained using assays available at the time. Here, we report updated progression-free survival and overall survival results, and a prespecified examination of the effect of PI3K/AKT/PTEN pathway alterations identified by an expanded genetic testing panel on treatment outcomes. This randomised, multicentre, double-blind, placebo-controlled, phase 2 trial recruited postmenopausal adult women aged at least 18 years with ER-positive, HER2-negative, metastatic or locally advanced inoperable breast cancer and an Eastern Cooperative Oncology Group performance status of 0-2, who had relapsed or progressed on an aromatase inhibitor, from across 19 hospitals in the UK. Participants were randomly assigned (1:1) to receive intramuscular fulvestrant 500 mg (day 1) every 28 days (plus a 500 mg loading dose on day 15 of cycle 1) with either capivasertib 400 mg or matching placebo, orally twice daily on an intermittent weekly schedule of 4 days on and 3 days off, starting on cycle 1 day 15. Treatment continued until disease progression, unacceptable toxicity, loss to follow-up, or withdrawal of consent. Treatment was allocated by an interactive web-response system using a minimisation method (with a 20% random element) and the following minimisation factors: measurable or non-measurable disease, primary or secondary aromatase inhibitor resistance, PIK3CA status, and PTEN status. The primary endpoint was progression-free survival in the intention-to-treat population. Secondary endpoints shown in this Article were overall survival and safety in the intention-to-treat population, and the effect of tumour PI3K/AKT/PTEN pathway status identified by an expanded testing panel that included next-generation sequencing assays. Recruitment is complete. The trial is registered with ClinicalTrials.gov, number NCT01992952. Between March 16, 2015, and March 6, 2018, 183 participants were screened for eligibility and 140 (77%) were randomly assigned to receive fulvestrant plus capivasertib (n=69) or fulvestrant plus placebo (n=71). Median follow-up at the data cut-off of Nov 25, 2021, was 58·5 months (IQR 45·9-64·1) for participants treated with fulvestrant plus capivasertib and 62·3 months (IQR 62·1-70·3) for fulvestrant plus placebo. Updated median progression-free survival was 10·3 months (95% CI 5·0-13·4) in the group receiving fulvestrant plus capivasertib compared with 4·8 months (3·1-7·9) for fulvestrant plus placebo (adjusted hazard ratio [HR] 0·56 [95% CI 0·38-0·81]; two-sided p=0·0023). Median overall survival in the capivasertib versus placebo groups was 29·3 months (95% CI 23·7-39·0) versus 23·4 months (18·7-32·7; adjusted HR 0·66 [95% CI 0·45-0·97]; two-sided p=0·035). The expanded biomarker panel identified an expanded pathway-altered subgroup that contained 76 participants (54% of the intention-to-treat population). Median progression-free survival in the expanded pathway-altered subgroup for participants receiving capivasertib (n=39) was 12·8 months (95% CI 6·6-18·8) compared with 4·6 months (2·8-7·9) in the placebo group (n=37; adjusted HR 0·44 [95% CI 0·26-0·72]; two-sided p=0·0014). Median overall survival for the expanded pathway-altered subgroup receiving capivasertib was 38·9 months (95% CI 23·3-50·7) compared with 20·0 months (14·8-31·4) for those receiving placebo (adjusted HR 0·46 [95% CI 0·27-0·79]; two-sided p=0·0047). By contrast, there were no statistically significant differences in progression-free or overall survival in the expanded pathway non-altered subgroup treated with capivasertib (n=30) versus placebo (n=34). One additional serious adverse event (pneumonia) in the capivasertib group had occurred subsequent to the primary analysis. One death, due to atypical pulmonary infection, was assessed as possibly related to capivasertib treatment. Updated FAKTION data showed that capivasertib addition to fulvestrant extends the survival of participants with aromatase inhibitor-resistant ER-positive, HER2-negative advanced breast cancer. The expanded biomarker testing suggested that capivasertib predominantly benefits patients with PI3K/AKT/PTEN pathway-altered tumours. Phase 3 data are needed to substantiate the results, including in patients with previous CDK4/6 inhibitor exposure who were not included in the FAKTION trial. AstraZeneca and Cancer Research UK.

Impact of Duration of Neoadjuvant Aromatase Inhibitors on Molecular Expression Profiles in Estrogen Receptor-positive Breast Cancers.

Aromatase inhibitor (AI) treatment is the standard of care for postmenopausal women with primary estrogen receptor-positive breast cancer. The impact of duration of neoadjuvant endocrine therapy (NET) on molecular characteristics is still unknown. We evaluated and compared changes of gene expression profiles under short-term (2-week) versus longer-term neoadjuvant AIs. Global gene expression profiles from the PeriOperative Endocrine Therapy for Individualised Care (POETIC) trial (137 received 2 weeks of AIs and 47 received no treatment) and targeted gene expression from 80 patients with breast cancer treated with NET for more than 1 month (NeoAI) were assessed. Intrinsic subtyping, module scores covering different cancer pathways and immune-related genes were calculated for pretreated and posttreated tumors. The differences in intrinsic subtypes after NET were comparable between the two cohorts, with most Luminal B (90.0% in the POETIC trial and 76.3% in NeoAI) and 50.0% of HER2 enriched at baseline reclassified as Luminal A or normal-like after NET. Downregulation of proliferative-related pathways was observed after 2 weeks of AIs. However, more changes in genes from cancer-signaling pathways such as MAPK and PI3K/AKT/mTOR and immune response/immune-checkpoint components that were associated with AI-resistant tumors and differential outcome were observed in the NeoAI study. Tumor transcriptional profiles undergo bigger changes in response to longer NET. Changes in HER2-enriched and Luminal B subtypes are similar between the two cohorts, thus AI-sensitive intrinsic subtype tumors associated with good survival might be identified after 2 weeks of AI. The changes of immune-checkpoint component expression in early AI resistance and its impact on survival outcome warrants careful investigation in clinical trials.

Randomised, open-label, multicentric phase III trial to evaluate the safety and efficacy of palbociclib in combination with endocrine therapy, guided by ESR1 mutation monitoring in oestrogen receptor-positive, HER2-negative metastatic breast cancer patients: study design of PADA-1

IntroductionThe combination of a CDK4/6 inhibitor with an aromatase inhibitor (AI) has recently become the gold standard for AI-sensitive first line treatment of oestrogen receptor-positive (ER+) HER2-negative (HER2−) advanced breast...

Abstract PD1-04: Estrogen-mediated mechanisms in estrogen receptor-positive breast cancer at the single cell level

Abstract Background: Estrogen typically promotes the progression of hormone-dependent breast cancer through activation of estrogen receptor (ER)-α encoded by ESR1. While estrogen-induced tumor suppression in ER+ breast cancer has been clinically observed as an unexpected outcome of aromatase inhibitor (AI)-resistance, the molecular mechanisms have not yet been fully defined. Methods: Characterization of estrogen regulation in the two ER+ breast cancer patient-derived xenograft (PDX) models with opposite responses to estrogen offered us an unprecedented opportunity to assess how 17β-estradiol (E2) modulates ER+ cancer. We succeeded in establishing estrogen-stimulating (SC31) and estrogen-suppressing (GS3) PDX models. In vivo tumor promotion or suppression by estrogen was confirmed through experiments by implanting E2 pellets in mice carrying SC31/GS3, and then single cell analysis was performed using SC31/GS3 tumors. To investigate whether GS3 would change its response to E2, we performed intermittent E2 treatment every 28 days (E2 pellet on/off every 28 days) and whole-genome RNA sequencing using GS3 tumors. Results: SC31 and GS3 behaved oppositely regarding estrogen-mediated tumor growth. Immunohistochemistry indicated that the number of ERα+ cells and Ki-67+ cells were increased in SC31 and decreased in GS3 after E2 treatment, but progesterone receptor+ cells appeared in both SC31 and GS3 after E2 treatment at the protein level. The E2-induced suppression of GS3 involves ERα, not ERβ, which was wild-type and not amplified. Single cell RNA sequencing analysis of these PDXs had revealed that E2 upregulated the expression of estrogen-regulated genes (e.g., PGR and AREG) in both SC31 and GS3. However, E2 treatment induced cell cycle promotion in SC31, while E2 induced cell cycle arrest in GS3. These gene-expression changes occurred in both ESR1+ cells and ESR1- cells, demonstrating for the first time the influence of estrogen on ESR1- cells in ER+breast tumors. This result suggests that 100% ER positivity is not essential for endocrine response. E2 also upregulated a tumor suppressor gene, IL24, only in GS3. More IL24+ cells were ESR1+ and in G1 phase than IL24- cells. Hallmark apoptosis gene sets were upregulated and the hallmark G2M checkpoint gene set was downregulated in IL24+ cells after E2 treatment. After three rounds of intermittent E2 treatment on GS3, an E2 independent growth developed. Approximately 60% of genes in the intermittent E2-treated sample had the same trend as E2-treated samples (mainly ER target genes), in which 40% of the genes behaved similarly to the placebo-treated sample (mainly cell cycle progression genes). Furthermore, lower levels of IL24 were linked to estrogen independence. Conclusions: Estrogen/ERα signaling increases the expression of estrogen-regulated genes, but it can modulate HR+ tumor growth in different manners. While E2 should activate ERα and regulate ESR1+ cells, our findings regarding ESR1- cells were new and suggested crosstalk between ESR1+ cells and ESR1- cells in both estrogen-stimulating and -suppressing ER+ tumors. Furthermore, our studies demonstrate the potential roles of tumor suppressor gene, IL24 in HR+ cancer. Our findings point to the need to identify biomarkers for patients with estrogen-suppressing tumors who can benefit from E2 treatment after AI resistance; measurements of ER and PR expression are insufficient. Expression of IL24 in AI-resistant tumors may be one such indicator for favorable response to E2 therapy. Citation Format: Hitomi Mori, Kohei Saeki, Gregory Chang, Jinhui Wang, Xiwei Wu, Pei-Yin Hsu, Noriko Kanaya, George Somlo, Masafumi Nakamura, Andrea Bild, Shiuan Chen. Estrogen-mediated mechanisms in estrogen receptor-positive breast cancer at the single cell level [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD1-04.

Abstract P2-07-04: Analysis of host inflammatory and estrogen biomarkers in JAKEE: A phase II trial of the JAK inhibitor ruxolitinib in combination with exemestane for estrogen receptor-positive metastatic breast cancer

Abstract Background: Circulating IL-6, an activator of JAK/STAT signaling, is associated with poor outcomes and aromatase inhibitor (AI) resistance in hormone-receptor positive (HR+) metastatic breast cancer (MBC). We previously presented clinical outcomes of JAKEE, a single arm, phase II, Simon two-stage clinical trial that tested Ruxolitinib (Rux), an oral selective inhibitor of JAK1/2, and exemestane (EXE) in 25 participants (pts) with HR+ MBC that relapsed/progressed on non-steroidal AI (NSAI); specifically, the primary endpoint of safety was met, but there were no complete or partial responses and 6/25 (24%) achieved stable disease (SD) for ≥6 cycles. We investigated whether host circulating inflammatory markers, IL-6 genotypes, and estrogen levels were associated with differential response to therapy.Methods: Responders (R) were defined as having achieved SD≥6 cycles. Flow cytometry was performed on baseline and on-treatment peripheral blood samples to assess downstream CD3+ T-cell phosphoSTAT3 inhibition by Rux. Serum concentrations of C-reactive protein (CRP), IL-6, serum amyloid A (SAA), Estrone (E1) and Estradiol (E2) were measured at baseline and serially on treatment. Sanger sequencing was performed to assess for three functional variants of the IL-6 promoter: −572G&amp;gt;C (rs1800796), −597G&amp;gt;A (rs1800797), and −174G&amp;gt;C (rs1800795), with high-risk polymorphisms being -597G/G and/or -174G/G. Non-parametric median testing was employed to test for differences in circulating markers by response groups given non-normal distribution, with a two-sided alpha of 0.05.Results: The cohort was heavily pre-treated: 28% received ≥2 lines of chemotherapy for MBC and 20% had CNS disease at enrollment. Among 17/25 pts with samples for pharmacodynamic assessment, Rux exhibited a 25% median inhibition (range 0-77%) of phosphoSTAT3. There was no differential effect in R vs non-responders (NR) (median inhibition 20% vs 29%, p=0.15). Frequency of high-risk IL-6 genotypes and distribution of baseline serum CRP, IL-6, SAA, E1 and E2 are depicted in the table. 15/25 (60%) harbored a high-risk IL-6 promoter polymorphism, with no significant difference in frequency between R and NR (50% vs 63%, p=0.65). 19 pts had samples for inflammatory biomarker analysis. 16/19 had baseline CRP≥10mg/L. While median levels of baseline CRP, SAA, and IL-6 were above upper limit of normal, there was no difference between R and NR (table). The proportion of pts with baseline undetectable E1 and E2 were similar between R and NR (E1: 36.9% vs 33.3%, p=1.0, E2: 52.6% vs 50%, p=1.0); notably, a significantly lower proportion with high-risk IL-6 genotype had undetectable baseline E1 (20% vs 60%, p=0.041), while no difference was noted for baseline E2, nor in the % change in E1 or E2 levels from baseline to cycle 4 by responder status.Conclusions: The JAKEE cohort represents an inflamed population with elevated circulating inflammatory markers and a high proportion with high-risk IL-6 genotypes. Examination of host inflammatory markers, IL-6 genotypes and estrogen levels did not reveal a differential response to the combination of Rux and EXE in patients with HR+ MBC that had progressed on prior NSAI. At tolerable dosing, Rux exhibited only a modest inhibition of phosphoSTAT3. Further work is needed to optimize strategies for targeting inflammation and JAK/STAT signaling in HR+ MBC. IL-6 GenotypeFrequencyFrequency by ResponderSignificanceHigh-Risk -174G/G and/or -597G/G15/25 (60%)Non-responder12/19 (63.2%)Responder3/6 (50%)p-value0.56Pretreatment Inflammatory Biomarkers [Upper limit of normal]Median (Range)Median Level by Responder GroupSignificanceCRP [8mg/L]SAA [10mg/L]IL-6 [2pg/mL]24.0 (0.2-146.8)12.8 (2.8-162.5)4.2 (1.8-11.5)Non-responder23.219.84.5Responder24.7 12.42.7p-value0.510.150.15Baseline Estrogen BiomarkersMedian (Range)Non-responderResponderp-valueEstrone (E1) (pg/mL)Estradiol (E2) (pg/mL)79.5.0 (0.2-1039.0)0.2 (0.2-44.1)79.8 0.247.8 4.20.410.91 Citation Format: Igor Makhlin, Nicholas McAndrew, E. Paul Wileyto, Amy Clark, Robin Holmes, Lisa N Bottalico, Grace R Jeschke, Kevin R Fox, Susan M Domcheck, Jennifer M Matro, Angela R Bradbury, Natalie Shih, Michael D Feldman, Elizabeth O Hexner, Jacqueline F Bromberg, Angela DeMichele. Analysis of host inflammatory and estrogen biomarkers in JAKEE: A phase II trial of the JAK inhibitor ruxolitinib in combination with exemestane for estrogen receptor-positive metastatic breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-07-04.

Abstract PD15-02: HER2-enriched subtype and novel molecular subgroups drive aromatase inhibitor resistance and an increased risk of relapse in early ER+/HER2+ breast cancer

Abstract Background: Approximately 10% of estrogen receptor positive (ER+) early breast cancers (EBC) are human epidermal growth factor positive/overexpressed (HER2+). Resistance to aromatase inhibitors (AIs) is poorly understood among ER+HER2+ BC. We aimed to identify biomarkers of response and resistance using the HER2+ tumours in the POETIC trial. Methods: POETIC was a phase III trial of post-menopausal women with ER+ EBC randomized 2:1 to 2-weeks of peri-surgical AI vs control, followed by standard-of-care. Baseline formalin-fixed paraffin-embedded samples from 342 ER+HER2+ BC (237 treated/105 controls) were gene expression profiled using Breast cancer 360™ panel (NanoString) covering intrinsic subtypes and 40 key biological signatures. New molecular subgroups were identified using consensus clustering (CC).Proliferation rate was estimated as percentage of cancer-cells staining for Ki67. Ki67 response was defined by percentage-changes from baseline to surgery: poor (PR; reduction &amp;lt;50%), intermediate (IR; 50-75%) and good (GR; &amp;gt;75%). Residual Ki67 at 2 weeks timepoint (Ki672w) was defined as High (H) (≥ 10%) vs Low (L) (&amp;lt;10%). Association of molecular features with Ki67 response was assessed using logistic regression models and with time to recurrence (TTR) using multivariable Cox regression models adjusted for post-surgery clinicopathological variables: grade, tumour size, nodal status and age. Results: Ki672w was measured for all treated samples (n = 227, 48% H, 52% L) and a fraction of controls (n = 50, 96% H, 4% L). At baseline, 45% of tumours were HER2-enriched (HER2-E), 53% Luminal A/B and 2% Basal-Like. HER2-E associated with high Ki672W (84% H, 16% L) compared to non-HER2-E (29% H, 71% L, p&amp;lt;0.001). Amongst the 40 BC360 signatures, 9 were significantly associated with Ki67 response (Table). High expression of endocrine-related and apoptosis signatures associated with GR while HRD and TP53 with PR. These features were also related to Ki672W, with additional signatures involved in immune-checkpoint component and immune-enrichment (IM) (IDO1, PD-L1, IFN-gamma and T-reg) associated with high Ki672W.. Using CC, 4 molecular subgroups predicting differential response to AI were identified. Cluster (C) 1 was characterized by overexpression of IM features and low ER signaling (42%); C2 low IM but highest ERBB2 expression (15%); C3 ESR1 high and PgR low (4%) and C4 high endocrine signaling and lowest ERBB2 expression (39%). Distribution of Intrinsic subtypes varied across these molecular subgroups with the majority of C1 being HER2-E (62%) and Luminal B (27%); C2 HER2-E (74%); C3 Luminal B (64%) and C4 Luminal B (55%) and Luminal A (30%). HER2-E had significantly poorer TTR (HR 2.14; 95%CI1.11-4.17; p=0.0224) compared to Luminal tumours, remaining significant (HR 2.55, 95%CI1.14-5.69; p=0.0222) in the multivariable analysis. C2 (HR 4.67, 95%CI 1.90-11.51; p&amp;lt;0.001) and C4 (HR 2.42, 95%CI1.05-4.5.58; p=0.0385) were independent predictors of shorter TTR compared to C1, adding significant value beyond intrinsic subtypes (Likelihood ratio test, p=0.00429). Conclusions: Our results confirm that HER2-E, TP53 mutation, HRD and IM features are the main components driving poor early response to AI in ER+/HER2+ EBC. Novel molecular subgroups identify additional non-HER-E tumours not responding to AI with an increased risk of relapse. The appropriate additional treatment warrants further investigation. List of significant gene expression signatures associated with response as defined by reduction of Ki67. Odds ratio (OR) were determined by ordinal logistic regression analyses. OR: &amp;lt;1 indicating associated with good response and &amp;gt; 1.0 associated with worse response.Odds Ratiop-valuep-adjusted (False Discovery Rate)ESR10.65&amp;lt; 0.001&amp;lt; 0.001Estrogen Receptor Signaling0.44&amp;lt; 0.001&amp;lt; 0.001ERBB21.52&amp;lt; 0.001&amp;lt; 0.001TP53 mutant signature2.65&amp;lt; 0.001&amp;lt; 0.001FOXA10.42&amp;lt; 0.001&amp;lt; 0.001Apoptosis0.26&amp;lt; 0.0010.00228Homologous Recombination Deficiency (HRD)2.33&amp;lt; 0.0010.00349PgR0.820.001480.00737Hypoxia2.180.001660.00737 Citation Format: Milana A Bergamino Sirvén, Elena López-Knowles, Gabriele Morani, Holly Tovey, Lucy Kilburn, Chris Holcombe, Anthony Skene, Ian Smith, John Robertson, Gene Schuster, Judith M Bliss, Mitch Dowsett, Maggie CU Cheang, POETIC investigators. HER2-enriched subtype and novel molecular subgroups drive aromatase inhibitor resistance and an increased risk of relapse in early ER+/HER2+ breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD15-02.

Mitochondrial stress adaptation promotes resistance to aromatase inhibitor in human breast cancer cells via ROS/calcium up-regulated amphiregulin-estrogen receptor loop signaling.

Many breast cancer patients harbor high estrogen receptor (ER) expression in tumors that can be treated with endocrine therapy, which includes aromatase inhibitors (AI); unfortunately, resistance often occurs. Mitochondrial dysfunction has been thought to contribute to progression and to be related to hormone receptor expression in breast tumors. Mitochondrial alterations in AI-resistant breast cancer have not yet been defined. In this study, we characterized mitochondrial alterations and their roles in AI resistance. MCF-7aro AI-resistant breast cancer cells were shown to have significant changes in mitochondria. Low expressions of mitochondrial genes and proteins could be poor prognostic factors for breast cancer patients. Long-term mitochondrial inhibitor treatments-mediated mitochondrial stress adaptation could induce letrozole resistance. ERα-amphiregulin (AREG) loop signaling was activated and contributed to mitochondrial stress adaptation-mediated letrozole resistance. The up-regulation of AREG-epidermal growth factor receptor (EGFR) crosstalk activated the PI3K/Akt/mTOR and ERK pathways and was responsible for ERα activation. Moreover, mitochondrial stress adaptation-increased intracellular levels of reactive oxygen species (ROS) and calcium were shown to induce AREG expression and secretion. In conclusion, our results support the claim that mitochondrial stress adaptation contributes to AI resistance via ROS/calcium-mediated AREG-ERα loop signaling and provide possible treatment targets for overcoming AI resistance.

Enhanced IFNα Signaling Promotes Ligand-Independent Activation of ERα to Promote Aromatase Inhibitor Resistance in Breast Cancer.

Simple SummaryInterferon alpha (IFNα) signaling is highly upregulated in ER+ breast cancers that become resistant to estrogen deprivation therapy. This study uncovers how enhanced (IFNα)/JAK-STAT signaling directly influences estrogen receptor (ERα) activation in the absence of estrogen. We found that inhibiting IFNα signaling downregulates expression and activation of ERα. In addition, STAT1 and ERα directly interact and regulate a key interferon-stimulated gene, IFITM1, in AI-resistant breast cancer cells. We demonstrate that crosstalk occurs between IFNα and ERα pathways, which contributes to aggression and survival of AI-resistant breast cancer, thus representing a novel mechanism of acquired AI resistance.Aromatase inhibitors (AIs) reduce estrogen levels up to 98% as the standard practice to treat postmenopausal women with estrogen receptor-positive (ER+) breast cancer. However, approximately 30% of ER+ breast cancers develop resistance to treatment. Enhanced interferon-alpha (IFNα) signaling is upregulated in breast cancers resistant to AIs, which drives expression of a key regulator of survival, interferon-induced transmembrane protein 1 (IFITM1). However, how upregulated IFNα signaling mediates AI resistance is unknown. In this study, we utilized MCF-7:5C cells, a breast cancer cell model of AI resistance, and demonstrate that these cells exhibit enhanced IFNα signaling and ligand-independent activation of the estrogen receptor (ERα). Experiments demonstrated that STAT1, the mediator of intracellular signaling for IFNα, can interact directly with ERα. Notably, inhibition of IFNα signaling significantly reduced ERα protein expression and ER-regulated genes. In addition, loss of ERα suppressed IFITM1 expression, which was associated with cell death. Notably, chromatin immunoprecipitation experiments validated that both ERα and STAT1 associate with ERE sequences in the IFITM1 promoter. Overall, hyperactivation of IFNα signaling enhances ligand-independent activation of ERα, which promotes ER-regulated, and interferon stimulated gene expression to promote survival in AI-resistant breast cancer cells.