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Background: Approximately 10% of breast cancers (BC) are hormone receptor positive (HR+) and HER2 positive (HER2+). Despite treatment advances in the modern era of HER2 targeted therapies for early-stage disease, there remains a risk for late relapses. However, the role of adjuvant endocrine therapy (ET) to reduce recurrence in this BC subtype is unclear. Oncologists employ clinical judgment given lack of consensus, resulting in differences in treatment patterns. The ideal endocrine agent including the role of adding ovarian suppression (OS) in premenopausal women is unknown. These patients are largely underrepresented in clinical trials such as the phase III SOFT and TEXT studies. Additionally, these trials were initiated prior to the widespread use of trastuzumab with chemotherapy, which is now standard of care for HER2+ disease. We aimed to describe real world patterns surrounding choice of adjuvant ET and clinicopathologic features which predicted treatment with OS in premenopausal women with HR+/HER2+ BC. Methods: We performed a multi-institutional retrospective analysis of premenopausal women with non-metastatic HR+/HER2+ BC in the American Society of Clinical Oncology CancerlinQ® Discovery database from January 2010 to May 2020. Electronic health record data was obtained from 74 participating academic and community oncology sites. We collected clinical data on women less than 50 years who received chemotherapy, anti-HER2 therapy (trastuzumab with or without pertuzumab), and ET. Adjuvant OS was defined as receipt of at least 6 months of goserelin or leuprolide or surgical bilateral oophorectomy. Demographics, clinical characteristics, and treatment history was collected. Patients were categorized into 1 of 4 groups based on type of adjuvant ET prescribed at treatment initiation: aromatase inhibitor (AI) + OS, OS, tamoxifen + OS, or tamoxifen. Multivariable logistic regression was conducted to assess the association between clinicopathologic features and OS use. Results: Out of 360,540 patients with invasive breast cancer in the database, 937 met inclusion criteria. Mean age was 41.7 (SD 5.9) years; 83% had stage 1 or 2 BC and 78% had node positive disease. The majority (n=818, 87%) were prescribed tamoxifen whereas only 4 (0.4%), 50 (5.3%), and 65 (6.9%) received OS, tamoxifen + OS, and AI + OS, respectively. Table 1 includes demographic and clinical characteristics of the cohort. No clinicopathologic features predicted OS use apart from age; patients ≥35 years were less likely to receive OS compared with those < 35 (p< 0.001) (table 2). Conclusion: To our knowledge, this is the first real world study evaluating OS treatment in HR+/HER2+ BC. The use of OS was uncommon; this suggests a perception of its limited benefit when added to HER2-targeted therapy. Most patients received tamoxifen as the ET of choice. Age was the only factor to predict OS treatment; high risk features including node positivity and higher stage was not associated with its use. This highlights the wide variability in real world practice surrounding the clinical indications for OS. Further investigation is warranted to characterize the utility of ET including addition of OS to prevent recurrence in premenopausal HR+/HER2+ BC. This will better inform a personalized approach to tailor therapy for optimal outcomes in this distinct BC subtype. Table 1. Demographic and clinical characteristics of study participants by endocrine therapy treatment group. Table 2. Multivariable logistic regression model of clinicopathologic characteristics to predict use of ovarian suppression. Citation Format: Jasmine S. Sukumar, Sagar Sardesai, Andy Ni, Nicole Williams, Bhuvaneswari Ramaswamy, Robert Wesolowski, Mathew A. Cherian, Daniel Stover, Margaret Gatti-Mays, Ashley C. Pariser, Preeti K. Sudheendra, Mridula A. George, maryam lustberg. Real World Treatment Patterns of Adjuvant Endocrine Therapy and Ovarian Suppression in Premenopausal HR+/HER2+ 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 P1-02-03.

BackgroundAmong women, breast cancer is the leading cause of cancer-related death worldwide. Estrogen receptor α-positive (ERα+) breast cancer accounts for 70% of all breast cancer subtypes. Although ERα+ breast cancer initially responds to estrogen deprivation or blockade, the emergence of resistance compels the use of more aggressive therapies. While ERα is a driver in ERα+ breast cancer, ERβ plays an inhibitory role in several different cancer types. To date, the lack of highly selective ERβ agonists without ERα activity has limited the exploration of ERβ activation as a strategy for ERα+ breast cancer.MethodsWe measured the expression levels of ESR1 and ESR2 genes in immortalized mammary epithelial cells and different breast cancer cell lines. The viability of ERα+ breast cancer cell lines upon treatments with specific ERβ agonists, including OSU-ERb-12 and LY500307, was assessed. The specificity of the ERβ agonists, OSU-ERb-12 and LY500307, was confirmed by reporter assays. The effects of ERβ agonists on cell proliferation, cell cycle, apoptosis, colony formation, cell migration, and expression of tumor suppressor proteins were analyzed. The expression of ESR2 and genes containing ERE-AP1 composite response elements was examined in ERα+ human breast cancer samples to determine the correlation between ESR2 expression and overall survival and that of putative ESR2-regulated genes.ResultsIn this study, we demonstrate the efficacy of highly selective ERβ agonists in ERα+ breast cancer cell lines and drug-resistant derivatives. ERβ agonists blocked cell proliferation, migration, and colony formation and induced apoptosis and S and/or G2/M cell-cycle arrest of ERα+ breast cancer cell lines. Also, increases in the expression of the key tumor suppressors FOXO1 and FOXO3a were noted. Importantly, the strong synergy between ERβ agonists and ERα antagonists suggested that the efficacy of ERβ agonists is maximized by combination with ERα blockade. Lastly, ESR2 (ERβ gene) expression was negatively correlated with ESR1 (ERα gene) and CCND1 RNA expression in human metastatic ERα+/HER2- breast cancer samples.ConclusionOur results demonstrate that highly selective ERβ agonists attenuate the viability of ERα+ breast cancer cell lines in vitro and suggest that this therapeutic strategy merits further evaluation for ERα+ breast cancer.

Among women worldwide, breast cancer has the highest incidence and is the leading cause of cancer-related death. Patients with the triple-negative breast cancer (TNBC) subtype have an inferior prognosis in comparison to other breast cancers because the current therapies do not facilitate long-lasting responses. Thus, there is a critical need for novel therapeutic strategies that induce stronger responses. In our previous research, we discovered that augmenting the concentration of extracellular ATP (eATP) greatly enhances the chemotherapeutic response of TNBC cell lines by activating purinergic receptors (P2RXs), leading to cell death through the induction of non-selective membrane permeability. However, eATP levels are limited by several classes of extracellular ATPases. One endogenous molecule of interest that can inhibit multiple classes of extracellular ATPases is heparan sulfate. Heparan sulfate itself is degraded by heparanase, an enzyme that is known to be highly expressed in various cancers, including breast cancer. Heparan sulfate has previously been shown to regulate several cancer-related processes such as fibroblast growth factor signaling, neoangiogenesis by sequestering vascular endothelial growth factors in the extracellular matrix, hedgehog signaling and cell adhesion. In this project, we identified an additional mechanism for a tumor suppressor role of heparan sulfate: inhibition of extracellular ATPases, leading to augmented levels of eATP. Several heparanase inhibitors have been previously identified, including OGT 2115, suramin, PI-88, and PG 545. We hypothesized that heparanase inhibitors would augment eATP concentrations in TNBC by increasing heparan sulfate in the tumor microenvironment, resulting in increased cell death in response to chemotherapy. We treated TNBC cell lines MDA-MB 231, Hs 578t, and MDA-MB 468 and non-tumorigenic immortal mammary epithelial MCF-10A cells with increasing concentrations of the chemotherapeutic agent paclitaxel in the presence of heparan sulfate and/or OGT 2115 with eATP content and cell viability being assessed. Moreover, to confirm that the effects of OGT 2115 are mediated through eATP, we utilized specific antagonists to the purinergic receptors P2RX4 and P2RX7. In addition, the protein expression of heparanase was compared in all the examined cell lines via Western blot analysis. We also evaluated the effects of this combination on the breast cancer-initiating cell population in the treated cells using flow cytometry and tumorsphere formation efficiency assays. These results demonstrate that inhibiting the degradation of heparan sulfate in the tumor microenvironment enhances the susceptibility of TNBC cell lines to chemotherapy by augmenting extracellular ATP concentrations. This strategy could potentially be used to induce deeper and more durable responses in triple-negative breast cancer patients. Citation Format: Jasmine M. Manouchehri, Jharna Datta, Mathew A. Cherian. The role of heparan sulfate in enhancing the chemotherapeutic response in triple-negative breast cancer [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 397.

The leading cause of cancer-related death among women worldwide is breast cancer. The triple-negative breast cancer (TNBC) subtype has limited therapeutic options that produce durable responses. As a result, it is associated with an especially poor prognosis compared to other breast cancer subtypes. Hence, the development of innovative therapeutic strategies that produce deeper and more durable responses is critically needed. We found that increasing extracellular ATP (eATP) can enhance the susceptibility of TNBC cell lines to chemotherapy through the activation of purinergic receptors (P2RXs) P2RX4 and P2RX7. However, eATP levels are limited by several different classes of eATPases, making the design of a single molecule that effectively inhibits all classes of eATPase complicated. Heparan sulfate, a carbohydrate moiety that is covalently linked to several cell surface and extracellular matrix proteins has previously been identified as an endogenous inhibitor of several classes of eATPases. Heparan sulfate is desulfated by sulfatases 1 and 2 at the 6-O-hydroxyl groups of glucosamine residues in heparan sulfate. Sulfatase 2 has been determined to be highly expressed in a variety of cancers including breast cancer, whereas sulfatase 1 is not. Several sulfatase inhibitors have been previously developed, such as OKN-007, which is a specific inhibitor of sulfatase 2. We hypothesized that OKN-007 would augment eATP levels in TNBC cell lines exposed to chemotherapy by increasing levels of heparan sulfate in the tumor microenvironment, leading to intensified cell death. TNBC cell lines MDA-MB 231, Hs 578t, and MDA-MB 468 and non-tumorigenic immortal mammary epithelial MCF-10A cells were treated with increasing concentrations of the chemotherapeutic agent paclitaxel in the presence of heparan sulfate and/or OKN-007, and eATP content and cell viability were evaluated. Inhibitors to the purinergic receptors P2RX4 and P2RX7 were used to further confirm that the mechanism of enhanced cell death induced by OKN-007 was through these receptors. In addition, basal protein and cell surface expression of sulfatases 1 and 2 were determined in all the examined cell lines via ELISA, Western blot, and flow cytometry analysis. To assess the effects on cancer-initiating cell properties, TNBC cell lines were treated with paclitaxel in the presence of heparan sulfate and/or OKN-007, and the tumorsphere formation efficiency assay used to assess for cancer-initiating cells. In parallel, we assessed for effects on cancer-initiating cells in treated cells using flow cytometry. Our results showed that inhibiting the 6-O desulfation of heparan sulfate increases eATP accumulation, thereby, enhancing TNBC cell lines’ response to chemotherapy. These findings may lead to therapies for TNBC based on sulfatase 2 inhibition that results in more effective responses with fewer side effects. Citation Format: Jasmine M. Manouchehri, Jharna Datta, Mathew A. Cherian. The inhibition of sulfation of heparan sulfate augments the chemotherapeutic response in triple-negative breast cancer [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 398.

Background: Hormone receptor (HR) low (1-10%) HER2-negative breast cancer (BC) is emerging as a distinct subtype with similarities in clinical outcomes to triple-negative BC. However, there is a lack of consensus on treatment recommendations for chemo-immunotherapy and endocrine therapy in this subset. Here, we present results from a US National Cancer Database (NCDB) analyses of patients with HER2-negative BC evaluating response to neoadjuvant chemotherapy (NAC) and patterns of care by HR expression. Methods: Patients with stage I-III HER2-negative BC diagnosed in 2018 were identified in NCDB, a nationwide oncology outcomes database in the US. Quantitative HR expression was unavailable prior to 2018. Data were categorized into four groups by estrogen receptor (ER) and progesterone receptor (PR) expression: ER< 1% & PR< 1% (HR-Neg), ER 1-10% and/or PR 1-10% (HR-Low), ER >11-30% and/or PR>11-30% (HR-Int), ER> 30% and/or PR > 30% (HR-High). Those with undocumented HR status (3%) or without curative intent surgery (5%) were excluded. The primary outcome was pathologic complete response (pCR) by HR expression in those undergoing neoadjuvant chemotherapy. Key secondary objectives included assessment of clinicopathologic characteristics and practice patterns. The categorical variables were compared between the four groups using a Chi-square test. Age was compared using a Kruskal-Wallis test. Results: Out of 104,205 incident cases, 2541 (2.4%) were HR-Low and 1241 (1.2%) were HR-Int. Significant differences were found between HR groups with higher grade, clinical stage, and Ki-67 in HR-Low vs. HR-Int or HR-High groups (Table 1). Patients with HR-Low and HR-Int BC were more likely to receive chemotherapy than HR-High (74%, 70% vs. 20%; p < 0.001) and the use of adjuvant endocrine therapy correlated with HR expression. Only half of patients in the HR-Low group received any endocrine therapy compared to higher rates in the HR-Int and HR-High groups (52% vs. 74%, 92%; p< 0.001). pCR rates in those receiving neoadjuvant chemotherapy were significantly different by HR status, with higher pCR rates in HR-Low vs. HR-High groups (p< 0.001) (Table 2). NAC utilization significantly differed between groups. A higher proportion of patients with HR-Low BC received NAC than other HR-positive groups (p < 0.001). Additionally, there was an increased use of NAC in patients with HR-Low BC treated at academic vs. community cancer centers (p< 0.001). Conclusions: This is one of the largest real-world analyses comparing key differences in biology and practice patterns of HR-Low, HER2-negative BC. Consistent with prior studies, we report HR-Low BC to be a rare and distinct subtype with higher pCR rates compared to HR-High BC. Practice patterns show wide variability in utilization of neoadjuvant chemotherapy and endocrine therapy for these patients. Future studies should address this disparity and enhance representation of patients with HR-Low BC in clinical trials to improve long-term outcomes. Table 1: Patient demographics Table 2: Neoadjuvant chemotherapy response amongst differing HR expression levels Citation Format: Dionisia Quiroga, Michael Grimm, Julie Stephens, Kai Johnson, Nicole Williams, Preeti K. Sudheendra, Mathew A. Cherian, Daniel Stover, Ashley C. Pariser, Margaret Gatti-Mays, Robert Wesolowski, Jose G. Bazan, Sasha Beyer, Ko Un Park, Bridget Oppong, Bhuvaneswari Ramaswamy, Julia White, Sachin R. Jhawar, Sagar Sardesai. Impact of low hormone receptor expression on neoadjuvant chemotherapy response and patterns of care in early-stage HER2-negative breast cancer: a US National Cancer Database analysis [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 P4-02-17.

IntroductionBreast cancer affects two million patients worldwide every year and is the most common cause of cancer-related death among women. The triple-negative breast cancer (TNBC) sub-type is associated with an especially poor prognosis because currently available therapies fail to induce long-lasting responses. Therefore, there is an urgent need to develop novel therapies that result in durable responses. One universal characteristic of the tumor microenvironment is a markedly elevated concentration of extracellular adenosine triphosphate (eATP). Chemotherapy exposure results in further increases in eATP through its release into the extracellular space of cancer cells via P2RX channels. eATP is degraded by eATPases. Given that eATP is toxic to cancer cells, we hypothesized that augmenting the release of eATP through P2RX channels and inhibiting extracellular ATPases would sensitize TNBC cells to chemotherapy.MethodsTNBC cell lines MDA-MB 231, Hs 578t and MDA-MB 468 and non-tumorigenic immortal mammary epithelial MCF-10A cells were treated with increasing concentrations the chemotherapeutic agent paclitaxel in the presence of eATPases or specific antagonists of P2RXs with cell viability and eATP content being measured. Additionally, the mRNA, protein and cell surface expressions of the purinergic receptors P2RX4 and P2RX7 were evaluated in all examined cell lines via qRT-PCR, western blot, and flow cytometry analyses, respectively.ResultsIn the present study, we observed dose-dependent declines of cell viability and increases in eATP of paclitaxel-treated TNBC cell lines in the presence of inhibitors of eATPases, but not of the MCF-10A cell line. These effects were reversed by specific antagonists of P2RXs. Similar results, as those observed with eATPase inhibitors, were seen with P2RX activators. All examined cell lines expressed both P2RX4 and P2RX7 at the mRNA, protein and cell surface levels.ConclusionThese results reveal that eATP modulates the chemotherapeutic response in TNBC cell lines, which could be exploited to enhance the efficacy of chemotherapy regimens for TNBC.

TPS12139 Background: Obesity at diagnosis is associated with an increase in breast cancer (BC) specific mortality. Behavioral interventions targeting obesity-related health choices offer a scalable approach to improve patient engagement in the community. Episodic Future Thinking (EFT) is a novel remotely delivered behavioral intervention which engages the science of prospection where participants mentally simulate positive, detailed, and personal events that can occur in the future. EFT targets a key behavioral economic measure – Delay Discounting (DD). As DD rate rises, an individual devalues future outcomes to a greater degree and has a higher bias for immediate gratification. EFT can decrease DD in obese patients, leading to improved diet quality and weight loss. However, valuation of the future may impact cancer survivors differently. Herein, we propose the first randomized Phase II trial evaluating adherence and changes in DD and body weight with 12-week remotely delivered EFT vs control in overweight or obese BC survivors. Methods: Eligible patients have a history of DCIS or stage 1 to 3 BC, BMI > 25 kg/m2, have completed surgery, radiation, and chemotherapy, and are motivated to lose weight. They are randomized 1:1 to a 12-week EFT intervention or control (Episodic Recent Thinking; ERT). ERT is a validated control in which patients imagine events in the recent past. Randomization will be stratified by baseline DD rate. There is an optional 12 week follow up period where patients can continue to receive EFT or ERT cues. DD will be measured at baseline and every 4 weeks for 24 weeks. The primary endpoint is adherence, measured by percentage of thrice daily smartphone prompts participants open and attend to during the 12-week trial. Secondary endpoints are changes in body weight and DD rate at 12 and 24 weeks and change in PROs (PROMIS scales), insulin resistance (HOMA-IR), hs-CRP, and diet quality (HEI 2015) at 12 weeks from baseline. With 20 subjects per arm, the study has 80% power to detect deviation of 14 points from a target adherence rate of 80% using a 1 sample t-test. The primary endpoint is evaluable for all subjects, regardless of drop-out. Secondary endpoints will be evaluated using a linear mixed effects model. Hypothesis tests and confidence intervals will be two-sided at 5% significance/95% confidence level. Since trial activation in November 2021, 9 of the planned 46 patients are enrolled. Accrual is closely integrated with the dedicated Ohio State University Comprehensive Cancer Center Survivorship Program. This work is supported by the Alliance Cancer Control Program Pilot Award (5UG1CA189823-08). Clinical trial information: NCT05012176.

1504 Background: Trial recruitment that requires specific actionable mutations based on next-generation sequencing (NGS) is challenging. Barriers can include competing studies, physician study awareness, site proximity, mutation incidence, among other concerns. Methods: This study (NCT04432454) opened clinical sites using two methods during the COVID-19 pandemic. The “Traditional” approach included site selection, IRB and contract approval, and trial activation prior to a patient being identified for enrollment. The second approach used the Tempus “TIME” Trials network that would only open a site after identifying a patient with a mutation of interest and eligible for the trial. Results: The first patient enrolled was on 10/12/20 and the last patient was on 6/24/21. A total of 16 sites (6 Traditional and 10 TIME) participated. All Traditional sites, and none of the TIME sites, were affiliated with major academic institutions. Duration for full CTA execution for Traditional sites averaged 200.5 days (range 142 to 257) and for TIME sites averaged 7.6 days (range 2 to 14). IRB approval time average for Traditional sites was 27.5 days (range 12 to 71) and TIME sites was 3.0 days (range 1 to 12 days). Days from site selection to activation letter for Traditional sites was on average 250.0 days (range 187 to 281) and for TIME sites was 131.6 days (range 22 to 248). Time from study activation to first consent was 33.3 days (range 18 to 58) for Traditional sites and 8.8 days (range 1 to 35) for TIME sites. The first patient on-study was at a TIME site 115 days prior to a Traditional site and the first 7 patients enrolled were at TIME sites. Traditional sites consented 23 and enrolled 16 patients while the TIME sites consented 16 and enrolled 13. The trial enrolled all 29 patients in 8 months with the anticipated enrollment duration being 12 to 18 months. Conclusions: Although the Traditional and TIME programs had different operational models, they both contributed a significant number of patients and reduced the projected enrollment timeline. TIME sites enrolled the initial patients. These results demonstrate that the “Just-in-Time model,” in conjunction with a Traditional model, can reduce projected overall time to enrollment in biomarker-driven studies. [Table: see text]

532 Background: Limited compelling prospective and retrospective data regarding the added benefit of anti-HER2 therapy in the management of small, node-negative HER2-positive breast cancer (HER2+BC) exists, in part due to differences in outcome reporting, unmatched analyses, and a lack of head-to-head comparisons. As a result, national guideline committees find themselves unable to confidently recommend anti-HER2 therapy and clinicians are left to exercise clinical judgement on whether the use of anti-HER2 therapy should be considered for such patients. Methods: Our team performed a multi-institutional retrospective analysis using the ASCO CancerLinQ database, with a focus on clinical data from small, node-negative HER2+BC patients diagnosed between 2010 to 2021. We compared clinical outcomes between those who received adjuvant trastuzumab therapy, with or without chemotherapy, to those who did not, with our primary outcomes being invasive disease-free survival (iDFS) and overall survival (OS). We performed both a univariate and multivariate analysis, using a Cox proportional hazard model to control for factors including age, ethnicity, body mass index, hormone status, tumor grade, histology type, BRCA status, region, and smoking history. Additionally, a three-arm univariate analysis was performed comparing untreated patients to trastuzumab alone versus combination therapy. Results: In total, 1206 patients met inclusion criteria, including 779 patients who received trastuzumab with or without chemotherapy. We found a statistically significant improvement in both iDFS (HR 0.73, p = 0.01) and OS (HR 0.63, p = 0.027) on univariate analysis for those receiving anti-HER2 therapy. Similarly on multivariate analysis, iDFS (HR 0.75, p = 0.030) and OS (HR 0.61, p = 0.029) were improved in those who received therapy, regardless of tumor size. Our three-arm univariate analysis involving no treatment (n = 427), trastuzumab monotherapy (n = 169), and combination therapy (n = 578) found that iDFS was significantly improved for both treatment arms compared to observation alone (p = 0.006), whereas OS trended towards significance in the treatment arms but did not reach this target (p = 0.061). No significant difference was noted between treatment arms. Conclusions: Our analysis found a statistically significant improvement in iDFS and OS when patients with small, node negative, HER2+BC received adjuvant anti-HER2 therapy with or without chemotherapy as compared to observation. From our univariate three-arm comparison, it appears that trastuzumab provides the majority of benefit to patients in terms of DFS, but this result is exploratory. Further investigation is warranted, including meta-analyses to better characterize the degree of benefit seen with anti-HER2 treatment. For now, this data adds to evidence suggesting added benefit with therapy over observation.

Estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) belong to a superfamily of nuclear receptors called steroid hormone receptors, which, upon binding ligand, dimerize and translocate to the nucleus where they activate or repress the transcription of a large number of genes, thus modulating critical physiologic processes. ERβ has multiple isoforms that show differing association with prognosis. Expression levels of the full length ERβ1 isoform are often lower in aggressive cancers as compared to normal tissue. High ERβ1 expression is associated with improved overall survival in women with breast cancer. The promise of ERβ activation, as a potential targeted therapy, is based on concurrent activation of multiple tumor suppressor pathways with few side effects compared to chemotherapy. Thus, ERβ is a nuclear receptor with broad-spectrum tumor suppressor activity, which could serve as a potential treatment target in a variety of human cancers including breast cancer. Further development of highly selective agonists that lack ERα agonist activity, will be necessary to fully harness the potential of ERβ.