Selective Estrogen Receptor Modulators Research Articles

Contemporaneous symptom networks and correlates during endocrine therapy among breast cancer patients: A network analysis.

Endocrine therapy-related symptoms are associated with early discontinuation and quality of life among breast cancer survivors. Although previous studies have examined these symptoms and clinical covariates, little is known about the interactions among different symptoms and correlates. This study aimed to explore the complex relationship of endocrine therapy-related symptoms and to identify the core symptoms among breast cancer patients. This is a secondary data analysis conducted based on a multicenter cross-sectional study of 613 breast cancer patients in China. All participants completed the 19-item Chinese version of the Functional Assessment of Cancer Therapy-Endocrine Subscale (FACT-ES). Multivariate linear regression analysis was performed to identify significant factors. A contemporaneous network with 15 frequently occurring symptoms was constructed after controlling for age, payment, use of aromatase inhibitors, and history of surgery. Network comparison tests were used to assess differences in network structure across demographic and treatment characteristics. All 613 participants were female, with an average age of 49 years (SD = 9.4). The average duration of endocrine therapy was 3.6 years (SD = 2.3) and the average symptom score was 18.99 (SD = 11.43). Irritability (n = 512, 83.52%) and mood swings (n = 498, 81.24%) were the most prevalent symptoms. Lost interest in sex (mean = 1.95, SD = 1.39) and joint pain (mean = 1.57, SD = 1.18) were the most severe symptoms. The edges in the clusters of emotional symptoms ("irritability-mood swings"), vasomotor symptoms ("hot flashes-cold sweats-night sweats"), vaginal symptoms ("vaginal discharge-vaginal itching"), sexual symptoms ("pain or discomfort with intercourse-lost interest in sex-vaginal dryness"), and neurological symptoms ("headaches-dizziness") were the thickest in the network. There were no significant differences in network structure (P = 0.088), and global strength (P = 0.330) across treatment types (selective estrogen receptor modulators vs. aromatase inhibitors). Based on an evaluation of the centrality indices, irritability and mood swings appeared to be structurally important nodes after adjusting for the clinical covariates and after performing subgroup comparisons. Endocrine therapy-related symptoms are frequently reported issues among breast cancer patients. Our findings demonstrated that developing targeted interventions focused on emotional symptoms may relieve the overall symptom burden for breast cancer patients during endocrine therapy.

Z-Endoxifen prevents aggressive mammary cancers in mice by inhibiting cell proliferation and creating a tumor suppressive microenvironment.

Aggressive estrogen receptor (ER) positive breast cancer is frequently tamoxifen-resistant; alternative endocrine approaches exist for therapy, but not for prevention, particularly in premenopausal women. We examined the efficacy of the selective ER modulator (Z-endoxifen) as monotherapy and in combination with the selective progesterone receptor modulators (onapristone and ulipristal acetate) in the tamoxifen-insensitive C3(1)/SV40TAg mouse mammary tumorigenesis model. Unlike tamoxifen at human equivalent dose (HED) 101 mg/day, endoxifen at HED 24 mg/day significantly increased latency and reduced tumor growth relative to untreated controls. Ulipristal acetate (UPA) at HED 81 mg/day also significantly increased latency however failed to inhibit tumor growth, while onapristone (HED 98 mg/day) had no tumor prevention efficacy in this model. Addition of UPA to endoxifen did not enhance preventive efficacy over endoxifen alone. The expression of genes associated with cell cycle, cell proliferation and extracellular matrix remodeling was similarly repressed by endoxifen and UPA however only endoxifen significantly downregulated prominent genes associated with poor prognosis (Col11a1, Il17b, Pdgfa, Tnfrsf11a). Our results indicate that endoxifen can prevent breast cancers, even when tamoxifen-resistant, through its role in favorable tissue remodeling and immunomodulation.

Corifolitropin-Alfa plus Five Days Letrozole Versus Daily Recombinant-FSH in Expected Normo-Responder Patients: A Retrospective Comparative Study

Background: In recent times, different novel GnRH-antagonist protocols with various combinations of gonadotropins and other molecules (e.g., aromatase inhibitors, selective estrogen receptor modulators) have been proposed for expected normal ovarian responders undergoing assisted reproductive treatments. The purpose of this study was to evaluate the effectiveness of a novel ovarian stimulation protocol based on the combination of corifollitropin-alfa plus five days of letrozole in E-NOR women undergoing IVF as compared with a daily recombinant-FSH regimen. Methods: We conducted a retrospective-controlled study on 182 couples undergoing their first IVF attempt. In Group A (experimental), letrozole (2.5 mg daily) was administered from day 2 (up to day 6 of the cycle), followed by corifollitropin-alfa on day 3 and daily recombinant FSH from day 10. In Group B, recombinant FSH from day 2 were administered (150 IU-225 IU daily). Statistical analysis was completed using SPSS Statistics. The primary outcome was the total number of MII oocytes retrieved. Results: Group A showed similar results compared to Group B in terms of MII oocytes, live birth, implantation, and clinical pregnancy rates (p = ns). Nevertheless, the experimental group was associated with a trend towards a higher number of developing follicles, total oocytes, and embryos (p < 0.05) with lower estradiol and progesterone values at ovulation induction compared to Group B, resulting in an increased chance of performing a fresh embryo transfer (p < 0.05). Conclusions: The combination of CFα plus five days of letrozole was associated with a trend towards a higher number of developing follicles, total oocytes, and obtained embryos. Moreover, the experimental protocol resulted in lower estradiol and progesterone values at ovulation induction compared to daily rFSH, with an increased chance of performing a fresh embryo transfer (with no OHSS occurrence). Given the observational design of our study, further well-conducted RCTs are needed.

Ormeloxifene, a nonsteroidal antifertility drug promotes megakaryocyte differentiation in leukemia cell line K562.

Ormeloxifene (ORM) (3,4-trans-2,2-dimethyl-3-phenyl-4-p-(β-pyrrolidinoethoxy) phenyl-7-methoxychroman), world's first nonsteroidal selective estrogen receptor modulatorapproved for contraception in India has been shown to have potential anticancer activities. Here, we show that ORMcan induce megakaryocyte and myeloid (granulocytic) but not erythroid differentiation in multipotent human myeloid leukemia cell line K562. We show that ORM at an IC50 of 7.5 µM can induce morphological changes similar to megakaryocytes in K562 cells. ORM led to increase in levels of megakaryocytic differentiation markers (CD41 and CD61) as well as key transcription factors GATA1 and AML1. We further show that ORM induces megakaryocytic differentiation in K562 cells through ERK activation and induction of autophagy in a fashion similar to other known inducers of megakaryocytic differentiation such as phorbol esters. In addition, as shown earlier, we yet again observed that ORM led to activation of caspases since their inhibition through pan-caspase inhibitor mitigated megakaryocytic differentiation as they led to significant decrease in CD41 and CD61. Because induction of megakaryocytic differentiation in K562 involves growth arrest and exit from cell cycle, we also observed an increase in levels of p21 and p27 with decrease in c-Myc protein levels in K562 cells treated with 7.5 µM ORM for 24 and 48 h,respectively. Taken together, these findings indicate that ORM can markedly induce megakaryocytic differentiation in K562 cells.

Cyclopia extracts act as selective estrogen receptor subtype downregulators in estrogen receptor positive breast cancer cell lines: Comparison to standard of care breast cancer endocrine therapies and a selective estrogen receptor agonist and antagonist.

Breast cancer is the most diagnosed type of cancer amongst women in economically developing countries and globally. Most breast cancers express estrogen receptor alpha (ERα) and are categorized as positive (ER+) breast cancer. Endocrine therapies such as, selective estrogen receptor modulators (SERMs), aromatase inhibitors (AIs), and selective estrogen receptor downregulators (SERDs) are used to treat ER+ breast cancer. However, despite their effectiveness, severe side-effects and resistance are associated with these endocrine therapies. Thus, it would be highly beneficial to develop breast cancer drugs that are as effective as current therapies, but less toxic with fewer side effects, and less likely to induce resistance. Extracts of Cyclopia species, an indigenous South African fynbos plant, have been shown to possess phenolic compounds that exhibit phytoestrogenic and chemopreventive activities against breast cancer development and progression. In the current study, three well characterized Cyclopia extracts, SM6Met, cup of tea (CoT) and P104, were examined for their abilities to modulate the levels of the estrogen receptor subtypes, estrogen receptor alpha and estrogen receptor beta (ERβ), which have been recognized as crucial to breast cancer prognosis and treatment. We showed that the Cyclopia subternata Vogel (C. subternata Vogel) extracts, SM6Met and cup of tea, but not the C. genistoides extract, P104, reduced estrogen receptor alpha protein levels while elevating estrogen receptor beta protein levels, thereby reducing the ERα:ERβ ratio in a similar manner as standard of care breast cancer endocrine therapies such as fulvestrant (selective estrogen receptor downregulator) and 4-hydroxytamoxifen (elective estrogen receptor modulator). Estrogen receptor alpha expression enhances the proliferation of breast cancer cells while estrogen receptor beta inhibits the proliferative activities of estrogen receptor alpha. We also showed that in terms of the molecular mechanisms involved all the Cyclopia extracts regulated estrogen receptor alpha and estrogen receptor beta protein levels through both transcriptional and translational, and proteasomal degradation mechanisms. Therefore, from our findings, we proffer that the C. subternata Vogel extracts, SM6Met and cup of tea, but not the C. genistoides extract, P104, selectively modulate estrogen receptor subtypes levels in a manner that generally supports inhibition of breast cancer proliferation, thereby demonstrating attributes that could be explored as potential therapeutic agents for breast cancer.

Effects of ospemifene on overactive bladder in postmenopausal women with vulvovaginal atrophy

Objective Overactive bladder (OAB) is a complex and multifactorial syndrome associated with urinary frequency, urgency and incontinence. The menopause-associated hormonal changes play a role in the development of this condition. Vaginal estrogens are effective in improving OAB in postmenopausal women (PMW) with vulvovaginal atrophy (VVA). Ospemifene is a selective estrogen receptor modulator licensed for the treatment of VVA. This study aimed to evaluate the effects of ospemifene on OAB symptoms in PMW with VVA. Methods Forty PMW suffering from OAB and VVA received oral ospemifene (60 mg/day) for 12 weeks. All patients were assessed with a urodynamic study, a 3-day bladder diary and validated questionnaires (International Consultation on Incontinence Questionnaire – Urinary Incontinence Short Form [ICIQ-UI SF] and International Consultation on Incontinence Questionnaire – Overactive Bladder [ICIQ-OAB]) at enrollment and at the end of the study. Results Cytometric capacity, bladder compliance and verbal sensory threshold responses during bladder filling were improved after treatment. The voiding diary showed a significant reduction of daily voids, urge urinary incontinence episodes and nocturnal events. The median overall scores of the ICIQ-UI and ICIQ-OAB were also significantly improved. Conclusions Our study suggest that treatment with ospemifene in PMW suffering from OAB is associated with a reduction of OAB symptoms due to a decreased bladder sensitivity and with an improvement in quality of life.

Development and validation of a liquid chromatography-mass spectrometry assay for quantification of Z- and E- isomers of endoxifen and its metabolites in plasma from women with estrogen receptor positive breast cancer

The selective estrogen receptor modifier tamoxifen (TAM) is widely used for the treatment of women with estrogen receptor positive (ER+ ) breast cancer. Endoxifen (ENDX) is a potent, active metabolite of TAM and is important for TAM’s clinical activity. While multiple papers have been published regarding TAM metabolism, few studies have examined or quantified the metabolism of ENDX. To quantify ENDX and its metabolites in patient plasma samples, we have developed and validated a rapid, sensitive, and specific liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the quantitative determination of the E- and Z-isomers of ENDX (0.5–500 ng/ml) and the ENDX metabolites norendoxifen (1–500 and 0.5–500 ng/ml E and Z, respectfully), ENDX catechol (3.075–307.5 and 1.92–192 ng/ml E and Z, respectfully), 4′-hydroxy ENDX (0.33–166.5 and 0.33–333.5 ng/ml E and Z, respectfully), ENDX methoxycatechol (0.3–300 and 0.2–200 ng/ml E and Z, respectfully), and ENDX glucuronide (2–200 and 3–300 ng/ml E and Z, respectfully) in human plasma. Chromatographic separation was accomplished on a HSS T3 precolumn attached to an Poroshell 120 EC-C18 analytical column using 0.1 % formic acid/water and 0.1 % formic acid/methanol as eluents followed by MS/MS detection. The analytical run time was 6.5 min. Standard curves were linear (R2 ≥ 0.98) over the concentration ranges. The intra- and inter-day precision and accuracy, determined at high-, middle-, and low-quality control concentrations for all analytes, were within the acceptable range of 85 % and 115 %. The average percent recoveries were all above 90 %. The method was successfully applied to clinical plasma samples from a Phase I study of daily oral Z-ENDX.

Managing Estrogen Therapy in the Pituitary Patient

Growth hormone (GH) regulates metabolic and physical health in the adult human. Because the GH system is regulated by estrogens, therapeutic estrogen compounds are likely to affect metabolic health. Estrogens are available for oral and parenteral use in natural, prodrug, and synthetic formulations including selective estrogen receptor modulators (SERMs). This review covers the pharmacology of estrogen and the effects on GH action to inform judicious use in the pituitary patient. The effects on the GH system are route dependent due to first-pass hepatic metabolism. Oral but not parenteral estrogen compounds inhibit GH action, reducing hepatic insulin-like growth factor-1 (IGF-1) production, protein anabolism, and fat utilization. In patients with GH deficiency, oral estrogen therapy exacerbates the degree of hyposomatotrophism and attenuates the beneficial effects of GH replacement therapy, effects that are greater with contraceptive than replacement doses. Surveys report that less than one-fifth of hypopituitary women are appropriately replaced by a transdermal route and up to half on oral therapy are inappropriately treated with contraceptive steroids. In acromegaly, however, estrogens, especially synthetic formulations of greater potency, reduce IGF-1, improving disease control, an effect also observed in men treated with SERMs. The route-dependent effects and potency of estrogen formulations are important considerations for optimizing the management of hypogonadal patients with pituitary disease, in particular GH deficiency and acromegaly. For hypopituitary women, estrogens should be replaced by a nonoral route. For acromegaly, oral estrogen formulations can be considered as simple adjuvant therapy for disease control.

Abstract P3-03-14: Comparing risk of breast cancer and cardiovascular disease and uptake of chemoprevention and statins among racially/ethnically diverse women with atypical hyperplasia or lobular carcinoma in situ

Abstract Background: Chemoprevention with selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) has been shown in randomized controlled trials to decrease breast cancer incidence by 50-65% among women at high risk for breast cancer. However, chemoprevention uptake remains low among high-risk women. Women with atypical hyperplasia (AH) or lobular carcinoma in situ (LCIS) derive the greatest benefit from SERMs and AIs for breast cancer risk reduction. A potential barrier to chemoprevention uptake is competing comorbidities, including atherosclerotic cardiovascular disease (ASCVD). We calculated risk of breast cancer and ASCVD among women with AH or LCIS and assessed uptake of chemoprevention and statins among women who met high-risk criteria for both breast cancer and ASCVD. Methods: We conducted a retrospective cohort study among women, age 40-74 years, with AH or LCIS diagnosed in 2007-2015 at Columbia University Irving Medical Center (CUIMC) in New York City. Eligible women had sufficient data in the electronic health record (EHR) to calculate 5 and 10-year invasive breast cancer risk according to the Breast Cancer Surveillance Consortium (BCSC) risk calculator, including age, race/ethnicity, first-degree family history of breast cancer, breast biopsy results, and mammographic density. We calculated 10-year ASCVD risk using the 2013 American College of Cardiology (ACC)/American Heart Association (AHA) ASCVD risk calculator using additional EHR data, including systolic blood pressure, total and HDL cholesterol, history of diabetes, treatment for hypertension, and current smoking status. High-risk criteria to determine eligibility for SERMs/AIs and statins was defined as a 5-year invasive breast cancer risk 1.67% and 10-year ASCVD risk 7.5%, respectively. We compared mean 10-year risk of breast cancer vs. ASCVD using a paired t-test and uptake of SERMs/AIs vs. statins among women at high risk for breast cancer and ASCVD, respectively, using McNemar’s test. Results: Among 298 evaluable women, mean age was 58.2 years (standard deviation [SD], 8.34), with 33% non-Hispanic White, 41% Hispanic, 9% non-Hispanic Black, 6% Asian, and 11% other/unknown race/ethnicity. About 98% of women met high-risk criteria for breast cancer and 30% were high risk for ASCVD. Mean 10-year risk of breast cancer was higher than mean 10-year risk of ASCVD (9.14% vs. 6.69%, p&amp;lt; 0.001). Among women who met high-risk criteria for both breast cancer and ASCVD, uptake of statins was higher compared to SERMs/AIs (58% vs. 21%, p&amp;lt; 0.001). Comparing non-Hispanic Whites vs. racial/ethnic minorities, mean 10-year breast cancer risk was higher (12.12% vs. 7.71%, p&amp;lt; 0.001), but there were no statistically significant differences in ASCVD risk or uptake of chemoprevention or statins. Conclusions: Among women with AH or LCIS, mean absolute risk of breast cancer was higher compared to risk of ASCVD, however, uptake of statins was higher compared to chemoprevention with SERMs or AIs. To address under-utilization of chemoprevention among high-risk women, use of SERMs or AIs should be placed in the context of medications used for other chronic diseases, such as statins for ASCVD. Citation Format: Luisa Nilan, Jacquelyn N. Amenta, Julia E. McGuinness, Rita Kukafka, Katherine D. Crew, Kehinde Lawal. Comparing risk of breast cancer and cardiovascular disease and uptake of chemoprevention and statins among racially/ethnically diverse women with atypical hyperplasia or lobular carcinoma in situ [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 P3-03-14.

Abstract P4-03-23: Comparison of Breast Cancer vs Cardiovascular Disease Risk and Uptake of Chemoprevention vs Statins in a Cohort of Predominantly Hispanic Women Undergoing Screening Mammography

Abstract Background: Atherosclerotic cardiovascular disease (ASCVD) and breast cancer are two of the most diagnosed chronic diseases among women in the U.S. Although prevention of ASCVD with statins is widely practiced, breast cancer chemoprevention with selective estrogen receptor modulators (SERMs) or aromatase inhibitors (AIs) is underutilized in the primary care setting, despite significant evidence in randomized controlled trials demonstrating its clinical benefits. We compared the risk of ASCVD and breast cancer among predominantly Hispanic women undergoing screening mammography, as well as uptake of statins and SERMs/AIs for ASCVD and breast cancer risk reduction, respectively, among high-risk women. Methods: We conducted a retrospective cohort study among 1,655 English or Spanish-speaking women, age 40-79 years, with no prior history of breast cancer, who underwent screening mammography from 2014 to 2016 at Columbia University Irving Medical Center in New York City. Participants completed a survey collecting data on sociodemographic and breast cancer risk factors and had available data in the electronic health record (EHR) for calculating ASCVD risk, including systolic blood pressure, total and HDL cholesterol, history of diabetes, treatment for hypertension, and current smoking status. The main outcomes included 5-year and lifetime invasive breast cancer risk according to the Gail model, and 10-year and lifetime ASCVD risk score according to the 2013 American College of Cardiology (ACC)/American Heart Association (AHA) ASCVD risk calculator. High-risk was defined as a 5-year invasive breast cancer risk 1.67% and 10-year ASCVD risk 7.5%. Secondary outcomes included uptake of chemoprevention with SERMs or AIs and statins among women at high-risk for breast cancer and ASCVD, respectively, based upon medication lists in the EHR. We compared mean lifetime risk of breast cancer vs ASCVD for the entire cohort using a paired t-test, and the proportion of high-risk women taking statins vs chemoprevention using McNemar’s test. Results: Among 1,655 evaluable women, mean age was 58 years (SD=10.1 years), with 76% Hispanic, 6% non-Hispanic White, 3% non-Hispanic Black, 2% Asian, and 13% other. About half (48%) of women met high-risk criteria for ASCVD compared to 15% who met high-risk criteria for breast cancer. Among all women, mean lifetime ASCVD risk was higher than mean lifetime breast cancer risk (10.71% vs. 5.46%, p&amp;lt; 0.001). Among women at high risk for ASCVD or breast cancer, respectively, statin uptake was higher compared to SERM/AI uptake for breast cancer chemoprevention (84% vs. 7%, p&amp;lt; 0.001). Overall, fewer Hispanic compared to non-Hispanic women met high-risk criteria for ASCVD (47% vs. 51%, respectively) and breast cancer (9% vs. 34%, respectively). Conclusions: In a population of predominantly Hispanic women undergoing screening mammography, we found that more women met high-risk criteria for ASCVD compared to breast cancer. Among women at high risk for ASCVD, statin uptake was about 12-fold higher compared to uptake of breast cancer chemoprevention among women at high risk for breast cancer. Given significant underutilization of breast cancer chemoprevention, placing this in the context of prevention of other chronic diseases, such as statins for ASCVD, may enhance uptake of SERMs or AIs in the primary care setting. Citation Format: Luisa Nilan, Mary M. McDermid, Jacquelyn N. Amenta, Julia E. McGuinness, Katherine D. Crew, Rita Kukafka. Comparison of Breast Cancer vs Cardiovascular Disease Risk and Uptake of Chemoprevention vs Statins in a Cohort of Predominantly Hispanic Women Undergoing Screening Mammography [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 P4-03-23.

Abstract PD10-01: PD10-01 Impact of ESR1 mutations on Selective Estrogen Receptor Degraders and Modulators: an integrated liquid-biopsy and pharmacodynamics approach

Abstract Background: ESR1 hotspot mutations (HS) (i.e. 380, 536, 537, and 538) are important drivers of resistance to aromatase inhibitors, but the differential impact of genomic variants (HS vs non-HS) on response to endocrine therapies (ET) under clinical development, such as novel oral Selective Estrogen Receptor Degraders and Modulators (SERDs and SERMs), is not known. The aim of the study was to evaluate the impact of non-HS ESR1 mutations on the pharmacodynamics of SERDs and SERMs as an additional ET resistance mechanism. Materials and Methods: The study analyzed a multi-institutional cohort of 1008 patients with hormone receptor positive metastatic breast cancer characterized by circulating tumor DNA (ctDNA). Pathway classification was defined based on previous work (i.e. RTK, RAS, RAF, MEK, NRF2, ER, WNT, MYC, p53, Cell Cycle, Notch, PI3K). Single nucleotide variations (SNVs) were annotated through OncoKB; co-occurrence was tested by Fisher’s exact test. A structure-based computational strategy was used to create 3D-models of ESR1 mutants and predict changes in binding affinity (dAff) across approved and experimental drugs. A positive dAff reflects a lower affinity of the drug for mutant ESR1 compared with wild type and thus a potential for a reduced response. Results: Among the total 680 detected ESR1 mutations, 633 were missense, and 631 were gain-of-function. The most frequent mutations were in codon 537 (N=305), followed by 538 (N=224). No significant MAF differences were observed across ESR1 variants (P=0.0829). The L391F mutation resulted in an increased binding affinity for Lasofoxifene (LAS) (dAff -0.34), Giredestrant (GIR) (dAff -0.18), Elacestrant (ELA) (dAff -0.08) and Amcenestrant (AMC) (dAff -0.41), while a decreased binding affinity was observed for 4OH-Tamoxifen (TAM) (dAff 0.01), Imlunestrant (IML) (dAff 0.15), Fulvestrant (FUL) (dAff 0.43), and Camizestrant (CAM) (dAff 0.02). V392F decreased binding affinity for TAM (dAff 0.05), LAS (dAff 0.13), IML (dAff 0.11), GIR (dAff 0.11), FUL (dAff 0.04), CAM (dAff 0.05), AMC (dAff 0.06) but not for ELA (dAff -0.01). F404L decreased binding affinity for FUL (dAff 0.07), ELA (dAff 0.73), and CAM (dAff 0.26), while it increased binding affinity for TAM (dAff -0.27), LAS (dAff -0.02), IML (dAff -0.05), GIR (dAff -0.69), and AMC (dAff -2.01). G415E increased binding affinity for LAS, (dAff -0.15) GIR (dAff -0.02) and ELA (dAff -0.08), while it decreased binding affinity for TAM (dAff 0.11), IML (dAff 0.09), FUL (dAff 0.29), CAM (dAff 0.19) and AMC (dAff 0.10). Mutations in codon 537 did not affect dAff for TAM, GIR, and ELA; a significant decrease in binding affinity was observed for FUL and AMC, whereas it was increased for LAS. Mutational co-occurrence was tested between ESR1 mutations in FUL docking sites and oncogenic pathways. Significant associations were observed for cell cycle SNVs (P=0.047), Notch SNVs (P=0.020), and ER SNVs (P&amp;lt; 0.001). Within these pathways, significant single-gene associations were observed for FBXW7 SNVs (P=0.020), ESR1 SNVs (P&amp;lt; 0.001), and GATA3 SNVs (P= 0.016). Given the highly significant co-occurrence of non-HS with other ESR1 mutations, combined models were examined. The Y537/F404 combination resulted in decreased binding affinity for FUL and increased binding affinity for LAS, while L536/F404 decreased binding affinity for TAM and increased binding affinity for IML, ELA, and AMC. Notably, L540/F404 restored the FUL-ESR1 interaction resulting in an increased binding affinity (dAff -2.1). Conclusions: The study suggests that genomic variability in drug targets detectable through ctDNA may modulate therapeutic response. Preclinical models are under development to investigate the combined endocrine resistance mechanism suggested by the significant co-occurrence between ESR1 mutations in SERDs/SERMs docking sites and ESR1 hotspot mutations and provide valuable additional insights for drug development and future treatment algorithms. Citation Format: Lorenzo Gerratana, Rossana Roncato, Mattia Sturlese, Andrew A. Davis, Marko Velimirovic, Carolina REDUZZI, Katherine K. Clifton, Whitney L. Hensing, Ami N. Shah, Charles S. Dai, Paolo D’Amico, Arielle J. Medford, Alessandra Franzoni, Linda Cucciniello, Firas Wehbe, Seth A. Wander, Barbara Belletti, William Gradishar, Amir Behdad, Giuseppe Damante, Cynthia Ma, Fabio Puglisi, Aditya Bardia, Massimo Cristofanilli. PD10-01 Impact of ESR1 mutations on Selective Estrogen Receptor Degraders and Modulators: an integrated liquid-biopsy and pharmacodynamics approach. [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 PD10-01.

Abstract P2-24-04: The ER antagonist giredestrant induces profound chromatin remodeling including activation of cis-regulatory elements bound by FOXA1 and GATA3 in HR+ breast cancer models

Abstract The progression of HR+ breast cancers is governed by ER signaling which makes the development of optimized ER-targeting agents a high priority. Among current therapeutics, selective ER antagonists and degraders (SERDs) have emerged as a particularly attractive class. Fulvestrant is the only SERD that is currently approved, but its clinical potential is hypothesized to be somewhat limited by poor physicochemical properties, dosage and exposure. We identified giredestrant as an orally bioavailable, highly potent non-steroidal ER antagonist, immobilizer and degrader with robust anti-proliferative activity. Giredestrant is currently being evaluated in phase III clinical trials for the treatment of ER+ breast cancer, after showing evidence of clinical activity in phase I and II trials. Here, we set out to investigate the impact of giredestrant on the transcriptome and chromatin landscape of ER+ breast cancer cells in vitro and in vivo. RNA-seq data from MCF-7 cells in vitro and three patient-derived xenograft (PDX) models in vivo showed that giredestrant inversely regulates the RNA levels of E2-regulated genes. Transcriptome wide, the expression of genes affected by giredestrant are strongly correlated (r2=0.8) with those affected by siRNA-mediated silencing of ESR1, consistent with on-target ER antagonism. Performing ATAC-seq in the three PDX models, including one ESR1 mutant model, reveals that giredestrant reduces accessibility in up to 18,173 chromatin regions that are associated with genes induced by E2. These regions are also enriched in the ER binding motif. In addition, we found up to 17,127 chromatin regions with increased accessibility upon giredestrant treatment. These regions are associated with genes suppressed by E2, and are enriched in the FOXA1 binding motif. To probe giredestrant’s role in functional modulation of cis-regulatory elements (CREs), we performed ChIP-seq targeting FOXA1 and GATA3, as key pioneer factors for ER, as well as ER itself and H3K27ac as a marker of active enhancers in MCF-7 cells. We identified 3,195 regions with significantly reduced H3K27ac signals that also exhibit significant reduction of both ER and FOXA1 binding upon giredestrant treatment, indicating that giredestrant deactivates these CREs concomitantly with suppressing ER and FOXA1 binding. ChIP-seq analysis further revealed 2,292 regions with a significant increase in H3K27ac signals upon giredestrant treatment. Interestingly, the GATA3 motif was identified as the most enriched motif in these activated CREs. Further, GATA3 and FOXA1 ChIP-seq signals were stronger in the activated CREs compared to the deactivated ones. Similar profiling of both tamoxifen, an approved selective ER modulator (SERM), as well an GNE-858, a SERM from the same chemical series as giredestrant, showed their epigenetic effects to be highly distinct from those observed for giredestrant, suggesting that the high degree of chromatin remodeling is likely related to giredestrant’s mechanism of action. Our studies here show that giredestrant profoundly alters ER binding and remodels the chromatin landscape, thus changing the transcriptional profile of cells in a manner that is associated with an anti-proliferative response. While deactivation of CREs with ER binding is perhaps in line with expectations, giredestrant treatment also leads to the activation of a number of regions associated with pioneer factors, potentially through their redistribution. This raises the question of the functional consequences of CRE activation and if this is relevant for the anti-proliferative effects of giredestrant. Citation Format: Musaddeque Ahmed, Jane Guan, Wei Zhou, Xiaosai Yao, Ciara Metcalfe, Marc Hafner. The ER antagonist giredestrant induces profound chromatin remodeling including activation of cis-regulatory elements bound by FOXA1 and GATA3 in HR+ breast cancer models [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 P2-24-04.

Abstract P2-03-22: Assessment of the relationship between duration of treatment with hormonal therapy and the detection of ESR1 mutations in hormone-receptor positive, HER2 negative, breast cancer

Abstract Background Hormonal therapeutic agents such as the selective estrogen receptor modulator, tamoxifen, as well as the aromatase inhibitors (AIs), anastrozole, exemestane, and letrozole, are a mainstay in the treatment of patients with hormone-receptor positive (HR+) breast cancer. Despite the effectiveness of hormonal therapy in the treatment of patients with HR+ breast cancer, acquired resistance to both AIs and tamoxifen does occur, often associated with mutations of the estrogen receptor 1 (ESR1). Although the acquisition of ESR1 mutations in patients with HR+ breast cancer treated with hormonal therapy has been established as a mechanism of resistance, there is a paucity of data on the relationship between the duration of treatment with hormonal therapies and the development of ESR1 mutations. Using a cohort of patients with HR+ breast cancer who underwent next generation sequencing (NGS) as part of routine clinical care, we aim to assess the relationship between the duration of exposure to aromatase inhibitors and the detection of ESR1 mutations. The results of this study will further our understanding of ESR1-mutations and provide insight into strategies to optimize therapy for patients with HR+ breast cancers. Methods This is a retrospective analysis of patients 18 years of age or older treated at a single academic cancer center in the USA, diagnosed with HR+, HER2-negative breast cancer and who have received somatic NGS as part of clinical care over a six-year period through July 2019. Demographic, disease-related, and treatment related variables were collected along with genomic testing variables (ie. ESR1-mutation status) for each patient who met the inclusion criteria. Two patient groups were formed on the basis of ESR1-mutation status (present vs. absent) and compared on the basis of duration of treatment with AI, and other clinical and demographic categories. Results A total of 247 patients who were diagnosed with HR-positive, HER2-negative breast cancer and received NGS were identified and are included in this analysis. A total of 138 (56%) patients were assessed via tissue based NGS and 109 (44%) by liquid biopsy. The median age at time of sequencing was 58 years [57 years (ESR1-absent) vs. 60 years (ESR1-present); p =0.009]. The majority of patients were white (83%) and post-menopausal (57%). Invasive ductal carcinoma was the most common histological type (82%). Nearly all (96%) patients had stage IV disease at time of sequencing and ESR1 mutations were found exclusively in those with stage IV disease. ESR1 mutations were identified in 62 patients (25%). The median duration of treatment with aromatase inhibitors prior to testing was 19.3 months in both groups (ESR1 mutations absent vs. detected). There was no difference in prior tamoxifen use between groups (absent 43%, present 48%; p=0.48). In patients tested by tissue based NGS, ESR1 mutations were more frequently detected in metastatic sites (28% vs. 13%; p=0.03). Twenty-two patients for whom NGS did not identify an ESR1 mutation prior to July 2019 received subsequent NGS testing. Of these 22 patients, six (27%) had ESR1 mutations detected by subsequent NGS. In these six patients, ESR1-mutations were detected by NGS a median of 2.2 years (range 1.7 years to 2.6 years) after prior NGS. Conclusion There was no difference in duration of treatment with aromatase inhibitor therapy in patients with ESR1 mutations vs. those without ESR1 mutations detected by NGS. Patients with ESR1 mutations tended to be older. Although the detection of ESR1 mutations is associated with acquired therapy resistance and progression on treatment, a longer duration of treatment may make the emergence of ESR1 mutations more likely. These counterbalancing factors may have contributed to the results reported here. Citation Format: Hannah Ayettey-Anie, Kamran A. Ahmed, Iman Washington, Kosj Yamoah, Todd Knepper. Assessment of the relationship between duration of treatment with hormonal therapy and the detection of ESR1 mutations in hormone-receptor 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 P2-03-22.

Abstract P2-24-03: Z-Endoxifen Allosterically Inhibits PKCβI and its Paradoxical Membrane Translocation

Abstract Z-Endoxifen Allosterically Inhibits PKCβI and its Paradoxical Membrane Translocation Sayantani Sarkar Bhattacharya1, Taylor L. Witter1, Anh Q. T. Cong1, Elizabeth Bruinsma2, Swaathi Jayaraman2, Matthew P. Goetz2, John R. Hawse1, and Matthew J. Schellenberg1 1Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, MN USA 55905 2Department of Oncology, Mayo Clinic, Rochester, MN USA 55905 Background: Z-Endoxifen (ENDX), the active metabolite of tamoxifen (TAM) and a selective estrogen receptor modulator (SERM), exhibited high antitumor activity in endocrine-resistant hormone receptor-positive breast and other gynecologic cancer. ENDX has also been shown to be a protein kinase C (PKC) inhibitor. PKCs participate in diverse cellular functions and their activity is often elevated in breast tumors. Guided by mechanistic insights from our recently determined crystal structure of PKCβI, we sought to determine the effects of ENDX on PKCβI in a breast cancer cell line model. Methods: To determine how ENDX regulates PKC activity, we used a Z’LYTE kinase activity assay. This Fluorescence Resonance Energy Transfer (FRET) based biochemical method can detect differential sensitivity of phosphorylated and non-phosphorylated peptides to proteolytic cleavage. We probed changes in activity for conventional and novel PKCs, as well as the purified catalytic domain of conventional PKCs in vitro. Alongside, as kinase activity of PKC relies on its spatial assembly, therefore we studied its intracellular localization using live cell confocal imaging. MCF7 cells expressing YFP-tagged PKCβI were grown in a glass bottom chamber and treated with ENDX and other modulators for relevant time and doses for this study. Images were taken using Zeiss LSM 780 confocal laser scanning microscope and analyzed in Zeiss-ZEN microscope software and GraphPad Prism 9. Results: Our data from an in vitro kinase assay indicates that ENDX inhibits the kinase activity of conventional (PKCβI) and novel (PKC𝛿) PKC isoforms with a similar IC50, however PKCβI catalytic domain is less sensitive to ENDX. We also identified a multi-domain mechanism of PKC inhibition through an allosteric inhibitory mechanism. Our live cell imaging study demonstrated that ENDX promotes the recruitment of PKCβI to the cell membrane in both a dose and time-dependent manner. Moreover, this translocation can also be mitigated by co-treatment with inhibitors of the PKC-regulator PHLPP phosphatases. Conclusion: Taken together, these results suggest the allosteric effects of ENDX trigger PKCβI recruitment to the cell membrane, yet since ENDX also inhibits kinase activity it suggests that ENDX triggers a non-productive interaction with the enzyme and "breaks" the well-known mechanism of PKC activation upon binding the cell membrane. Furthermore, ENDX is likely to trigger dephosphorylation and ultimately degradation, suggesting that ENDX represents a new mechanistic basis for targeting and downregulating PKC in cancer cells. Hence, the current study provides an integrated pattern of highly specific treatments regimen that can exploit PKCβI as a repurposing clinical target in breast cancer. Citation Format: Sayantani Sarkar Bhattacharya, Taylor L. Witter, Anh T. Cong, Elizabeth S. Bruinsma, Swaathi Jayaraman, Matthew P. Goetz, John Hawse, Matthew Schellenberg. Z-Endoxifen Allosterically Inhibits PKCβI and its Paradoxical Membrane Translocation [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 P2-24-03.

Abstract OT1-15-01: SWOG 1904: Cluster-randomized controlled trial of patient and provider decision support to increase chemoprevention informed choice among women with atypical hyperplasia or lobular carcinoma in situ (MiCHOICE)

Abstract Background: Despite evidence of substantial breast cancer risk reduction, few high-risk women adopt chemopreventive medications such as selective estrogen receptor modulators (SERMs) or aromatase inhibitors (AIs). Women with benign breast disease, such as atypical hyperplasia (AH) or lobular carcinoma in situ (LCIS), have an increased risk of developing breast cancer and derive a greater benefit from antiestrogens compared to other high-risk women. Reasons for low uptake of chemoprevention include insufficient patient and clinician knowledge about antiestrogens, time constraints during the clinical encounter, and concerns about side effects. To address these barriers, we have developed patient and provider web-based decision support tools to improve informed choice about breast cancer chemoprevention among women with AH or LCIS. Study design: We are conducting a cluster-randomized controlled trial of clinical decision support to improve chemoprevention informed choice among women with AH or LCIS and their treating providers. Twenty-six U.S. sites through the SWOG Cancer Research Network were randomly assigned 1:1 to standard educational materials alone or in combination with the patient-centered decision aid (RealRisks) and provider decision support tool (BNAV). A total of 415 patients and 200 healthcare providers will be recruited from these sites. RealRisks consists of interactive modules to calculate personalized breast cancer risk and elicit preferences on chemoprevention. The modules are available in English and Spanish. BNAV is comprised of self-directed case-based learning modules on breast cancer risk assessment and chemoprevention. Patients complete questionnaires at baseline, 6 and 12 months. Providers complete surveys at baseline and after their enrolled patient’s 6-month clinical encounter. The primary endpoint is chemoprevention informed choice at 6 months, using a measure combining knowledge, attitude, and intention scales. Secondary endpoints include perceived breast cancer risk/worry, chemoprevention knowledge/intention, decision conflict/regret, shared decision-making, and chemoprevention uptake. For patients who begin chemoprevention, adherence and reasons for discontinuation are assessed annually for up to 5 years. Barriers and facilitators to implementing RealRisks and BNAV into clinic workflow will be assessed by conducting patient and provider interviews at baseline and mid-implementation. Eligibility criteria: Eligible patients include women, age 35-74 years, with AH or LCIS, no history of breast cancer, no prior use of SERMs or AIs, no bilateral mastectomies, English or Spanish-speaking, and access to the internet. Eligible providers include breast surgeons, medical oncologists, primary care providers, and physician extenders who see patients with AH or LCIS. Statistical methods: We have 90% power to detect a 15% increase in the frequency of chemoprevention informed choice with a 1-sided 0.025 level test, assuming an intraclass correlation (ICC) of 0.02 to account for clustering, roughly equal accrual at each site, 10% loss to follow-up, and ≤10% event rate in the control arm. Current/target accrual: The trial was activated on 9/1/2020. As of 7/7/2022, all 26 sites have been randomized, 157/200 providers and 184/415 patients have been enrolled. Discussion: Our hybrid effectiveness/implementation study seeks to evaluate the effectiveness of a multi-level intervention in promoting informed decision-making about breast cancer chemoprevention. Study results will provide valuable insights on how the decision support tools are integrated in diverse clinical settings. Citation Format: Katherine D. Crew, Garnet Anderson, Kathryn Arnold, Andrew Stieb, Jacquelyn N. Amenta, Cynthia Law, Ana Sandoval-Leon, Sarah Colonna, Tari King, Debra Mangino, Sandhya Pruthi, Maria Grosse Perdekamp, Christa Braun-Inglis, Stacy Krisher, Lisa Yee, Danielle Bertoni, Samantha Seaward, Kari B. Wisinski, Justin Floyd, Corrine Zarwan, Tarah J. Ballinger, Lindi VanderWalde, Masey M. Ross, Preston Steen, Shelly Lo, Alison Conlin, Kathleen Yost, John Ellerton, Erin Lin, Holly J. Pederson, Sagar Sardesai, Cheryl Jernigan, Dawn Hershman, Marian L. Neuhouser, Banu K. Arun, Rita Kukafka. SWOG 1904: Cluster-randomized controlled trial of patient and provider decision support to increase chemoprevention informed choice among women with atypical hyperplasia or lobular carcinoma in situ (MiCHOICE) [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 OT1-15-01.

Abstract P5-05-04: Estrogen receptor 1 (ESR1) mutations in circulating tumor DNA (ctDNA) from patients with ER+/HER2- metastatic breast cancer (mBC) treated with lasofoxifene or fulvestrant in the ELAINE 1 study

Abstract Introduction: Acquired ESR1 mutations (mutESR1) after long-term endocrine therapy drive treatment resistance, metastasis, and poor prognosis for patients (pts) with ER+/HER2- metastatic breast cancer (mBC). Lasofoxifene (LAS), a selective estrogen receptor modulator, alone or with a CDK4/6 inhibitor (CDK4/6i) reduced tumor growth better than fulvestrant (Fulv) in mutESR1 BC xenograft models. ELAINE 1 is a randomized trial of LAS vs Fulv in pts with mutESR1 and prior progression on aromatase inhibitor and CDK4/6i. Preliminary results (ESMO 2022) showed that LAS prolonged median progression-free survival (mPFS) compared with Fulv with a favorable safety profile. Here, we report changes in ESR1 ctDNA mutant allele frequency (MAF) from baseline to 8 wks and their associations with clinical benefit (CB) and mPFS. Methods: ELAINE 1 pts were randomized to oral LAS 5 mg daily or IM Fulv 500 mg on days 1, 15, and 29, then every 4 wks, until disease progression or severe toxicity. ctDNA mutESR1 mutations (baseline and 8 wks) were assessed using the Sysmex Inostics OncoBeam or SafeSeq assays—which detect mutESR1 at low allele fractions. MAF changes from baseline to wk 8 were characterized as decreased (decrease in ESR1 MAF or fully cleared), increased (increase in MAF), or equivocal (polyclonal patients [&amp;gt;1 mutESR1] with some increasing and decreasing MAF trends); correlations with PFS and CB were explored. Efficacy measures included objective response rate (ORR), PFS, and CB at 24 wks (CB defined as response or stable disease ≥24 wks). Results: 103 pts received LAS (n=52) or Fulv (n=51). Most common baseline ESR1 variants detected were D538G (56%), Y537S (39%), Y537N (29%), E380Q (22%); 56 (54%) pts were polyclonal. Of the 61 pts with evaluable baseline and wk 8 ctDNA, LAS decreased mutESR1 MAF in 29/35 pts (83% [11 complete clearance]) while Fulv decreased mutESR1 MAF in 16/26 pts (61.5% [6 complete clearance]) (Table). mPFS with LAS was 8 and 4 mos for pts with decreased/cleared MAF and increased MAF, respectively, and with Fulv was 4.5 and 2.8 mos, respectively (Table). LAS decreased the common mutESR1 variants more frequently than Fulv (median relative change -87.1% vs -14.7%). In pts with decreased MAF, CB was observed in 16/29 LAS pts (55%) and 4/16 Fulv pts (25%). The predictiveness of ESR1 MAF clearance for CB was also explored. Of 11 pts with ESR1 MAF clearance taking LAS, 10 achieved CB, yielding a positive predictive value (PPV) of 90.9%. In contrast, 2/6 pts with ESR1 MAF clearance taking Fulv had CB for a PPV of 33.3%. Sensitivity for predicting CB based on direction of ESR1 MAF change was 94% with LAS and 80% with Fulv. In pts with Y537S MAF (n=33), LAS decreased Y537S in 13/15 (87%), with a median relative MAF decrease of 89%. In marked contrast, Fulv increased Y537S MAF in 11/18 pts (61%), corresponding to an MAF relative increase of 82%. LAS and Fulv resulted in complete clearance of Y537S MAF in 33% and 6% of pts, respectively. Conclusion: Our data demonstrate that LAS more effectively decreased or cleared mutESR1 than Fulv. Further, mutESR1 clearance was associated with prolonged PFS and more CB in LAS but not Fulv pts, suggesting that LAS results in robust mutESR1 target engagement. Taken together, our data suggest mutESR1 as a potential liquid biomarker for predicting response to LAS in mutESR1, endocrine-resistant mBC pts. Table. Change from baseline to week 8 in ESR1 MAF and clinical benefit at 24 weeks. CI, confidence interval; MAF, mutant allele fraction; ND, none detected; PFS, progression-free survival. Citation Format: Matthew P. Goetz, Einav Gal-Yam, Daniel Stover, Sarah L. Sammons, Stephanie L. Graff, Grace Wang, Massimo Cristofanilli, Gary Riordan, Hillary S. Sloane, Dominic Carroll, Paul V. Plourde, David J Portman. Estrogen receptor 1 (ESR1) mutations in circulating tumor DNA (ctDNA) from patients with ER+/HER2- metastatic breast cancer (mBC) treated with lasofoxifene or fulvestrant in the ELAINE 1 study [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 P5-05-04.

Radiation-Induced Localized Bullous Pemphigoid.

Radiation-Induced Localized Bullous Pemphigoid.

Abstract P1-02-05: Endocrine-targeted therapies modify the gut microbiome affecting responsiveness in ER+ breast cancer

Abstract Background: The microbiome consists of the totality of microorganisms (bacteria, fungi, protist, viruses, and phages) that live on and within the body. Studies implicate the gut bacterial microbiome as a risk factor for estrogen receptor-positive (ER+) breast cancer. While diet is the main contributor to the gut microbiome, medications also shift the bacterial microbiome. We currently do not know whether oral endocrine targeting therapies, such as aromatase inhibitors or tamoxifen shift the gut microbiome. Furthermore, we do not know whether gut microbiome populations can influence drug efficacy. Methods: Fecal samples from human donors were placed into ex-vivo colonic bioreactors for stabilization (n=3). Bioreactors were untreated or treated with letrozole or tamoxifen citrate for 48 hours. Samples were collected, DNA isolated, and metagenomic sequencing performed to determine direct drug-bug interactions. C57BL/6 mice were placed on a healthy control (HC; 21% kcal from fat derived from olive oil and fish oil) or a Western diet (45% kcal from fat derived from corn oil, lard, and milkfat) for 6 weeks. Mice within dietary patterns were randomized and administered control, tamoxifen (TAM; 37 ppm tamoxifen citrate), or an aromatase inhibitor (AI; 5 ppm letrozole) for 16 weeks. Metagenomics sequencing were performed on fecal DNA samples at study endpoint. Female BALB/c mice fed a HC or Western diet were injected with bone metastatic 4T1.2ER+ breast cancer cells. Mice were administered tamoxifen citrate, oral probiotics, or a combination of TAM + probiotics for 3-weeks. Tumor volume, tumor weight, and lung weight were recorded at the end of the study. Hindlimbs were analyzed for metastatic lesions. Results: Metagenomic sequencing from the ex-vivo colonic bioreactors treated with aromatase inhibitors or tamoxifen display differential shifts in several β-glucuronidase-expressing and obesity-associated bacterial species suggesting AI and selective estrogen receptor modulators have varying effects in the gut microbiome that may influence estrogen bioavailability and metabolic parameters. C57BL/6 mice on HC or Western diet treated with AI or TAM also display differences in the microbiota and phage populations with TAM elevating Lactobacillus johnsonii and letrozole increasing Lactococcus lactis bacterial proportional abundance. In the tumor-bearing model, combination of oral Lactobacillus probiotics and TAM significantly reduced tumor weight when compared with the tumor weight in control, TAM, or probiotic treated mice fed a Western diet. Combination of probiotics and TAM also prevented the development of ER+ bone metastatic lesions. Conclusions: Our study indicates oral endocrine therapies differentially affect the gut microbiome and these drug-bug interactions are sensitive to dietary-influenced baseline microbiota populations, which may influence drug efficacy and metabolic outcomes. Furthermore, our preclinical studies suggest oral probiotic supplements may enhance tamoxifen efficacy to reduce tumor growth and metastatic development. Further clinical studies are needed on this topic. Citation Format: Alana Arnone, Adam S. Wilson, Akiko Chiba, Bethany Kerr, David R. Soto-Pantoja, Alexandra Thomas, Katherine L. Cook. Endocrine-targeted therapies modify the gut microbiome affecting responsiveness in ER+ 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 P1-02-05.

Abstract P5-05-02: Estrogen receptor 1 (ESR1) mutations in circulating tumor DNA (ctDNA) from patients with ER+/HER2- metastatic breast cancer (mBC) treated with lasofoxifene plus abemaciclib in the ELAINE 2 study

Abstract Introduction: Use of long-term endocrine therapy (ET) for ER+ breast cancer often leads to acquired ESR1 mutations (mutESR1), causing endocrine resistance, tumor progression, and poor prognosis. An unmet clinical need exists for treating ER+ mBC patients with mutESR1, particularly after progression on CDK4/6 inhibitors (CDK4/6i). ELAINE 2 is an open-label, phase 2, multicenter trial evaluating safety and efficacy of lasofoxifene (LAS [selective estrogen receptor modulator]) plus abemaciclib (Abema [CDK4/6i], provided by Eli Lilly) in patients with ER+/HER2- and mutESR1 mBC who progressed after prior ET. Preliminary data with LAS plus Abema showed median progression-free survival of 55.7 wks, objective response rate of 50%, and 24-wk clinical benefit (CB) rate of 69%, with an acceptable safety and tolerability profile. Here, we report ESR1 ctDNA mutant allele frequency (MAF) and correlations of ESR1 MAF changes with CB. Methods: ELAINE 2 patients with detectable ctDNA mutESR1 at baseline (BL) were analyzed. Oral LAS 5 mg/day and Abema 150 mg BID were taken until disease progression, death, unacceptable toxicity, or withdrawal from the study. ctDNA was assessed by the Sysmex-Inostics SafeSeq assay—which detects mutESR1 at low allele fractions—at BL, every 4 wks, and end of treatment. MAF changes from BL to wk 4 were characterized as decreased (decrease in ESR1 MAF or none detected [ND]), increased (increase in MAF), or equivocal (in polyclonal patients [&amp;gt;1 mutESR1] with some MAF increasing and decreasing trends). Correlations of MAF change at 4 wks with CB at 24 wks were explored. Results: 29 patients (median of 2 prior metastatic therapies: 97% CDK4/6i, 79% fulvestrant, 48% chemotherapy) had BL mutESR1 of Y537S (66%), D538G (45%), Y537N (28%), and other less frequently detected mutations; 14 (48.3%) patients were polyclonal. 26 of 29 patients had evaluable BL and wk-4 ctDNA results: 21 patients had decreased MAF (81% [54% with ND]), 3 (12%) had increased, and 2 (8%) had equivocal ESR1 MAF changes (Table). CB at 24 wks was seen in 17 patients with a decrease, 2 with an increase, and 1 with equivocal MAF change. A sensitivity of 89.5% and specificity of 20% were calculated for predicting CB based on direction of ESR1 MAF change. The positive predictive value (PPV) for CB with decreased MAF was 81% and the negative predictive value (NPV) for an increased MAF was 33%. Of the 14 (54%) patients with ND ESR1 MAF, 13 had CB resulting in 87% sensitivity, 50% specificity, 93% PPV, and 33% NPV. Conclusion: In ELAINE 2, 81% of patients had decrease/cleared (ND) mutESR1 after 4 wks of LAS plus Abema, which correlated with clinical benefit. All mutESR1 detected appear targeted with this therapy. High sensitivity and favorable PPV were observed in patients with decreased MAF, and even more so in those with ND MAF; however, increased MAF was less specific and not as predictive of treatment failure. Our results indicate that ESR1 liquid biopsy evaluation may be an adequate non-invasive surrogate marker for monitoring patients on treatment. Further study in a larger population of women with endocrine-resistant mBC and acquired mutESR1 is warranted to explore this potential for monitoring treatment response or resistance to this novel LAS-Abema combination. Table. Change from baseline to week 4 in ESR1 MAF and clinical benefit at 24 weeks. CI, confidence interval; MAF, mutant allele fraction; ND, none detected; NPV, negative predictive value; PPV, positive predictive value. *Sensitivity and specificity analyses do not include equivocal results. Citation Format: Senthil Damodaran, Halle Moore, Ciara C. O’Sullivan, Paul V. Plourde, Gary Riordan, Hillary S. Sloane, Debu Tripathy, Dominic Carroll, David J Portman. Estrogen receptor 1 (ESR1) mutations in circulating tumor DNA (ctDNA) from patients with ER+/HER2- metastatic breast cancer (mBC) treated with lasofoxifene plus abemaciclib in the ELAINE 2 study [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 P5-05-02.

Abstract P4-01-19: Real World Statistics on CDK4/6 Inhibitor Use in Metastatic Hormone Receptor Positive and HER2-Negative Breast Cancer with a Focus on Age

Abstract Background: Combining CDK 4/6 inhibitors with hormone directed therapy using aromatase inhibitors (AI) or selective estrogen receptor down-regulators (SERD) is considered first line therapy for treatment of hormone receptor positive (HR+) human epidermal receptor 2 negative (HER2-) metastatic breast cancer (mBC). Here we aim to evaluate first line CDK 4/6 inhibitor practice patterns both nationally and locally (Wisconsin) in HR+ HER2- mBC patients and evaluate age as a potential factor impacting practice patterns. Methods: A retrospective analysis was performed utilizing IQVIA Anonymized Patient Longitudinal Data. The data base captures 60-85% of cancer patients across all 50 states in the United States and relies on diagnostic coding used in service or treatment claim filing. Patients queried were greater than 18 years old and were identified by a code indicating CDK4/6 inhibitor therapy or a combination of codes indicating mBC, HR+ status and HER- status. First line treatment was categorized as CDK4/6 inhibitor combination therapy, AI monotherapy, SERD, selective estrogen receptor modulator (SERM) monotherapy, chemotherapy, or other. Patients were further filtered by year, location, practice type (academic vs community), and age. Results: A total of 313,978 patients with mBC receiving first-line therapy were identified nationally between 2015 and 2021. The proportion of patients on first line CDK4/6 inhibitor combination therapy nationally significantly improved from 20% in 2015 (N=27,063) when palbociclib was first approved, to 53% in 2021 (N=54,023) (Table 1.) In 2021, first line use of CDK 4/6 inhibitor combination therapy was higher at 59% both in WI and our institution Froedtert Hospital and the Medical College of Wisconsin Cancer Center (Table 2.) At our center in 2021, the use of first line CDK4/6 inhibitor combination therapy in patients greater than age 65 was significantly lower at 35% (N=39) compared to patients less than 65 at 75% (N=64.) Further delineating the age groups at our center (Table 3,) use of CDK4/6 inhibitor combination therapy in patients aged 70-79 was 35% (N=20) and fell to 20% in patients greater than 80 in age (N=8.) Conclusion: Despite significant improvement in long-term outcomes with first line CDK4/6 inhibitor combination therapy in HR+ HER2- mBC patients, overall national usage is suboptimal. At our center, this was particularly apparent in patients older than 70. Identifying barriers to optimal use is critical to improve utilization of this important and effective class of drugs. National Percentages of HR+ HER2- mBC Patients Stratified by First Line Treatment Between 2015-2021 HR+ HER2- mBC patients Nationally treated with first line therapy (N=313,978.) National and Local Percentages of HR+ HER2- mBC Patients Stratified by First Line Treatment in 2021 HR+ HER2- mBC patients treated with first line therapy in 2021 Nationally (N=54,026,) in Wisconsin (N=694,) and at Froedtert Hospital and the Medical College of Wisconsin Cancer Center (N=103.) Local Percentages HR+ HER2+ mBC Patients Stratified by First Line Treatment and Age Group HR+ HER2- mBC patients treated with first line therapy in 2021 at Froedtert Hospital and the Medical College of Wisconsin Cancer Center (N=103.) Citation Format: Kathleen Monahan, Sailaja Kamaraju, Yee Chung Cheng, Janet Retseck, Deepika Sriram, John Burfeind, Christopher Chitambar, Lubna N. Chaudhary. Real World Statistics on CDK4/6 Inhibitor Use in Metastatic Hormone Receptor Positive and HER2-Negative Breast Cancer with a Focus on Age [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 P4-01-19.