Oral Selective Estrogen Receptor Degrader Research Articles

A review of endocrine therapy for hormone-dependent breast cancer

Purpose of the study: to provide current data on pharmacotherapy of hormone-dependent breast cancer (hdBC) and to consider the feasibility of introducing new hormone therapy drugs for breast cancer into clinical practice. Material and Methods. We analyzed 80 publications available Pubmed, Springer, Cochrane Library, etc. concerning the study of pharmacological characteristics of various groups of drugs for the treatment of hdBC, of which 49 were included in this review. Results. Currently, there are several approaches to the treatment of hdBC. Selective estrogen receptor modulators and aromatase inhibitors are the most studied and frequently used drugs. The cyclin-dependent kinase 4/6 inhibitors can be present in both the first- and second-line therapy. Currently, close attention is paid to the development of new drugs based on genomic profiling of the tumor, which is the standard of treatment for hdBC, and contributes to the personalization of therapy. Conclusion. Further development of drugs holds great promise for increasing overall survival and more accurate prognosis, response to conventional systemic therapy, and individualization of pharmacotherapy for hdBC. However, further research and development of new drugs is required. In this regard, the introduction of oral selective estrogen receptor degraders into practice and the development of new drugs that block estrogen-dependent and independent signaling to estrogen receptors are the most promising trends.

Imlunestrant, an oral selective estrogen receptor degrader (SERD), in combination with human epidermal growth factor receptor 2 (HER2) directed therapy, with or without abemaciclib, in estrogen receptor (ER) positive, HER2 positive advanced breast cancer (aBC): EMBER phase 1a/1b study.

1027 Background: Imlunestrant is a next-generation, oral SERD designed to deliver continuous ER-target inhibition. In the first-in-human Phase 1a/b EMBER study (NCT04188548), imlunestrant demonstrated favourable safety, PK, and clinical benefit when administered as monotherapy or with targeted therapy in ER+, HER2- aBC. Here we present clinical data of imlunestrant with HER2 targeted therapy, in patients (pts) with ER+, HER2+ aBC enrolled in EMBER. Methods: Pts were randomized to imlunestrant (400mg po daily) + trastuzumab (H), with or without (±) abemaciclib (Part C) or received maintenance treatment with imlunestrant + H + pertuzumab (P) (Part E), at standard doses in 21-day cycles. Key eligibility (Part C): ≥ 2 prior HER2 directed regimens, no prior CDK4/6i or fulvestrant; (Part E): received 1st line induction taxane chemotherapy (any duration) + H + P, without disease progression and ≤1 prior therapy for aBC. Key endpoints included safety, PK, ORR, CBR and PFS. Results: Overall, 45 pts with ER+/HER2+ aBC received the following combinations: imlunestrant + H (n=18), imlunestrant + H + abemaciclib (n=21; 1 pt received 800 mg imlunestrant), and imlunestrant + H + P (n=6). Baseline characteristics, safety, and preliminary efficacy are presented below (Table). Most TEAEs were grade 1-2. Common TEAEs with imlunestrant + H were fatigue (33%) and nausea (28%); with imlunestrant + H + abemaciclib were diarrhea (100%), neutropenia (57%), fatigue (52%) and nausea (48%); and with imlunestrant + H + P were diarrhea (50%) and arthralgia (50%). Imlunestrant PK was consistent with those previously reported with no drug-drug interactions observed with the combinations. From Part C, sequencing of baseline plasma ctDNA samples (n=37) identified prevalent mutations in TP53 (49%), PIK3CA (30%), ESR1 (24%), and GATA3 (14%). Clinical activity of imlunestrant in this ER+/HER2+ population, while based on small numbers, is promising. Conclusions: Imlunestrant in combination with trastuzumab ± abemaciclib or pertuzumab was well tolerated, showed no drug-drug interactions, and demonstrated preliminary antitumor activity in pts with ER+/HER2+ aBC. Clinical trial information: NCT04188548 . [Table: see text]

Artificial intelligence (AI) –powered H&E whole-slide image (WSI) analysis to predict recurrence in hormone receptor positive (HR+) early breast cancer (EBC).

571 Background: The recurrence risk (RS) based on transcriptomic profiles or Ki-67 level of HR+ EBC has been used for adjuvant treatment decisions and may also aid selecting patients who could benefit from novel treatments including CDK4/6 inhibitor or oral selective estrogen receptor degraders (SERDs). Artificial intelligence-powered H&E whole slide image (WSI) models can be a practical approach for this biomarker without the need of additional tissue samples. We developed ML-based models of histopathologic features from an H&E WSI to approximate RS of 21-gene assay (OncotypeDx) using various algorithms of different complexities. Methods: The development dataset was composed of 1,009 cases from TCGA-BRCA with gene expression profiles and 483 EBC cases from Samsung Medical Center with RS results. The prediction model was developed in two-stages. First, Lunit SCOPE, an AI-powered H&E WSI analyzer, detects various cell types and segments tissue areas resulting in various cell densities in tissue regions of interest. Next, four traditional ML models were applied on top of the first stage results. In addition, a deep learning model based on multiple instance learning (DL-MIL) was developed using three features: a supervised convolutional neural network, a supervised cell detection model, and an AI-based pathology profiling analyzer, for extracting semantic contents. The model performance was validated using two independent EBC test sets: 248 cases for comparison with the RS results (set 1), and 708 cases with DFS (set 2). Results: The cross-validation performance of the four traditional ML algorithms had area under the curve of receiver operating characteristics curve (AUROC) ranging from 0.728 to 0.779, where mitotic cell density, tumor cell density, and fibroblast density in cancer area were the variables with the highest importance for all four models. The DL-MIL model had cross validation performance of AUROC 0.831. In test set 1, DL-MIL model showed the highest discrimination with AUROC of 0.828 compared to traditional ML models where logistic regression showed the highest discrimination with AUROC of 0.786 (Table). The DL-MIL model showed the highest performance among the models to predict DFS with hazard ratio (HR) of 2.48 (1.47-4.18). Conclusions: Histopathomic models can accurately predict the RS of high risk as well as poor DFS from only H&E WSIs. These AI models can be a practical tool for treatment selection including emerging drugs such as SERDs. [Table: see text]

Next generation selective estrogen receptor degraders in postmenopausal women with advanced-stage hormone receptors-positive, HER2-negative breast cancer.

Breast cancer is the most prevalent malignancy in women, and is characterized by its heterogeneity; exhibiting various subgroups identifiable through molecular biomarkers that also serve as predictive indicators. More than two thirds of breast tumors are classified as luminal with positive hormone receptors (HR), indicating that cancer cells proliferation is promoted by hormones. Endocrine therapies play a vital role in the effective treatment of breast cancer by manipulating the signaling of estrogen receptors (ER), leading to a reduction in cell proliferation and growth rate. Selective estrogen receptor modulators (SERMs), such as tamoxifen and toremifene, function by blocking estrogen's effects. Aromatase inhibitors (AI), including anastrozole, letrozole and exemestane, suppress estrogen production. On the other hand, selective estrogen receptor degraders (SERDs), like fulvestrant, act by blocking and damaging estrogen receptors. Tamoxifen and AI are widely used both in early- and advanced-stage disease, while fulvestrant is used as a single agent or in combination with other agents like the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors (palbociclib, abemaciclib, ribociclib) or alpelisib for advanced-stage disease. Currently, SERDs are recognized as an effective therapeutic approach for the treatment of ER-positive breast cancer, showing proficiency in reducing and blocking ER signaling. This review aims to outline the ongoing development of novel oral SERDs from a practical therapeutic perspective, enhancing our understanding of the mechanisms of action underlying these compounds.

Abstract PS15-01: A first-in-human phase 1 study of SIM0270, a brain-penetrant oral selective estrogen receptor degrader (SERD), in patients with ER+/HER2- locally advanced or metastatic breast cancer

Abstract Background: ER+/HER2- breast cancer is the most prevalent subtype, and endocrine-based therapy is the standard treatment. SIM0270 is a brain-penetrant and highly potent oral SERD that has demonstrated ER degradation and robust antitumor activity across various preclinical models, including those with intracranial xenograft tumors. Methods: This Phase 1 study evaluates the safety, pharmacokinetics and preliminary anti-tumor activity of SIM0270 as monotherapy and/or in combination with palbociclib or everolimus in patients with HR+/HER2- advanced or metastatic breast cancer. The primary objective of the phase 1a study, which consisted of the dose escalation stage and the dose expansion stage, is to determine the maximum tolerated dose (MTD) and/or recommended phase 2 dose (RP2D) of SIM0270 as monotherapy. A Bayesian Optimal Interval design (BOIN) was adopted. SIM0270 was administered orally daily in 28-day cycles. Here we report preliminary monotherapy data. Results: As of May 22nd 2023, 45 female patients were enrolled, receiving SIM0270 at dose levels of 10 mg (n=4), 30 mg (n=4), 60 mg (n=9), 90 mg (n=6), 120 mg (n=11), 200 mg (n=5), or 300 mg (n=6). The median age was 56 years (range, 40-73) and ECOG performance status was 0 (18%) or 1 (82%). Most patients were heavily pretreated, with a median of 2 prior endocrine-based therapies (range, 0-5) and a median of 2 prior chemotherapies (range, 0-8). Thirty-one patients (69%) received prior fulvestrant, 42 (93%) received prior aromatase inhibitors, and 28 (62%) received prior CDK4/6 inhibitors. Visceral metastases were present in 33 patients (72%), and brain metastases in 7 patients (16%). The most common TEAEs were sinus bradycardia (51%), anemia (38%), hypalbuminaemia (31%), hypercholesterolaemia (24%), urinary tract infection (24%), asthenia (24%), electrocardiogram (ECG) QT prolonged (24%), and dizziness (22%). Most of the TEAEs were Grade 1-2. Grade 3 TRAEs were ECG QT prolonged, gamma-glutamyltransferase(γ-GT) increased and dizziness in 2 patients each. All AEs were manageable with dose interruption or reduction, except only 1 patient discontinued treatment of SIM0270 due to TRAE (Grade 3 γ-GT increased). Four patients experienced dose-limiting toxicities (DLTs): 1 with Grade 3 ECG QT prolonged at 200mg dose level, 1 with Grade 3 ECG QT prolonged and 2 with Grade 3 dizziness at 300mg dose level. All DLTs were resolved with dose interruption then reduction. The MTD of single-agent SIM0270 was established as 200mg QD. Pharmacokinetic analysis revealed a T1/2 of approximately 70 hours, and the AUC and Cmax increased in an approximately linear manner with dose escalation. Concentration in cerebrospinal fluid was measured in 1 patient after 41 days treatment of SIM0270, which is consistent with preclinical data and suggested high concentration of SIM0270 in the brain of patients. Among 31 response evaluable patients (per RECIST 1.1 criteria), 4 PRs (3 unconfirmed) were observed, yielding an ORR of 12.9%. Brain lesions were all stable in 4 evaluable patients per RANO-BM criteria. In patients with ESR1 mutations at baseline and samples available for analysis, 4/6(67%) exhibited a reduction or loss of mutant ESR1 upon SIM0270 treatment. Conclusions: Single-agent SIM0270 was well tolerated and showed favorable antitumor activity in heavily pretreated advanced or metastatic ER+/HER2- breast cancer patients, including those previously treated with CDK4/6 inhibitors and fulvestrant. Phase 1a dose expansion with single-agent SIM0270 is ongoing, and the RP2D will be determined based on further accumulative data of tolerability and efficacy. (NCT05293964) Citation Format: Jiong Wu, Jian Zhang, Qingyuan Zhang, Yuping Sun, Hongtao Li, Yongmei Yin, Yehui Shi, Wenfeng Li, Yunjiang Liu, Min Yan, Chen Yang, Lili Zhu, Yang Yang, Liting Xue. A first-in-human phase 1 study of SIM0270, a brain-penetrant oral selective estrogen receptor degrader (SERD), in patients with ER+/HER2- locally advanced or metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS15-01.

Abstract PO3-19-09: SERENA-1: Results from a Phase 1 study (Parts I/J) testing the next-generation oral selective estrogen receptor degrader (SERD) camizestrant (AZD9833) in combination with capivasertib in women with ER+, HER2 advanced breast cancer

Abstract Background: SERENA-1 (NCT03616587) is a Phase 1, multi-part, open-label study of camizestrant in women with ER+/HER2− advanced breast cancer. Data for camizestrant as monotherapy and in combination with palbociclib or abemaciclib have been presented previously.1–3 Here we present data from parts I and J (dose ranging and expansion, respectively), which examined camizestrant in combination with the pan-AKT inhibitor capivasertib. Methods: The primary objective was to determine the safety and tolerability of camizestrant 75 mg once daily (QD) in combination with capivasertib 400 mg twice daily (BID; intermittent: 4 days on, 3 days off). Secondary objectives included investigation of anti-tumor response and pharmacokinetics (PK). Participants were women of any menopausal status (pre-menopausal women received concomitant ovarian function suppression). Prior treatment with an endocrine therapy (ET) in the advanced setting was required with no limit on the number of lines of prior ET. Prior treatment with ≤2 lines of chemotherapy in the advanced setting was permitted. Prior treatment with CDK4/6 inhibitors (CDK4/6i) and/or fulvestrant was also permitted. Results: As of 21 March 2023, 29 patients had received camizestrant in combination with capivasertib. Patients were heavily pretreated in the advanced setting (48% prior chemotherapy, 90% prior CDK4/6i, 55% prior fulvestrant) and 72% had visceral metastases. The safety and tolerability profile of the combination of camizestrant and capivasertib was broadly consistent with that of each drug individually.4,5 In the 29 patients, adverse events considered related to camizestrant and/or capivasertib in >15% of patients (%; n grade 1/n Grade 2/n Grade >3) included diarrhea (66%; 14/2/3), visual effects (62%; all Grade 1), bradycardia (34%; all Grade 1), nausea (34%; 6/4/0), fatigue (21%; 4/2/0) maculopapular rash (17%; 0/3/2) and aspartate aminotransferase increased (17%; 3/0/2). The PK and safety data indicated no clinically relevant drug–drug interactions between camizestrant and capivasertib. The objective response rate was 29.6% (8/27), the clinical benefit rate at 24 weeks was 51.7% (15/29), and median progression–free survival was 8.3 months. Overall, 12 patients had a detectable ESR1 mutation (ESR1m) at baseline (Cycle 1, Day 1) and an evaluable Cycle 2, Day 1 (C2D1) result; of these, ESR1m was reduced by >50% at C2D1 in 91.7% of patients (11/12), including 66.7% (8/12) of patients where ESR1m was cleared. Conclusions: Camizestrant 75 mg QD in combination with capivasertib 400 mg BID was well tolerated and associated with encouraging clinical and pharmacodynamic activity. These data support further evaluation of this combination in women with ER+/HER2− advanced breast cancer.

Abstract PO1-05-08: Enhanced ER+ tumor growth inhibition of fulvestrant from effective oral delivery yielding elevated plasma concentrations

Abstract Fulvestrant has been a backbone of endocrine therapy (ET) for ER+/HER2- metastatic breast cancer (mBC) for over 20 years due to its nanomolar potency, purely antagonistic behavior, and well-tolerated adverse event (AE) profile. It remains the only selective estrogen receptor degrader (SERD) approved in ER+/HER2- mBC regardless of ESR1 mutation status. While ESR1 mutations reduce fulvestrant binding affinity, they do not implicitly lead to acquired resistance as with aromatase inhibitors (AIs) and can be overcome by increasing exposure.1 Further, preclinical evidence suggests that higher exposure to fulvestrant correlates with higher efficacy in rodents.2 Unfortunately, fulvestrant is currently delivered at its maximum feasible dose due to its poor solubility and a lack of effective drug delivery technologies. In response, several novel oral SERDs have been brought into clinical development to improve upon the efficacy of fulvestrant. However, multiple have failed to prove superior to fulvestrant in its intramuscular (IM) injection formulation and improvements from elacestrant (EMERALD) and camizestrant (SERENA-2) are largely attributed to patients with ESR1 mutations after prior ETs. Therefore, enabling higher exposure of fulvestrant could prolong progression-free survival and improve quality of life for ET-naïve mBC patients and those that have progressed on ETs. VeraMorph has leveraged a proprietary oral drug delivery technology to overcome the limitations of fulvestrant’s poor solubility and potentially help mitigate acquired resistance by improving exposure via daily oral dosing. The oral dosages, which consist of a novel polymer-based soft gummy, stabilize fulvestrant in simulated intestinal media up to 5 mg/mL without influencing fulvestrant permeability or causing cytotoxic effects. Pharmacokinetic (PK) studies of oral fulvestrant in rodents have demonstrated maximum plasma concentrations up to 450 ng/mL, a level roughly 20 times that of the average exposure from the monthly IM formulation in clinical settings. PK studies also demonstrated that oral exposure increased with a roughly dose-proportional relationship, with a reduction in AUC from day 1 to day 8 upon repeat dosing. In a cell-derived xenograft (CDX) study utilizing ER+ MCF-7 tumors implanted in athymic nude mice, once daily dosing via oral gavage of oral fulvestrant at 125 mg/kg was able to achieve a statistically significant 55% reduction in tumor growth by day 42 relative to a 50 mg/kg dose of the IM formulation delivered subcutaneously once per week, which was determined to create an equivalent exposure to the monthly IM human dose in separate studies. Oral fulvestrant yielded no observed adverse effects up to 125 mg/kg in both the 42-day CDX study in mice or an 8-day safety study in rats. Further, onset of action and acquired resistance are being tested by monitoring cell viability (via the MTT assay) with MCF-7 cells exposed to time profiles of fulvestrant exposure resembling oral and monthly intramuscular delivery. The study demonstrated that oral dosing reaches minimal cell viability 1-2 weeks faster than IM dosing. Also, preliminary data suggests that the onset of acquired resistance (as demonstrated by a resumption of cell growth) may occur faster from the steady exposure of IM dosing relative to the modulation of daily oral dosing. These results suggest that oral fulvestrant has the potential to finally realize the full potential of fulvestrant as an effective ET for metastatic breast cancer with a well-tolerated AE profile, enhanced efficacy, and reduced susceptibility to acquired resistance with applicability across all mBC patients. 1 Brett JO, Spring LM, Bardia A, Wander SA (2021) Breast Cancer Research, 23(1):1–15. 2 Wardell SE, et. al. (2020) Breast Cancer Research and Treatment, 179(1):67–77. Citation Format: Doug Godfrin, Matthew Butchbach. Enhanced ER+ tumor growth inhibition of fulvestrant from effective oral delivery yielding elevated plasma concentrations [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-05-08.

Abstract PO4-27-07: A Phase 3, randomized, open-label study of upfront camizestrant vs standard endocrine therapy as adjuvant treatment for ER-positive/HER2-negative early breast cancer with intermediate-high or high risk of recurrence (CAMBRIA-2)

Abstract Background: Camizestrant is a next-generation oral selective estrogen receptor (ER) degrader and pure ER antagonist being investigated in early and advanced breast cancer (BC). In postmenopausal women with advanced ER-positive/human epidermal growth factor receptor 2 (HER2)-negative BC and disease recurrence or progression on or after ≤1 endocrine therapy (ET) in the advanced setting, camizestrant significantly prolonged progression-free survival compared with fulvestrant in the Phase 2 SERENA-2 trial. Camizestrant may also provide benefit in the early BC setting. Following locoregional therapy (surgery ± radiotherapy), standard adjuvant treatment of ET with or without chemotherapy and/or a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) has shown significant benefit in decreasing the risk of recurrence of stage I–III ER-positive/HER2-negative BC. However, BC recurrence as incurable metastatic disease is still common. Thus, adjuvant therapeutic options with improved clinical outcomes are needed. The CAMBRIA-2 study (NCT05952557) aims to assess whether upfront camizestrant can improve outcomes compared with standard ET as adjuvant treatment in patients with ER-positive/HER2-negative early BC with an intermediate-high or high risk of recurrence after definitive locoregional therapy. Trial design: This ongoing Phase 3, randomized, open-label study is enrolling women (pre-, peri-, or postmenopausal) and men with ER-positive/HER2-negative (HER2-negative status defined as immunohistochemistry 0, or 1+, or in situ hybridization-negative) early BC who are at intermediate-high or high risk of recurrence (as defined in the protocol) after having completed definitive locoregional therapy (surgery ± radiotherapy) ± (neo)adjuvant systemic chemotherapy, and who have no evidence of disease. Patients may have received up to 12 weeks of (neo)adjuvant ET prior to randomization. Patients are randomized (1:1) to receive camizestrant 75 mg ± abemaciclib ± luteinizing hormone-releasing hormone (LHRH) agonist or standard ET of the investigator’s choice (any aromatase inhibitor or tamoxifen) ± abemaciclib ± LHRH agonist for up to 7 years. The primary endpoint is invasive BC-free survival (IBCFS) (Standardized Definitions for Efficacy End Points [STEEP] 2.0 criteria). Secondary endpoints include invasive disease-free survival (IDFS) and distant relapse-free survival (DRFS) (STEEP 2.0 criteria), overall survival, safety, and health-related quality of life. Primary endpoint analysis will use a stratified log-rank test adjusting for stratification factors, assuming a two-sided significance level of 5%. Approximately 5500 patients will be randomized. Clinical trial identification: NCT05952557 Editorial acknowledgment: Writing assistance was provided by Clare Davis of BOLDSCIENCE Inc., funded by AstraZeneca. Legal entity responsible for the study: AstraZeneca Funding: This study was supported by AstraZeneca. Citation Format: Sibylle Loibl, Yeon Park, Sara Tolaney, Ioanna Gioni, Simon Johnston, Teresa Klinowska, Ingrid A. Mayer, Raquel Nunes, Barbara Pistilli, Mary Stuart, Angela Quintana, Andrew Walding, Michael Gnant. A Phase 3, randomized, open-label study of upfront camizestrant vs standard endocrine therapy as adjuvant treatment for ER-positive/HER2-negative early breast cancer with intermediate-high or high risk of recurrence (CAMBRIA-2) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-27-07.

Abstract PS17-01: Key drivers of therapeutic response and resistance to giredestrant from GO39932: a Phase Ia/b study in patients with estrogen receptor-positive, HER2-negative, locally advanced or metastatic breast cancer

Abstract BACKGROUND Several investigational oral selective estrogen receptor antagonists and degraders (SERDs) are under investigation. These were partly developed to benefit patients (pts) with breast cancer (BC) who acquired resistance to standard-of-care (SoC) endocrine therapies (ET); e.g., through gain of ESR1 mutations that enable estrogen-independent ER activity. One such SERD, giredestrant (G), demonstrates activity in pts with ESR1-wild type (WT) or -mutant (m) tumors, and in pts who progressed on other ETs. Results from Phase I/III trials in ER-positive, HER2-negative, locally advanced/metastatic BC (ER+, HER2– LA/mBC) showed that next-generation SERDs had increased activity in ESR1m tumors vs SoC ETs. However, enthusiasm was hindered by heterogeneous responses, driven in part by study design differences. Across acelERA BC (NCT04576455), EMERALD (NCT03778931), SERENA-2 (NCT04214288), and AMEERA-3 (NCT04059484), 30–50% of pts progressed rapidly (< 2 months) when treated with SoC ETs or next-generation SERDs, while others benefited for ≥ 1 year. We present an exploratory biomarker analysis aimed at understanding the mechanistic basis for heterogeneous responses to SERDs in ER+, HER2– LA/mBC, by assessing a Phase Ia/b G cohort (NCT03332797) with paired baseline and on-treatment (tx) biopsies. METHODS Pts had ≤ 2 prior therapies for LA/mBC; disease recurrence/progression while on adjuvant ET for ≥ 24 months and/or ET for ER+, HER2– LA/mBC; and tumor response/stable disease for ≥ 6 months. Single-agent dose-escalation stage: 10, 30, 90, or 250 mg G once daily (QD) on Days (D) 1–28 of 28-D cycles (C). Dose-expansion stage: 30, 100, or 250 mg G QD. 100 mg G + 125 mg palbociclib on a 21-D on/7-D off schedule was also explored. Pre-/perimenopausal pts received LHRH agonists. Paired pre- and on-tx (C2D8) tumor biopsies (n = 29) were immunolabeled for ER, progesterone receptor, and Ki67, and assessed via bulk RNA-sequencing. Pre-tx liquid biopsies (n = 85) were evaluated by FoundationOne Liquid CDx assay. To validate our clinical observations, we generated a SERD-resistant MCF7 cell line and tested its sensitivity to other therapies. RESULTS We compared fast-progressing pts (FP; progression-free survival [PFS] < 2 months) with those experiencing long-term benefit (LTB; PFS > 12 months). LTB was significantly associated with high tumor baseline ER pathway activity. Although the benefit of G was of larger magnitude among pts with ESR1m tumors compared with SoC ET in acelERA BC, here pts with predominantly ESR1WT tumors received LTB on G tx. At the molecular level, G acted on LTB tumors by suppressing cell cycle- and ER-associated genes. In contrast, G had no effect on the transcriptome of FP tumors, which had lower baseline ER activity than LTB tumors. FP tumors were instead enriched for multiple cancer-associated pathways, e.g., RAS/MAPK and PI3K, which may drive resistance to G and thus enable fast progression. By liquid biopsy, most FP pts (> 90%) did not harbor a mutation in the associated pathway (e.g., NF1, KRAS), and thus could not be identified by genomic profiling alone. FP tumors were instead enriched for multiple cancer-associated pathways by gene expression. We explored these mechanisms further in the SERD-resistant cell line, which was similarly enriched for EGFR and MAPK activation vs standard MCF7 cells. Although cells became resistant to ER-targeted drugs, they acquired sensitivity to EGFR/MAPK inhibitors e.g., gefitinib, cobimetinib. CONCLUSIONS We identified molecular features associated with LTB to G and revealed a set of oncogenic pathways associated with FP pts on tx; demonstrating that these pathways represent acquired dependencies and potential therapeutic targets for SERD-resistant tumors and FP pts. Citation Format: Jackson Liang, Christy Ong, Jennifer Giltnane, Junko Aimi, Ching-Wei Chang, Mary Gates, Jennifer Eng-Wong, Pablo Perez-Moreno, Komal Jhaveri, Elgene Lim, Nicholas Turner, Heather Moore. Key drivers of therapeutic response and resistance to giredestrant from GO39932: a Phase Ia/b study in patients with estrogen receptor-positive, HER2-negative, locally advanced or metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS17-01.

Abstract RF01-01: SERENA-3: A randomized pre-surgical window of opportunity study assessing dose and duration of camizestrant treatment in post-menopausal women with ER-positive, HER2-negative primary breast cancer

Abstract Background Camizestrant, a next-generation oral selective estrogen receptor (ER) degrader (SERD) and pure ER antagonist, has demonstrated statistically significant and clinically meaningful progression-free survival (PFS) benefit over fulvestrant at both 75 and 150 mg once-daily doses in the Phase 2 SERENA-2 (NCT04214288) study in post-menopausal women with ER+ HER2- advanced breast cancer. SERENA-3 (NCT04588298) was established to explore the biological effects of three camizestrant dose levels in post-menopausal women with ER+ HER2- primary breast cancer. Methods Post-menopausal women with ER+ HER2- primary breast cancer scheduled to undergo curative intent surgery were randomly assigned in Stages 1 and 2 to pre-surgical treatment with camizestrant at 75, 150 or 300 mg once daily for 5 to 7 days or, in a sequential Stage 3, randomly assigned to camizestrant at 75 or 150 mg once daily for 12 to 15 days. Image-guided tumor biopsies were accessed as pre-treatment samples, while on-treatment ultrasound-guided tumor biopsies were obtained on the last day of camizestrant treatment. The primary objective was to explore the effect of camizestrant on ER expression by comparison of pre-treatment and on-treatment biopsies as assessed by immunohistochemistry (IHC) H-score. Secondary objectives included assessment of Ki67 and progesterone receptor (PgR) expression, and also of safety and tolerability, with exploratory pharmacodynamic (PD) assessment using other transcriptomic and proteomic technologies. The study was undertaken in 17 centers across 3 countries (Georgia, Mexico and the United Kingdom). Results 135 patients were randomized and received treatment. 76 patients received camizestrant for 5 to 7 days at doses of 75 mg (n=30), 150 mg (n=33), and 300 mg (n=13); 59 received camizestrant for 12 to 15 days at doses of 75 mg (n=30) and 150 mg (n=29). Baseline disease characteristics, and ER, Ki67 and PgR levels were well balanced across arms and stages. Pharmacokinetic (PK) observations in all stages were commensurate with steady-state predictions derived from previous studies. PD data for ER and Ki67 are shown in Table 1. Camizestrant reduced ER H-score by approximately 65% irrespective of dose or duration. While the 75 mg dose reduced Ki67 score to a lesser degree than 150 mg and 300mg after 5–7 days exposure, after 12–15 days exposure 75 and 150 mg reduced Ki76 score to a similar substantial degree (82% reduction). PD results assessed by additional transcriptomics and proteomics assays were concordant with the IHC analyses. No serious adverse events were reported. All treatment-emergent adverse events were grade 1, apart from one patient with a grade 2 visual impairment, and one patient with grade 3 diarrhea. Conclusions SERENA-3 is the first study investigating the effect of multiple dose levels of camizestrant with different treatment durations on ER, Ki67 and PgR expression in post-menopausal women with primary ER+ HER2˗ breast cancer. SERENA-3 demonstrated that the 75 mg dose of camizestrant achieves maximal levels of ER degradation and Ki67 suppression. Together with SERENA-2, this provides strong evidence supporting 75 mg as the optimal dose of camizestrant, including for the early disease setting, and highlights the additive value of a specifically designed multi-dose pre-surgical PD study for optimal dose selection. Table 1 Citation Format: John Robertson, Teimuraz Gogitidze, Zaza Katashvili, Juan Enrique Bargallo Rocha, Ekaterine Arkania, Iain Moppett, Kwok-Leung Cheung, Gia Nemsadze, Maxine Ajimi, Itziar Irurzun-Arana, Justin Lindemann, Teresa Klinowska, Alastair Mathewson, Christopher Morrow, Myria Nikolaou, Giorgi Dzagnidze. SERENA-3: A randomized pre-surgical window of opportunity study assessing dose and duration of camizestrant treatment in post-menopausal women with ER-positive, HER2-negative primary breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr RF01-01.

Abstract PO2-16-05: ESR1 mutations (ESR1mut) in HR(+)HER2(-)patients with metastatic breast cancer (MBC): prevalence along treatment course and predictive value for endocrine therapy (ET) resistance in real-world practice

Abstract Background: ET combined with CDK4/6 inhibitors (CDKi) is the standard of care for HR(+)HER2(-) patients with MBC. ESR1mut is an acquired resistance mechanism to ET, especially aromatase inhibitors (AI). The prevalence and behavior of ESR1mut in patients receiving the selective estrogen receptor degrader (SERD) fulvestrant remains in question. This study aimed to characterize the prevalence of ESR1mut at start of successive lines of therapy and to evaluate clinical outcomes of ET by ESR1 status in 1st line therapy in real-world practice. Methods: This study included patients with HR(+)HER2(-) MBC who underwent genomic testing using Foundation Medicine tissue or liquid comprehensive genomic profiling (CGP) assays, with specimens collected within 60 days of therapy initiation. Patient clinical data was obtained by the nationwide (US-based) de-identified Flatiron Health and Foundation Medicine real-world clinicogenomic breast database (FH-FMI CGDB), originated from ~280 US cancer clinics (~800 sites of care) between 01/2011 and 04/2023. ESR1mut prevalence was calculated in tissue and liquid samples collected in the 1st, 2nd, and 3rd lines of therapy. Real-world progression free-survival (rwPFS), time to treatment discontinuation (rwTTD), and overall survival (rwOS) were compared between patients who received AI + CDKi and between patients receiving fulvestrant + CDKi 1st line therapy [ESR1mut vs. ESR1 wild-type (wt) by tissue CGP] by Cox models. Multivariable analyses adjusted for adjuvant therapy, age, ECOG, menopausal status, histology, stage at diagnosis, metastatic site, and specific ESR1mut (Y537S, D538G, E380W, or others) were performed. Results: The prevalence of ESR1mut in tissue samples collected in 1st, 2nd, and 3rd lines of therapy was 7.9% (n=159/2004), 26.4% (n=66/250), and 34.0% (n=68/200), respectively. The prevalence of ESR1mut in liquid samples collected in the 1st, 2nd, and 3rd lines of therapy was 11.6% (n=30/259) 34.4% (n=63/183), and 35.9% (n=42/117), respectively. Among patients receiving AI + CDKi 1st line therapy (n=485), those with ESR1mut vs. ESR1wt had less favorable rwPFS [median 6.1 vs. 20.8 months, hazard ratio (HR) 2.36, 95% CI 1.35-4.14, p=0.003]. rwTTD and rwOS assessments were consistent with the magnitude of effect of rwPFS. Among patients receiving fulvestrant + CDKi 1st line therapy (n=302), those with ESR1mut vs. ESR1wt had less favorable rwPFS (median 10.5 vs. 14.5 months, HR 1.5, 95% CI 1.05-2.15, p=0.025), but no difference was observed for rwTTD and rwOS (p >0.05). Notably, the multivariable analyses found the independent association of ESR1mut and less favorable rwPFS (HR 2.18, 95% CI 1.15-4.1, p=0.018), rwTTD (HR 3.34, 95% CI, 1.84-6.1, p< 0.001), and rwOS (HR 2.46, 95% CI 1.04-5.9, p=0.041) in patients receiving AI + CDKi. The multivariable analyses confirmed no association between ESR1mut and outcomes in patients receiving fulvestrant + CDKi (p >0.05). No substantial differential associations were observed between specific ESR1mut and clinical outcomes in the multivariable analyses. Conclusion: In our dataset, ESR1mut is prevalent in about 8-11% of baseline tissue and liquid samples of patients with HR(+)HER2(-) MBC. Baseline ESR1mut is associated with less favorable outcomes in patients receiving AI + CDKi 1st line therapy, but not in patients receiving fulvestrant + CDKi. Specific ESR1mut do not seem to be associated with different outcomes to ET. ESR1mut prevalence increased during treatment course. Further investigation is warranted to determine optimal endocrine partner with CDKi based upon ESR1 status, particularly with development of oral SERDs. Citation Format: Manali Bhave, Julia Quintanilha, Ryon Graf, Takara Scott, Hanna Tukachinsky, Gerald Li, Alexa Schrock, Geoffrey Oxnard, Mia Levy, Kevin Kalinsky. ESR1 mutations (ESR1mut) in HR(+)HER2(-)patients with metastatic breast cancer (MBC): prevalence along treatment course and predictive value for endocrine therapy (ET) resistance in real-world practice [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-16-05.

Abstract PO3-04-13: SUMIT-ELA: Phase 1b/2 combination of cyclin-dependent kinase 7 inhibitor (CDK7i) samuraciclib and selective estrogen receptor degrader (SERD) elacestrant in advanced hormone receptor positive (HR+) breast cancer after CDK4/6i

Abstract Background: CDK7 has 3 critical roles in cancer: enhanced transcription of oncogenes and upregulation of anti-apoptotic genes, acceleration of the cell cycle through phosphorylation of other CDKs and driving estrogen receptor (ER) resistance to hormonal therapy. Inhibition of CDK7 is therefore a potential novel anti-cancer therapeutic strategy. Samuraciclib (CT7001), a once daily oral CDK7 inhibitor has demonstrated a favorable safety profile and clinical activity in combination with fulvestrant (SERD) in patients with HR+/HER2- advanced breast cancer who have previously been treated with a CDK4/6 inhibitor [1]. Extended benefit was observed in patients with no TP53-mutation per circulating tumor DNA (ctDNA) analysis detectable at baseline. Limitations in the pharmacokinetic (PK) properties and route of administration (intramuscular only) of fulvestrant mean that oral alternatives are desirable. Recently, the EMERALD phase 3 study of the oral SERD elacestrant demonstrated a significant PFS benefit over standard of care endocrine treatment in the ER+/HER2- advanced or metastatic breast cancer population previously treated with a CDK4/6 inhibitor. [2]. Non-clinical data indicate that the underlying biology driving samuraciclib combination with endocrine therapy translates from fulvestrant to elacestrant, clinical evaluation is warranted [3]. This open-label Phase 1b dose escalation and Phase 2 dose expansion study will evaluate the safety, efficacy and pharmacokinetics of samuraciclib in combination with elacestrant. ctDNA analysis is informative for both elacestrant and samuraciclib, via ESR1-mutation status and TP53-mutation status respectively, and is an integral part of the study design. Trial Design: Eligible patients will provide written informed consent, be aged 18 years or older, have histologically or cytologically confirmed ER+/HER2- advanced or metastatic breast cancer not amenable to resection or radiotherapy of curative intent, have received an aromatase inhibitor in combination with a CDK4/6 inhibitor in either the adjuvant or advanced setting, be receiving a luteinizing hormone-releasing hormone agonist if pre/perimenopausal and have RECIST v1.1 evaluable disease. Prior SERD, mammalian target of rapamycin inhibitor (mTORi) or chemotherapy for advanced breast cancer are not permitted. The study will enrol ~48 patients. All patients will undergo baseline Guardant360 ctDNA analysis to establish their ESR-1 and TP53-mutation status. Patients will undergo RECIST v1.1 evaluation at baseline every 8-weeks until week 48, followed by every 12-weeks thereafter. Initially both samuraciclib and elacestrant will be administered once daily in a dose escalation/de-escalation approach under supervision of the study Safety Review Committee. The primary endpoint in Phase 1b is the identification of tolerable combination dose levels for both samuraciclib and elacestrant and in Phase 2 expansion to quantify the progression free survival benefit of the combination. Secondary endpoints are tolerability, clinical benefit response at 24 weeks, overall response rate, duration of response, best percent change in tumor size, pharmacokinetics and correlations between ctDNA detectable ESR1 and TP53 mutations and efficacy/safety finding in this patient population. The study opened for recruitment in June 2023.

Abstract PO4-27-05: A phase I open-label dose escalation trial of FWD1802 as monotherapy and in combination with palbociclib in patients with ER+/HER2- unresectable locally advanced or metastatic breast cancer with or without ESR1 mutations

Abstract Background: Estrogen receptor (ER) signaling sits at center of the tumor biology in ER+/HER2- breast cancers. Current standard of care involves hormone therapy with agents including selective estrogen receptor modulators (SERMs), selective estrogen receptor degraders (SERDs), and aromatase inhibitors (AIs). Unfortunately, most patients eventually develop resistance to these therapies. Mutations in estrogen receptor 1 (ESR1) are partially responsible for the acquired resistance. Mutated estrogen receptors are still sensitive to fulvestrant but with a much less sensitivity. In January 2023, elacestrant received its first approval for the treatment of postmenopausal women or adult men with breast cancer harboring ESR1 mutations in US. However, there is still great unmet medical need for this population. FWD1802 is a third-generation oral SERD for ER-positive breast cancer patients. It binds competitively to ER, with a much higher affinity than that of fulvestrant. Preclinically, FWD1802 alone and in combination with palbociclib exhibits significant anti-tumor activity in the ER+/HER2- models and in ESR1-mutated models in vitro and in vivo. This ongoing phase I study is to assess the safety, tolerability, pharmacokinetics (PK) and clinical activity of FWD1802 as monotherapy and in combination with palbociclib in ER+/HER2- unresectable locally advanced or metastatic breast cancer with or without ESR1 mutations. Methods: This clinical study (CTR20232130) consists of three parts: monotherapy dose escalation (Part A), dose escalation in combination with palbociclib (Part B) and monotherapy dose expansion in ESR1-mutated patients (Part C). A conventional “3+3” design is used for this study. Approximately a total of 129 patients will be enrolled. FWD1802 is administered orally daily. In monotherapy dose escalation study (Part A), at each dose level, a single ascending dose (SAD) is followed by a 28-day continuous multiple ascending dose (MAD) period (defined as 1 cycle). In Part B dose, palbociclib (125 mg daily) is administered on a 28-day treatment cycle with 21-day on treatment and 7-day off treatment, along with the 28-day treatment cycle of FWD1802. The primary objectives include dose limiting toxicity (DLT), maximum tolerated dose (MTD), safety, tolerability, and the recommended phase 2 dose (RP2D) of FWD1802. Secondary objectives include pharmacokinetics characteristics of FWD1802 and its major metabolite as monotherapy and in combination with palbociclib. Preliminary anti-tumor activity including objective response rate (ORR), duration of response (DoR), disease control rate (DCR) and progression-free survival (PFS) will also be evaluated. DLT evaluation will be conducted after the first cycle of multi-dose period, and tumor assessment will be conducted every 8 weeks by investigators according to RECIST version 1.1. Patients with ER+/HER2- unresectable locally advanced or metastatic breast cancer with or without ESR1 mutation are eligible for the enrollment. Enrolled patients should have progressed disease, or failed from previous therapy, or intolerable or inaccessible to standard therapy. In addition, patients with ECOG score ≤1 and with adequate renal, cardiac, hepatic, and hematologic function are eligible for enrollment. The monotherapy dose escalation (Part A) is currently ongoing. The dose selection for Part B and Part C will be based on the data from the ongoing Part A. Clinical trial information: CTR20232130. Research Sponsor: Shenzhen Forward Pharmaceuticals Co., Ltd. (FORWARD PHARMA). Citation Format: Ling Zeng, Jin Liu, Dan Wu, Huiwen Ge, Zirui Bian, Chenggang Zhu. A phase I open-label dose escalation trial of FWD1802 as monotherapy and in combination with palbociclib in patients with ER+/HER2- unresectable locally advanced or metastatic breast cancer with or without ESR1 mutations [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-27-05.

The value of oral selective estrogen receptor degraders in patients with HR-positive, HER2-negative advanced breast cancer after progression on ≥ 1 line of endocrine therapy: systematic review and meta-analysis

BackgroundCurrently, the value of oral selective estrogen receptor degraders (SERDs) for hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer (aBC) after progression on ≥ 1 line of endocrine therapy (ET) remains controversial. We conducted a meta-analysis to evaluate progression-free survival (PFS) and safety benefits in several clinical trials.Materials and methodsCochrane Library, Embase, PubMed, and conference proceedings (SABCS, ASCO, ESMO, and ESMO Breast) were searched systematically and comprehensively. Random effects models or fixed effects models were used to assess pooled hazard ratios (HRs) and 95% confidence intervals (CIs) for treatment with oral SERDs versus standard of care.ResultsA total of four studies involving 1,290 patients were included in our analysis. The hazard ratio (HR) of PFS showed that the oral SERD regimen was better than standard of care in patients with HR+/HER2- aBC after progression on ≥ 1 line of ET (HR: 0.75, 95% CI: 0.62-0.91, p = 0.004). In patients with ESR1 mutations, the oral SERD regimen provided better PFS than standard of care (HR: 0.58, 95% CI: 0.47-0.71, p < 0.00001). Regarding patients with disease progression following previous use of CDK4/6 inhibitors, PFS benefit was observed in oral SERD-treatment arms compared to standard of care (HR: 0.75, 95% CI: 0.64-0.87, p = 0.0002).ConclusionsThe oral SERD regimen provides a significant PFS benefit compared to standard-of-care ET in patients with HR+/HER2- aBC after progression on ≥ 1 line of ET. In particular, we recommend oral SERDs as a preferred choice for those patients with ESR1m, and it could be a potential replacement for fulvestrant. The oral SERD regimen is also beneficial after progression on CDK4/6 inhibitors combined with endocrine therapy.

481P Second generation oral selective estrogen receptor degraders (SERDs) in breast cancer: A systematic review and meta-analysis of clinical trials

481P Second generation oral selective estrogen receptor degraders (SERDs) in breast cancer: A systematic review and meta-analysis of clinical trials

399P Results from a first-in-human phase Ia/b study of LX-039, an oral selective estrogen receptor (ER) degrader (SERD), in postmenopausal patients with ER+, HER2- advanced breast cancer (ABC)

399P Results from a first-in-human phase Ia/b study of LX-039, an oral selective estrogen receptor (ER) degrader (SERD), in postmenopausal patients with ER+, HER2- advanced breast cancer (ABC)

The Next-Generation Oral Selective Estrogen Receptor Degrader Camizestrant (AZD9833) Suppresses ER+ Breast Cancer Growth and Overcomes Endocrine and CDK4/6 Inhibitor Resistance.

Camizestrant, a next-generation oral SERD, shows promise in preclinical models of ER+ breast cancer alone and in combination with CDK4/6 and PI3K/AKT/mTOR inhibitors to address endocrine resistance, a current barrier to treatment.

Identification of an Imidazopyridine-based Compound as an Oral Selective Estrogen Receptor Degrader for Breast Cancer Therapy.

An imidazopyridine that selectively degrades ERαand is orally bioavailable has been identified for the development of ER+ breast cancer therapeutics. This compound also activates wild-type p53 and disrupts the gain-of-function tumorigenic activity of mutant p53, resulting in cell-cycle arrest and the induction of apoptosis.

Modeling the novel SERD elacestrant in cultured fulvestrant-refractory HR-positive breast circulating tumor cells

PurposeMetastatic hormone receptor-positive (HR+) breast cancer initially responds to serial courses of endocrine therapy, but ultimately becomes refractory. Elacestrant, a new generation FDA-approved oral selective estrogen receptor degrader (SERD) and antagonist, has demonstrated efficacy in a subset of women with advanced HR+breast cancer, but there are few patient-derived models to characterize its effect in advanced cancers with diverse treatment histories and acquired mutations.MethodsWe analyzed clinical outcomes with elacestrant, compared with endocrine therapy, among women who had previously been treated with a fulvestrant-containing regimen from the recent phase 3 EMERALD Study. We further modeled sensitivity to elacestrant, compared with the currently approved SERD, fulvestrant in patient-derived xenograft (PDX) models and cultured circulating tumor cells (CTCs).ResultsAnalysis of the subset of breast cancer patients enrolled in the EMERALD study who had previously received a fulvestrant-containing regimen indicates that they had better progression-free survival with elacestrant than with standard-of-care endocrine therapy, a finding that was independent estrogen receptor (ESR1) gene mutations. We modeled elacestrant responsiveness using patient-derived xenograft (PDX) models and in ex vivo cultured CTCs derived from patients with HR+breast cancer extensively treated with multiple endocrine therapies, including fulvestrant. Both CTCs and PDX models are refractory to fulvestrant but sensitive to elacestrant, independent of mutations in ESR1 and Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunit Alpha (PIK3CA) genes.ConclusionElacestrant retains efficacy in breast cancer cells that have acquired resistance to currently available ER targeting therapies. Elacestrant may be an option for patients with HR+/HER2- breast cancer whose disease progressed on fulvestrant in the metastatic setting.Translational Relevance.Serial endocrine therapy is the mainstay of management for metastatic HR+breast cancer, but acquisition of drug resistance highlights the need for better therapies. Elacestrant is a recently FDA-approved novel oral selective estrogen receptor degrader (SERD), with demonstrated efficacy in the EMERALD phase 3 clinical trial of refractory HR+breast cancer. Subgroup analysis of the EMERALD clinical trial identifies clinical benefit with elacestrant in patients who had received prior fulvestrant independent of the mutational status of the ESR1 gene, supporting its potential utility in treating refractory HR+breast cancer. Here, we use pre-clinical models, including ex vivo cultures of circulating tumor cells and patient-derived xenografts, to demonstrate the efficacy of elacestrant in breast cancer cells with acquired resistance to fulvestrant.

ESR1 mutational landscape and impact of co-existing resistance variants on clinical outcomes in patients with metastatic breast cancer.

1074 Background: Mutations in estrogen receptor 1 ( ESR1) confer resistance to aromatase inhibitors but may retain sensitivity to selective estrogen receptor degraders (SERD). Recently, elacestrant, an oral SERD, was approved for patients with HR+/HER2- ESR1mut metastatic breast cancer (MBC). In this study, we evaluated the genomic landscape of ESR1 alterations from a genomic database and also evaluated the impact of resistance-associated alterations on clinical outcomes. Methods: A large de-identified database of mutations detected in plasma cell-free DNA (cfDNA) from patients with HR+/HER2- MBC was assessed for ESR1 mutations and resistance-associated co-alterations. We used the Guardant 360 assay, a 70- to 74-gene targeted next generation sequencing panel and applied a breast cancer subtype classifier based on detected mutations (Bardia A, SABCS 2020). In addition, clinical outcomes were evaluated from a clinically annotated institutional dataset, linking genomic alterations with progression-free survival (PFS) and overall survival (OS) to single-agent SERD therapy for patients with MBC. Results: Among 11,456 patients with MBC and detectable cfDNA, 4,694 were classified as likely HR+/HER2-, of which 2,708 (58%) had ESR1 missense mutations, the majority of which occurred in the ligand binding domain (n = 2,690, 99%). Among the latter, alterations included 204 unique mutations, of which 55 were variants of uncertain significance (VUS). Multiple ESR1 mutations were detected in 979 (36%) patients. Among 567 patients classified as HR+/HER2- that underwent serial sampling, 136 (24%) had detectable ESR1 missense mutations that were previously undetectable. Within the ESR1mut population, 1,990 (74%) had co-existing non-synonymous, non-VUS alterations at genes associated with SERD resistance: PIK3CA (n = 1,330; 49%), ARID1A (n = 314; 12%), PTEN (n = 201; 7%), ERBB2 (n = 180; 7%), AKT1 (n = 139; 5%), and NF1 (n = 103; 4%); top amplifications were FGFR1 (n = 522; 19%), EGFR (n = 277; 10%), and MYC (n = 177; 7%). Among a clinically annotated HR+/HER2- subgroup (n = 350), 51 patients received SERD monotherapy, either fulvestrant (n = 9) or oral SERD on clinical trial (n = 42); among 28 patients with cfDNA variants associated with SERD resistance, median PFS and OS were lower compared to the 23 patients without resistance mutations (PFS: 2.4 vs 11.3 mo; HR = 0.44; 95% CI, 0.23-0.81; p = 0.008; and OS: 21.1 vs 31.4 mo; HR 0.47; 95% CI, 0.22-0.98; p = 0.045). Conclusions: ESR1 mutations that qualify patients to receive approved SERD therapy are detectable by plasma-based genotyping and can associate with various genomic co-alterations, including variants that may impact clinical outcomes. Further research is needed to confirm the impact of co-alterations in additional datasets and evaluate the role of SERD-based combination therapy to further improve clinical outcomes of patients with MBC.