Selective Estrogen Receptor Degrader Research Articles

A phase 1b/2 dose escalation and expansion study of OP-1250 in combination with ribociclib or alpelisib in patients with advanced and/or metastatic estrogen receptor–positive (ER+)/HER2-negative (HER2-) breast cancer.

TPS1127 Background: Therapies targeting the estrogen receptor (ER) are standard of care in the management of ER+/HER2- metastatic breast cancer (MBC) and when administered concurrently with cyclin-dependent kinase (CDK) inhibitors are the most effective first-line therapy. CDK 4/6 inhibitors plus endocrine therapy improve progression-free survival and, for some agents, overall survival for patients with MBC. However, most patients will acquire resistance, highlighting an unmet need for more effective endocrine options in ER+ disease. OP-1250 is a small molecule oral complete estrogen receptor antagonist (CERAN) and selective estrogen receptor degrader (SERD) that binds the ligand binding domain of ER and completely blocks ER-driven transcriptional activity. OP-1250 potently inactivates both wild-type and mutant forms of ER, the latter of which confers ligand independent activity as a mechanism of resistance to standard of care endocrine therapies. In preclinical studies, OP-1250 in combination with CDK4/6 inhibitors demonstrated synergistic activity in models of both wild-type ER and those with ESR1 (the gene that encodes ER) activating mutations, and in brain metastasis. OP-1250 is orally bioavailable with a favorable pharmacokinetic (PK) profile supportive of once-daily dosing, and a low probability of drug:drug interactions, making it an attractive agent for patients with ER+/HER2- MBC. Methods: The study (NCT05508906) is an open-label, multicenter, 2-part study of OP-1250 in combination with either the CDK4/6 inhibitor ribociclib or the PI3K inhibitor alpelisib for patients with MBC. Eligibility includes adults with evaluable (measurable or non-measurable) ER+/HER2- recurrent, locally advanced or MBC confirmed by histology/cytology, ≤2 prior hormonal regimens (prior CDK4/6 inhibitors allowed) and ≤ 1 prior line of chemotherapy. Patients in the alpelisib arm must have a PIK3CA mutation. A total of 30 patients will be enrolled in each arm. The dose escalation cohorts will evaluate the safety and PK of varied OP-1250 doses (30mg, 60mg, or 120mg) administered orally (PO) every day (QD) in combination with approved dosages of either ribociclib 600 mg PO QD or alpelisib 300 mg PO QD to identify the recommended phase 2 doses (RP2D). The dose expansion cohorts will assess additional safety and PK parameters and further explore the anti-tumor activity of OP-1250 in combination with ribociclib or alpelisib. Tolerability, safety and PK of OP-1250 in combination with ribociclib or alpelisib constitute the primary endpoints. Overall response rate, clinical benefit rate and duration of response are key secondary endpoints. Enrollment at approximately 20 sites in the United States and Australia is ongoing. Clinical trial information: NCT05508906 .

Comprehensive genomic profiling of Taiwanese patients with breast cancer using a novel targeted panel: Preliminary analyses from a prospective triple-negative cohort.

e12553 Background: Breast cancer is one of the leading causes of cancer-related deaths in women, and there is a demand in developing an Asian-based genetic profiling database to improve treatment outcomes. This study aims to determine the molecular alterations and identify potential therapeutic targets from a cohort of Taiwanese triple negative breast cancers using a novel next-generation sequencing (NGS) targeted panel. Methods: The VGH-TAYLOR study was designed to understand the genetic profiling of different subtypes of breast cancer in Taiwan. The study population comprised both early- (first-line surgery or neoadjuvant therapy) and late-stage (early/late relapse or de novo stage IV disease) of breast cancers, and we conducted molecular profiling with the illumina TruSight Oncology 500 assay using both DNA and RNA extracted from FFPE tissues from a subset of 101 prospectively enrolled breast cancers, which were previously assayed with a commercialized panel of 161 genes. Results: A total of 100 breast cancers from VGH-TAYLOR study were successfully assayed, with the majority (n = 97) being triple-negative, two HER2 over-expressed and the remaining one being HER2-low phenotypically. The most prevalent actionable alterations (more than 5% of study cohort) came from BRCA2 (24%), BRCA1 (12%), PALB2 (5%), PIK3CA (38%, both mutations and amplifications), ERBB2 (14%, both mutations and amplifications), ERBB3 (10%) and PTEN (12%). One-quarter of assayed samples were categorized into tumor mutation burden (TMB)-high cancers ( > 10 mutations per million bases), which came from 15 out of 74 (20%) early-stage and 10 out of 26 (38%) late-stage patients (chi-square test p-value: 0.06). In general, the novel panel with 500 targeted genes reported more actionable alterations than the former one with limited interrogated genes ( BRCA2: 7%, BRCA1: 12%, PALB2: 7%, PIK3CA: 45%). Conclusions: Our study ascertained the clinical applicability and feasibility of comprehensive genomic profiling from a cohort of Taiwanese breast cancers. Overall, the successful rate for NGS with QC metrics are around 90%. The novelty benefits from more investigated genes as well as multi-gene biomarkers such as TMB and microsatellite instability (MSI) status. Patients with a positive finding from the genomic assay may benefit from an immune checkpoint inhibitor (TMB-high and MSI), poly ADP- ribose polymerase (PARP) inhibitor (homologous recombination deficiency genes: BRCA1/2 and PALB2), selective estrogen receptor degrader ( ESR1), tyrosine kinase inhibitor ( ERBB2/3), phosphoinositide 3-kinase inhibitor ( PIK3CA) and so on. Further studies with more samples enrolled and longer follow up are warranted to elucidate the true value of comprehensive genomic profiling for breast cancer.

VERITAC-2: A global, randomized phase 3 study of ARV-471, a proteolysis targeting chimera (PROTAC) estrogen receptor (ER) degrader, vs fulvestrant in ER+/human epidermal growth factor receptor 2 (HER2)- advanced breast cancer.

TPS1122 Background: ARV-471 is an oral PROTAC ER degrader that binds to and degrades wild-type ER and clinically relevant mutants. ARV-471 directly recruits the ubiquitin-proteasome system to degrade ER, whereas selective ER degraders (SERDs) indirectly cause ER degradation. In a first-in-human phase 1/2 study, ARV-471 monotherapy was well tolerated and showed clinical activity in heavily pretreated patients with ER+/HER2- advanced breast cancer. The phase 3 monotherapy dose (200 mg once daily [QD]) was chosen due to comparable efficacy and favorable tolerability relative to 500 mg QD and robust ER degradation in paired tumor biopsies. The randomized phase 3 VERITAC-2 study (NCT05654623) will compare efficacy and safety of ARV-471 vs the SERD fulvestrant in patients with ER+/HER2- advanced breast cancer after prior combination cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor therapy and endocrine therapy (ET). Methods: Eligible patients (aged ≥18 years) have a confirmed diagnosis of ER+/HER2- locoregional recurrent or metastatic breast cancer not amenable to surgical resection or radiation; 1 prior line of combination CDK4/6 inhibitor therapy and ET; ≤1 additional line of ET; most recent ET given for ≥6 months before disease progression; and radiological disease progression during or after the last line of therapy. Prior chemotherapy in the locally advanced or metastatic setting and prior fulvestrant are not permitted. Patients (N~560) are randomized 1:1 to receive 200 mg ARV-471 orally QD continuously or fulvestrant intramuscularly on days 1 and 15 in the first 28-day cycle and on day 1 in subsequent cycles; patients are stratified by ESR1 mutation status and presence of visceral disease. The primary endpoint, progression-free survival, will be assessed by blinded independent central review in the intention-to-treat population and the ESR1 mutation sub-population. Secondary outcome measures include overall survival, antitumor activity (objective response rate, duration of response, and clinical benefit rate), safety, and quality of life assessments. Clinical trial information: NCT05654623 .

Tolerability and effectiveness of fulvestrant as first-line endocrine therapy in unselected patients with advanced breast cancer (ABC): Results of a multicenter, single arm, non-interventional study.

e13061 Background: Fulvestrant, a selective estrogen receptor (ER) down-regulator, is a standard of care for first-line treatment in hormone receptor-positive (HR+) advanced breast cancer. This study is to assess the safety profile and effectiveness of fulvestrant 500mg as the first-line endocrine therapy of post-menopausal women with HR+ locally advanced or metastatic breast cancer. Methods: The study is an open-label, multicentre, prospective, observational study in China. Chinese patients receiving fulvestrant 500mg as the first endocrine agent after relapse according to investigator’s clinical practice were enrolled. Primary variable is the frequency and severity of adverse events (AEs). Secondary variables were objective response rate (ORR), clinical benefit rate (CBR), progress free survival (PFS) and overall survival (OS). Predictive effects of endocrine resistance, chemotherapy for advanced disease and adjuvant endocrine therapy on the effectiveness were prespecified as exploratory variables. Results: From November 2017 to December 2020, 461 patients were enrolled by 29 Chinese centers. Median age was 56 years and the ECOG score was: 0, 165 (35.8%); 1, 281 (60.9%); 2, 12 (2.6%); and 3, 3 (0.7%). 20 (4.3%) patients had the Kaplan-Feinstein index ≥2. Majority (77.0%) of the patients were ER+ and PgR+, and 190 (41.2%) had visceral metastasis. In total, 91 (19.7%) patients were primary endocrine resistant, 202 (43.8%) were secondary endocrine resistant and 123 (26.7%) were endocrine-therapy naïve. 147 (31.9%) patients received previous chemotherapy for advanced disease, while 101 (21.9%) patients had anti-estrogen drugs as adjuvant therapy. 54 (11.7%) patients had concurrent CDK4/6 inhibitors. After a median follow-up of 26.8 months (IQR 18.7-38.2), median PFS was 13.0 months (95% CI 9.8-16.4) and median OS was 41.2 months (95% CI 37.3-NE). ORR was 11.3% (95% CI 8.2-15.1) and CBR was 60.6% (95%CI 55.2-65.8). Patients with endocrine-therapy naïve tumor, no previous chemotherapy for advanced disease and adjuvant anti-estrogen treatment had a longer PFS of first-line fulvestrant 500mg. 67 (14.5%) patients had at least one AE, with 1 (0.2%) patient withdrawing because of AEs. Drug-related serious AEs (SAEs) were rare, occurring in 2 (0.4%) patients. No patient died as a result of a drug-related AE. Neutropenia, increased ALT/AST, and anemia were the most commonly reported AEs in this study. Conclusions: This is the first prospective real-world study confirming that fulvestrant 500mg is a well-tolerated treatment option in unselected ABC in China. In the real clinical practice, fulvestrant 500mg can achieve the consistent anti-tumor activity as in clinical trials. Patients with endocrine-therapy naïve status and no prior chemotherapy tend to have a longer control time. Clinical trial information: NCT02909361 .

Clinical activity of camizestrant, a next-generation SERD, versus fulvestrant in patients with a detectable ESR1 mutation: Exploratory analysis of the SERENA-2 phase 2 trial.

1066 Background: Camizestrant, a next-generation oral selective estrogen receptor antagonist and degrader (ngSERD), was compared at two dose levels to fulvestrant 500 mg (F) in post-menopausal women with advanced ER+, HER2˗ breast cancer with disease recurrence or progression after ≤1 endocrine therapy in the advanced setting in the Phase 2 randomized SERENA-2 study (NCT04214288). Camizestrant demonstrated statistically significant and clinically meaningful benefit vs F in progression-free survival (PFS) in the overall study population (Oliveira M et al. SABCS 2022 Annual Meeting. Abstract GS3-02.). Methods: Baseline circulating tumour DNA was collected at screening and/or Cycle 1 Day 1 and analysed by next-generation sequencing. A post hoc exploratory analysis compared investigator assessed PFS with camizestrant (data from 75 and 150 mg combined) vs F in patients with detectable baseline ESR1 (the gene that encodes ERα) hotspot mutations ( ESR1m). Patients were divided into subgroups based on whether 1 or >1 ESR1m variants were detected, the specific ESR1m, and whether mutations were detected in BRCA1/2 or the MAPK pathway. The most prevalent mutations are presented. A Cox proportional hazards model was used to compare PFS. Results: 48/147 (32.7%) patients treated with camizestrant and 35/73 (47.9%) treated with F had a detectable ESR1m in at least 1 baseline sample. ESR1m were detected in 6/9 (67%) patients with a BRCA1/2 mutation and 5/26 (19%) with a MAPK pathway alteration; however, the numbers were too small to analyse efficacy in these subgroups. Conclusions: Camizestrant showed improved outcome vs F in patients with a detectable ESR1m at baseline and in the subgroups tested. The greatest median PFS improvement was seen in patients where a single ESR1m variant was detected, suggesting that early intervention upon detection of an ESR1m may provide the maximum patient benefit for camizestrant, a hypothesis that is being tested in the SERENA-6 clinical trial (NCT04964934). Clinical trial information: NCT04214288 . [Table: see text]

Interim analyses (IA) of the giredestrant (G), G + abemaciclib (A), and G + ribociclib (R) arms in MORPHEUS Breast Cancer (BC): A phase I/II study of G treatment (tx) combinations in patients (pts) with estrogen receptor-positive, HER2-negative locally advanced/metastatic BC (ER+, HER2– LA/mBC).

1061 Background: Limitations of current approved endocrine therapies (ETs), a mainstay tx for ER+ BC, include incomplete ER signaling inhibition. Novel ETs, such as selective estrogen receptor antagonists and degraders (SERDs), may help overcome this. G, a potent, nonsteroidal, oral (PO) SERD, is well tolerated and has shown robust ER occupancy and encouraging antitumor activity as monotherapy and in combination with the cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) palbociclib (P). MORPHEUS BC (NCT04802759) is evaluating the safety and efficacy of G tx combinations in ER+, HER2– LA/mBC. We present results from the 16-week IA of G and G + CDK4/6i (A or R). Methods: Eligible pts had disease progression on 1–2 lines of ET (including a CDK4/6i) for LA/mBC. Pts were randomized 1:6 (planned n = 15 per arm) to receive G (30 mg PO QD; control arm), G + A (150 mg PO BID), or sequentially, G + R (600 mg PO QD) until disease progression/unacceptable toxicity. The study was not designed to make explicit power and type I error considerations for a hypothesis test. Primary endpoints were safety and objective response rate; other endpoints included progression-free survival, overall survival, clinical benefit rate, disease control rate (DCR), duration of response, and pharmacokinetics. Genetic alterations were defined using baseline circulating tumor DNA. Results: As of Feb 4, 2022, 15 pts were enrolled in the G + A arm; 73% received one prior line of tx in the LA/mBC setting; 27% received prior fulvestrant; 73% had liver metastases at baseline (BL). As of Sept 22, 2022, 11 and 16 (of whom 14 were evaluable) pts were enrolled in the G and G + R arms, respectively; 64%/86% received one prior line of tx in the LA/mBC setting; 73%/36% received prior fulvestrant; 73%/57% had liver metastases at BL. Three pts had a partial response (PR; G + A, n = 1; G + R, n = 2); 19 had stable disease (G, n = 5; G + A, n = 7; G + R, n = 7). DCRs were 36% (G), 40% (G + A), and 50% (G + R). Safety is shown. Conclusions: G combined with a CDK4/6i (A or R) was well tolerated, with no unexpected safety signals. Three PRs were seen in this heavily pretreated population of pts with disease progression post-CDK4/6i tx. This study provides the first data supporting the combinability of G with the CDK4/6is A and R, in addition to P as seen in prior studies. G can therefore be combined with all three approved CDK4/6is. Clinical trial information: NCT04802759 . [Table: see text]

Inhibition of FASN as a potential treatment of advanced endocrine therapy-resistant breast cancer.

e13077 Background: The past decade has seen significant advancement in increasing survival estrogen receptor alpha (ERα) positive breast cancer. The use of selective estrogen receptor down-regulators and modulators (SERMs) and cyclin-dependent kinase inhibitors aided to extend overall survival of breast cancer patients. However, resistance to endocrine therapy is a common occurrence and patients will eventually succumb to their disease. Fatty acid synthase (FASN), a key enzyme in lipid biosynthesis, is overexpressed in more aggressive and therapy-resistant tumors, including breast cancers. FASN inhibitor, TVB-2640, has been evaluated in multiple tumor cell lines and in a phase 1 clinical study, and showed partial responses in 5 patients and multiple patients with prolonged stable disease (≥16 weeks). Methods: We generated therapy-resistant cells by long term exposure to tamoxifen (MCF7/TamR cells), and fulvestrant (MCF7/FR cells). Palbociclib-resistant (MCF7/RB1Crispr and ZR75/RB1Crispr) cells were generated through CRISPR/Cas9 knockout of the retinoblastoma (RB) gene. We assessed the impact of TVB-3166 inhibitor in these cells on proliferation, viability, cell cycle, apoptosis, ERα expression in patient derived explants, and tumor growth in xenografts. RNA sequencing of tamoxifen- and fulvestrant-resistant cells was performed to investigate gene expression. Subcellular localization of ERα was assessed using subcellular fractionations. Palmitoylation and ubiquitination of ERα were assessed by immunoprecipitation. ERα and p-eIF2α protein levels were analyzed by western blot after treatment with TVB with or without the addition of palmitate or BAPTA. Results: TVB treatment leads to a marked inhibition of proliferation in tamoxifen-, fulvestrant- and palbociclib-resistant cells compared to the parental cells. RNA sequencing of explants of patients with ERα positive disease showed down regulation of ESR1 related genes and genes involved in invasiveness. RNA sequencing of fulvestrant-resistant cells showed that treatment with TVB results in down regulation of EMT and E2F target genes, cholesterol homeostasis genes and mTORC1 signaling. Additionally, TVB significantly inhibited tumor growth in mice and decreased proliferation of primary tumor explants compared to untreated controls. FASN inhibition significantly reduced ERα levels most prominently in endocrine resistant cells and altered its subcellular localization. Furthermore, we showed that the reduction of ERα expression upon TVB treatment is mediated through the induction of endoplasmic reticulum stress. Conclusions: Our preclinical data provide evidence that FASN inhibition presents a promising therapeutic strategy for treatment of endocrine-resistant breast cancer. Further clinical development of FASN inhibitors for endocrine resistant breast cancer should be considered.

Update Breast Cancer 2023 Part 2 – Advanced-Stage Breast Cancer

AbstractIn recent years, a number of new therapies have led to advances in the treatment of patients with advanced breast carcinoma. These substances are mainly CDK4/6 inhibitors and other substances that can overcome endocrine resistance, oral selective estrogen receptor degraders, antibody drug conjugates (ADCs), and PARP inhibitors. This review summarizes and evaluates the latest study results that have been published in recent months. This includes the overall survival data of the Destiny-Breast03 study, the first analysis of the CAPItello-291 study, the comparison of CDK4/6 inhibitor treatment with chemotherapy in the first line of therapy (RIGHT Choice study), the first analysis of the Destiny-Breast02 study in the treatment setting after T-DM1 treatment, and the first analysis of the Serena-2 study.Most of these studies have the potential to significantly change the therapeutic landscape for patients with advanced breast carcinoma and show that the continued rapid development of new therapies is always producing new results.

Elacestrant: a new FDA-approved SERD for the treatment of breast cancer

Elacestrant (RAD-1901), a selective estrogen receptor degrader, was approved by USFDA on January 27, 2023, for the treatment of breast cancer. It has been developed by Menarini Group under the brand name Orserdu®. Elacestrant showed anticancer activity both in vitro and in vivo in ER+ HER2-positive breast cancer models. The present review delebrates the development stages of Elacestrant, with its medicinal chemistry, synthesis, mechanism of action, and pharmacokinetic studies. Clinical data and safety profile has also been discussed, including data from randomized trials.

Discovery of novel covalent selective estrogen receptor degraders against endocrine-resistant breast cancer

Endocrine-resistance remains a major challenge in estrogen receptor α positive (ERα+) breast cancer (BC) treatment and constitutively active somatic mutations in ERα are a common mechanism. There is an urgent need to develop novel drugs with new mode of mechanism to fight endocrine-resistance. Given aberrant ERα activity, we herein report the identification of novel covalent selective estrogen receptor degraders (cSERDs) possessing the advantages of both covalent and degradation strategies. A highly potent cSERD 29c was identified with superior anti-proliferative activity than fulvestrant against a panel of ERα+ breast cancer cell lines including mutant ERα. Crystal structure of ERα‒29c complex alongside intact mass spectrometry revealed that 29c disrupted ERα protein homeostasis through covalent targeting C530 and strong hydrophobic interaction collied on H11, thus enforcing a unique antagonist conformation and driving the ERα degradation. These significant effects of the cSERD on ERα homeostasis, unlike typical ERα degraders that occur directly via long side chains perturbing the morphology of H12, demonstrating a distinct mechanism of action (MoA). In vivo, 29c showed potent antitumor activity in MCF-7 tumor xenograft models and low toxicity. This proof-of-principle study verifies that novel cSERDs offering new opportunities for the development of innovative therapies for endocrine-resistant BC.

202P A phase Ib/II study of OP-1250, an oral complete estrogen receptor (ER) antagonist (CERAN) and selective ER degrader (SERD) with palbociclib in patients (pts) with advanced or metastatic ER-positive, HER2-negative breast cancer (MBC)

OP-1250 is small molecule CERAN/SERD that binds to and completely blocks transcriptional activity of wild-type and mutant ER. OP-1250 was well tolerated in a phase I/II monotherapy study (OP-1250-001), and the recommended phase II dose is 120 mg once a day (qd). OP-1250 with palbociclib showed synergistic activity in preclinical models. Here we report updates of pharmacokinetics (PK), drug-drug interactions (DDI), safety and efficacy from a study of OP-1250 with palbociclib (OP-1250-002). Pts with advanced or MBC with progression on or after ≤1 line of endocrine therapy (prior CDK4/6 inhibitors and chemotherapy were allowed) were enrolled into sequential cohorts to receive escalating doses of OP-1250 PO qd with palbociclib 125 mg PO qd for 21 of 28 days, using a 3+3 design, followed by dose expansion. As of January 23, 2023, 20 pts have been treated with palbociclib and OP-1250 doses of 30/60/90/120 mg (n=3/3/3/11). Fourteen received prior CDK4/6 inhibitor; 11 received prior palbociclib. No DLTs occurred. The most common (≥4 pts) treatment emergent adverse events (AEs) were neutropenia, nausea, vomiting, anemia, gastroesophageal reflux, constipation, and thrombocytopenia (all were Grade 1-2, except neutropenia). Grade 3 neutropenia occurred in 11 pts (55%). No Grade 4 AEs occurred. The exposure of OP-1250 (n=18) was consistent with the monotherapy study. Palbociclib exposure at steady state was comparable to published monotherapy data when combined with OP-1250 at all dose levels tested. Anti-tumor activity has been observed including partial responses. OP-1250 did not affect palbociclib PK and no DDIs have been observed with this combination. OP-1250 and palbociclib combination was well tolerated, safety was consistent with individual profiles of each drug as a monotherapy. Tumor responses were observed in this heavily pretreated population. Expanding on our previous report (SABCS 2022), these data provide rationale to continue exploring OP-1250 with the approved dose of palbociclib. Updated data will be presented. (NCT0526610).

Physiologically-based pharmacokinetic/pharmacodynamic modeling to predict tumor growth inhibition and the efficacious dose of selective estrogen receptor degraders in humans.

GDC-9545 (giredestrant) is a highly potent, nonsteroidal, oral selective estrogen receptor antagonist and degrader that is being developed as a best-in-class drug candidate for early-stage and advanced drug-resistant breast cancer. GDC-9545 was designed to improve the poor absorption and metabolism of its predecessor GDC-0927, for which development was halted due to a high pill burden. This study aimed to develop physiologically-based pharmacokinetic/pharmacodynamic (PBPK-PD) models to characterize the relationships between oral exposure of GDC-9545 and GDC-0927 and tumor regression in HCI-013 tumor-bearing mice, and to translate these PK-PD relationships to a projected human efficacious dose by integrating clinical PK data. PBPK and Simeoni tumor growth inhibition (TGI) models were developed using the animal and human Simcyp V20 Simulator (Certara) and adequately described each compound's systemic drug concentrations and antitumor activity in the dose-ranging xenograft experiments in mice. The established PK-PD relationship was translated to a human efficacious dose by substituting mouse PK for human PK. PBPK input values for human clearance were predicted using allometry and in vitro in vivo extrapolation approaches and human volume of distribution was predicted from simple allometry or tissue composition equations. The integrated human PBPK-PD model was used to simulate TGI at clinically relevant doses. Translating the murine PBPK-PD relationship to a human efficacious dose projected a much lower efficacious dose for GDC-9545 than GDC-0927. Additional sensitivity analysis of key parameters in the PK-PD model demonstrated that the lower efficacious dose of GDC-9545 is a result of improvements in clearance and absorption. The presented PBPK-PD methodology can be applied to support lead optimization and clinical development of many drug candidates in discovery or early development programs.

Abstract CT073: Efficacy of novel oral SERD elacestrant in fulvestrant-refractory hormone receptor-positive (HR+) breast cancer: a translational investigation

Abstract Purpose: Metastatic HR+ breast cancer initially responds to serial courses of endocrine therapy but ultimately becomes refractory Elacestrant, a new generation oral selective estrogen receptor degrader (SERD) and antagonist, has demonstrated efficacy in a subset of women with advanced HR+ breast cancer (Bidard FC, et al J Clin Oncol 2022;40:3246-3256), but there are few patient-derived models to characterize its effect in advanced cancers with diverse treatment histories and acquired mutations In this translational study, we tested the efficacy of elacestrant on ex vivo circulating tumor cells (CTCs) and patient-derived xenograft (PDX) mouse models with serial exposures to fulvestrant and conducted a reanalysis of the EMERALD trial to evaluate the efficacy of elacestrant vs standard endocrine therapy among patients who had received prior fulvestrant. Methods: Using PDX models and cultured CTCs isolated from metastatic breast cancer patients, we analyzed sensitivity to elacestrant, compared with the currently approved SERD, fulvestrant We further analyzed clinical responses to elacestrant, compared with endocrine therapy, among women who had previously been treated with a fulvestrant-containing regimen from the recent phase 3 EMERALD Study (NCT03778931), Results: In a patient-derived HR+ breast cancer PDX model, we demonstrate that elacestrant, at a clinically relevant dose, is highly effective in tumors extensively pretreated with fulvestrant In cultured CTCs isolated from HR+ breast cancer patients who had received multiple prior treatments, including fulvestrant, elacestrant is active, independent of mutations in the estrogen receptor (ESR1) or Phosphatidylinositol-4,5-Bisphosphate 3-Kinase Catalytic Subunit Alpha (PIK3CA) genes In these cells, elacestrant suppresses estrogen-receptor (ER) signaling at concentrations below those required for protein degradation Post-hoc analysis of the EMERALD clinical trial confirms that elacestrant is effective in patient populations previously treated with fulvestrant-containing regimens, irrespective of ESR1 mutation status. Conclusion: In the EMERALD study, elacestrant showed significantly prolonged PFS compared with SOC (including fulvestrant) Using a PDX model and CTCs isolated from patients with MBC from the EMERALD study, this translational investigation demonstrates that elacestrant 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 while receiving fulvestrant in the metastatic setting. Citation Format: Taronish D. Dubash, Aditya Bardia, Brian Chirn, Brittany A. Reeves, Joseph A. LiCausi, Risa Burr, Ben S. Wittner, Sumit Rai, Hitisha Patel, Teeru Bihani, Heike Arlt, Francois-Clement Bidard, Virginia G. Kaklamani, Philippe Aftimos, Javier Cortés, Simona Scartoni, Monica Binaschi, Nassir Habboubi, A. John Iafrate, Mehmet Toner, Daniel A. Haber, Shyamala Maheswaran. Efficacy of novel oral SERD elacestrant in fulvestrant-refractory hormone receptor-positive (HR+) breast cancer: a translational investigation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT073.

A Basic Review on Estrogen Receptor Signaling Pathways in Breast Cancer.

Breast cancer is the most common cancer and the deadliest among women worldwide. Estrogen signaling is closely associated with hormone-dependent breast cancer (estrogen and progesterone receptor positive), which accounts for two-thirds of tumors. Hormone therapy using antiestrogens is the gold standard, but resistance to these treatments invariably occurs through various biological mechanisms, such as changes in estrogen receptor activity, mutations in the ESR1 gene, aberrant activation of the PI3K pathway or cell cycle dysregulations. All these factors have led to the development of new therapies, such as selective estrogen receptor degraders (SERDs), or combination therapies with cyclin-dependent kinases (CDK) 4/6 or PI3K inhibitors. Therefore, understanding the estrogen pathway is essential for the treatment and new drug development of hormone-dependent cancers. This mini-review summarizes current literature on the signalization, mechanisms of action and clinical implications of estrogen receptors in breast cancer.

Structure-guided identification of novel dual-targeting estrogen receptor α degraders with aromatase inhibitory activity for the treatment of endocrine-resistant breast cancer

Drug resistance is a major challenge in conventional endocrine therapy for estrogen receptor (ER) positive breast cancer (BC). BC is a multifactorial disease, in which simultaneous aromatase (ARO) inhibition and ERα degradation may effectively inhibit the signal transduction of both proteins, thus potentially overcoming drug resistance caused by overexpression or mutation of target proteins. In this study, guided by the X-ray structure of a hit compound 30a in complex with ER-Y537S, a structure-based optimization was performed to get a series of multiacting inhibitors targeting both ERα and ARO, and finally a novel class of potent selective estrogen receptor degraders (SERDs) based on a three-dimensional oxabicycloheptene sulfonamide (OBHSA) scaffold equipped with aromatase inhibitor (AI) activity were identified. Of these dual-targeting SERD-AI hybrids, compound 31q incorporating a 1H-1,2,4-triazole moiety showed excellent ERα degradation activity, ARO inhibitory activity and remarkable antiproliferative activity against BC resistant cells. Furthermore, 31q manifested efficient tumor suppression in MCF-7 tumor xenograft models. Taken together, our study reported for the first time the highly efficient dual-targeting SERD-AI hybrid compounds, which may lay the foundation of translational research for improved treatment of endocrine-resistant BC.

Abstract 3894: The role of ELF3 in acquired resistance to endocrine therapy in ER-positive breast cancer

Abstract Endocrine therapies targeting the estrogen receptor (ER) are the major treatments of ER+ breast cancer, however acquired resistance to endocrine therapy is almost unavoidable and is the main cause of death with breast cancer. The only FDA-approved selective estrogen receptor degrader (SERD) fulvestrant has been used in the second line treatment to overcome aromatase inhibitor- or tamoxifen-resistance. However, fulvestrant has poor pharmacokinetic properties, which has inspired the development of a new generation of oral SERDs including amcenestrant and giredestrant. To understand the mechanisms of acquired resistance to SERDs in ER+ breast cancer, we established ER+ breast cancer cell lines (MCF7 and T47D) resistant to fulvestrant, amcenestrant or giredestrant. We found that all of our MCF7 and T47D SERD-resistant cell lines showed cross-resistance to each of the three SERDs. We then performed multiple genome-wide profiling assays including CRISPR-KO screen, whole exome sequencing (WES), RNA-seq, ATAC-seq and H3K27ac ChIP-seq on the parental and SERD resistant cell lines. Our findings from the CRISPR-KO screens suggest that ETS transcription factor ELF3 is a potential driver of resistance to SERDs. ELF3 expression is negatively regulated by estrogen signaling. Cells that developed acquired resistance to different SERDs consistently show depletion of ER expression but also a dramatic increase of ELF3 expression, while knockdown of ELF3 in the resistant cells leads to cell growth inhibition, suggesting that ELF3 can drive cell survival in the SERD resistant cells. When comparing the open chromatin regions and active enhancer regions between SERD resistant and parental cells via ATAC-seq and H3K27ac ChIP-seq respectively, ELF3 DNA binding motif was found significantly enriched in SERD resistant cells versus parental cells further indicating that ELF3 may play an important role in the SERD resistant cells. ELF3 ChIP-seq was also performed to identify genes enriched with ELF3 binding sites in SERD resistant and parental cells. Pathway analysis of ELF3-targeted genes reveals that ELF3 may regulate cell metabolism specifically in SERD resistant cells but not in parental cells. Our study suggests that ELF3 may be a novel driver of acquired resistance to SERDs in ER+ breast cancer. Citation Format: Na Zhang, Rongbin Zheng, Myles Brown. The role of ELF3 in acquired resistance to endocrine therapy in ER-positive breast cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3894.

Abstract 1619: Generation of a novel estrogen receptor degrader

Abstract Fulvestrant is an effective therapy to treat post-menopausal women with estrogen receptor (ER)-positive metastatic breast cancer. The compound works as a selective estrogen receptor degrader (SERD). Structurally similar to estrogen, fulvestrant binds to the ligand binding domain (LBD) of the ER, altering the stability of the protein, leading to its accelerated proteasomal degradation. While effective, fulvestrant must be given intramuscularly, which has limited its clinical adoption. Based on the success of fulvestrant, multiple attempts have been made to generate orally available SERDs. These attempts have focused on compounds that, like fulvestrant, bind within the LBD of the ER. Overall, these intensive efforts have generated limited success. We reasoned that it may be possible to identify potential SERDs that bind to the ER in regions outside the LBD. Such an approach would be useful for tumors bearing wildtype ER, and likely for those bearing a mutant form of the ER, since most mutations cluster in the LBD. We have recently developed a high throughput platform that allows for the identification of ligands that can bind to wide variety of protein targets. Using this approach, we screened a diverse chemical library of 100,000 small molecules to identify compounds that bind outside of the LBD, and in particular to the N-terminal region of the ER. This strategy identified CL-ER-100, a small molecule that directly bound to the N-terminal region of the ER and functioned as a SERD. While CL-ER-100 rapidly induced ER proteasomal-dependent degradation, it had no effect on the protein levels of other, structurally related nuclear receptors. Based on its structure and size (~ MW 300 Da), CL-ER-100 is expected to have excellent bioavailability. Consistent with its binding outside of the LBD, addition of tamoxifen had no effect on the ability of CL-ER-100 to induce ER protein degradation. In cells, CL-ER-100 was able to markedly inhibit the growth of breast cancer cell lines such as MCF7 and T47D (IC50 of ~ 100 nM) while not inhibiting the growth of a wide array of normal and non-breast cancer tumor cell lines. Taken together, these results identify a novel strategy to develop small molecule SERDs that induce ER protein degradation by binding to previously untargeted regions of the estrogen receptor. Citation Format: Yuan Liu, Jason R. Kennerdell, Bo Lin, Mads B. Larsen, Irene Alfaras, Daniel P. Camarco, Ferhan Tuncer, Toren Finkel, Bill B. Chen. Generation of a novel estrogen receptor degrader [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1619.

Abstract 4919: Estrogen regulates the immune microenvironment and immunotherapy response of colorectal liver metastases

Abstract Liver metastases (LM) remain a major cause of cancer-related death and many cancers preferably metastasize to the liver due to its unique anatomical location, rich blood supply and immune-tolerant microenvironment. Up to 50% of colorectal carcinoma patients develop LM during their disease and this is associated with a poor prognosis. Our laboratory previously identified a sexual dimorphism in the regulation of the immune microenvironment (IME) of LM and showed that estrogen could promote the accumulation of myeloid-derived suppressor cells (MDSCs) and Tregs in the liver in response to invading cancer cells. The objective of this study was to elucidate the role of estrogen in the recruitment and polarization of other immune cells and to determine the therapeutic potential of Selective Estrogen Receptor Degrader (SERD) therapy in CRCLM bearing female mice, and evaluate if SERD would improve anti PD-1 immunotherapy efficacy. In estrogen-competent female mice bearing CRCLM, we found increased gene and protein expression of the immunosuppressive cytokine TGF-β, and inversely a decrease in the pro-inflammatory cytokine TNF-alpha. Furthermore, we identified a significant decrease of immunosuppressive CD68+CD163+ M2 macrophages, and an increase of the pro-inflammatory CD68+TNF-alpha+ M1 macrophages in estrogen-depleted (ovariectomized) mice as compared to estrogen-competent mice. This observation was reversed upon estradiol supplementation. Moreover, treatment of female mice with the SERD Fulvestrant, markedly reduced the number of CRCLM compared to vehicle-treated mice, reduced the cell frequencies and counts of M2 macrophages, significantly increased NK cells recruitment in the liver. The addition of SERD to immunotherapy has a better therapeutic effect than both therapies alone, and increased CD8+ T-cells, NK cells, and M1/M2 macrophages ratio. Taken together, our results identify estrogen as a critical regulator of an immunosuppressive TME in the liver and a potential therapeutic target to enhance immunotherapy efficacy. Citation Format: Yasmine Benslimane, Sarah Lapin, Julien Chambon, Stephanie Perrino, Pnina Brodt. Estrogen regulates the immune microenvironment and immunotherapy response of colorectal liver metastases. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4919.

Abstract 5977: Evaluation of the potential combination regimens for q901, a clinical stage selective cdk7 inhibitor, as a DNA damage repair inhibitor

Abstract Background; Q901 is an extremely selective CDK7 inhibitor with potent single agent activity in the multiple in vivo tumor models including small cell lung cancer, cholangiocarcinoma, colon cancer, pancreatic cancer, hormone receptor positive breast cancer, castrate-resistant prostate cancer, and ovarian cancer. Q901 is currently in Phase 1/2 clinical trial for patients with selected advanced solid tumors (ClinicalTrials.gov: NCT05394103). In the preclinical models, Q901 treatment induces DNA damage response (DDR) impairment markers, particularly ones involved in the double-strand break repair mechanisms. This opens up broad range of possibilities for new combination regimen that could benefit from the accumulated DNA damages caused by Q901 treatment. Methods: Various combinations have been tested to evaluate potential clinical regimens in multiple solid tumor models. Two different BRCA wild-type ovarian cancer A2780 and OVCAR3 models, an estrogen receptor positive MCF7 breast cancer model, or a syngeneic RENCA mouse renal cancer model was tested with Q901 in combination with poly ADP-ribose polymerase (PARP) inhibitor, selective estrogen receptor degrader (SERD), an estrogen receptor (ER) PROTAC degrader, or an anti-PD-1 antibody, respectively. Results: In the A2780 model, the PARPi alone group showed 15% TGI (tumor growth inhibition), but significant tumor regression (104% TGI) was observed in the Q901 and PARPi combination. In the OVCAR3 model, the Q901 and PARPi combination group showed 105% TGI, whereas the PARPi alone group showed 38% TGI. In the MCF7 model, SERD alone showed 64% TGI and Q901 combination with SERD showed 94% TGI. Remarkable tumor regression (111% TGI) was observed in the MCF7 model combining ER PROTAC and Q901, whereas ER PROTAC alone group showed 55% TGI. In the RENCA model, the anti-PD-1 antibody alone group showed 13.2% TGI and the combination of Q901 and anti-PD-1 antibody showed 66% TGI. In summary, these preclinical model studies demonstrated significant additive/synergistic effects of the Q901 combination on tumor growth inhibition. Conclusion: The non-clinical data demonstrate that Q901 not only has a potential as a single agent,but can also be used in combination with other therapeutic strategies to develop next-generation cancer therapeutics to potentially address unmet medical needs. Citation Format: Donghoon Yu, Yeejin Jeon, YoonJi Lee, Seung-Joo Lee, Jaeseung Kim, Hyerim Jung, Tae-Kyung Kim, Kiyean Nam. Evaluation of the potential combination regimens for q901, a clinical stage selective cdk7 inhibitor, as a DNA damage repair inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5977.

Abstract 474: Preclinical development of an oral selective estrogen receptor degrader (SERD) TFX06 for the treatment of ER+HER2− breast cancer

Abstract Breast cancer (BC) has recently surpassed lung cancer to become the most commonly diagnosed cancer in women worldwide. Approximately 70% of BC was ER+, in which estrogen receptor α (ERα, encoded by ESR1 gene) drives dysregulated cell proliferation. In clinic, ER+ BC has been effectively treated by endocrine therapy targeting estrogen or ER. As the first approved ERα degrader, fulvestrant has proven to be effective for locally advanced or metastatic BC. However, its clinical utilization is hampered by inconvenience of intramuscular injection, as well as its poor PK/PD profile and limited efficacy, especially in the patient population that developed ESR1 mutation-conferring drug resistance after the earlier line of treatment with an aromatase inhibitor. Thus, developing an oral and more effective therapy for ER+ BC remains an unmet medical need. We report herein discovery and preclinical investigation of a novel orally bioavailable SERD TFX06. TFX06 demonstrated potent binding affinity to ERα with a Ki of 0.10 nM and preferably antagonized ERα to ERβ with an IC50 value of 0.086 nM and 2.83 nM, respectively. TFX06 potently induced ERα degradation in ER+ BC cells and substantially inhibited cell proliferation in BC cell lines with ER wild type (WT), and MCF-7 cells expressing Y537S or D538G mutants. Oral administration of TFX06 demonstrated impressive antitumor efficacy in ER WT (TGI = 101%) and ER D538G-expressing (TGI = 99%) MCF-7 cell line-derived xenograft (CDX) models in mice. Similarly, in an ER+ BC patient-derived xenograft (PDX) model (HCI-013) with ER Y537S mutation, TFX06 also exerted excellent antitumor activity (TGI = 99%). Furthermore, TFX06 in combination with palbociclib achieved synergistic activity in MCF-7 CDX model. Moreover, TFX06 exhibited an excellent correlation between systemic/local drug exposure, pharmacodynamic modulation (i.e., ERα downregulation) and antitumor activity. Collectively, these findings demonstrate that TFX06 is a novel, orally bioavailable SERD. TFX06 demonstrates substantial antitumor activity in both in vitro and in vivo preclinical tumor models, including those expressing ESR1 mutations, through downregulation of ERα. The preclinical data warranted the clinical evaluation of TFX06 in human with an IND application submitted to the China NPMA. Citation Format: Jinping Wu, Ke An, Douglas D. Fang, Jianyu Lu, Lihong Hu, Jing Wang, Kaili Zhang, Yuanfeng Xia, Charles Ding, Shuhui Chen, Sha Wei. Preclinical development of an oral selective estrogen receptor degrader (SERD) TFX06 for the treatment of ER+HER2− breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 474.