Selective Estrogen Receptor Degrader Research Articles

OR11-02 Structural Determinants of Pure Antiestrogenic Activity

Abstract Disclosure: A. Vallet: None. M. Diennet: None. K. Thiombane: None. A. Vivet: None. S. Weber: None. M. El Ezzy: None. F. Shaikh: None. J. Poupart: None. R. Mendoza-Sanchez: None. D. Schuetz: None. A. Marinier: None. G.L. Greene: None. S.W. Fanning: None. S.N. Mader: None. Inhibitory activities of antiestrogens on estrogen receptor alpha (ERα) range from mixed antagonism/agonism (selective ER modulators; SERMs) to complete antiestrogenicity associated with accelerated ERα turnover (selective ER degraders; SERDs). Using a panel of SERMs, SERDs and PROTACs, we show that antiestrogens induce variable degrees of SUMOylation of ERα and that efficient induction of SUMOylation, rather than increased suppression of coactivator recruitment or accelerated ERα degradation, correlates with suppression of ERα basal and induced transcriptional activity in reporter assays. Antiestrogens did not induce SUMOylation of ERβ and did not suppress its basal transcriptional activity in the same assays. Chimeras between the two receptors identified ERα-specific residues required for induction of SUMOylation by antiestrogens and revealed differences according to the antiestrogen structure. In addition, induction of ERα SUMOylation by antiestrogens depended on the hydrophobicity of N-terminal residues of ligand binding domain (LBD) helix H12. L536 mutations, including those occurring in endocrine therapy-resistant breast cancer, abolished both induction of ERα SUMOylation and pure antiestrogenicity. Structures of the L536S human ERα LBD bound to fulvestrant analogs and molecular dynamics simulations revealed dynamic side chain interactions with ERα H12 in the coactivator-binding groove, and predict the impact of SUMOylation-suppressing L536 mutations on these interactions. Together, our results show that antiestrogen-induced ERα SUMOylation is associated with complete suppression of its transcriptional activity and provide insights into the structural determinants of pure antiestrogenicity. Presentation Date: Friday, June 16, 2023

Interplay between ESR1/PIK3CA codon variants, oncogenic pathway alterations and clinical phenotype in patients with metastatic breast cancer (MBC): comprehensive circulating tumor DNA (ctDNA) analysis

Backgroundalthough being central for the biology and druggability of hormone-receptor positive, HER2 negative metastatic breast cancer (MBC), ESR1 and PIK3CA mutations are simplistically dichotomized as mutated or wild type in current clinical practice.MethodsThe study analyzed a multi-institutional cohort comprising 703 patients with luminal-like MBC characterized for circulating tumor DNA through next generation sequencing (NGS). Pathway classification was defined based on previous work (i.e., RTK, RAS, RAF, MEK, NRF2, ER, WNT, MYC, P53, cell cycle, notch, PI3K). Single nucleotide variations (SNVs) were annotated for their oncogenicity through OncoKB. Only pathogenic variants were included in the models. Associations among clinical characteristics, pathway classification, and ESR1/PIK3CA codon variants were explored.ResultsThe results showed a differential pattern of associations for ESR1 and PIK3CA codon variants in terms of co-occurring pathway alterations patterns of metastatic dissemination, and prognosis. ESR1 537 was associated with SNVs in the ER and RAF pathways, CNVs in the MYC pathway and bone metastases, while ESR1 538 with SNVs in the cell cycle pathway and liver metastases. PIK3CA 1047 and 542 were associated with CNVs in the PI3K pathway and with bone metastases.ConclusionsThe study demonstrated how ESR1 and PIK3CA codon variants, together with alterations in specific oncogenic pathways, can differentially impact the biology and clinical phenotype of luminal-like MBC. As novel endocrine therapy agents such as selective estrogen receptor degraders (SERDS) and PI3K inhibitors are being developed, these results highlight the pivotal role of ctDNA NGS to describe tumor evolution and optimize clinical decision making.

382MO Updated results from the phase I/II study of OP-1250, an oral complete estrogen receptor (ER) antagonist (CERAN) and selective ER degrader (SERD) in patients (pts) with advanced or metastatic ER-positive, HER2-negative breast cancer

382MO Updated results from the phase I/II study of OP-1250, an oral complete estrogen receptor (ER) antagonist (CERAN) and selective ER degrader (SERD) in patients (pts) with advanced or metastatic ER-positive, HER2-negative breast cancer

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)

Optimization of small molecule degraders and antagonists for targeting estrogen receptor based on breast cancer: current status and future.

The estrogen receptor (ER) is a classical receptor protein that plays a crucial role in mediating multiple signaling pathways in various target organs. It has been shown that ER-targeting therapies inhibit breast cancer cell proliferation, enhance neuronal protection, and promote osteoclast formation. Several drugs have been designed to specifically target ER in ER-positive (ER+) breast cancer, including selective estrogen receptor modulators (SERM) such as Tamoxifen. However, the emergence of drug resistance in ER+ breast cancer and the potential side effects on the endometrium which has high ER expression has posed significant challenges in clinical practice. Recently, novel ER-targeted drugs, namely, selective estrogen receptor degrader (SERD) and selective estrogen receptor covalent antagonist (SERCA) have shown promise in addressing these concerns. This paper provides a comprehensive review of the structural functions of ER and highlights recent advancements in SERD and SERCA-related small molecule drugs, especially focusing on their structural optimization strategies and future optimization directions. Additionally, the therapeutic potential and challenges of novel SERDs and SERCAs in breast cancer and other ER-related diseases have been discussed.

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.

Effectiveness of Selective Estrogen Receptor Modulators in Breast Cancer Therapy: An Update.

Breast cancer is considered to be 2nd most common cancer subtype investigated worldwide. It is mainly prevalent in postmenopausal women. Estrogen Receptor (ER) is a primary transcription factor for the survival and growth of tumors. Around 80% BCs of all classes are ER-positive (ER+). Powerful evidence for estrogen proved to be involved in BC pathogenesis both exogenously and endogenously. It brings the concept of ER inhibitors to treat BC with distinct mechanisms into focus and ER PROTACs (Proteolysis-Targeting Chimeras), AIs (Aromatase inhibitors), SERMs (Selective estrogen receptor modulators), and SERDs (Selective estrogen receptor degrader) were developed. For over 30 years, Tamoxifen, a triphenylethylene SERM, was the drug of choice solely to treat ER+BC patients. Although several SERMs got approval by US FDA after tamoxifen, complicacies remain because of dangerous adverse effects like endometrial carcinoma, hot flashes, and VTE (Venous thromboembolism). In addition to that, drug-resistant tumors put a surging need for novel, potent candidates with no or low adverse effects for ER+ BC prevention. This article explores the possibilities of SERMs as effective BC agents. A detailed literature survey of the history and recent advancements of SERMs has been carried out, taking BC as the primary target. This review provides information about ER structure, signaling, pharmacological action, chemical classification with SAR analysis, and benefits and adverse effects of SERMs as potential BC agents. Exhaustive literature studies suggested that SERMs having an agonistic, antagonistic or mixed activity to ER could efficiently inhibit BC cell proliferation. Each chemical class of SERMs comprises some salient features and potentials, which may be further investigated to obtain novel effective SERMs in BC therapy.

Fulvestrant increases the susceptibility of enzalutamide-resistant prostate cancer cells to NK-mediated lysis

BackgroundEnzalutamide, a next-generation antiandrogen agent, is approved for the treatment of metastatic castration-resistant prostate cancer (CRPC). While enzalutamide has been shown to improve time to progression and extend overall survival...

Place of fulvestrant in the treatment of patients with metastatic luminal Her2-negative breast cancer

Hormone therapy alone or in combination with targeted agents (CDK4/6 inhibitors, alpelisib, everolimus) is currently the standard treatment of metastatic luminal Her2-negative breast cancer. Aromatase inhibitors and fulvestrant are the main hormone therapy agents. Fulvestrant belongs to a special class of antiestrogens – selective estrogen receptor degraders (SERD) and does not have estrogen receptor agonist activity, unlike tamoxifen. In addition, the efficacy of fulvestrant does not depend on the presence of ESR1 mutations in the tumor. The combination of aromatase inhibitors with CDK4/6 inhibitors is the standard first-line treatment in patients with hormone-sensitive tumors, that is, with progression of more than 1 year after the end of adjuvant hormone therapy. Whereas fulvestrant ± CDK4/6 inhibitors are used for disease progression on adjuvant hormonal therapy in the 1st line or as 2nd line for progression on aromatase inhibitor therapy for metastatic cancer. The choice of treatment for patients with a PIK3CA mutation with progression on the 1st line of fulvestrant with a CDK4/6 inhibitor is difficult. This article presents a clinical example of the use of a combination of fulvestrant and alpelisib in a patient with secondary hormone resistance (progression on the adjuvant therapy with aromatase inhibitors) and progression on the first-line therapy with fulvestrant and palbociclib. Carrying out therapy with fulvestrant and alpelisib as the 2nd line provided a long-term (for 14 months) stabilization of the tumor process.

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.

UBR5 forms ligand-dependent complexes on chromatin to regulate nuclear hormone receptor stability

Nuclear hormone receptors (NRs) are ligand-binding transcription factors that are widely targeted therapeutically. Agonist binding triggers NR activation and subsequent degradation by unknown ligand-dependent ubiquitin ligase machinery. NR degradation is critical for therapeutic efficacy in malignancies that are driven by retinoic acid and estrogen receptors. Here, we demonstrate the ubiquitin ligase UBR5 drives degradation of multiple agonist-bound NRs, including the retinoic acid receptor alpha (RARA), retinoid x receptor alpha (RXRA), glucocorticoid, estrogen, liver-X, progesterone, and vitamin D receptors. We present the high-resolution cryo-EMstructure of full-length human UBR5 and a negative stain model representing its interaction with RARA/RXRA. Agonist ligands induce sequential, mutually exclusive recruitment of nuclear coactivators (NCOAs) and UBR5 to chromatin to regulate transcriptional networks. Other pharmacological ligands such as selective estrogen receptor degraders (SERDs) degrade their receptors through differential recruitment of UBR5 or RNF111. We establish the UBR5 transcriptional regulatory hub as a common mediator and regulator of NR-induced transcription.

Revisiting Estrogen for the Treatment of Endocrine-Resistant Breast Cancer: Novel Therapeutic Approaches.

Estrogen receptor (ER)-positive breast cancer is the most common subtype, representing 70-75% of all breast cancers. Several ER-targeted drugs commonly used include the selective estrogen receptor modulator (SERM), tamoxifen (TAM), aromatase inhibitors (AIs) and selective estrogen receptor degraders (SERDs). Through different mechanisms of action, all three drug classes reduce estrogen receptor signaling. Inevitably, resistance occurs, resulting in disease progression. The counterintuitive action of estrogen to inhibit ER-positive breast cancer was first observed over 80 years ago. High-dose estrogen and diethylstilbestrol (DES) were used to treat metastatic breast cancer accompanied by harsh side effects until the approval of TAM in the 1970s. After the development of TAM, randomized trials comparing TAM to estrogen found similar or slightly inferior efficacy but much better tolerability. After decades of research, it was learned that estrogen induces tumor regression only after a period of long-term estrogen deprivation, and the mechanisms of tumor regression were described. Despite the long history of breast cancer treatment with estrogen, this therapeutic modality is now revitalized due to the development of novel estrogenic compounds with improved side effect profiles, newly discovered predictive biomarkers, the development of non-estrogen small molecules and new combination therapeutic approaches.

Targeting the Estrogen Receptor for the Treatment of Breast Cancer: Recent Advances and Challenges.

Estrogen receptor alpha (ERα) is a well-established therapeutic target for the treatment of ER-positive (ER+) breast cancers. Despite the tremendous successes achieved with tamoxifen, a selective ER modulator, and aromatase inhibitors (AIs), resistance to these therapies is a major clinical problem. Therefore, induced protein degradation and covalent inhibition have been pursued as new therapeutic approaches to target ERα. This Perspective summarizes recent progress in the discovery and development of oral selective ER degraders (SERDs), complete estrogen receptor antagonists (CERANs), selective estrogen receptor covalent antagonists (SERCAs), and proteolysis targeting chimera (PROTAC) ER degraders. We focus on those compounds which have been advanced into clinical development.

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.

Selenium Analogue of LSZ102 as a Highly Potent ER‐α Binder

AbstractNowadays, selective estrogen receptor modulators (SERMs) and downregulators (SERDs) are used as the first‐line medical treatment for estrogen receptor positive (ER+) tumors. Herein we report synthesis, ER‐α binding activity, and cytotoxicity of LSZ102 selenium analogues 11 and 28. TR‐FRET competitive ER‐α binding experiments and cytotoxicity assays have shown that the selenium analogues exhibit closely related activity to LSZ102. Furthermore, the prepared selenium analogues are not toxic in rat cardiomyoblasts (H9C2), indicating that substituted benzo[b]selenophene is a prospective scaffold for the development of ER‐α modulators and downregulators for the treatment of ER+ cancers.

Estrogen receptor blockade and radiation therapy cooperate to enhance the response of immunologically cold ER+ breast cancer to immunotherapy

BackgroundMost patients with estrogen receptor positive (ER+) breast cancer do not respond to immune checkpoint inhibition (ICI); the tumor microenvironment (TME) of these cancers is generally immunosuppressive and contains few tumor-infiltrating lymphocytes. Radiation therapy (RT) can increase tumor inflammation and infiltration by lymphocytes but does not improve responses to ICIs in these patients. This may result, in part, from additional effects of RT that suppress anti-tumor immunity, including increased tumor infiltration by myeloid-derived suppressor cells and regulatory T cells. We hypothesized that anti-estrogens, which are a standard of care for ER+ breast cancer, may ameliorate these detrimental effects of RT by reducing the recruitment/ activation of suppressive immune populations in the radiated TME, increasing anti-tumor immunity and responsiveness to ICIs.MethodsTo interrogate the effect of the selective estrogen receptor downregulator, fulvestrant, on the irradiated TME in the absence of confounding growth inhibition by fulvestrant on tumor cells, we used the TC11 murine model of anti-estrogen resistant ER+ breast cancer. Tumors were orthotopically transplanted into immunocompetent syngeneic mice. Once tumors were established, we initiated treatment with fulvestrant or vehicle, followed by external beam RT one week later. We examined the number and activity of tumor infiltrating immune cells using flow cytometry, microscopy, transcript levels, and cytokine profiles. We tested whether fulvestrant improved tumor response and animal survival when added to the combination of RT and ICI.ResultsDespite resistance of TC11 tumors to anti-estrogen therapy alone, fulvestrant slowed tumor regrowth following RT, and significantly altered multiple immune populations in the irradiated TME. Fulvestrant reduced the influx of Ly6C+Ly6G+ cells, increased markers of pro-inflammatory myeloid cells and activated T cells, and augmented the ratio of CD8+: FOXP3+ T cells. In contrast to the minimal effects of ICIs when co-treated with either fulvestrant or RT alone, combinatorial treatment with fulvestrant, RT and ICIs significantly reduced tumor growth and prolonged survival.ConclusionsA combination of RT and fulvestrant can overcome the immunosuppressive TME in a preclinical model of ER+ breast cancer, enhancing the anti-tumor response and increasing the response to ICIs, even when growth of tumor cells is no longer estrogen sensitive.

Efficacy of oral selective estrogen receptor degraders (SERD)s in the treatment of estrogen receptor positive (ER+), HER2-negative metastatic breast cancer (MBC): A stratified analysis of the ESR1 wild type (wt) and mutant (mt) subgroups.

1096 Background: Oral SERDs are a novel drug class developed to counteract endocrine resistance due to ESR1 mutations. Several SERDs have emerged from phase 2 and 3 trials, and the FDA has most recently limited approval for Elacestrant to patients with ESR1mt tumours despite PFS benefit in the overall population. Questions remain on whether patients with ESR1wt tumours stand to benefit from SERDs. We conducted an IPD meta-analysis of the trials EMERALD, SERENA2, ACELERA and AMEERA3 to compare survival outcomes of SERDs in the 2nd or 3rd line setting against endocrine therapy (ET), using KMSubtraction to unveil HRs of unreported ESR1wt subgroups from AMEERA3 and EMERALD. Meta-analysis results were stratified by ESR1 status. Methods: RCTs investigating the efficacy of novel SERDs versus ET for ER+, HER2- MBC, and which reported the Kaplan Meier (KM) plots of progression free survival (PFS) were selected after a systematic search of Embase and PubMed from inception until January 21,2023. A graphical reconstructive algorithm was applied to estimate time-to-event outcomes from reported KM plots in all overall and reported subgroup cohorts, deriving IPD. A bipartite matching algorithm, KMSubtraction was used to subtract patients in the reported subgroup from the overall cohort, to derive survival data for unreported ESR1wt subgroups. An IPD meta-analysis was then performed, pooling data by ESR1 mutation. Results: A total of 1216 patients were included in this analysis. In the pooled analysis of the overall cohort, PFS benefit was observed with SERDs compared to treatment of physician's choice (TPC) (HR 0.787, 95%CI 0.683-0.908, p<0.001). In the ESR1mt subgroup SERDs demonstrated improved PFS (HR 0.557, 95%CI 0.440-0.705, p<0.001) compared with TPC. In the ESR1wt subgroup, SERDs demonstrated no significant PFS benefit (HR 0.948, 95%CI 0.790-1.137, p=0.565) when compared to TPC. KMSubtraction was implemented on AMEERA3 to reveal survival data in ESR1wt subgroup. Amcenestrant revealed no significant PFS difference in the ESR1wt population (HR 1.197, 95% CI 0.857-1.670, p=0.291) when compared to ET. Additionally, a separate analysis of EMERALD was performed using KMSubtraction to derive survival data of the ESR1wt subgroup comparing Elacestrant vs Fulvestrant. No significant PFS difference was observed between the 2 agents in the ESR1wt population (HR 0.832, 95% CI 0.587-1.178, p=0.299). Conclusions: Our results suggest that PFS benefit in the overall population is mainly driven by the ESR1mt subgroup. These findings are in line with the recent FDA approval of Elacestrant for patients with ESR1mt tumours. [Table: see text]

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.

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 .