ER Y537S

Baseline ESR1 mutations in EV-DNA were associated with shorter progression-free survival (PFS) across the cohort, with the Y537S mutation showing a particularly strong impact on the outcome of fulvestrant-treated patients. In contrast, PIK3CA mutations in EV-DNA did not significantly correlate with PFS, whereas in ctDNA, they were linked to poor outcomes. Altogether, this study positions EV-DNA as a valuable biomarker alongside ctDNA, enriching the understanding of different analytes in liquid biopsy and supporting strategies for HR+/HER2-mBC in precision oncology.

ERD-12310A achieved a DC50 value of 47 pM and is 10 times more potent than ARV-471...Importantly, ERD-12310A achieved strong tumor growth inhibition in MCF-7 xenograft tumors harboring the clinically relevant ESR1Y537S mutation, which confers resistance to traditional antiestrogens. Our data position ERD-12310A as a promising candidate for further development as a potential therapy for ER+ breast cancer.

YS mutant BC cells induced the conversion of fibroblasts into CAFs, via YAP, which represent a potential therapeutic target which interrupt the functional interactions between mutant cells/TME and to be implemented in the novel therapeutic strategy of a subset of metastatic BC patients carrying the frequent Y537S mutations.

The APIS ESR1 Mutations Kit demonstrated high sensitivity and specificity as a qualitative qPCR assay for detecting ESR1 mutations in varying WT backgrounds. Its performance with the SensID ESR1 Reference Set 1% AF cfDNA validated the kit's LoD at ≤1% MAF with external samples. The APIS kit is a valuable tool for assessing ESR1 mutations in both clinical and research settings, offering a more accessible alternative to traditional next-generation sequencing (NGS) and dPCR assays.

P3, N=400, Not yet recruiting, Fudan University Shanghai Cancer Center; Fudan University Shanghai Cancer Center

In 1 pt, eribulin and abraxane did not have the same effect; ESR1 polyclonality was maintained. One pt with 1 ESR1 clone after 2.5 years of fulvestrant+palbociclib developed 15 additional clones after 1.5 years of an experimental SERD+everolimus. Another pt with 1 ESR1 clone after anastrozole+palbociclib developed 9 additional ESR1 clones after 10 months of AKT inhibition+anastrozole... This is the first investigation of dynamic changes in extreme ESR1 polyclonality in association with treatment outcomes in pts with ER+ MBC. Capecitabine was associated with a decrease in detectable ESR1-mutant clones, while increased polyclonality was found at progression on SERD+everolimus, AKT-inhibitor+AI, and an ADC. Although extreme ESR1 polyclonality is rare, understanding mutational dynamics will improve our understanding of polyclonality more broadly and its impact on treatment resistance.

None of the patients received ESR1-targeted therapy with Elacestrant. Our findings indicate that the level changes of ctDNA ESR1 mutations have implications with regards to treatment sensitivity and resistance to various therapies in patients with HR+/HER2- mBC. These results suggest a potentially effective method for monitoring treatment response and guiding future therapy by providing new insights into targeting ESR1 pathways to overcome resistance.

Elacestrant demonstrated a significant improvement in mPFS vs SOC in patients with both high and low ESR1 VAF. The clinical benefit and activity of elacestrant vs SOC was maintained regardless of type of ESR1 mutation variant and the abundance/quantity of ESR1 mutations in patients with ER+/HER2- mBC. In all patients with ER+/HER2, ESR1-mut mBC, elacestrant may replace fulvestrant-based combinations, and delay chemotherapy or ADC-based regimens.

ERD-1233 was developed based on Lasofoxifene as the ER binding moiety and a novel cereblon ligand through extensive optimization of the linker...Oral administration of ERD-1233 effectively reduces ER protein in ER+ tumors and achieves tumor regression in the ER wild-type MCF-7 xenograft tumor model and strong tumor growth inhibition in the ESR1Y537S mutated model in mice. Our data demonstrate that ERD-1233 is a promising ER PROTAC degrader for extensive evaluation as a new therapy for the treatment of ER+ human breast cancer.

Taken together, these data indicate that ESR1 mutations confer chemoresistance through activation of the JNK/MDR1 axis. These finding suggest a novel treatment option for BC tumors expressing ESR1 mutations.

These findings suggest a potential reduction in lasofoxifene efficacy due to the dual mutation. Further experimental validation is required to confirm these results and fully understand the impact of dual mutations on lasofoxifene's effectiveness in ERα + metastatic BC.

The developed novel multi-marker liquid bead array assay for the simultaneous detection of PIK3CA and ESR1 hotspot mutations in single CTCs is highly specific, highly sensitive, high-throughput, and sample-, cost-, and time-saving. This multi-marker liquid bead array assay can be extended to detect up to 100 mutations in many genes at once and can be applied for bulk CTC and ctDNA analysis.

No such effect was observed for ESR1 mutations, with a rate ratio of 1.15 (confidence interval 0.67-1.95). We conclude that ER loss and ESR1 mutation together account for 30% of the resistance to endocrine therapy.

Molecular profiling allowed the identification of ESR1Y537S, in an aggressive prolactinoma. ESR1Y537S was not detected early in the course of the disease and is likely conferring tumor aggressiveness. This finding emphasizes the significance of estrogen receptor signaling in prolactinomas. It also allowed the use of targeted therapy with successful control of disease progression.

Elacestrant, a second-line ER degrader, increases overall free-survival in patients with resistant breast cancer and ESR1Y537S...Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.

P2, N=100, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Trial completion date: Feb 2024 --> Dec 2024 | Trial primary completion date: Feb 2024 --> Dec 2024

Additionally, protein expression and phosphorylation patterns, while under different regulation, still recapitulated the estrogen-independent phenotype of ER mutant cells. Our study is the first proteome-centric characterization of ESR1 mutant models, out of which we confirm estrogen independence of ER mutants and reveal the enrichment of immune signaling pathways at the proteomic level.

P2, N=29, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Trial completion date: Jan 2024 --> May 2024 | Trial primary completion date: Jan 2024 --> May 2024

According to the findings of this meta-analysis, the assessment for plasma ESR1 mutations may provide prognostic and clinical guidance regarding subsequent endocrine therapy decisions for ER-positive, metastatic BC patients who had received prior therapy with AIs.

Ultimately, we identified five peptides derived from the three most common ESR1 mutations (D538G, Y537S, and E380Q) and their associated wild-type peptides, which were the most immunogenic. Overall, these data confirm the immunogenicity of epitopes derived from ESR1 and highlight the potential of these peptides to be targeted by novel immunotherapy strategies.

Our findings establish PMM2 as an innovative therapeutic target for metastatic BC expressing the ERα Y537S variant, offering alternative strategies for managing and treating this disease.

To investigate the basis of this lack of recognition we performed immunopeptidomics analysis of a mutant-overexpressing lymphoblastoid cell line and found that the ESR1 Y537S neopeptide was not endogenously processed, despite binding to HLA-B*40:02 when exogenously pulsed onto the target cell. These results indicate that stimulation of T cells that likely derive from the naïve repertoire with pulsed minimal peptides may lead to the expansion of clones that recognize non-processed peptides, and highlights the importance of using methods that selectively expand T cells with specificity for antigens that are efficiently processed and presented.

P1/2, N=206, Completed, Eisai Inc. | Active, not recruiting --> Completed

Of these, IRS1 stood out as a gene of interest, and ERα and PR occupancy at chromatin binding sites along IRS1 were uniquely altered in the context of ERα Y537S. Furthermore, siRNA knockdown of IRS1 or treatment with the IRS1 inhibitor NT-157 had a significant anti-proliferative effect in ERα Y537S cell lines, implicating IRS1 as a potential therapeutic target for restoring treatment sensitivity to patients with breast cancers harboring ERα Y537S mutations.

The over-activation of ERα signaling is regarded as the major driver for luminal breast cancers, which could be effective controlled via selective estrogen receptor modulators (SERM), such as tamoxifen...Interestingly, ChIP assay shows that ERα could bind to the promoter region of PSMD14 and facilitate its gene transcription, which indicates PSMD14 is both the upstream modulator and downstream target for ERα signaling in breast cancer. In general, we identified a novel positive feedback loop between PSMD14 and ERα signaling in breast cancer progression, while blockade of PSMD14 could be a plausible strategy for luminal breast cancer.

P2, N=29, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Trial completion date: Feb 2023 --> Jan 2024 | Trial primary completion date: Feb 2023 --> Jan 2024

Several oral ERa degraders were active against compound mutant models. We have identified a resistance mechanism specific to fulvestrant, that can be targeted by treatments in clinical development.

We found that continuous dosing of palbociclib is more effective for lowering overall tumor burden than the standard, pulsed-dose palbociclib treatment. Importantly, our mathematical modeling and statistical analysis platform provides a rational method for comparing treatment strategies in search of optimal combination dosing strategies of other cell-cycle inhibitors in ER+ breast cancer.

ESR1 mutations do not seem to be involved in the mechanisms of resistance to AI or M in HR+ endometrial cancer. The clinical relevance of their detection is not demonstrated.

Combining C3aR inhibitors with fulvestrant or CDK4/6 inhibitors had a synergistic inhibitory effect on ESR1 mutant cells...Our findings suggest that upregulation of adipsin from ESR1 mutant cells and adipocytes in crosstalk increases tumor cell growth. Future studies will investigate the critical role of adipsin's auto- and paracrine effects in breast cancer with ESR1 mutations.

However, the lack of prospective cohorts comparing the distinctions among palbociclib (PAL), ribociclib (RIB) and abemaciclib (ABE) poses a significant clinical knowledge gap. Out of the 114 pts enrolled, 59 received PAL, 48 RIB, and 16 ABE. In the de novo subgroup, 51.4 % of pts were treated with PAL, 42.9% with RIB, and 5.7% with ABE. Among the pts who received PAL, 7 harbored an ESR1 mutation at BL (D538G: 2, H377R: 1, Y537N: 1, Y537S: 3).

We have previously identified exportin 1 (XPO1) to be more highly expressed in breast cancers with acquired TAM resistance compared to TAM-sensitive counterparts and showed that a combination treatment of TAM with an XPO1 inhibitor, selinexor, is an effective method of tumor suppression in preclinical models. We did not observe weight changes in either model. Our findings suggest that eltanexor may be a relevant treatment for further exploration in the context of a TAM combination therapy for treatment of ER+ breast cancer.

The ESR1 Mutations Kit demonstrated high sensitivity and performance as a qualitative qPCR assay to detect eleven ESR1 mutations. Mutations detected with the APIS ESR1 Mutations Kit

Unlike Y537S, cells harboring mutations at the dimer interface maintain their sensitivity to Selective Estrogen Receptor Degraders (SERDs), including Fulvestrant, recently FDA-approved Elacestrant, and Camizestrant, as well as Selective Estrogen Receptor Modulators (SERMs) including Tamoxifen and Raloxifene. Collectively, our finding unveiled a new class of ER mutations that enforce receptor dimerization and activation of the ER signaling pathway. The discovery opens up a new therapeutic interventional possibility, suggesting that targeting dimerization could emerge as a new strategy to combat these malignancies.

When treated with palbociclib + fulvestrant or palbociclib + estrogen-free medium (mimicking estrogen suppression in patients treated with aromatase inhibitors), cells harboring the Y537S and D538G mutations displayed an 11.2 to 46.9-fold increase in the IC50 of palbociclib. ESR1 mutations are enriched following treatment with CDK4/6i in a cohort of 3,958 patients with ER+/HER2- metastatic breast cancer. Mutations in ESR1 in ER+ breast cancer cells directly promote resistance to CDK4/6i alone or CDK4/6i + antiestrogens in vitro and in vivo.

CGP detects a wide spectrum of mutations in ESR1, including missense and indel mutations and fusions. When LBx TF is < 1%, sensitivity for ESR1mut is reduced and repeat testing should be considered. ESR1mut can coexist with complementary or competing resistance mechanisms, particularly when ESR1 is a minor allele, which could impact benefit from novel ET approved for patients with ESR1 mutations.

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.

We observed concordance and heterogeneity in the status of actionable mutations between the CSF, peripheral blood, and systemic metastatic tumor tissue. Larger studies are needed to assess the clinical utility of these observations, particularly with the development of several novel targeted agents that are CNS-penetrant.