ER Y537N

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

However, clinical trials have been predominantly carried out with selected populations and single drugs (Palbociclib, Ribociclib or Abemaciclib). At baseline, and considering the pre-defined criteria, ctDNA analysis detected PIK3CA mutations in 32.25% and 44.44% of patients by AVENIO and ddPCR, respectively. ESR1 mutations were detected in 3.22% and 9.37% by AVENIO and ddPCR, respectively. For PIK3CA mutations, the Kappa value was 0.62 (p-value: 0.0004) and 0.47 for ESR1 mutations (p-value: 0.0039).

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.

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.

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

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

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.

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).

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

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.

Conclusions The PACE trial offers one of the largest clinical trial cohorts describing the genomic landscape of HR+/HER2- MBC post-CDK4/6 exposure. Results from this correlative study provide a comprehensive mutational landscape post-CDK4/6i, demonstrate associations between mutations and clinical outcomes, and suggest the potential value of early ctDNA dynamics in this setting.

The APIS ESR1 Mutations Kit demonstrated high sensitivity and performance as a qualitative qPCR assay to detect 11 ESR1 mutations.

Our assays have proven to be robust and highly sensitive and are very well-suited for monitoring ESR1 mutations in the plasma of MBC patients.

P2, N=100, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Trial completion date: Feb 2023 --> Feb 2024 | Trial primary completion date: Feb 2023 --> Feb 2024

P2, N=29, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Trial completion date: Nov 2022 --> Feb 2023 | Trial primary completion date: Nov 2022 --> Feb 2023

Differential gene expression analysis demonstrates up-regulation of estrogen-responsive genes, as expected, but also reveals that enrichment for interferon-regulated gene expression is a common feature of all mutations. Finally, a new gene signature developed from the gene expression profiles in ER mutant cells predicts clinical response in breast cancer patients with ER mutations.

P2, N=100, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Trial completion date: Feb 2022 --> Feb 2023 | Trial primary completion date: Feb 2022 --> Feb 2023

Our data demonstrate that LAS more effectively decreased or cleared mutESR1 than Fulv. Further, mutESR1 clearance was associated with prolonged PFS and more CB in LAS but not Fulv pts, suggesting that LAS results in robust mutESR1 target engagement. Taken together, our data suggest mutESR1 as a potential liquid biomarker for predicting response to LAS in mutESR1, endocrine-resistant mBC pts.

29 patients (median of 2 prior metastatic therapies: 97% CDK4/6i, 79% fulvestrant, 48% chemotherapy) had BL mutESR1 of Y537S (66%), D538G (45%), Y537N (28%), and other less frequently detected mutations; 14 (48.3%) patients were polyclonal. In ELAINE 2, 81% of patients had decrease/cleared (ND) mutESR1 after 4 wks of LAS plus Abema, which correlated with clinical benefit. All mutESR1 detected appear targeted with this therapy. High sensitivity and favorable PPV were observed in patients with decreased MAF, and even more so in those with ND MAF; however, increased MAF was less specific and not as predictive of treatment failure.

METHODS Patients were post- and pre- or peri-menopausal women, or men, with ER+, HER2– aBC who had progressed after 1–2 lines of systemic therapy in the advanced setting (≤1 targeted agent; ≤1 chemotherapy regimen; prior fulvestrant allowed). Overall, these data support continued investigation of giredestrant to advance and improve treatment outcomes in hormone receptor+ BC. Exploratory subgroup analyses

Patients with mutated ESR1 and PIK3CA or CCND1 and FGFR1 amp at baseline had shorter rwPFS than patients with wild-type genes . Genotyping analysis of progression ctDNA highlights the emergence of mutations in estrogen receptor and cell cycle pathways under selective therapeutic pressure and could guide monitoring and therapeutic sequencing for patients with HR+/HER2– MBC.

The aim of this study was to explore the incidence of circulating ESR1 mutations in patients treated by AA or megestrol acetate (M) for advanced endometrial carcinoma.Methodology: This single-center retrospective study was performed at the Henri Becquerel Center (Rouen) and looked for circulating ESR1 gene mutations by droplet digital PCR (E380Q, L536R, Y537S, Y537N, Y537C, D538G, S463P) in patients with advanced HR+ endometrial carcinoma treated between 2008 and 2020 for at least 30 days by AA or M. Timepoints were before exposure and at progression/during endocrine therapy. 22 patients were included: 13 were treated with AA, 12 of whom progressed; 9 patients were treated with M, 8 of whom progressed... ESR1 mutations do not seem to be involved in the mechanisms of resistance to AA or M in HR+ endometrial cancer. The clinical relevance of their detection is not demonstrated.

Furthermore, de novo mutational signatures derived from CTCs described patient-specific biological processes. These data suggest that tumor tissue and CTCs provide complementary clinically relevant information to map tumor heterogeneity and tumor evolution.

P2, N=29, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Trial primary completion date: May 2022 --> Nov 2022

We present a low cost, highly specific, sensitive and robust assay for blood-based ESR1 profiling. The developed assay is fast, with a comparable sensitivity to ddPCR but has lower cost relative to ddPCR, and thus can be used as a fast screening method to classify patients as positive or negative for ESR1 mutations. Our results are consistent with reports that indicate that ESR1 mutations (especially D538G, Y537S) are associated with more aggressive disease.

Patients with HR+ MBC were evaluated before endocrine therapy (Anastrozole, Exemestane and Letrozole ) +/- CDK4/6 inhibitors (Palbociclib, Ribociclib, Abemaciclib), and every six months after initiation of therapy. The outcome of this study demonstrated that patients with mutations on known ESR1 hotspots (E380Q, L536H, Y537N/C/S and D538G) develop clinical resistance to endocrine therapy. Evaluation of these ESR1 hotspots mutations by ctDNA could be reliably used to monitor the response to endocrine therapy and predict the prognosis for MBC patients.

P2, N=100, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Recruiting --> Active, not recruiting

Amcenestrant combined with palbociclib showed encouraging OR and CB in pts with ER+/HER2- aBC, irrespective of ESR1 mutation status.

P2, N=29, Active, not recruiting, Sermonix Pharmaceuticals Inc. | Recruiting --> Active, not recruiting

The total concordance rate between ESR1 status on tumor tissue and plasma was 91%. Our results confirm the potential role of liquid biopsy as a non-invasive alternative to tissue biopsy for ESR1 mutation assessment in mBC patients.

Of those, 31 (41.3%) received letrozole with palbociclib, and 28 (37.3%) received exemestane and everolimus (EverX). In contrast, PIK3CA mutations were significantly associated with longer TTP in patients receiving EverX treatment (median TTP of EverX: 15.9 months vs. 5.2 months, p = 0.01) and remained a significant factor in multivariable analysis for TTP of EverX in this subgroup (hazard ratio = 0.2, 95% CI = 0.1- 0.8, p = 0.03). ESR1 and PIK3CA mutations in cfDNA were associated with clinical efficacies of ET in HR+ MBC patients.

Fulvestrant, a pure antagonist and selective estrogen receptor degrader (SERD) is approved for the treatment of HR+/HER2- metastatic breast cancer. LY3484356 has shown synergy or additivity in combination with CDK4/6 inhibitor abemaciclib, mTOR inhibitor everolimus and PIK3CA inhibitor alpelisib in inhibiting cell proliferation in ER+ breast cancer cell lines in vitro and tumor growth inhibition in relevant xenograft or PDX models in vivo. The first-in-human Phase 1/2 clinical trial of LY3484356 (EMBER, ClinicalTrials.gov NCT04188548) is currently ongoing and a window-of-opportunity study evaluating the pharmacodynamic effects of LY3484356 in early stage breast cancer is expected to begin early 2021.

We present a low cost, highly specific, sensitive and robust assay for blood-based ESR1 profiling. The clinical performance of the ESR1 NAPA assay will be prospectively evaluated in a large number of well-characterized patient cohorts.

P2, N=100, Recruiting, Sermonix Pharmaceuticals Inc. | Trial completion date: Sep 2020 --> Feb 2022 | Trial primary completion date: Jul 2020 --> Feb 2022

Furthermore, the monitoring of circulating tumor DNA using this technique during treatment follow-up predicts the clinical benefit of palbociclib-fulvestrant. The multiplex ESR1-ddPCR detects, in a single reaction, the most frequent ESR1 activating mutations with good sensitivity. This method allows real-time liquid biopsy for ESR1 mutation monitoring in large cohorts of patients.

P2, N=24, Recruiting, Sermonix Pharmaceuticals Inc. | Not yet recruiting --> Recruiting | Initiation date: Jul 2020 --> Sep 2020