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.

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