ER D538G

P2, N=4, Terminated, M.D. Anderson Cancer Center | Completed --> Terminated; Despite modifications, accrual was poor and the closed

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 conclusion, employing enrichment techniques for low-allele frequency variants can improve the sensitivity of ESR1 mutation detection in breast cancer. This approach has the potential to enhance our ability to monitor and respond to disease progression dynamically, ultimately supporting more effective, individualized patient care.

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).

A dilution series of DNA fragments, ranging from 5 to 10,000 copies per reaction, was analyzed to determine linearity. The kit's performance was further assessed using the SensID ESR1

Methods The phase II PELOPS trial (NCT02764541) enrolled pts with clinical stage I-III HR+, HER2-negative BC and randomized to NET plus palbociclib or NET alone for 24 weeks prior to surgery; plasma samples were collected at baseline (BL) and prior to surgery (PS)...These results are limited by the sample size but warrant further investigation of the role of ctDNA as a tool to predict sensitivity to ET and long term outcomes. Such a tool could be valuable in tailoring treatment approaches, particularly given the ongoing development of novel ETs and the addition of CDK4/6i in the adjuvant setting.

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.

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.