ER D538G

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

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.

This analysis identified acquired gene alterations in patients with HR+/HER2- advanced breast cancer treated with ribociclib plus ET or placebo plus ET. These data may support further studies on acquired resistance mechanisms and inform future systemic interventions in the post-CDK4/6 inhibitor setting.

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.

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

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.

Our results strongly suggest that metastatic driver mutations are sequentially acquired and selected within the same clonal lineage both before, but more commonly after, dissemination from the primary tumor, and that these mutations are biologically consequential. Despite inherent limitations in sampling archival primary tumors, our findings indicate that tumor cells in most patients continue to undergo clinically relevant genomic evolution after their dissemination from the primary tumor. This provides further evidence that metastatic recurrence is a multi-step, mutation-driven process that extends beyond primary tumor dissemination and underscores the importance of longitudinal tumor assessment to help guide clinical decisions.

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.

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.

In summary, our results indicate that CSF-cfTNA analysis with the Genexus-OPA can provide clinically relevant information in patients with brain metastases with short TAT.

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

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.

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.

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.

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).

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.

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

She received nab-paclitaxel and radiation to the right breast, regional lymph nodes, and T11 bone...With objective evidence of evaluable disease, exemestane was discontinued and fulvestrant and ribociclib were initiated...Incorporating FES-PET in routine staging should be considered in patients with metastatic breast cancer that is low-grade and ER-positive. Further evidence of response prediction in patients with ESR1 mutations will enhance the utility of FES-PET in the clinical management of these patients.

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.

Prior anticancer treatments included one or two lines of endocrine therapies, including fulvestrant, and/or one or two lines of chemotherapies. Based on the preliminary results, oral administration of TFX06 at 60 or 100 mg once daily had favorable safety and PK profiles. TRAEs, such as gastrointestinal toxicities, bradycardia, and visual disturbances were not observed. Pleasingly, TFX06 demonstrated early extracranial and intracranial antitumor activity (PR) in a patient with a brain metastasis.

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.

METHODS Pts (n = 303) were randomized 1:1 to GIR or PCET (75% of pts had fulvestrant [FUL]; 25%, an aromatase inhibitor). ESR1 MAF declined to a greater degree with GIR compared with cTF, which is consistent with the larger magnitude of PFS benefit seen with GIR in pts with ESR1m tumors. The specific ESR1 variants D538G and Y537X showed greater sensitivity to GIR compared with PCET or FUL.

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

Two pts each had homozygous deletions in ATM, BRCA2 and CHEK2; another had FGFR2 fusion (FGFR2-KIAA1598). Conclusions Comprehensive ctDNA NGS can identify ESR1 mutations along with co-alterations that may inform therapeutic decisions for patients with ABC in AME.

P2, N=4, Completed, M.D. Anderson Cancer Center | Recruiting --> Completed | N=124 --> 4 | Trial completion date: Apr 2024 --> Sep 2023 | Trial primary completion date: Apr 2024 --> Sep 2023

Inhibition of these regulatory proteins resulted in decreased growth and migration, representing potential novel treatment strategies for ER mutant endometrial cancer. Implications: This study provides insight into mutant ER activity in endometrial cancer and identifies potential therapies for women with ER mutant endometrial cancer.

Collectively, our data support evidence for a stronger RXR response associated with FOXA1 reprogramming in ESR1 mutant cells, suggesting development of therapeutic strategies targeting RXR pathways in breast tumors with ESR1 mutation. Implications: It provides comprehensive characterization of the role of FOXA1 in ESR1 mutant breast cancer and potential therapeutic strategy through blocking RXR activation.

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.

Two mutations with a variant allele frequency (VAF) of ≥0.1% and twenty-six mutations with a VAF of <0.1% were found. By using this LNA-clamp ddPCR, this study demonstrated the presence of minor clones with a VAF of <0.1% in primary breast cancer.

P2, N=124, Recruiting, M.D. Anderson Cancer Center | Trial primary completion date: Apr 2023 --> Apr 2024

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

Our results suggest that RRM2 and CDC6 downstream of XBP1 contribute to endocrine resistance in ER-positive breast cancer. XBP1-gene signature is associated with poor outcome and response to tamoxifen in ER-positive breast cancer.

P1/2, N=206, Active, not recruiting, Eisai Inc. | N=151 --> 206