Quantifying the clinical impact of tissue reflex testing for liquid biopsy <i>ESR1</i> mutation–negative cases with low ctDNA tumor fraction (TF) in HR(+)HER2(-) breast cancer.

Abstract

1065 Background: ESR1 mutations ( ESR1 mut) commonly drive acquired resistance to estrogen deprivation by aromatase inhibitors, a first-line standard of care for HR(+)HER2(-) metastatic breast cancer (MBC). We previously published that approximately 63% of patients with HR(+)HER2(-) MBC at progression have a liquid biopsy (LBx) negative for ESR1 mut. The absence of an ESR1 mut may either accurately reflect the tumor genotype (true negative) or represent a false negative due to insufficient ctDNA shedding, with the risk of missing an actionable mutation. Among these patients, 40% exhibit a high ctDNA TF ≥1% (informative negative), while 60% have a low ctDNA TF &lt; 1% (indeterminate negative). This suggests that up to 38% of all patients with HR(+)HER2(-) MBC in this context could potentially benefit from reflex tissue biopsy (TBx) for ESR1 mut in cases deemed indeterminate negative by LBx due to low ctDNA shedding. The goal of this study is to determine the rate of ESR1 mut detection in a new TBx after an indeterminate negative result from FoundationOne Liquid CDx (F1LCDx). Methods: This study included a cohort of patients with BC who underwent tissue and liquid Foundation Medicine comprehensive genomic profiling (CGP) within an interval of up to 90 days during routine clinical care. Clinical data of a subset of patients with confirmed HR(+)HER2(-) MBC was obtained from the US-wide deidentified Flatiron Health-Foundation Medicine MBC clinicogenomic database (CGDB). The data originated from ~280 cancer clinics (~800 sites of care) between 01/2014-09/2024. False negative rate (FNR) and positive percent agreement (PPA) for ESR1 mut detection were calculated with tissue CGP as reference. Results: A total of 522 BC patients underwent TBx and LBx. Among these, 229 (43.9%) had ctDNA TF &lt; 1%. Without accounting for TF, the overall FNR for ESR1 mut was 6.3% and the PPA was 67.1%. In LBx with TF ≥1%, the FNR for ESR1 mut was 0.9% and PPA was 96.0%. In contrast, for TF &lt; 1% samples, the FNR was 12.0% and the PPA 25.7%. 101 patients were included in the CGDB and had a confirmed HR(+)HER2(-) MBC, in which 56 (55.4%) had LBx with ctDNA TF &lt; 1%. The overall FNR for ESR1 mut in this subset of patients was 9.5% and the PPA was 61.9%. In LBx with TF ≥1%, the FNR was 0% and PPA was 100%. And for TF &lt; 1%, the FNR was 15.1% and PPA 20.0%. Conclusions: BC patients with informative negative ESR1 mut (defined as LBx ESR1 mut negative with TF ≥1%) are unlikely to have ESR1 mut detected on tissue CGP testing. However, patients with indeterminate negative ESR1 mut (defined as LBx ESR1 mut negative with TF &lt; 1%), 12-15% were found to be false negatives. This suggests that approximately 5% of all HR(+)HER2(-) MBC patients with ESR1 mut could be missed without reflex testing with a TBx. ctDNA TF levels offer critical guidance in deciding when a reflex to tissue is warranted, ensuring accurate treatment.