Real-world estrogen receptor alpha 1 (ESR1) testing patterns and results for ER+/HER2- metastatic breast cancer in...

To understand historical and recent ESR1 testing rates, and when ESR1 mutations emerge during first-line (1 L) treatment.

This retrospective, observational cohort study used the Flatiron Health Research Database (FHRD) and the Flatiron Health-Foundation Medicine metastatic breast cancer (mBC) Clinico-Genomic Database (CGDB). Adult patients with a confirmed diagnosis of hormone receptor-positive/human epidermal growth factor receptor 2-negative mBC from 1/1/2018 to 6/30/2024 were included. ESR1 testing patterns and test results were descriptively analyzed.

Among 7772 patients with mBC in the FHRD who initiated 1 L therapy, tumor ESR1 mutation status was evaluated for 222 (3%) patients at baseline (≤ 90 days before 1 L) and 1355 (17%) during 1 L. The percentage of patients who had an ESR1 test result reported during 1 L increased over time (11% in 2018-19, 19% in 2020-21, 22% in 2022-24). Median time from 1 L start to first ESR1 test was 7.4 months (mos) among tested patients. A positive test result was reported for 29/222 (13%) patients tested at baseline and 240/1355 (18%) tested during 1 L. Most (60%) tests during 1 L used tissue specimens, while the remaining 40% were liquid biopsies, and the median time from specimen collection to result reporting in 1 L was 28 (IQR:10-84) days. Focusing on time periods wherein specimens were provided, ESR1 test positivity was 6.7% (76/1,127) for specimens provided at baseline, 23% (15/65) for specimens provided 9 to 12 months into 1L therapy, 38% (26/69) for those provided 15 to 18 months into 1L, and 40% (38/94) for those provided from 18 to 24 months into 1L.

ESR1 mutations can be detected at any time interval during 1L.

Not applicable.