<i>ESR1</i> mutation longitudinal dynamics in RWD cohort of HR+/HER2− metastatic breast cancer patients treated with standard-of-care hormonal therapy.

3062 Background: ESR1 mutations ( ESR1 m) are known drivers of resistance to aromatase inhibitors. While ESR1 m has been linked to worse survival outcomes, clonal complexity over time has not been well-evaluated in a real-world context. Here we characterized longitudinal dynamics of ESR1 m, including rare point mutation emergence and polyclonality in hormone receptor-positive/HER2-negative metastatic breast cancer (HR+/HER2- mBC) patients (pts) treated with aromatase inhibitor (AI) plus CDK4/6 inhibitor (CDK4/6i). Methods: We identified a cohort for longitudinal analysis from the Tempus real-world database (RWD). Eligible pts had HR+/HER2- mBC (diagnosed before 01/2023), received AI+CDK4/6i therapy in mBC, and had comprehensive genomic profiling from tissue/liquid biopsy (Tempus xT/Tempus xF) within 2 yrs of treatment, with ≥1 on-treatment xF tests. We analyzed ESR1 clonal dynamics, starting from first ESR1 m detection in a subset of pts who had multiple xF tests. Polyclonality was also assessed, defined as presence of at least 2 distinct ESR1 m in a sample. Results: Of 301 HR+/HER2- mBC pts on AI+CDK4/6i therapy, 102 (33.9%) developed an ESR1 m. In the ESR1 m group, 72 pts (70.6%) had 2+ on-treatment liquid biopsies (Tempus xF/xF+), comprising the final cohort for observation. At ESR1 m detection, 64.8% pts had a single detectable ESR1 m, most commonly D538G (31.9%) and Y537S (12.5%). In contrast, 34.7% had polyclonal mutations (19.7% with 2 distinct mutations; 15.3% with 3+). Recurrent detection of D538G (52.8%), Y537S (20.8%), and Y537N (16.7%) was noted in repeat xF testing. Detection of additional ESR1 m (26.4%) over time confirmed known common mutations such as Y537N (8.3%), D538G (5.6%), and Y537S (5.6%). However, longitudinal testing also found rare mutations such as V422del and H524L. Mutational dynamics shifts were also revealed by longitudinal testing. In 36.1% pts, a previously detected ESR1 m was no longer found, often in those who initiated a SERD. D538G (5.6%) and Y537N (4.2%) were the most commonly lost mutations. Recurrent and new ESR1 m detection occurred in pts who continued AI+CDK4/6i (44.4%). Many pts showed a combination of these dynamics. Conclusions: Multimodal RWD analyses demonstrate valuable clinical insights on ESR1 m patterns detected from longitudinal molecular surveillance testing in HR+/HER2- mBC pts treated with AI+CDK4/6i. We observed shifting dynamics in ESR1 m burden, including detection of rare mutations and polyclonality. These findings show the value of monitoring ESR1 m longitudinally over disease course and prompt further research into clinical impacts of clonal dynamics.