Extreme ESR1 Polyclonality, Mutation Dynamics, and Effects on Treatment Outcomes in Patients with ER+ Metastatic...

ESR1 mutations commonly emerge on endocrine therapy (ET) during breast cancer treatment and contribute to therapeutic resistance. However, the impact of increasing degrees of polyclonality is not well understood. We conducted a retrospective, descriptive review of patients with estrogen receptor-positive (ER+) metastatic breast cancer (MBC) and ≥10 concurrent ESR1 variants defined as extreme ESR1 polyclonality, assessed on circulating tumor DNA (ctDNA) between August, 2020 and July, 2025. Patient and tumor characteristics, frequency of ESR1 variants, co-occurring mutations, and clonal changes in ESR1 in response to treatment were explored. Nine patients met criteria for extreme ESR1 polyclonality. Patients received a median of 1 line of ET in the metastatic setting before their initial ctDNA assessment, which identified 0-31 ESR1 clones (mean, 6). The most frequent ESR1 clonal mutations were Y537N, Y537S, and D538G. Common co-alterations included TP53 and RB1 mutations and CCND1 amplification. Tumor mutational burden (TMB) at the time extreme ESR1 polyclonality emerged ranged from 6.7-56.5 mutations per megabase (mut/Mb); all assessable tumors were microsatellite stable (MSS). This represents the first case series to explore extreme ESR1 polyclonality. Characterizing the mutational dynamics associated with this phenomenon may improve understanding of ESR1 mutations and related impacts on treatment resistance.