Abstract PS4-03-11: Uncovering ctDNA-detected co-mutational patterns in HR+/HER2− metastatic breast cancer to inform treatment sequencing

Abstract Introduction: Clinically actionable mutations in ESR1 and genes within the PI3K/AKT/PTEN pathway inform the use of selective estrogen receptor degraders (SERDs), PI3K inhibitors, and AKT inhibitors in patients (pts) with hormone receptor-positive, HER2-negative (HR+/HER2−) metastatic breast cancer (mBC). While the prevalence of such mutations detected through circulating tumor DNA (ctDNA) varies by treatment type and line, the broader landscape of co-occurring actionable mutations remains poorly characterized. Methods: Clinical and genomic data were retrospectively collected from patients with HR+/HER2− mBC who underwent ctDNA testing at the European Institute of Oncology (IEO) between November 2023 and June 2025, upon progression to endocrine therapy plus CDK4/6 inhibitors. Genomic profiling was performed using a 77-gene liquid biopsy (LB)-based next-generation sequencing (NGS) assay. Progression-free survival (PFS) and overall survival (OS) were compared between groups using Cox proportional hazards models. Results: Among the 256 patients who underwent ctDNA testing, 53% were tested after first-line (1L) therapy, 24% after second-line (2L), and 23% after third-line or beyond (≥3L). PIK3CA mutations were detected in 33% of pts (n=84), ESR1 in 32% of pts (n=82), PTEN in 5.9% of pts (n=15), and AKT1 in 3.9% of pts (n=10). PIK3CA mutations were more frequently identified in patients tested at ≥3L, while ESR1, PTEN, and AKT1 alterations were more frequently observed in patients tested at 1L. However, these distributional differences across treatment lines did not reach statistical significance. Other clinically relevant genes mutations were also observed in TP53 (n=47, 18.4%), ERBB2 (n=16, 6.3%), and RB1 (n=10, 3.9%). Co-mutations in both ESR1 and PIK3CA were identified in 30.4% (n=25) of ESR1-mutant patients, more frequently when tested after the 1L (31.2%) and ≥3L (42.1%). ESR1 co-mutation with AKT1 and PTEN was less frequent, occurring in 6.1% and 3.6% of ESR1-mutant patients, respectively. To explore the clinical relevance of these molecular profiles, PFS and OS were compared across three molecular subgroups: (i) ESR1-mutant/PI3K-pathway-wild-type, (ii) ESR1-wild-type/PI3K-pathway-mutant (PIK3CA, AKT1, or PTEN), and (iii) double-mutant (ESR1-mutant/PI3K-pathway-mutant). No associations between co-mutations and clinical outcomes were found. Conclusions: ctDNA-guided molecular profiling is a fundamental tool for guiding clinical decisions among patients with HR+/HER2− who experience progression on ET. In our cohort, co-mutations occurred in almost 1/3 of patients and seems not to influence prognosis. More data are needed to clarify the prognostic role of co-mutations and to determine the most appropriate molecular driver for therapeutic choices. Citation Format: K. Venetis, A. Marra, D. Presti, D. Trapani, V. Peruzzo, R. Adorisio, G. Castellano, A. Ranghiero, A. Borghi, D. Vacirca, E. Giordano, S. Perazzo, K. Qeraj, P. Zagami, M. Lombardi, E. Munzone, G. Curigliano, E. Guerini Rocco, N. Fusco. Uncovering ctDNA-detected co-mutational patterns in HR+/HER2− metastatic breast cancer to inform treatment sequencing [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-03-11.