Abstract PS2-08-22: Clinical Relevance of ctDNA-Guided Molecular Profiling After Progression on CDK4/6 Inhibitor plus Endocrine Therapy in HR+/HER2- Metastatic Breast Cancer

Abstract Background: Circulating tumor DNA (ctDNA)-based molecular profiling is used to guide therapeutic decision-making in hormone receptor-positive/HER2-negative (HR+/HER2-) metastatic breast cancer (mBC) following progression on cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i) and endocrine therapy (ET). The predictive value of post-progression genomic alterations for subsequent lines of treatment remains unclear. Methods: We analyzed all consecutive patients with HR+/HER2- mBC treated at our Institution with ctDNA testing after progression on CDK4/6i-based therapy between November 2023 and June 2025. ctDNA sequencing was performed using a 77-gene liquid biopsy next-generation sequencing assay. Clinical, pathological, treatment, and outcome data were extracted from medical records. Oncogenic alterations were assessed both individually and by oncogenic pathways, as described by Sanchez-Vega et al. (Cell, 2018). Real-world progression-free survival (PFS) was defined as the time from each treatment initiation to disease progression or death, whichever occurred first. The log-rank test was used to compare PFS among groups, and Cox proportional-hazards models were applied to compute hazard ratios (HRs) and 95% confidence intervals (CIs). Results: A total of 256 patients were included (median age: 49 years, interquartile range IQR 43-57; 51% premenopausal). ctDNA was collected following progression on CDK4/6i in combination with either aromatase inhibitors (51%) or fulvestrant (49%). The median time from metastatic diagnosis to ctDNA testing was 36 months (IQR 18-65), with most patients tested after first- (n=135, 53%) and second-line treatment (n=62, 24%). Based on CDK4/6i treatment duration, 11% showed primary resistance (PFS 6 months), 47% intermediate response (6-24 months), and 42% prolonged benefit (24 months). Oncogenic alterations associated with reduced benefit from CDK4/6i plus ET were BRCA2 (HR 2.54, 95% CI 1.44-4.49, p0.001) and of the receptor tyrosine kinase (RTK) pathway (HR 2.96, 95% CI 1.66-5.26, p0.001). ctDNA analysis revealed PIK3CA (33%) and ESR1 (32%) as the most common actionable alterations, followed by PTEN (6%) and AKT1 (4%), in line with prior evidence. Following ctDNA profiling, patients received oral selective estrogen receptor degraders (SERDs; 10.5%), Pi3k/Akt pathway inhibitors with ET (7%), everolimus with exemestane (17.6%), oral chemotherapy (51.2%), or intravenous chemotherapy (19.1%). Patients with 6 months of exposure to CDK4/6i received oral SERDs and Pi3k/Akt pathway inhibitors only in 2.6% and 0% of cases, respectively, while patients with 24 months of exposure to CDK4/6i in 6.1% and 24.1% of cases, respectively. Genomic alterations of the Pi3k/Akt pathway were associated with a trend toward reduced PFS on oral SERDs (HR 4.95, 95% CI 0.99-24.75, p=0.052), while MAPK pathway alterations to reduced benefit from everolimus with exemestane (HR 2.98, 95% CI 1.00-8.88, p=0.050). Oncogenic alterations of SR1 were correlated with shorter PFS on single-agent chemotherapy (HR 1.99, 95% CI 1.19-3.32, p=0.008). Conclusions: ctDNA-guided molecular profiling after progression on CDK4/6i and ET reveals clinically-relevant genomic alterations that may influence subsequent treatment outcomes in HR+/HER2- mBC. These findings further support integrating ctDNA analysis into treatment planning beyond first-line progression in HR+/HER2- mBC. Citation Format: A. Marra, D. Presti, K. Venetis, G. Castellano, V. Peruzzo, R. Adorisio, A. Ranghiero, A. Borghi, D. Vacirca, E. Giordano, S. Perazzo, K. Qeraj, M. Lombardi, P. Zagami, D. Trapani, E. Munzone, E. Guerini Rocco, N. Fusco, G. Curigliano. Clinical Relevance of ctDNA-Guided Molecular Profiling After Progression on CDK4/6 Inhibitor plus Endocrine Therapy in HR+/HER2- Metastatic Breast Cancer 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 PS2-08-22.