PS5-09-09: Ctc-express: a prospective observational study for ctc biomarker expression and cfdna/cfrna assessment in metastatic breast cancer

Abstract Background Metastatic breast cancer (MBC) remains an incurable disease with a median overall survival (OS) depending on prognostic factors and systemic therapy. As of yet, prospective real-world data on circulating tumor cell (CTC) phenotyping such as expression of HER2, estrogen receptor (ER), and PD-L1 on CTCs and response to therapy is limited. Furthermore, adding cfDNA/cfRNA to CTC phenotyping may provide additional clinical information, for which prospective data are also not yet available. This prospective study in a real-world clinical setting assesses the impact on treatment decisions and response assessment of CellSearch CTC enumeration, CTC biomarker expression, and cfDNA/cfRNA results in subjects with MBC. Trial design CTC EXPRESS is a prospective, multicenter, observational study of CellSearch CTC enumeration, CTC HER2, ER, and PD-L1 expression, and cfDNA/cfRNA results in subjects with histologically proven MBC. Treatment for MBC will be chosen and performed by treating physicians/providers. Subjects will be followed until disease progression. The study will simultaneous enroll two MBC cohorts: Cohort 1 will include HR+/HER2- subjects before starting 1st line therapy and Cohort 2 will enroll triple-negative subjects starting any line of therapy and HR+/HER2- or HER2+ subjects starting 2nd line therapy or beyond. CTCs and cfDNA/cfRNA will be collected minimally at Baseline and before the first re-staging visit. In Cohort 1, additional cfDNA/cfRNA will be collected at week 4, 8 and 12. Physicians will complete a short questionnaire regarding the impact of CTCs and cfDNA/cfRNA on treatment decisions at several timepoints. A total of 187 subjects will be enrolled in up to 10 US centers. The main objectives are: 1. To prospectively assess the impact on treatment decisions and response assessment of CTC enumeration and CTC biomarker expression, and cfDNA/cfRNA results in subjects with MBC. 2. To evaluate the correlation of CTC HER2, ER and PD-L1 expression with response to therapy and PFS. 3. To evaluate the correlation of cfDNA/cfRNA genomic alterations with response to therapy and PFS. 4. To evaluate the correlation of early cfDNA/cfRNA dynamics with response to 1st line therapy and PFS in HR+/HER2- MBC. Clinical trial identification NCT NCT06833853 Citation Format: M. Balic, D. Riseberg, T. Pluard, C. Tweed, J. Foldi, F. de Snoo, L. Stork-Sloots, F. K. Kuhr, S. Lazaro, D. S. Spinner, R. Prendergast, C. Dorris, M. Albitar, A. M. Brufsky. Ctc-express: a prospective observational study for ctc biomarker expression and cfdna/cfrna assessment in 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 PS5-09-09.