Abstract PS4-01-07: Prospective Validation of HER2DX in Early HER2+ Breast Cancer: Operational Feasibility of Centralized Genomic Screening in the DEFINITIVE Trial. Real-Time Insights from the First 80 Screened Patients

Abstract Background: HER2-positive breast cancer (BC) accounts for approximately 20% of all breast tumors. Despite major advances in HER2-targeted therapies, many patients with early-stage HER2-positive BC remain overtreated, leading to unnecessary toxicity and increased healthcare costs. HER2DX is a genomic diagnostic tool that stratifies patients by risk of recurrence and likelihood of achieving pathological complete response (pCR) after neoadjuvant therapy. The DEFINITIVE trial (www.thedefinitivetrial.eu) evaluates the clinical utility of HER2DX to guide treatment de-escalation or escalation in early-stage HER2-positive BC. We report preliminary feasibility and performance metrics from the first 80 screened patients. Methods: DEFINITIVE (NCT06446882) is an international, multicenter, randomized, open-label, two-arm clinical trial designed to evaluate the impact of HER2DX-guided treatment versus standard of care on health-related quality of life, safety, and efficacy outcomes. Eligible patients include premenopausal or postmenopausal women and men with stage II-IIIA HER2-positive BC suitable for neoadjuvant therapy. This interim analysis includes all patients screened from study initiation on October 30, 2024, through June 19, 2025. All patients underwent centralized screening with HER2DX using formalin-fixed paraffin-embedded (FFPE) tumor samples shipped to a reference laboratory. Operational endpoints included the number of samples received, HER2DX testing success rate, and turnaround time (TAT) from sample receipt to result delivery. All data were collected prospectively as part of the trial workflow. Results: As of the data cutoff, 80 patients had been screened across 11 sites in 2 countries. A total of 80 samples were received by the central laboratory, and HER2DX testing was successfully completed in 78 cases, yielding a success rate of 97.5%. Test failures were due to insufficient tumor content or low RNA purity. The median TAT was 8 working days (interquartile range [IQR] 7-10; range 3-16), confirming the feasibility of rapid centralized testing. The median time from tumor biopsy to sample reception at the central lab was 29.5 days (IQR 22-42), reflecting real-world variability in local processing and logistics. Among the 78 patients with available HER2DX results, 42.3% were classified as having low pCR likelihood, 30.8% as intermediate, and 26.9% as high. Based on HER2DX risk score, 47.4% were considered low-risk and 52.6% high-risk. ERBB2 mRNA expression levels also reflected biological heterogeneity, with 11.6% classified as low, 25.6% as intermediate, and 62.8% as high expression. Conclusions: Centralized HER2DX-based screening in the DEFINITIVE trial has proven operationally feasible on an international scale. High testing success rates, acceptable TAT, and biologically informative distributions support the integration of genomic classifiers into real-time treatment decision-making in prospective clinical trials. Citation Format: M. Gonzalez Rodriguez, K. Amilliano, S. Pernas, P. Sánchez, I. Blancas, E. Gal-Yam, M. Lopez-Flores, E. Lopez-Miranda, T. Curiel, C. Rodriguez, A. Pous, E. Galve-Calvo, E. Sanfeliu, M. Pujol, C. Polo, C. Guardia, L. Paré, M. Marín-Aguilera, A. Herrera, A. Sebastian, A. Prat, O. Martínez-Sáez, T. Pascual. Prospective Validation of HER2DX in Early HER2+ Breast Cancer: Operational Feasibility of Centralized Genomic Screening in the DEFINITIVE Trial. Real-Time Insights from the First 80 Screened Patients [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 PS4-01-07. Funding: This project has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No. 101136953.