Abstract GS1-06: Circulating tumor DNA (ctDNA) in human epidermal growth factor receptor 2-positive (HER2+) Early Breast Cancer (EBC): Translational analysis of PHERGain neoadjuvant tailored treatment study

Abstract Introduction HER2-directed therapies have dramatically improved the outcome of patients (pts) with HER2+EBC, leading to the investigation of different de-escalation approaches. ctDNA is an emerging tool for risk stratification and real-time monitoring in EBC, offering the potential to personalize treatment decisions. The PHERGain study showed the feasibility of a FDG positron emission tomography (PET)-guided, pathological complete response (pCR)-adapted strategy to safely omit chemotherapy (CT) in pts with stage I-IIIA HER2+ EBC undergoing neoadjuvant dual HER2 blockade with trastuzumab and pertuzumab (HP). The PET- and pCR-guided approach allowed omission of CT in 37.9% of pts, achieving a 3-year invasive-disease free survival (iDFS) rate (94.8%) in the overall adaptive group. In this sub-study, we assess a tumor-uninformed epigenomic-based ctDNA assay for minimal residual disease detection to improve the prediction of pCR and 3-year iDFS, thereby enabling more tailored (neo)adjuvant treatment strategies for HER2+ EBC pts within the framework of the PHERGain study. Methods Details of the trial design and study population have been previously reported. Out of 356 randomized pts, 63 in group A (standard treatment) and 267 in group B (adaptive treatment) proceeded to surgery. The primary objective of the PHERGuide sub-study was to assess the correlation between ctDNA clearance after two treatment cycles and pCR (ypT0/is ypN0) in all included pts. Secondary objectives evaluated the association between ctDNA levels and patient outcomes. Blood samples were collected at baseline, after cycle 2 of neoadjuvant treatment, and pre-surgery. A total of 932 samples from 351 pts (336 baseline, 311 cycle 2, and 285 pre-surgery) were analyzed using Guardant RevealTM, a tumor-uninformed epigenomic assay that provides a binary ctDNA detection result along with an estimated tumor fraction. ctDNA clearance was defined as detectable ctDNA at baseline but undetectable at a subsequent time point. Categorical variables were analyzed using logistic regression models, and survival outcomes were assessed with Cox proportional hazards regression. Results Of the 932 samples, 801 were eligible for this sub-study (161 in group A and 640 in group B). ctDNA was detected in 204 of 288 (71%) baseline samples suitable for analysis. Detection rates were significantly correlated with disease stage: 33% (9/27) in stage I, 73% (154/217) in stage II, and 93% (41/44) in stage III tumors (p 0.001). A total of 126/167 (75.4%) and 124/149 (83.2%) pts with detectable baseline ctDNA showed ctDNA clearance after two treatment cycles and prior to surgery, respectively. ctDNA clearance after two treatment cycles (p = 0.003) and at the pre-surgical time point (p 0.001) was significantly associated with achieving a pCR. No significant correlation was observed between baseline ctDNA status and pCR (p = 0.583). No patient with detectable ctDNA prior to surgery (n = 25) achieved a pCR. ctDNA positivity at baseline was associated with worse 3-year iDFS (92.5% vs. 100%, HR 0.20; 95%CI 0.02-0.98; p = 0.046). Conclusions These findings demonstrate a significant correlation between ctDNA clearance and pCR in HER2+ EBC pts undergoing neoadjuvant HER2-targeted therapy. Moreover, detectable ctDNA at baseline is associated with inferior 3-year outcomes. Further prospective studies are needed to confirm these results. Citation Format: A. Llombart-Cussac, J. Pérez-García, M. Ruiz-Borrego, A. Stradella, B. Bermejo, S. Escrivá-de-Romaní, C. Reboredo, N. Ribelles, A. Cortés-Salgado, C. Albacar, M. Colleoni, G. Antonarelli, G. Notini, M. Gion, J. García-Mosquera, L. Sanz, E. Martínez-García, P. González-Alonso, A. Amaya-Garrido, J. Guerrero, J. Rodríguez-Morató, L. Mina, G. Martrat, M. Quintana, F. Riva, D. Dustin, H. Zhang, M. Mancino, J. Cortés. Circulating tumor DNA (ctDNA) in human epidermal growth factor receptor 2-positive (HER2+) Early Breast Cancer (EBC): Translational analysis of PHERGain neoadjuvant tailored treatment study 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 GS1-06.