Abstract PS2-08-09: Baseline HER2 expression on circulating tumor cells in metastatic breast cancer

Abstract Background: The predictive role of human epidermal growth factor receptor 2 (HER2) in metastatic breast cancer (mBC) is well established. Recently, trastuzumab deruxtecan (T-DXd) has shown efficacy even in tumors with low HER2 expression, prompting a reassessment of how HER2 levels are evaluated. Circulating tumor cells (CTCs) can provide a more granular, non-invasive and real-time assessment of HER2 status. In this study, we aim to investigate baseline (BL) HER2 expression on CTCs and test its potential impact on treatment response and survival in a real-world cohort of patients (pts) treated with trastuzumab-based therapy (antiHER2) or T-DXd. Methods: Pts with mBC who received either antiHER2 or T-DXd and underwent CTCs enumeration and HER2 staining with CellSearch® before treatment started were eligible for analysis. BL counts of total and HER2-positive (HER2+) CTCs were recorded and CTC status was stratified accordingly into indolent (ind; 5 total CTCs), aggressive-HER2-negative (agg-HER2-; ≥ 5 CTCs, no HER2+) and aggressive-HER2+ (agg-HER2+; ≥ 5 CTCs, ≥ 1 HER2+). The HER2+/total CTC ratio was also calculated and dichotomized based on the overall median (high vs low ratio). Differences in median values by treatment type were assessed using the Mann-Whitney U test. Associations with clinical benefit rate (CBR, dichotomized for this analysis) and progression-free survival (PFS) were evaluated using logistic regression and Kaplan-Meier curves with log-rank test, respectively. Results: A total of 23 pts were included: 16 (69.6%) had de novo mBC and 14 (60.9%) were HER2+ on tissue biopsy. Median follow-up from diagnosis of stage IV disease was 17.1 months. From September 2023 to February 2025, 19 anti-HER2 and 11 T-DXd treatments were initiated (6 pts received 1 treatment). Of these, 10 were administered in the first-line setting (all antiHER2), 8 in the second-line (2 antiHER2, 6 T-DXd) and 12 in third or later lines (7 antiHER2, 5 T-DXd). AntiHER2 was administered in combination in 18/19 cases (94.7%), mostly with chemotherapy (12/19, 63.2%). BL CTC status was classified as ind in 12 occasions (40.0%), while agg-HER2- and agg-HER2+ phenotypes were observed in 9 cases (30.0%) each. No significant differences were documented in BL median counts of total or HER2+ CTCs between antiHER2 and T-DXd. The median HER2+/total CTC ratio was 0.13% (interquartile range, 0-42.9%) overall, with no significant differences between treatment groups. The overall CBR was 50.0%, with rates of 57.9% for antiHER2 and 36.4% for T-DXd. Compared to ind CTC status, the agg-HER2- phenotype was associated with a significantly lower likelihood of clinical benefit (OR 0.09, p = 0.046), while a higher HER2+/total CTC ratio was strongly linked to increased odds of clinical benefit (OR 28, p = 0.012), regardless of treatment received. Interestingly, although CTC count was not associated with PFS, agg-HER2+ and ind status showed similar and significantly better outcomes than agg-HER2-, both in the overall cohort (log-rank p = 0.027) and in the antiHER2 subgroup (log-rank p = 0.002), but not for T-DXd. Similarly, a higher HER2+/total CTC ratio was significantly associated with improved PFS, both overall and in the antiHER2 group (log-rank p = 0.010 and 0.001, respectively). Conclusions: Agg-HER2+ CTC phenotype and a higher HER2+/total CTC ratio were both associated with greater benefit from antiHER2, remarking the predictive role of HER2 and suggesting validity of CTC-based HER2 assessment. Further research is warranted to identify reliable predictors of response to T-DXd, which may require alternative cutoffs or distinct metrics to more accurately reflect HER2 expression levels. Despite limitations due to the sample size, these preliminary findings provide a rationale for developing CTC-based scoring systems to guide treatment selection with HER2-targeted agents. Citation Format: B. Pastò, G. Guimarães, E. Molteni, N. Bayou, M. Serafini, E. Nicolò, L. Pontolillo, K. L. Eng, E. Andreopoulou, O. Elemento, L. Gerratana, F. Puglisi, C. Reduzzi, M. Cristofanilli. Baseline HER2 expression on circulating tumor cells 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 PS2-08-09.