Abstract PS2-10-08: Quantitative measurement of HER2 and TROP2 predicts outcomes with trastuzumab deruxtecan in metastatic breast cancer

Abstract Background: Antibody-drug conjugates (ADCs) with targets of TROP2 (sacituzumab govitecan) and HER2 (trastuzumab deruxtecan, T-DXd) have expanded treatment options for metastatic breast cancer (mBC), including HR-positive/HER2-non-amplified and triple-negative subtypes. Nevertheless, the optimal selection of patients and treatment sequencing for these ADC therapies remains a clinical challenge. While both drugs are thought to be targeted therapies, the clinical value of assessment of HER2 and TROP2 expression has not been established. Here, we evaluated whether high-sensitivity, simultaneous, quantitative immunofluorescence (QIF) measurement of HER2 and TROP2 (TROPLEX™ assay) predicts clinical outcomes in T-DXd treated patients in a Yale retrospective cohort. Methods: We identified 65 patients with mBC who received T-DXd at Yale between 4/2020 to 2/2025 and retrieved available FFPE specimens. Sections were stained on a Leica BOND RX autostainer for HER2, TROP2, and cytokeratin using the TROPLEX™ assay. Samples were imaged with the CyteFinder II HT multiplex fluorescent microscope (Rarecyte). Image analysis was performed in QuPath using an image processing plugin developed for automated QIF/IHC analysis (Qymia). Tumor regions were annotated by a pathologist, and QuPath with the Qymia plugin quantified protein concentration (attomoles/mm2) for HER2 and TROP2 against a mass-spectrometry-calibrated cell line standard curve. Time to treatment failure (TTF) was defined as time from T-DXd initiation to radiographic progression, clinical progression, or treatment discontinuation due to toxicity up to 2 years of follow-up. Biomarker expression was dichotomized at the cohort median for Kaplan-Meier analysis with log-rank testing. Results: Samples from 65 T-DXd treated patients (19 HER2+, 35 HER2-/HR+, 11 TNBC) were analyzed with the TROPLEX™ assay. Specimens included metastatic biopsies in 51 patients (78.5%), local recurrences in 8 (12.3%), and primary tumor in 6 (9.2%). Median interval from tissue sampling to T-DXd was 18.4 months (range 0.7-30.2). For this specimen cohort, HER2 median expression was 3,554 amol/mm2 (range 15 - 8,232) and TROP2 median expression was 2,868 amol/mm2 (range 0 - 23,383). For HER2 expression, patients with above-median HER2 had significantly longer TTF (median 12.8 vs. 7.5 months; log-rank p = 0.019) and a hazard ratio (HR) of 0.51 (95% CI 0.29-0.91). Additionally, there was an inverse association between TROP2 expression and TTF, where patients with above-median TROP2 had significantly shorter TTF (median 5.5 vs. 12.8 months; log-rank p = 0.050) and HR of 1.74 (95% CI 1.01-3.04). In a continuous analysis of TROP2 expression, each 2,000-unit increase was associated with shorter TTF (HR 1.17, 95% CI 1.07-1.29, p = 0.0010). These associations for TROP2 remained consistent in non-triple-negative (n = 54; HR 1.18, 95% CI 1.05-1.32; p = 0.0039) and non-HER2-amplified (n = 46; HR 1.14, 95% CI 1.04-1.28; p = 0.0056) subgroups. Conclusions: Quantitative assessment of HER2 in amol/mm2 is correlated with TTF of T-DXd for mBC using the TROPLEX™ assay. Surprisingly, in this cohort, there appears to be an association between TROP2 amol/mm2 and TTF for T-DXd. Due to the small size of the cohort and the lack of known biological relationship between the biomarkers, this finding requires further validation with larger patient cohorts. Citation Format: C. J. Robbins, W. Wei, N. N. Chan, M. He, J. Benanto, D. B. Doroshow, I. E. Krop, D. L. Rimm. Quantitative measurement of HER2 and TROP2 predicts outcomes with trastuzumab deruxtecan 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 PS2-10-08.