Abstract PO4-15-03: Quantitative Measurement of TROP2 via Chromogenic Immunohistochemistry in Breast Cancer

Abstract Sacituzumab govitecan (SG) is an antibody-drug conjugate that targets human trophoblast cell-surface antigen 2 (TROP2), which is expressed in over 90% of breast cancer cases (Zaman et al, 2019). By delivering the cytotoxic SN-38 (topoisomerase I inhibitor) to TROP2 expressing breast cancer cells, SG showed promising antitumor activities in clinical trials and is now an approved treatment for triple-negative breast cancer (TNBC) and hormone receptor (HR)-positive HER2-negative breast cancer in the metastatic setting. Despite the prevalent expression of TROP2 in breast cancer, the objective response rates reported from clinical trials were about 30%, indicating the need for a diagnostic test that can identify which patients are likely to benefit from therapy. Although the drug is approved without a companion diagnostic assay, recent data from the TROPiCS-02 trial has shown that TROP2-low (H-score 100) showed a non-significant hazard ratio for benefit from SG. In the same study, patients treated with SG with H-score 100 showed a significant hazard ratio for benefit compared to physician’s choice (Tolaney et al, ASCO 2023). This suggests that TROP2 expression level is associated with SG response and that a threshold for TROP2 expression level may aid patient selection. Although over 90% of breast cancer patients were considered TROP2-positive by IHC, the expression level of TROP2 was never quantitatively assessed in large breast cancer cohorts. Here we describe a quantitative chromogenic immunohistochemistry (IHC) assay with a cell line standard. Mass spectrometry was used to measure TROP2 peptide concentrations in the six standard cell lines, and the correlation between the mass spectrometry and IHC data for each cell line was then used to convert chromogenic signals to concentrations of TROP2 protein (fmol/mm2). The antibody used in this assay was rigorously validated, and the antibody concentration was optimized for the best signal-to-noise ratio. The optical density (OD) of chromogenic staining was measured using QuPath 0.4.3 (Qymia extension) by identifying the tumor area via object classifiers and manual editing and then calculating the area-normalized sum of OD. Collectively, this assay can measure up to 29.1 fmol/mm2 of TROP2. By applying this assay to two serial retrospective primary breast cancer cohorts from Yale University, we quantitatively measured TROP2 expression levels in 332 clinical cases. Not surprisingly, over 90% of cases showed some chromogenic signal, with a median TROP2 concentration of 2.1 fmol/mm2 and a maximum of 10.5 fmol/mm2. TROP2 expression levels showed no significant association with clinicopathologic characteristics including race, stage, BRCA mutation status, molecular subtype, HER2 IHC levels, and outcome. Further work is underway to optimize the assay toward the goal of determination of whether there is a quantitatively definable threshold for TROP2 expression below which patients are unlikely to benefit from SG or other TROP2-targeted therapies. Citation Format: Mengni He, Matthew Liu, Thazin Aung, Sneha Burela, Katherine Bates, Charles Robbins, David Rimm. Quantitative Measurement of TROP2 via Chromogenic Immunohistochemistry in Breast Cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-15-03.