Quantitative Assays for TROP2 Measurement in Breast Cancer and Comparison to H-Score

Trophoblast cell surface antigen 2 (TROP2) is a popular current antibody-drug conjugate target due to its high expression in various solid tumor types. With the recent FDA-approval of sacituzumab govitecan and datopotamab deruxtecan in breast cancer, TROP2-targeting therapies showed promising clinical outcomes. Despite the prevalent expression of TROP2 in breast cancer (about 90%), the objective response rates from clinical trials are around 30% with nonsignificant hazard ratios for benefit from sacituzumab govitecan in low TROP2-expressing tumors. This suggests that there may be value in a companion diagnostic assay to measure TROP2 protein expression in tumor samples, but H-score assessment is not required. Here, we develop a quantitative immunofluorescence (QIF) assay and a quantitative hematoxylin-DAB (QH-DAB) assay for potential use as a future companion diagnostic test. TROP2 peptide concentrations were measured in cell lines using mass spectrometry to convert fluorescent signal or chromogen optical density to protein concentrations in amol/mm 2 . Coupled with QuPath-based image analysis tool Qymia, our QIF and QH-DAB assays have limits of detection of 90 amol/mm 2 and 667 amol/mm 2 , and limits of quantifications of 272 amol/mm 2 and 2021 amol/mm 2 , respectively. Using these assays, we measured the TROP2 expression in a breast cancer serial cohort (N=264) and a triple-negative breast cancer cohort (N=100) from Yale New Haven Hospital, identifying a broad dynamic range of TROP2 expression in breast cancer samples. Since the H-score method is the current standard of practice in the clinics, we selected 68 breast cancer biopsies and compared the measurements from our assays to H-scores evaluated by 5 certified pathologists. There is an overall agreement between our QIF and QH-DAB measurements and pathologists’ readings. However, the quantitative assay has better sensitivity and less subjectivity. In summary, these assays allow for an accurate and reproducible TROP2 protein measurement that could be incorporated into a clinical workflow. This work represents only analytic validation, but work on clinical validation has begun. In the future, this assay may help identify patients who are more likely to benefit from TROP2-targeted therapies.