Abstract PO3-13-11: Quantitative Multiplex Immunofluorescence Assay for TROP2 and HER2 Expression in Breast Cancer: Towards Guiding Patient Selection for Antibody Drug Conjugate Therapies

Abstract Emerging antibody drug conjugate (ADC) therapies targeting human trophoblast cell-surface antigen (TROP2) and human epidermal growth factor receptor 2 (HER2) are transforming the treatment landscape for breast cancer. Sacituzumab govitecan (SG) and trastuzumab deruxtecan (T-DXd) have gained approval for an overlapping set of "HER2-low" metastatic breast cancers, including hormone receptor (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. Clinical trial objective response rates to SG are approximately 30%, compared to 30-90% for T-DXd depending on HER2 expression levels. While both drugs are thought to be targeted therapies, the value of measuring the target and the best methods to do so are still not established. We believe that quantitative measurement of TROP2 and HER2 antigen expression levels could establish thresholds for responders, enabling more effective patient selection for ADC therapies. Here, we present a TROP2, high-sensitivity HER2, and cytokeratin (CK) quantitative immunofluorescence (QIF) multiplex assay. Using a ten-cell line standard array and proteomic mass spectrometry, we can convert tumor compartment QIF intensity to protein concentration in fmol/mm2 for tissue specimens. Anti-HER2, anti-TROP2 antibodies, and fluorescence detection systems were titrated and combined to maximize signal-to-noise ratio on our cell line standard array and breast cancer tissue microarrays (TMA). The multiplex assay was designed for automated slide stainers (Leica BOND Rx) and fluorescence slide scanning (Rarecyte CyteFinder II HT). We perform our analyses in QuPath using an image processing plugin developed for automated QIF/IHC analysis (Qymia). Reproducible TROP2 and HER2 QIF scoring (R² 0.95) was achieved across multiple staining batches using serial sections of breast cancer TMAs and cell standard arrays. This assay has a TROP2 linear range between 0.63 - 9.17 fmol/mm2 (about 1 million TROP2 receptors/cell) and HER2 linear range between 0.09 - 0.565 fmol/mm2 (about 60,000 HER2 receptors/cell). We then applied this multiplex assay to two serial retrospective primary breast cancer cohorts from Yale University to quantitatively measure TROP2 and HER2 expression (338 clinical cases). We find a weak negative correlation between TROP2 and HER2 expression in our breast cancer cohorts (Pearson r = -0.14, p = 0.0097, n = 338). TROP2 expression levels were above the limit of detection (LOD) in 90.2% of cases, with 4.1% exceeding the limit of linearity (LOL), and a mean TROP2 expression of 4.05 fmol/mm2. For HER2, 67.2% of cases were above the LOD, with 7.1% exceeding the LOL, and a mean HER2 expression of 0.186 fmol/mm2. Both TROP2 and HER2 were below the LOD in 3.0% of cases, which we define as “negative”. We found 29.9% expressed TROP2 and were HER2-negative, and 6.8% expressed HER2 and were TROP2-negative. Our future studies will aim to quantitatively define expression thresholds for T-DXd and SG response with the goal to produce a clinical grade assay for ADC patient selection and determine the value of the assay to help select which ADC to give first. HER2 and TROP2 protein expression summary in Yale breast cancer cohort Table 1: Summary of HER2 and TROP2 protein expression levels in serial retrospective primary breast cancer cohort of 338 cases using our high-sensitivity HER2 and TROP2 multiplex immunofluorescent assay Citation Format: Charles Robbins, Mengni He, Revekka Khaimova, Katherine Bates, Nay Nwe Nyein Chan, Daniel Liebler, Regan Fulton, David Rimm. Quantitative Multiplex Immunofluorescence Assay for TROP2 and HER2 Expression in Breast Cancer: Towards Guiding Patient Selection for Antibody Drug Conjugate Therapies 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 PO3-13-11.