Imaging Mass Spectrometry‐Based Assessment of ER, PR, and HER2 Protein Expression in Breast Cancer

Imaging mass spectrometry has made significant advancements in recent years owing to its ability to study the spatial localization and abundance of proteins, peptides, lipids, glycans, metabolites, and drugs. In this study, we employed an imaging mass spectrometry-based workflow called MALDI-IHC, which uses antibodies conjugated with photocleavable mass-tags, to investigate the detection of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) along with pan-cytokeratin in formalin-fixed paraffin-embedded (FFPE) sections from cases with breast cancer. The expression of ER, PR, and HER2 was consistent with the clinical diagnosis based on conventional immunohistochemistry, lacking signal in triple-negative cases and concordant signals in the cases that were classified as ER+/PR+/HER2+, ER+/PR+/HER2-, and ER-/PR-/HER2+ groups, respectively. In addition, the potential utility of a seconday antibody-based MALDI-IHC approach for HER2 expression was evaluated. In agreement with immunohistochemistry results, HER2 expression co-localized within tumor regions in a HER2+ breast cancer case, while no expression was observed in a HER2- case. Collectively, these results highlight the potential of imaging mass spectrometry-based MALDI-IHC workflow for multiplexed detection of proteins in clinically relevant tissue sections.