A Small Molecule Drug-Based Ru(II) Polypyridine Mass-Tag for Multimodal Imaging of Tissue Samples

Mass spectrometry imaging (MSI) is a powerful tool for spatially resolved multiomics analysis of tissue samples in clinical research. However, its proteomics application is still limited due to challenges such as low ionization efficiency and signal interference from complex tissue environments. On-tissue mass-tag labeling (OTMT) addresses these limitations using affinity-based imaging agents that incorporate cleavable, highly ionizable reporter groups known as mass-tags (MTs). The majority of existing MTs rely on antibodies as targeting elements and organic moieties as reporter groups. Here, we introduce a new class of MTs featuring small-molecule inhibitors as binding motifs. Specifically, we present PARPi-MT, composed of a photocleavable and luminescent Ru-(II)-based reporter and the poly-(ADP-ribose) polymerase (PARP) inhibitor Olaparib for the targeted bimodal imaging of PARP1 in H446 xenograft tumor and mouse brain sections, via desorption electrospray ionization (DESI)-MSI and fluorescence microscopy. Using small-molecule inhibitors as binding motifs expands the design versatility and potential applications of OTMT, while overcoming some of the challenges of antibody-based mass-tags. The Ru-(II)-based reporter group offers further advantages, including distinct isotopic signatures derived from the metal center and inherent multimodal imaging capabilities.