A photocleavable peptidic Ru(ii) mass-tag enabling targeted DESI and MALDI mass spectrometry imaging in cancer tissues

Mass spectrometry imaging (MSI) is a versatile technique for spatially resolved multi-omics analysis in clinical research. However, its proteomic applications remain constrained by issues such as poor ionisation efficiency and signal interference from complex tissue matrices. On-tissue mass-tag labelling (OTMT) overcomes these challenges by employing affinity-based imaging agents that carry cleavable, highly ionisable reporter groups known as mass-tags (MTs). In this work, we introduce TATE-Ru(ii)MT, which comprises a somatostatin receptor subtype 2 (SSTR2)-targeting cyclic peptide and a photocleavable Ru(ii) polypyridine complex. We demonstrate that this MT enables multimodal visualisation of SSTR2 in tissue using matrix-assisted laser desorption/ionisation (MALDI) and desorption electrospray ionisation (DESI) MSI, following a staining workflow fully compatible with the widely used, commercially available Miralys™ MTs from AmberGen. Unlike commercial MTs, which use peptide sequences as photocleavable reporter groups, Ru(ii)-based MTs are not restricted to MALDI-MSI, offering a flexible platform with variable MT design and broad methodological applicability.