The Role of MALDI-TOF Mass Spectrometry in Photodynamic Therapy: From Photosensitizer Design to Clinical Applications.

Photodynamic therapy (PDT) has evolved considerably over the past decades, progressing from first-generation porphyrins to second- and third-generation photosensitizers, including nanocarrier-based systems with improved selectivity and bioavailability. In parallel, matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) has become a gold standard for the characterisation of complex biomolecules, enabling precise determination of molecular mass, purity and stability. This narrative review explores the intersection of these two fields, focusing on how MALDI-TOF MS supports the development, characterisation and clinical application of photosensitizers used in PDT. Literature searches were performed across PubMed, Web of Science, Scopus and Base-search, followed by targeted retrieval of studies on MALDI and PDT applications. Findings indicate that MALDI-TOF MS plays a crucial role at multiple stages: confirming the synthesis and chemical integrity of novel photosensitizers, monitoring their metabolic stability in biological systems and characterising photodegradation products after PDT. Moreover, MALDI imaging mass spectrometry (MALDI-IMS) enables spatial mapping of photosensitizer distribution in tissues, while rapid pathogen identification by MALDI-TOF supports antimicrobial PDT applications. Collectively, the evidence highlights that MALDI-MS is not only a tool for molecular characterisation but also a versatile analytical platform with a direct translational impact on PDT. Its integration with other omics and multimodal imaging approaches is expected to enhance the personalization and clinical effectiveness of photodynamic therapy.