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Mass Spectrometry-based Evaluation of the Photobleaching Rate of Protoporphyrin IX used in the Photodynamic Diagnosis and Therapy of Cancer
Photodynamic diagnosis and therapy (PDD/PDT) are highly selective and minimally invasive cancer diagnosis and treatment techniques. The efficacy of PDD and PDT depends on the photobleaching rate of the photosensitizer, and the phenomenon has been previously investigated using fluorescence spectroscopy. In this study, we investigated the photobleaching of protoporphyrin IX (PpIX) using mass spectrometry (MS). The photobleaching of PpIX dissolved in dimethyl sulfoxide was measured during PDD and PDT conditions using 405 and 635 nm lasers, respectively, at power densities of 10, 50, and 100 mW/cm2. The obtained results were compared with that of fluorescence spectroscopy. Our MS results show faster photobleaching of PpIX in comparison with that of fluorescence. Additionally, there was variation in the MS-based photobleaching coefficient of PpIX with the power density, while there was no statistically significant variation in the fluorescence-based photobleaching with the power density. The difference may be attributed to the change in the fluorescence quantum yield of PpIX with its concentration and the effect of fluorescence emission from the PpIX photoproducts, which affect the fluorescence-based results. Since the photobleaching rate is used to estimate PDD and PDT dosimetry, our results highlight the potential significance of these discoveries in the PDD and PDT efficacy.