The decomposition of protoporphyrin IX by ultrasound is dependent on the generation of hydroxyl radicals

The ultrasound activation of certain drugs, such as porphyrins, could cause synergistic cytotoxic effects on cells. Both sonomechanical and sonochemical effects occur and the latter play a critical role because antioxidant agents could exert significant protective effects against the cytotoxicity. To investigate the reactive oxygen species involved in the sonochemical effects, aqueous protoporphyrin IX (PpIX) solutions were characterized under ultrasound sonication in this study. Inertial cavitation was indirectly evaluated using terephthalic acid dosimetry. The fluorescence intensity of the PpIX was measured using a fluorescence spectrophotometer. The effects of PpIX concentration, ultrasound parameters and free radical scavengers on the PpIX activation by ultrasound were investigated. Our results showed that the increase in PpIX decomposition was significantly correlated with cavitation activities (R = 0.9874, p < 0.05), and the decomposing effect increases with ultrasound intensity (0.6-1.5 W/cm2), initial PpIX concentration (1-5 μM), duty cycle (10-100%) and the sonication duration (2-10 min). The fluorescence and absorption spectra of PpIX showed a decrease in the peak intensity without spectral shifts or new peak build-up after sonication. The PpIX decomposition was significantly inhibited by hydroxyl radical scavengers, histidine, mannitol, acetone, methanol and ethanol, but the decomposition was not inhibited by sodium azide, catalase or superoxide dismutase. These results suggest that the decomposition of protoporphyrin IX by ultrasound is dependent on the generation of hydroxyl radicals, which sheds some light on the sonochemical effects of the interaction between ultrasound and porphyrins.