Sonodynamic therapy using water-dispersed TiO2-polyethylene glycol compound on glioma cells: Comparison of cytotoxic mechanism with photodynamic therapy

Sonodynamic therapy is expected to be a novel therapeutic strategy for malignant gliomas. The titanium dioxide (TiO2) nanoparticle, a photosensitizer, can be activated by ultrasound. In this study, by using water-dispersed TiO2 nanoparticles, an in vitro comparison was made between the photodynamic and sonodynamic damages on U251 human glioblastoma cell lines. Water-dispersed TiO2 nanoparticles were constructed by the adsorption of chemically modified polyethylene glycole (PEG) on the TiO2 surface (TiO2/PEG). To evaluate cytotoxicity, U251 monolayer cells were incubated in culture medium including 100 μg/ml of TiO2/PEG for 3 h and subsequently irradiated by ultraviolet light (5.0 mW/cm2) or 1.0 MHz ultrasound (1.0 W/cm2). Cell survival was estimated by MTT assay 24 h after irradiation. In the presence of TiO2/PEG, the photodynamic cytotoxic effect was not observed after 20 min of an ultraviolet light exposure, while the sonodynamic cytotoxicity effect was almost proportional to the time of sonication. In addition, photodynamic cytotoxicity of TiO2/PEG was almost completely inhibited by radical scavenger, while suppression of the sonodynamic cytotoxic effect was not significant. Results of various fluorescent stains showed that ultrasound-treated cells lost their viability immediately after irradiation, and cell membranes were especially damaged in comparison with ultraviolet-treated cells. These findings showed a potential application of TiO2/PEG to sonodynamic therapy as a new treatment of malignant gliomas and suggested that the mechanism of TiO2/PEG mediated sonodynamic cytotoxicity differs from that of photodynamic cytotoxicity.

Research highlights
► SDT using water-dispersed TiO2/PEG on glioma cells was evaluated. The differences between phototoxicity and sonotoxicity were also examined.
► The sonodynamic antitumor effect was strongly enhanced by TiO2/PEG.
► Mechanism of TiO2/PEG sonotoxicity differs from that of phototoxicity. It was confirmed by the difference of staining pattern by immunostainings. The role of generated oxidative radicals in SDT is also distinct from PDT.