Sonodynamic and Photodynamics Used as a Combined Therapy in the Treatment of Malignant Neoplasms: Facts and Open Questions

Photodynamic therapy (PDT) used in combination with sonodynamic therapy (SDT) is a new approach that aims to increase the effectiveness of tumor treatment when compared to the effect of each independent therapy. PDT is based on stimulating sensitizers with photons, while the most accepted theory for SDT is that sensitizers are stimulated by the sonoluminescence phenomenon. However, after the excitation of the sensitizer, both therapies follow a common path, leading to the generation of free radicals and inducing cell death. One of the positive aspects of this combination is the augmentation of anti-tumor activity with fewer side effects, since cell death may be induced using lower sensitizer concentrations or less exposure to ultrasound or light. Another benefit of combining PDT and SDT, especially with the use of low-frequency ultrasound is the induction of sonophoresis. For instance, on the skin, it may facilitate the absorption of the sensitizer. However, research involving both PDT and SDT exhibit many variants, including differences in irradiation sources and their intensities, among others. These aspects contribute to a lack of standardization, leading to result variations, hindering assessment on the real contribution that these combined therapies can offer in tumor treatment. Thus, further research in the pre-clinical and clinical areas are crucial.