Abstract
As physical methods used for the treatment of leukemia and solid cancers, radiotherapy, light irradiation and ultrasonic irradiation are well known. Ultrasonic irradiation, is superior to radiotherapy in that the applied waves can be focused solely on the cancer tissues to be treated with little effect upon normal tissues, and is better than light irradiation in the degree of penetration. Recently, researcher discovered that non-thermal ultrasound or cavitation can activate a group of chemicals that were originally used as light activated chemicals for cancer therapy. This new ultrasound therapy has been termed as sonodynamic therapy. Ultrasonic waves are known to show chemical actions; for example, irradiation of water causes a reaction to generate hydrogen peroxide. As a result, it was found that certain drugs, upon ultrasonic irradiation, create active oxygen such as superoxide radicals and singlet oxygen, and the active oxygen thus formed effectively destructs cancer tissues. The agents themselves have no antitumor activity and very low in toxicity, exhibit antitumor activity only by the chemical action caused by ultrasonic irradiation. Thus, there is significant less risk of causing any systemic disorder. In addition, these drug act exclusively upon tumor tissues when combined with ultrasonic irradiation, with no adverse effect upon normal tissues. An important factor involved in ultrasound irradiation is the chemical reactions induced during the course of violent microbubble collapse, known as acoustic cavitation. Short lived free radicals can be created by ultrasound that could alter various compounds leading to cell killing. Sonoluminescence may also be related the complex sonochemical or sonodynamic reactions. However, the exact mechanism related to cytotoxicity still remains to be solved. Acoustic cavitation can chemically activate photosensitive drugs specifically bound to malignant cell membrane which could result in cell surface disruption. Recent experiments with Adult T cell leukemia cells were specifically killed by low intensity ultrasound of 0.3W/cm2 in the presence of porfimer sodium. Research on the bioeffects of ultrasound in the presence of various drugs has only just begun. Most investigations are still highly experimental and far from practice in the clinical situation, however, there definitely exists an interesting phenomenon combining ultrasound and drugs that can not be ignored. Understanding the mechanism will eventually result in optimization of the acoustics and designing of the ultrasound device for clinical therapeutic application.
