Mechanism of ultrasound enhanced porphyrin cytotoxicity. Part I: A search for free radical effects.

Intense ultrasound beams may have the potential to treat malignant tumours when combined with sensitizers, often called sonodynamic agents. Some of these agents, e.g., the porphyrins, are currently used for photodynamic therapy. However, the experimental evidence for ultrasound activation of sensitizers is inconsistent. This paper attempts to discover whether they yield of free radicals such as .OH and .H, which are produced by transient cavitation, could explain the killing of Chinese hamster ovary (CHO) cells in vitro with and without sonodynamic agents. CHO cells were irradiated with ultrasound beams in phosphate-buffered saline or in growth medium, and the immediate cell lysis and loss of cell colony forming ability were measured. Under our specific conditions, in which the standing wave patterns were minimized, a general correlation was observed between the transient cavitation, free radical production, and cytotoxicity. However, the yield of free radicals was much too small to explain the cell killing observed. We conclude that cytotoxicity is not linked to attack from free radicals formed outside the cells. In our experiments, immediate cell lysis is closely linked to the transient cavitation, which is known to produce shear forces that disrupt cellular membranes. We hypothesize that the loss of cell colony forming ability is also linked to damage of cellular membranes.