Optimization of enhancement of therapeutic efficacy of ultrasound: Frequency-dependent effects on iodine formation from KI-starch solutions and ultrasound-induced killing of rat thymocytes

Influence of changing pulse repetition frequency on chemical and biological effects induced by low-intensity ultrasound in vitro

This study was undertaken to examine ultrasound (US) mechanisms and their impact on chemical and biological effects in vitro as a function of changing pulse repetition frequency (PRF) from 0.5 to 100Hz using a 1MHz-generator at low-intensities and 50% duty factor (DF). The presence of inertial cavitation was detected by electron paramagnetic resonance (EPR) spin-trapping of hydroxyl radicals resulting from sonolysis of water. Non-cavitational effects were evaluated by studying the extent of sucrose hydrolysis measured by UV spectrophotometry. Biological effects were assessed by measuring the extent of cell killing and apoptosis induction in U937 cells using Trypan blue dye exclusion test and flow cytometry, respectively. The results indicate significant PRF dependence with respect to hydroxyl radical formation, cell killing and apoptosis induction. The lowest free radical formation and cell killing and the highest cell viability were found at 5Hz (100ms pulse duration). On the other hand, no correlation was found between sucrose hydrolysis and PRF. To our knowledge, this is the first report to be devoted to study the impact of low PRFs at low-intensities on US-induced chemical and biological effects and the mechanisms involved. This study has introduced the role of "US streaming" (convection); a forgotten factor in optimization studies, and explored its importance in comparison to standing waves.

Contributions of mechanical and sonochemical effects to cell membrane damage induced by single-shot pulsed ultrasound with adjacent microbubbles

PURPOSE

The objective was to investigate the contributions of mechanical effects due to kinetic force induced by the dynamic behavior of microbubbles and sonochemical effects due to free radicals produced by inertial cavitation to cell membrane damage under sonoporation conditions in which cells with adjacent microbubbles were irradiated with single-shot pulsed ultrasound.

METHODS

The free radical scavenger cysteamine was used to control the occurrence of sonochemical effects, and the ratios of cells with membrane damage to intact cells were compared in the presence and absence of cysteamine. To determine the optimal dose of cysteamine, free radical production on exposure to burst pulse ultrasound was investigated using KI-starch solutions with different concentrations (0-5 mM) of cysteamine. High-speed observation of the dynamic behavior of Levovist microbubbles during ultrasound exposure was also carried out in the presence and absence of cysteamine, and the difference in the ratios of the maximum bubble diameter to the initial diameter was evaluated. Next, human prostate cancer cells with adjacent Levovist microbubbles were exposed to single-shot pulsed ultrasound with a center frequency of 1 MHz, a peak negative pressure of 1.1 MPa, and a pulse width of 3 μs, and the percentages of cells with membrane damage were evaluated by fluorescent microscopy using propidium iodide in the presence and absence of cysteamine.

RESULTS

It was confirmed that cysteamine at a concentration of 5 mM completely suppressed sonochemical effects without causing a change in the dynamic response of microbubbles to pulsed ultrasound. The percentages of cells with membrane damage in the presence and absence of cysteamine (5 mM) were 10.3% ± 4.1% (n = 13) and 8.7% ± 3.9% (n = 9), respectively. No significant difference was found (P = 0.36).

CONCLUSION

The results indicate that cell membrane damage induced by single-shot pulsed ultrasound with adjacent microbubbles was due mainly to mechanical effects, not to sonochemical effects.