Free radical activity and enhancement of chemotherapy by low-level ultrasound

The mechanism of enhancement of chemotherapy by low-level ultrasound (US) (1.62 MHz tone burst, 10% duty cycle, 0.25 W/cm
²
, 0.28 MPa, and 2.0 MHz center frequency pulsed, 8-cycle wave form, 1 KHz prf, 0.18 W/cm
²
, 1 MPa) was investigated with respect to free radical activity. The cytotoxicity of Adriamycin and diaziquone was enhanced by US; both these compounds by themselves act in part through free radical mechanisms. The compounds 5-fluorouracil and cisplatin, which do not exhibit free radical activity by themselves, were not potentiated by US. Electron spin resonance spin trapping measurements showed that US by itself caused hydroxyl (O˙H) radical production in water comparable to that of 0.5 Gy of X-rays, but had no appreciable effect on clonogenic survival of CHO cells. Added Adriamycin did not change the yield of hydroxyl radicals