Continuous wave 1-MHz ultrasound at the therapeutic intensity of 1 W/cm2 was found to enhance significantly the hydroxyl radical production from two clinically employed redox cycling drugs, viz., Adriamycin (Doxorubicin) and Mitomycin C, with respect to the control drug-free insonicated phosphate buffer suspension. Benzoic acid (Bz) was employed as a sensitive chemical probe to detect hydroxyl radicals (OH⋅). Bz is initially nonfluorescent and upon aromatic hydroxylation becomes permanently fluorescent. A series of time course studies up to one-half hour were performed on drug suspensions to characterize the OH⋅ generation in the presence and absence of ultrasound at 37 °C. Identical ultrasound treatments on non-redox cycling clinical drugs 5-Fluorouracil and Methotrexate did not yield any significant enhancement in the production of OH⋅ in comparison to the drug-free insonicated phosphate buffer suspension. One-half hour ultrasound exposures did not yield measurable changes in the chemical constitution of the four drugs as assessed through high-pressure liquid chromatography. Identical ultrasound treatments at 3 MHz did not produce at OH⋅ in the presence or in the absence of these four anti-cancer drugs. Free radical scavengers such as mannitol, superoxide dismutase, catalase, and a transition metal chelating agent were employed independently to discern the chemical species and pathways involved in the production of the OH⋅. The findings strongly implicate an active role of acoustically induced cavitation in potentiating redox cycling drugs via chemical reduction and, thereafter, production of the OH⋅ via Fenton’s pathway.

It has been reported that OH production in anticancer drug Adramycin and Mitomycin C suspensions is dramatically enhanced by applying 0.5-MHz cw ultrasound (US). Time course studies were performed to characterize the OH generation with and without US exposures. The results show that to a first approximation [OH] can be expressed as superposition of a rapid exponentially growing term and a slow linear varying term. The experiment indicated that the first term is due to US. In view of these observations it is concluded that US activated some shortly lived OH generating pathways.