Ultrasound-enhanced hydroxyl radical production from two clinically employed anti-cancer drugs, adriamycin and mitomycin C

Continuous-wave 1 MHz ultrasound at the therapeutic intensity of 1 W cm−2 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 (HO■). Bz is initially non-fluorescent and upon aromatic hydroxylation becomes permanently fluorescent. A series of time course studies up to 30 min were performed on drug suspensions to characterize the HO■ 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 HO■ in comparison to the drug-free insonicated phosphate buffer suspension. Ultrasound exposures of 30 min did not yield measurable changes in the chemical constitution of the four drugs as assessed through high-performance liquid chromatography. Identical ultrasound treatments at 3 MHz did not produce any HO■ in the presence or 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 elucidate the chemical species and pathways involved in the production of the HO■. 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 Fentonʹs pathway.