EPR SPIN-TRAPPING STUDY OF THE DECOMPOSITION OF AZO-COMPOUNDS IN AQUEOUS-SOLUTIONS BY ULTRASOUND - POTENTIAL FOR USE AS SONODYNAMIC SENSITIZERS FOR CELL-KILLING

Sonodynamic therapy, a promising new approach to cancer treatment, is based on synergistic cell killing by combination of certain drugs (son osensitizers) and ultrasound. Although the mechanism of sonodynamic ac tion is not understood, the role of free radicals produced from sonose nsitizers by ultrasound is im plicated. In this work, we studied forma tion of free radicals during the decomposition of several water-solubl e azo compounds by 50 kHz ultrasound in aqueous solutions. Using the s pin trap 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) tertiary carbo n-centered radicals from 2,2'-azobis (N,N'-dimethyleneisobutyramidine) dihydrochloride (VA-044), 2-(carbamoylazo)-isobutyronitrile (V-30), a nd 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH) and (.)CH3 ra dicals from 1,1'-azobis (N,N'-dimethylformamide) (AAMF) were detected in argon-saturated solutions and the corresponding oxygen-centered rad icals (alkoxyl and peroxyl) from VA-044, V-30, and AAPH were identifie d using the spin trap 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) in aera ted sonicated solutions. No free radicals from 4,4'-dihydroxyazobenzen e-3,3'-dicarboxyl acid, disodium salt (DHAB) could be found in either system. While VA-044 and AAPH could also be readily decomposed by heat (42.5 degrees C and 80 degrees C), V-30 decomposition only occurred i n the ultrasound-exposed solutions. The most likely mechanism of decom position of azo compounds by ultrasound is their thermolysis in the he ated shell of the liquid surrounding cavitating bubbles driven by ultr asound and/or by pyrolysis inside these bubbles. Experiments using sca vengers of (OH)-O-. and H-., which are produced by sonolysis in aqueou s solutions, demonstrated that these radicals are not involved in the ultrasound-mediated radical production from the azo compounds. Due to the known cytotoxic potential of free radicals produced from azo compo unds, the use of these compounds as ultrasound sensitizers appears to be a promising approach for sonodynamic cell killing.