Peroxyl radical formation in aqueous solutions of N,N-dimethylformamide, N-ethylformamide, and dimethylsulfoxide by ultrasound: Implications for sonosensitized cell killing

Sonodynamic therapy, which refers to a synergistic effect of drugs and ultrasound, is a promising new modality for cancer treatment. The sonodynamic effect was found for a number of structurally unrelated compounds, and the underlying mechanisms are still unknown. Recently, Jeffers et al. (J. Acoust. Soc. Am. 97:669–676; 1995) have shown that the sonodynamic action of nontoxic concentrations of N,N-dimethylformamide (DMF), N-methyl formamide (MMF), and dimethylsulfoxide (DMSO) combined with ultrasound, on killing of cultured HL-60 human promyelocytic leukemia cells, and attributed this toxic effect to unknown short lived reactive species produced from these solutes by ultrasonic cavitation. Using the spin trap 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) in nitrogen-saturated aqueous solutions of DMF, MMF, or DMSO exposed to 50 kHz ultrasound, we detected formation of ·CH3 and ·CH2N(CH3)CHO radical adducts for DMF, mostly ·CH2NHCHO adducts for MMF, and ·CH3 adducts for DMSO. These radicals were formed either by reactions of the solutes with ultrasound-generated ·H and ·OH radicals (such as ·CH2R-type radicals in DMF and MMF, and ·CH3 radicals in DMSO), or by direct pyrolysis of the weak bonds in the solute molecules (e.g., ·CH3 radicals from DMF). In air-saturated sonicated solutions these carbon centered radicals were converted to the corresponding peroxyl radicals and spin trapped with 5,5-dimethyl-l-pyrroline-N-oxide (DMPO); ·OOCH2N(·CH3)CHO radicals were identified in DMF, ·OOCH2NHCHO radicals in MMF, and ·OOCH3 radicals in DMSO solutions. We suggest that these radical species by virtue of their longer lifetimes and higher selectivity, compared to ·OH radicals, which are also formed in sonicated solutions, are the species responsible for sonodynamic cell killing by the combined effect of ultrasound with DMF, MMF, or DMSO.