In vivo spin trapping of nitric oxide from animal tumors

Spin trapping/electron paramagnetic resonance (EPR) spectroscopy allows specific detection of nitric oxide (NO) generation, in vivo. However, in order to detect an EPR signal in living organism, usually a stimulation of immune system with LPS is used to achieve higher than physiological NO levels. Here, we report non-invasive spin trapping of NO in tumors of non-treated, living animals. EPR spectroscopy was performed at S-band to detect NO in Cloudman S91 melanoma tumors growing in the tail of living, syngeneic hosts—DBA/2 mice. Iron (II) N-(dithiocarboxy)sarcosine Fe2+(DTCS)2 was used as the spin trap. The results were confirmed by X-band ex vivo study. A characteristic three-line spectrum of NO–Fe(DTCS)2 (AN = 13 G) was observed (n = 4, out of total n = 6) in non-treated tumors and in tumors of animals treated with l-arginine. Substrate availability did not limit the detection of NO by spin trapping. Half-life time of the NO–Fe(DTCS)2 in tumor tissue was about 60 min. The feasibility of non-invasive spin trapping/EPR spectroscopic detection of NO generated in tumor tissue in living animals, without additional activation of the immune system, was demonstrated for the first time.