Nitronyl Nitroxides as a Spin Probe in EPR Tomography In Vivo

Recently, new water- and blood-soluble nitronyl nitroxides, 2-(5-methyl-1H-imidazole-4-yl)-4,4,5,5-tetramethyl-4,5-dihydroimidazole-3-oxide-1-oxyl (NN1) and 2-(1H-imidazole-4-yl)-4,4,5,5-tetramethyl-4,5-dihydroimidazole-3-oxide-1-oxyl (NN2), (Fig. 1), were synthesized and used as contrast agents for MRI (Savelov et al. Dokl Academ Nauk 416(4): 493–495, 2007). Taking into account the high rate constants of NN’s reduction by ascorbic acid and other biologically relevant reductants, it is not clear which factors helped with the use of these nitroxides in vivo as a contrast reagent. Moreover, due to high solubility in an aqueous solution and low toxicity (Ovcharenko et al. in Dokl Academ Nauk 404(2):198–200, 2005, Eriksson et al. in Drug Metab Dispos 15(2):155–160, 1987, Afzal et al. in Polyhedron 22(14):1957–1964, 2003) of NNs, it seems possible to use them as a spin probe for NO in vivo with EPR tomography. In this paper, we studied reduction of NN1 and NN2 in model conditions (by ascorbic acid) and in vitro. In addition, the possibility of NN1 and NN2 to be used as paramagnetic probes for L-band EPR imaging in vivo was investigated. Nitric oxide (NO) expression in vivo leads to the decrease in concentrations of NN1, 2 upon the injection in a mouse body, that can be explained by the reaction of studied radicals with NO and fast transformation of the reaction products to diamagnetic species. Pharmacokinetics of NN1, 2 and limitations of their application as contrast agents in MRI are discussed also. Finally, the results of EPR tomography were compared with MRI data. It is shown that the fast reduction of the reaction product of NN with NO—imino nitroxides—is the main obstacle to use NN as a spin probe in vivo.