IN-VIVO TOPICAL EPR SPECTROSCOPY AND IMAGING OF NITROXIDE FREE-RADICALS AND POLYNITROXYL-ALBUMIN

Piperidine nitroxides have considerable clinical potential, both as an tioxidant therapeutic compounds and contrast agents in magnetic resona nce imaging. However, their development has thus far been limited by t heir rapid bioreduction in vivo. Recently, it was reported that polyni troxyl albumin (PNA) can reverse the bioreduction of the reduced 4-hyd roxy-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempol) in the rat heart, e nabling the performance of high resolution EPR imaging for prolonged t ime (Kuppusamy et al., Biochemistry 35, 7051-7057 (1996)), In this rep ort, the efficacy of PNA in maintaining Tempol concentrations in vivo in mice was demonstrated, using L-band (1.25 GHz) EPR spectroscopy and imaging, The EPR signal of intravenous Tempol had a half-life of 1.0 +/- 0.2 min and became undetectable within 6 min. Subcutaneous Tempol, however, decayed at a slower rate (half-life, 5.0 +/- 0.5 min) sugges ting that Tempol had been bioreduced to the corresponding hydroxylamin e form, Tempol-H, Subcutaneously injected PNA restored 20% of the Temp ol signal in the vicinity of the PNA deposit. In vivo topical EPR imag ing demonstrated that the Tempol signal was restored at the site of RN A injection, but not at locations remote from the PNA injection site. The ability of PNA to maintain Tempol in its paramagnetic state in viv o should enable a wide range of therapeutic and diagnostic application s of piperidinyl nitroxides.