Dynamic electron spin resonance (ESR) imaging of the distribution of spin labeled dextran in a mouse.

The nitroxide group 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was covalently bound to a dextran via an N-acetamido linkage to produce a novel spin probe, TEMPO-dextran (TEMPO-DX), which circulates for long time periods in an animal without metabolic degradation. TEMPO-DX was stable in mice, while small TEMPO analogs quickly disappeared after administration. Since dextran was reduced somewhat in size during synthesis, the resultant spin-labeled dextran could be excreted through the kidney. A strong L-band electron spin resonance signal was obtained shortly after intravenous administration of TEMPO-DX into the tail vein of a mouse, from which three-dimensional images of specific organs were calculated. The signal was found to persist for well over 1 h.