Diminished aqueous microviscosity of tumors in murine models measured within vivo radiofrequency electron paramagnetic resonance

Using very low frequency in vivo electron paramagnetic resonance (EPR), we have compared, for the first time, the average microviscosity of the total aqueous compartment of murine fibrosarcomas and that of normal leg tissue i n a living animal, EPR spectra from dissolved nitroxide spin probes report the solvent microviscosity, The tumor aqueous microviscosity, 1.8 +/- 0.1 c entipoise, was significantly lower than that of the corresponding normal ti ssue, 2.9 +/- 0.3 centipoise, a difference of 38 +/- 7%, These results conf irm the commonly observed increase in the water proton transverse relaxatio n times (T-2) in magnetic resonance imaging of hyperproliferative states, f or example, malignancy. The specificity of the localization of the EPR sign al indicates a substantial portion of the T-2 increase seen in magnetic res onance imaging derives from decreased bulk-water viscosity. The effect of t his microviscosity differences may be the basis of several physiological di fferences between tumors and normal tissues which could confer a growth rat e advantage to tumor tissue.