Pharmacological compartment viscosity and polarity measured with very low‐frequency EPR in tumors of living animals

Quantitative measurements of the viscosity of a non‐vascular pharmacological compartment in a living animal are presented. Using novel, very low‐frequency electron paramagnetic resonance spectroscopy, capable of measurements in humans, novel analytical techniques and partially deuterated spin probes, the viscosity of the body water of murine fibrosarcomas implanted in the leg of a living animal were measured. The average viscosity was 1.9 ± 0.2 cP. Variation of the viscosity with tumor size and histology did not appear to be significant.