Local pharmacokinetic analysis of a stable spin probe in mice by in vivo L-band ESR with surface-coil-type resonators

In vivo ESR spectroscopy using a low frequency microwave of approximately 1 GHz has been developed to measure non-invasive ESR spectra in animals given paramagnetic compounds, in which a loop-gap-type resonator was used and ESR spectra were measured at the animal's head or abdomen. Therefore, the concentrations of paramagnetic species in both the blood and organs were compositely contributed to the spectra. When we understand the kinetics of paramagnetic species in detail, it is essentially important to know how these kinetics are expressed in each organ. For this purpose, a surface-coil-type resonator, which enabled local ESR measurement in specific organs, has been developed. By using this method, we studied the real-time pharmacokinetics of spin clearance curves detected in the organs of mice given 4-hydroxy-2,2,6,6tetramethylpiperidine-1-oxyl (4-hydroxy-TEMPO) intravenously (i.v.), by monitoring the inferior vena cava, liver and kidney. Quantified clearance curves in the organs were analyzed on the basis of a two-compartment model, and pharmacokinetic parameters were estimated based on the curve-fitting. The obtained pharmacokinetic parameters were found to depend on the measurement site, and the distribution and elimination processes of the spin probe were successfully separated between the blood and organs of mice.