A novel approach to measure the time course of paramagnetic spin probe concentration in the circulating blood of a living mouse using X-band (9.4 GHz) electron paramagnetic resonance spectrometer is described. Using this technique, the pharmacokinetics of several
nitroxyl spin probes was examined. The decay profiles were also independently simulated using pharmacokinetic properties as well as redox-mediated factors responsible in converting the
nitroxyl radicals to the corresponding
hydroxylamines. Finally, suitability of
nitroxyl radicals as the probes of in vivo redox status and for radioprotection was described. The studies indicate that the six-member
piperidine nitroxyls are suitable for estimating redox status in the circulation, whereas the five-member
pyrrolidine nitroxyl radicals are suited for tissue redox status determination. For selective protection against radiation of normal tissues rather than
cancer/
tumor, efficient reoxidation of the
hydroxylamine in normal tissue is preferable. Simulation results showed that for carbamoyl-PROXYL, only administration of the radical form might give radioprotection and not the
hydroxylamine. However, the
hydroxylamine form of
TEMPOL, i.e.,
TEMPOL-H, may give similar radioprotection as the radical form due to efficient reoxidation in vivo.