Development of 3-actoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl as an electron paramagnetic resonance imaging reagent for in vivo mapping brain oxygen distribution and infarction in ischemic brain

Abstract

Measurement of oxygen concentration and distribution in brain is essential to understanding the pathophysiology of stroke. Although brain oxygen level is critical for brain tissue survival, in vivo no-invasive measurement of brain oxygen distribution and identification of infarction area remain technical challenge. Low-frequency electron paramagnetic resonance (EPR) spectroscopy with application of oxygen sensitive probe is an attractive imaging modality that can potentially map O2 concentration in the brain. We recently developed a paramagnetic probe, 3-actoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl (AM-CTP), for brain oxygen measurement with EPR spectroscopy. In this study, we demonstrated that after intraperitoneal administration of AM-CTP to mice this nitroxide diffused across the blood brain barrier where, after hydrolysis, 3-carboxy-2,2,5,5-tetramethyl- pyrrolidinyloxyl was liberated and entrapped. The probe is sensitive to the changes of oxygen concentration. Pharmacokinetic and pharmacodynamic studies showed that the probe was relatively stable enough for brain imaging study. By using AM-CTP and low-frequency EPR instrument, we obtained EPR brain oxygen map, for the first time, in mice of ischemic stroke. Furthermore, by co-registration with magnetic resonance imaging, we demonstrated that the EPR imaging technology could be potentially used for identification of infarction area in ischemic brain. This finding suggests that EPRI with specifically designed nitroxides is a promising imaging modality for visualization brain O2 distribution, especially in an ischemic brain. We believe that this imaging method can be used for monitoring the effects of therapeutic intervention, aimed at enhancing brain O2 supply, so crucial in minimizing brain injury following stroke.