Acetoxymethoxycarbonyl nitroxides as electron paramagnetic resonance proimaging agents to measure O₂ levels in mouse brain: A pharmacokinetic and pharmacodynamic study

Measurement of O₂ concentration and distribution in brain is essential to understanding the pathophysiology of stroke. Low-frequency electron paramagnetic resonance (EPR) spectroscopy with a paramagnetic probe is an attractive imaging modality that can potentially map O₂ concentration in the brain. In a previous study, we demonstrated that, after intraperitoneal administration of 3-acetoxymethoxycarbonyl-2,2,5,5-tetramethyl-1- pyrrolidinyloxyl (1) to mice, this nitroxide crossed the blood-brain barrier into brain tissue where, after hydrolysis, 3-carboxy-2,2,5,5-tetramethyl-1- pyrrolidinyloxyl (2) was liberated and entrapped. This pilot study suggested that nitroxide 1 is a proimaging agent that can deliver nitroxide 2 to brain tissue, where O₂ levels can be estimated. In the present study, we conducted a series of pharmacokinetic and pharmacodynamic experiments designed to assess the uptake of structurally disparate nitroxides into brain tissue and retention, after hydrolysis, of the anions of the corresponding nitroxide acids. From these findings, nitroxide 1 and trans-3,4-di(acetoxymethoxycarbonyl)-2,2, 5,5-tetramethyl-1-pyrrolidinyloxyl (5) meet the requirement as EPR proimaging agents for mapping O₂ distribution in the brain following stroke.