Noninvasive evaluation of in vivo free radical reactions catalyzed by iron using in vivo ESR spectroscopy

Nitroxyl Radical as a Theranostic Contrast Agent in Magnetic Resonance Redox Imaging

Significance:In vivo assessment of paramagnetic and diamagnetic conversions of nitroxyl radicals based on cyclic redox mechanism can be an index of tissue redox status. The redox mechanism of nitroxyl radicals, which enables their use as a normal tissue-selective radioprotector, is seen as being attractive on planning radiation therapy. Recent Advances:In vivo redox imaging using nitroxyl radicals as redox-sensitive contrast agents has been developed to assess tissue redox status. Chemical and biological behaviors depending on chemical structures of nitroxyl radical compounds have been understood in detail. Polymer types of nitroxyl radical contrast agents and/or nitroxyl radical-labeled drugs were designed for approaching theranostics. Critical Issues: Nitroxyl radicals as magnetic resonance imaging (MRI) contrast agents have several advantages compared with those used in electron paramagnetic resonance (EPR) imaging, while support by EPR spectroscopy is important to understand information from MRI. Redox-sensitive paramagnetic contrast agents having a medicinal benefit, that is, nitroxyl-labeled drug, have been developed and proposed. Future Directions: A development of suitable nitroxyl contrast agent for translational theranostic applications with high reaction specificity and low normal tissue toxicity is under progress. Nitroxyl radicals as redox-sensitive magnetic resonance contrast agents can be a useful tool to detect an abnormal tissue redox status such as disordered oxidative stress. Antioxid. Redox Signal. 36, 95-121.

Iron Speciation in Particulate Matter (PM2.5) from Urban Los Angeles Using Spectro-microscopy Methods

The speciation, oxidation states, and relative abundance of iron (Fe) phases in PM2.5 samples from two locations in urban Los Angeles were investigated using a combination of bulk and spatially resolved, element-specific spectroscopy and microscopy methods. Synchrotron X-ray absorption spectroscopy (XAS) of bulk samples in situ (i.e., without extraction or digestion) was used to quantify the relative fractions of major Fe phases, which were corroborated by spatially resolved spectro-microscopy measurements. Ferrihydrite (amorphous Fe(III)-hydroxide) comprised the largest Fe fraction (34-52%), with hematite (α-Fe2O3; 13-23%) and magnetite (Fe3O4; 10-24%) identified as major crystalline oxide components. An Fe-bearing phyllosilicate fraction (16-23%) was fit best with a reference spectrum of a natural illite/smectite mineral, and metallic Fe(0) was a relatively small (2-6%) but easily identified component. Sizes, morphologies, oxidation state, and trace element compositions of Fe-bearing PM from electron microscopy, electron energy loss spectroscopy (EELS), and scanning transmission X-ray microscopy (STXM) revealed variable and heterogeneous mixtures of Fe species and phases, often associated with carbonaceous material with evidence of surface oxidation. Ferrihydrite (or related Fe(III) hydroxide phases) was ubiquitous in PM samples. It forms as an oxidation or surface alteration product of crystalline Fe phases, and also occurs as coatings or nanoparticles dispersed with other phases as a result of environmental dissolution and re-precipitation reactions. The prevalence of ferrihydrite (and adsorbed Fe(III) has likely been underestimated in studies of ambient PM because it is non-crystalline, non-magnetic, more soluble than crystalline phases, and found in complex mixtures. Review of potential sources of different particle types suggests that the majority of Fe-bearing PM from these urban sites originates from anthropogenic activities, primarily abrasion products from vehicle braking systems and engine emissions from combustion and/or wear. These variable mixtures have a high probability for electron transfer reactions between Fe, redox-active metals such as copper, and reactive carbon species such as quinones. Our findings suggest the need to assess biological responses of specific Fe-bearing phases both individually and in combination to unravel mechanisms of adverse health effects of particulate Fe.

In vivo ESR imaging of redox status in mice after X-ray irradiation, measured by acyl-protected hydroxylamine probe, ACP

More detailed investigations on the in vivo redox status are needed to elucidate the mechanisms contributing to damage caused by ionizing radiation. In the present study, the in vivo redox status of mice was examined using in vivo electron spin resonance (ESR) imaging after an intraperitoneal injection of 1-acetoxy-3-carbamoyl-2,2,5,5-tetramethylpyrrolidine (ACP) as a probe. ACP is easily hydrolyzed to its hydroxylamine form in the mouse body, and the interconversion between hydroxylamine and the corresponding nitroxyl radical reflects the biological redox status. Liver damage, based on changes in liver weight and plasma aspartate aminotransferase levels, was detected in mice 4 days after X-ray irradiation at 7.5 Gy. ESR imaging showed that the signal intensity of the nitroxyl radical was high at the liver area in both damaged and healthy mice after administration of ACP. Whereas the signal decayed at the liver area for healthy mouse, the decay was negligible in damaged mice. Unlike healthy mouse, signal in the chest for damaged mouse increased with time. The distribution of the sum of hydroxylamine and the nitroxyl radical was similar in damaged and healthy mice. X-ray irradiation slightly lowered the reduction activity of the liver microsomal fraction for the nitroxyl radical. Thiobarbituric acid reactive substances in the liver were higher in damaged mice than in healthy mice; however, no significant differences were noted in reduced glutathione. The present results indicate that the redox status of mice exposed to X-ray irradiation is more oxidative than that in healthy mice.

Increased Electron Paramagnetic Resonance Signal Correlates with Mitochondrial Dysfunction and Oxidative Stress in an Alzheimer's disease Mouse Brain

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized clinically by cognitive decline and memory loss. The pathological features are amyloid-β peptide (Aβ) plaques and intracellular neurofibrillary tangles. Many studies have suggested that oxidative damage induced by reactive oxygen species (ROS) is an important mechanism for AD progression. Our recent study demonstrated that oxidative stress could further impair mitochondrial function. In the present study, we adopted a transgenic mouse model of AD (mAPP, overexpressing AβPP/Aβ in neurons) and performed redox measurements using in vivo electron paramagnetic resonance (EPR) imaging with methoxycarbamyl-proxyl (MCP) as a redox-sensitive probe for studying oxidative stress in an early stage of pathology in a transgenic AD mouse model. Through assessing oxidative stress, mitochondrial function and cognitive behaviors of mAPP mice at the age of 8-9 months, we found that oxidative stress and mitochondrial dysfunction appeared in the early onset of AD. Increased ROS levels were associated with defects of mitochondrial and cognitive dysfunction. Notably, the in vivo EPR method offers a unique way of assessing tissue oxidative stress in living animals under noninvasive conditions, and thus holds a potential for early diagnosis and monitoring the progression of AD.

Pharmacokinetics of lipophilically different 3-substituted 2,2,5,5-tetramethylpyrrolidine-N-oxyl radicals frequently used as redox probes in in vivo magnetic resonance studies

3-Carboxy-, 3-carbamoyl-, 3-hydroxymethyl, and 3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl radicals (CxP, CmP, HMP, and MCP, respectively) have been widely used as redox probes in in vivo magnetic resonance studies. Knowledge of the pharmacokinetics of these probes is essential for redox analyses. The apparent partition coefficient (Kp) of these probes at neutral pH was in the order of MCP>HMP>CmP>CxP. After these probes had been injected intravenously, their blood levels decayed in a bi-phasic manner, namely, fast decay followed by slow decay. The order of the area under the curve (AUC) was CxP»HMP>MCP≥CmP, which roughly coincided with that of Kp in the opposite direction, except for CmP. Decay in the slow phase largely affected the AUC of these probes. The reduction of these probes contributed to their decay in the slow phase. A two-compartment model analysis of blood levels, cyclic voltammetry, and magnetic resonance imaging provided the following pharmacokinetic information. The distribution of the probes between the central and peripheral compartments rapidly reached an equilibrium. In addition to lipophilicity, reduction potential may also be involved in the rate of in vivo reduction of the probes. Hydrophilic probes, such as CxP and CmP, were predominantly excreted in the urine. MCP was distributed to the peripheral tissues and then rapidly reduced. HMP was unique due to its moderate lipophilicity and slower reduction. Among the probes examined, the liver and kidney appear to be included in the central compartment in the two-compartment model analysis. MCP and HMP were rapidly distributed to the brain.

In vivo evaluation of different alterations of redox status by studying pharmacokinetics of nitroxides using magnetic resonance techniques

Free radicals, particularly reactive oxygen species (ROS), are involved in various pathologies, injuries related to radiation, ischemia-reperfusion or ageing. Unfortunately, it is virtually impossible to directly detect free radicals in vivo, but the redox status of the whole organism or particular organ can be studied in vivo by using magnetic resonance techniques (EPR and MRI) and paramagnetic stable free radicals - nitroxides. Here we review results obtained in vivo following the pharmacokinetics of nitroxides on experimental animals (and a few in humans) under various conditions. The focus was on conditions where the redox status has been altered by induced diseases or harmful agents, clearly demonstrating that various EPR/MRI/nitroxide combinations can reliably detect metabolically induced changes in the redox status of organs. These findings can improve our understanding of oxidative stress and provide a basis for studying the effectiveness of interventions aimed to modulate oxidative stress. Also, we anticipate that the in vivo EPR/MRI approach in studying the redox status can play a vital role in the clinical management of various pathologies in the years to come providing the development of adequate equipment and probes.

Redox evaluation in sepsis model mice by the in vivo ESR technique using acyl-protected hydroxylamine

In vivo electron spin resonance (ESR) spectroscopy is a noninvasive technique that measures the oxidative stress in living experimental animals. The rate of decay of the ESR signal right after an injection of nitroxyl radical has been measured to evaluate the oxidative stress in animals, although the probe's disposition could also affect this rate. Because the amount of probes forming the redox pair of hydroxyl amine and its corresponding nitroxyl radical was shown to be nearly constant in most organs or tissues 10min after the injection of 1-acetoxy-3-carbamoyl-2,2,5,5-tetramethylpyrrolidine (ACP) in mice, we evaluated the oxidative stress in sepsis model mice induced by lipopolysaccharide (LPS) by intravenously injecting ACP as a precursor of redox probes. The in vivo ESR signal increased up to 7-8min after the ACP injection and then decreased. Decay of the in vivo signal in LPS-treated mice was significantly slower than that in healthy mice, whereas no significant difference was observed in the rate of change in the total amount of redox probes in the blood and liver between these groups. ESR imaging showed that the in vivo signals observed at the chest and upper abdomen decayed slowly in LPS-treated mice. Suppression of the decay in LPS-treated mice was canceled by the administration of a combination of pegylated superoxide dismutase and catalase, or an inhibitor of nitric oxide synthase, or gadolinium chloride. These results indicate that the LPS-treated mouse is under oxidative stress and that reactive oxygen species, such as superoxide and peroxynitrite, related to macrophages are mainly involved in the oxidative stress.

Hepatic reduction of carbamoyl-PROXYL in ferric nitrilotriacetate induced iron overloaded mice: an in vivo ESR study

Reduction of a nitroxyl radical, carbamoyl-PROXYL in association of free radical production and hepatic glutathione (GSH) was investigated in iron overloaded mice using an in vivo L-band electron spin resonance (ESR) spectrometer. Significant increases in hepatic iron, lipid peroxidation and decrease in hepatic GSH were observed in mice intraperitoneally (i.p.) administrated with ferric nitrilotriacetate (Fe(III)-NTA, a total 45 µmol/mouse over a period of 3 weeks). Free radical production in iron overloaded mice was evidenced by significantly enhanced rate constant of ESR signal decay of carbamoyl-PROXYL, which was slightly reduced by treatment with iron chelator, deferoxamine. Moreover, the rate constant of ESR signal decay was negatively correlated with hepatic GSH level (r=-0.586, p<0.001). On the other hand, hepatic GSH-depletion (>80%) in mice through daily i.p. injection and drinking water supplementation of L-buthionine-[S,R]-sulfoximine (BSO) significantly retarded ESR signal decay, while there were no changes in serum aspartate aminotransferase and liver thiobarbituric acid-reactive substances levels. In conclusion, GSH plays two distinguish roles on ESR signal decay of carbamoyl-PROXYL, as an antioxidant and as a reducing agent, dependently on its concentration. Therefore, it should be taken into account in the interpretation of free radical production in each specific experimental setting.

Whole-body kinetic image of a redox probe in mice using Overhauser-enhanced MRI

Overhauser-enhanced MRI (OMRI) enables visualization of free radicals in animals based on dynamic nuclear polarization. Real-time data of tissue redox status gathered from kinetic images of redox-sensitive nitroxyl radical probes using OMRI provided both anatomic and physiological information. Phantom experiments demonstrated the linear correlation between the enhancement factor and the concentration of a membrane-impermeable probe, carboxy-PROXYL (3-carboxy-2,2,5,5-tetramethyl- pyrrolidine-1-oxyl). Whole-body OMRI images illustrated the in vivo kinetics of carboxy-PROXYL for 25 min. Initial distribution was observed in lung, heart, liver, and kidney, but not brain, corresponding to its minimal lipophilicity. Based on these images (pixel size, 1.33 × 1.33 mm; slice thickness, 50mm), a time-concentration curve with low coefficient of variance (<0.21) was created to assess pharmacokinetic behaviors. A biexponential curve showed a distribution phase from 1 to 10 min and an elimination phase from 15 to 25 min. The α rate constant was greater than the β rate constant in ROIs, confirming that its pharmacokinetics obeyed a two-compartment model. As a noninvasive technique, combining OMRI imaging with redox probes to monitor tissue redox status may be useful in acquiring valuable information regarding organ function for preclinical and clinical studies of oxidative diseases.

Unique oxidation of imidazolidine nitroxides by potassium ferricyanide: strategy for designing paramagnetic probes with enhanced sensitivity to oxidative stress

Potassium ferricyanide (PF), routinely employed for the oxidation of sterically-hindered hydroxylamines to nitroxides, is considered to be chemically inert towards the latter. In the present study, we report on an unexpected oxidative fragmentation of the imidazolidine nitroxides containing hydrogen atom in the 4-position of the heterocycle (HIMD) by PF resulting in the loss of the EPR signal. The mechanistic EPR, spectrophotometric, electrochemical and HPLC-MS studies support the assumption that the HIMD fragmentation is facilitated by the proton abstraction from the 4-position of the oxoammonium cation formed as a result of the initial one-electron HIMD oxidation. Increase in steric hindrance around the radical fragment by introducing ethyl substituents decreased the rate of ascorbate-induced HIMD reduction by more than 20 times, but did not affect the rate of ferricyanide-induced HIMD oxidation. This preferential sensitivity of HIMDs to oxidative processes has been used to detect peroxyl radicals in the presence of high concentration of the reducing agent, ascorbate. HIMD-based EPR probes capable to discriminate oxidative and reductive processes might find application in biomedicine and related fields for monitoring the oxidative stress and reactive radical species in biological systems.

Brain redox imaging

Nitroxyl contrast agents (nitroxyl radicals, also known as nitroxide) are paramagnetic species, which can react with reactive oxygen species (ROS) to lose paramagnetism to be diamagnetic species. The paramagnetic nitroxyl radical forms can be detected by using electron paramagnetic resonance imaging (EPRI), Overhauser MRI (OMRI), or MRI. The time course of in vivo image intensity induced by paramagnetic redox-sensitive contrast agent can give tissue redox information, which is the so-called redox imaging technique. The redox imaging technique employing a blood-brain barrier permeable nitroxyl contrast agent can be applied to analyze the pathophysiological functions in the brain. A brief theory of redox imaging techniques is described, and applications of redox imaging techniques to brain are introduced.

Enhanced intraarticular free radical reactions in adjuvant arthritis rats

One of the reasons of rheumatoid arthritis (RA) development is widely recognized the relation of free radical reactions in tissue injuries. The aim of this study was to evaluate the location where in vivo free radical reactions was enhanced in adjuvant arthritis (AA) model rats using in vivo electron spin resonance (ESR)/nitroxyl spin probe technique. The signal decay after intravenous injection of spin probe was enhanced in AA than that in control and suppressed by the pre-treatment of dexamethasone (DXT). Interestingly, the decay in joint cavity occurred prior to paw swelling of AA and suppressed by a simultaneous injection of free radical scavengers, indicating that the enhancement of free radical reactions in joint cavity of AA rats. This technique would be useful tool to determine the location of the enhanced free radical reactions and evaluate the activity of antioxidant medicine with non-invasive real-time measurement.

Non-invasive monitoring of redox status in mice with dextran sodium sulphate-induced colitis

Change of redox status is associated with colitis induced by dextran sodium sulphate (DSS). This study monitored redox status in DSS-induced colitis in mice using in vivo electron spin resonance (ESR) spectroscopy with nitroxyl probes. Colitis was induced in male ICR mice by supplementing their drinking water with 3% DSS for 3, 5 or 7 days. The ESR signal decay rate of carbamoyl-PROXYL administered into the rectum was enhanced by DSS treatment and the enhancement on day 7 was suppressed by membrane-permeable antioxidants, tiron and dimethylsulphoxide and a membrane-impermeable antioxidant, mannitol. The enhancement on day 5 was suppressed by tiron and dimethylsulphoxide, while that on day 3 was inhibited only by tiron. These results suggest that redox change occurs in or around of epithelial cells on day 7, but only intracellularly on day 5, and that redox change such as generation of less reactive radicals occurs only intracellularly on day 3.

Physical Methods of Fast Reactions Investigation

This review presents the basic concepts of the methods used for investigation of fast reactions kinetics, such as: flow methods, with particular emphasis on the stopped-flow approach, NMR, ESR, electrochemical methods, with particular emphasis on the time resolved Fourier Transform electrochemical impedance spectroscopy, flash photolysis, and several others. It offers a brief description of fast reactions commonly encountered in chemical systems, providing an insight into the possibilities of performing kinetic investigations of such reaction systems.

Effect of anaesthesia-induced alterations in haemodynamics on in vivo kinetics of nitroxyl probes in electron spin resonance spectroscopy

Although the advent of in vivo electron spin resonance (ESR) spectroscopy has allowed analysis of the redox status of living animals, whether the haemodynamic condition affects the signal decay rate remains unknown. Three kinds of haemodynamic conditions were generated by changing the anaesthetic dosage in mice. Haemodynamics was analysed (n=6 each) and in vivo ESR was performed to measure the signal decay rates of three nitroxyl spin probes (carbamoyl-, carboxy- and methoxycarbonyl-PROXYL) at the chest and head regions (n=6 for each condition and probe). Haemodynamic analysis revealed negative inotropic and chronotropic effects on the cardiovascular system depending on the depth of anaesthesia. Although signal decay rates differed among three probes, they were not affected by heart rate alteration. In this study we report the haemodynamics-independent signal decay rate of nitoxyl probes.

Application of in vivo ESR/spin-probe technique to monitor tumor in vivo in mouse footpad

The redox status of tumors inoculated into the footpads of mice was investigated by using an in vivo ESR/spin-probe technique. A single-cell suspension of a metastatic subclone of colon carcinoma NL-17 was inoculated into the footpads of Balb/c mice. At 12, 24, 48, and 96 h after the inoculation, a spin probe, either carbamoylor carboxy-PROXYL, was intravenously injected, and then the ESR spectra of each footpad were separately obtained under a one-dimensional magnetic-field gradient. The change in the ESR signal intensity of the spin probe was closely related to the tumor volume in the footpads, but no significant difference was observed between carbamoyl- and carboxy-PROXYL. The in vivo ESR signal decay of carbamoyl-PROXYL, which is related to the conversion of the nitroxyl radical to hydroxylamine, was enhanced in the inoculated footpads but not in the reference one. The ESR signal decay was not influenced by coadministration of radical scavengers, SOD, catalase, mannitol, or dimethylthiourea, suggesting that the redox status but not reactive oxygen species generation played a role in the enhanced signal decay.

Free radical-mediated pre-hemolytic injury in human red blood cells subjected to lead acetate as evaluated by chemiluminescence

The mechanisms by which Pb(2+) induces hemolysis are not completely understood. For this reason, the involvement of oxidative stress in the mechanism of Pb(2+)-induced pre-hemolytic lesion was investigated by exposing RBC to Pb(2+) in vitro and then separating the intact non-hemolysed RBC. Oxidative stress was investigated on human RBCs by tert-butyl hydroperoxide-initiated chemiluminescence method (CL). Our results revealed that lead-induced time and concentration-dependent hemolysis and CL time curves showed a very narrow correlation each other. GSH oxidation to GSSG and the stress index also increased significantly. Treatment of lead-exposed RBC with desferrioxamine, an iron-chelating agent or the chain-breaking antioxidant, Trolox, quenched light emission and inhibited hemolysis dramatically. Mannitol and sodium formate, (*)OH scavengers, on the contrary, did not inhibit CL or hemolysis, significantly. These data indicate that lead-induced lipid peroxide formation is mediated by a metal-driven Fenton reaction but do not support the direct involvement of hydroxyl radicals in this process. By contrast, our results revealed a decrease in light emission and decreased hemolysis in the presence of histidine, a singlet oxygen scavenger. Our results suggest that membrane damage and hemolysis of RBC are mediated by Pb(2+) through free radical reactions and that singlet oxygen plays a significant role in this process.

A neural network approach for image reconstruction in electron magnetic resonance tomography

An object-oriented, artificial neural network (ANN) based, application system for reconstruction of two-dimensional spatial images in electron magnetic resonance (EMR) tomography is presented. The standard back propagation algorithm is utilized to train a three-layer sigmoidal feed-forward, supervised, ANN to perform the image reconstruction. The network learns the relationship between the 'ideal' images that are reconstructed using filtered back projection (FBP) technique and the corresponding projection data (sinograms). The input layer of the network is provided with a training set that contains projection data from various phantoms as well as in vivo objects, acquired from an EMR imager. Twenty five different network configurations are investigated to test the ability of the generalization of the network. The trained ANN then reconstructs two-dimensional temporal spatial images that present the distribution of free radicals in biological systems. Image reconstruction by the trained neural network shows better time complexity than the conventional iterative reconstruction algorithms such as multiplicative algebraic reconstruction technique (MART). The network is further explored for image reconstruction from 'noisy' EMR data and the results show better performance than the FBP method. The network is also tested for its ability to reconstruct from limited-angle EMR data set.

Dynamic nuclear polarization properties of nitroxyl radicals used in Overhauser-enhanced MRI for simultaneous molecular imaging

DNP parameters relevant to Overhauser-enhanced magnetic resonance imaging (OMRI) are reported for a few nitroxyl radicals and their corresponding (15)N and (2)H enriched analogues, used in simultaneous imaging by OMRI. DNP enhancement was measured at 14.529 mT, using a custom-built scanner operating in a field-cycled mode, for different concentrations, ESR irradiation times and RF power levels. DNP enhancements increased with agent concentration up to 2.5 mM and decreased above 3 mM, in tune with ESR line broadening measured at X-band as a function of the agent concentration. The proton spin-lattice relaxation times (T(1)) measured at very low Zeeman field (14.529 mT) and the longitudinal relaxivity parameters were estimated. The relaxivity parameters were in good agreement with those independently computed from the linear region of the concentration dependent enhancement. The leakage factor showed an asymptotic increase with increasing agent concentration. The coupling parameters of (14)N- and (15)N-labeled carbamoyl-PROXYL showed the interaction between the electron and nuclear spins to be mainly dipolar in origin. Upon (2)H labeling, about 70% and 40% increases in enhancement for (15)N- and (14)N-labeled nitroxyl agents were observed, respectively. It is envisaged that the results reported here may enable better understanding of the factors determining DNP enhancement to design suitable 'beacons' for simultaneous molecular imaging by OMRI.

Importance of volume limitation for tissue redox status measurements using nitroxyl contrast agents: a comparison of X-band EPR bile flow monitoring (BFM) method and 300 MHz in vivo EPR measurement

Methods proposed for in vivo redox status estimation, X-band (9.4 GHz) electron paramagnetic resonance (EPR) bile flow monitoring (BFM) and 300 MHz in vivo EPR measurement, were compared. The spin probe 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (carbamoyl-PROXYL) was utilized for both methods, due to its suitable lipophilicity. EPR signal decay of a nitroxyl spin probe in the bile flow and in the liver region (upper abdomen) of several rat groups with different selenium status were measured by both the BFM and the in vivo EPR method, respectively. The nitroxyl radical clearance measured with in vivo EPR method may be affected not only by the redox status in the liver but also by information from other tissues in the measured region of the rat. On the other hand, the time course of nitroxyl radical level in the bile flow of rats was found to be a reliable index of redox status. Measurement site and/or volume limitation, which was achieved by the BFM method in this paper, is quite important in estimating reasonable EPR signal decay information as an index of tissue/organ redox status.

In vivo detection of free radicals induced by diethylnitrosamine in rat liver tissue

Diethylnitrosamine (DEN) is a well-known carcinogenic substance that requires microsomal activation before it can react with DNA to cause mutations and cancer. The aim of this study was to use in vivo spin trapping and spin probe techniques to investigate whether free radicals are generated in rat liver tissue during DEN activation. We used alpha-phenyl-n-tert-butylnitrone (PBN) as the spin trapping agent, which was delivered through an intraperitoneal injection before DEN administration. One hour after DEN administration, multicomponent PBN adducts in the bile were detected, and the intensities were diminished by the cytochrome P450 inhibitor SKF-525A. A computer simulation of the ESR signals revealed the presence of a lipid-derived radical. Using the in vivo spin probe/ESR technique, the signal decay rate of methoxycarbonyl-PROXYL was significantly increased in the DEN-treated group compared with the rate in the vehicle group. The enhanced signal decay rate was restored with PBN and/or SKF-525A pretreatment. These results suggested that lipid-derived free radicals were generated in the liver within 1 h after DEN administration.

Noninvasive Mapping of Reactive Oxygen Species by in Vivo Electron Spin Resonance Spectroscopy in Indomethacin-Induced Gastric Ulcers in Rats

Reactive oxygen species (ROS) are thought to be involved in the gastric ulcer formation induced by indomethacin, a typical nonsteroidal anti-inflammatory drug. However, the location and the time course of ROS generation remain unknown. To assess the sites of ROS generation, we applied the noninvasive measurement of ROS to indomethacin-treated rats. By giving orally a membrane-permeable or impermeable probe, the spectra were collected as a function of time by in vivo 300-MHz electron spin resonance (ESR) spectroscopy. The ESR signal-decay rates of membrane-permeable probes, hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and 3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine-1-oxyl, in the gastric mucosal region were significantly enhanced 1 h after indomethacin treatment, and they both caused the protection of ulcer formation; however, membrane-impermeable probes, carboxy- and trimethylammonium-TEMPO, which did not exhibit the enhanced signal decay, had no effect on ulcer formation. The enhanced signal decay in the gastric mucosa was suppressed by coadministration of the antioxidants tiron or dimethylthiourea with the nitroxyl probe. The results suggest that the enhanced signal-decay rates in the gastric ulcers observed by in vivo ESR are associated with protective effects. The enhanced signal decay caused by ROS generation in stomach, contributing to the ulcer formation induced by indomethacin, is also suggested to occur at the gastric mucus layer or the interface or the intracellular compartment of epithelial cells. Overall, these results show the potentials of noninvasive assessment of ROS production and the sites of damage by in vivo ESR using nitroxyl probes directed to specific subcellular regions.

Estimation of free radical formation by β-ray irradiation in rat liver

In vivo free radical reactions in rat liver as a result of exposure to low-dose beta-radiation was evaluated with electron paramagnetic resonance (EPR) spectroscopy by monitoring the reduction of the nitroxyl spin probe after intravenous administration. The EPR signal intensity of a nitroxyl probe as a function of time in bile flow was monitored by cannulating the bile duct through the cavity of an X-band EPR spectrometer. The results show that the rate of nitroxyl signal loss was higher in rats whose livers were exposed to beta-rays compared to unexposed rats. However, the rate of signal loss was lower in animals whose organs were exposed to air by opening the abdominal cavity. In vitro experiments also showed that the nitroxyl EPR signal loss was greater in an atmosphere of nitrogen than in air. Results suggest that under low levels of tissue oxygen, exposure to beta-rays results in nitroxyl signal loss, which may be mediated by free radical dependent pathways. When tissue oxygen were higher, hydrogen peroxide mediated oxidation of hydroxylamine may predominate resulting in a signal loss of smaller magnitudes. This study shows possible evidence of reactive oxygen species formation by low-dose beta-ray irradiation in a living animal.

In vivo imaging of oxidative stress in ischemia-reperfusion renal injury using electron paramagnetic resonance

Oxidative stress during ischemia-reperfusion acute renal failure (IR-ARF) was noninvasively evaluated with in vivo electron paramagnetic resonance (EPR) imaging. Female ICR mice underwent left nephrectomy and 30-min ischemia-reperfusion of the right kidney. Oxidative stress was evaluated as organ reducing activity with the half-lives of the spin probe 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (carbamoyl-PROXYL) using 1) conventional L-band EPR, which showed organ-reducing activity in the whole abdominal area; and 2) EPR imaging, which showed semiquantitative but organ-specific reducing activity. The results were compared with the reducing activity of organ homogenate and phosphatidylcholine hydroperoxide (PC-OOH) concentrations. Half-lives of carbamoyl-PROXYL in the whole upper abdominal area, measured by L-band EPR, were prolonged on day 3 after ischemia-reperfusion and recovered to the level of nontreated mice on day 7. This trend resembled closely that of serum creatinine and blood urea nitrogen concentration. The EPR imaging-measured carbamoyl-PROXYL half-life was also prolonged on day 3 in both the kidney and the liver. However, in the kidney this showed only partial recovery on day 7. In the liver, this convalescence was more remarkable. The ex vivo studies of organ reducing activity and PC-OOH agreed with the results from EPRI, but not with those from L-band EPR. These results indicate that renal reducing activity shows only partial recovery on day 7 after ischemia-reperfusion, when serum creatinine and blood urea nitrogen have recovered. EPR imaging is an appropriate and useful method for the noninvasive evaluation of oxidative stress in the presence of renal injury.

In vivo EPR spectroscopic imaging for a liposomal drug delivery system

We used the membrane-impermeable nitroxyl radical 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyliodide (CAT-1) as a model drug encapsulated in liposomes in order to separately map the 2D distribution of both liposomal-encapsulated CAT-1 and free CAT-1. Phantoms were prepared with a CAT-1 solution and a liposomal CAT-1 suspension. Spectral-spatial images were obtained along several polar-arranged spatial axes through the phantom. The 1D spatial distributions (projections) of each signal component, reflecting the concentration of CAT-1, were then extracted from the spectral-spatial images. 2D EPR images of liposomal-encapsulated CAT-1 and free CAT-1 were separately reconstructed from the resulting projection data sets. 2D mapping of each component exhibited good agreement with respect to the phantom. Separate maps were generated from separate injections of free CAT-1 and liposomal CAT-1 injected into the femoral muscle of a living mouse. The EPR signal of the free CAT-1 gradually decreased during data acquisition. Because of this decay, we calibrated the image intensity by extrapolating the signal intensity to that detected at the beginning of data sampling. Both the position and size of the individual images were in very good agreement with those of the mouse thigh obtained by MRI.

Effect of hydrogen peroxide in redox status estimation using nitroxyl spin probe

A procedure for estimating in vivo redox status using EPR and a hydrogen peroxide (H(2)O(2))-dependent spin probe method is described. The mechanism of decreasing spin clearance in the selenium-deficient (SeD) rat is discussed. The in vivo decay constant of the nitroxyl spin probe in the liver region of SeD rats appeared to be slightly lower that of the selenium-adequate control (SeC) group, and was significantly smaller than that of normal rats. Bile H(2)O(2) levels in normal rats were significantly lower than those in SeD rats. The in vivo decay constant of the spin probe in SeD rats depended on the bile H(2)O(2) level. Furthermore, H(2)O(2) was detected in the bile in all SeD rats, whereas bile H(2)O(2) could be detected in only half of the normal rats. It was found that the in vivo decay constant of the spin probe in normal rats also depended on whether bile H(2)O(2) was detected or not. In vivo decay constants were smaller in rats subjected to the surgical operation than in the nonoperated groups. The EPR signal of the nitroxyl radical in the liver homogenate was increased by addition of H(2)O(2), which was administered 30 min before the rat was killed. It appears that H(2)O(2) can oxidize the hydroxylamine formed following reduction of the spin probe in the liver.

Non-invasive analysis of reactive oxygen species generated in rats with water immersion restraint-induced gastric lesions using in vivo electron spin resonance spectroscopy

Reactive oxygen species (ROS) are reportedly associated with gastric ulcer. We previously reported the use of an in vivo 300-MHz electron spin resonance (ESR) spectroscopy/nitroxyl probe technique to detect *OH generation in the stomachs of rats with gastric ulcers induced by NH4OH. However, this is an acute ulcer model, and the relationship between in vivo ROS generation and lesion formation remains to be clarified. To address this question, the same technique was applied to a sub-acute water immersion restraint (WIR) model. A nitroxyl probe that was less membrane-permeable was orally administered to WIR-treated rats, and the spectra in the gastric region were obtained by in vivo ESR spectroscopy. The signal intensity of the orally administered probe was clearly changed in the WIR group, but no change occurred in the control group. Both enhanced signal decay and neutrophil infiltration into mucosa were observed 2h after WIR with little formation of any mucosal lesions. The enhanced signal decay was caused by *OH generation, based on the finding that the decay was suppressed by mannitol, desferrioxamine and catalase. Intravenous treatment with either anti-neutrophil antibody or allopurinol also suppressed the enhanced signal decay, and allopurinol depressed neutrophil infiltration into the mucosa. In rats treated with WIR for 6 h, lesion formation was suppressed by 50% with all antioxidants used in this experiment except anti-neutrophil antibody. These findings suggest that *OH, which is generated in the stomach via the hypoxanthine/xanthine oxidase system upon neutrophil infiltrated into the mucosa, induces mucosal lesion formation, but that it accounts for only half the cause of lesion formation.

Recent progress in in vivo ESR spectroscopy

The generation of free radicals and redox status is related to various diseases and injuries that are related to radiation, aging, ischemia-reperfusion, and other oxidative factors. In vivo electron spin resonance (ESR) spectroscopy is noninvasive and detects durable free radicals in live animals. ESR spectrometers for in vivo measurements operate at a lower frequency (approximately 3.5 GHz, approximately 1 GHz, 700 MHz, and approximately 300 MHz) than usual (9-10 GHz). Several types of resonators have been designed to minimize the dielectric loss of electromagnetic waves caused by water in animal bodies. In vivo ESR spectroscopy and its imaging have been used to analyze radical generation, redox status, partial pressure of oxygen and other conditions in various disease and injury models related to oxidative stress with probes, such as nitroxyl radicals. Through these applications, the clarification of the mechanisms related to oxidative diseases (injuries) and the accumulation of basic data for radiological cancer therapy are now ongoing. In vivo ESR measurement is performed in about 10 laboratories worldwide, including ours. To introduce in vivo ESR spectroscopy to life scientists, this article reviews the recent progress of in vivo ESR spectroscopy in instrumentation and its application to the life sciences.

Evidence for contribution of vascular NAD(P)H oxidase to increased oxidative stress in animal models of diabetes and obesity

It is well established that oxidative stress is enhanced in diabetes. However, the major in vivo source of oxidative stress is not clear. Here we show that vascular NAD(P)H oxidase may be a major source of oxidative stress in diabetic and obese models. In vivo electron spin resonance (ESR)/spin probe was used to evaluate systemic oxidative stress in vivo. The signal decay rate of the spin probe (spin clearance rate; SpCR) significantly increased in streptozotocin-induced diabetic rats 2 weeks after the onset of diabetes. This increase was completely normalized by treatment with the antioxidants alpha-tocopherol (40 mg/kg) and superoxide dismutase (5000 units/kg), and was significantly inhibited by treatment with a PKC-specific inhibitor, CGP41251 (50 mg/kg), and a NAD(P)H oxidase inhibitor, apocynin (5 mg/kg). Both obese ob/ob mice (10 weeks old) with mild hyperglycemia and Zucker fatty rats (11 weeks old) with normoglycemia exhibited significantly increased SpCR as compared with controls. Again, this increase was inhibited by treatment with both CGP41251 and apocynin. Oral administration of insulin sensitizer, pioglitazone (10 mg/kg), for 7 days also completely normalized SpCR values. These results suggest that vascular NAD(P)H oxidase may be a major source of increased oxidative stress in diabetes and obesity.

In Vivo Oxygen Radical Generation in the Skin of the Protoporphyria Model Mouse with Visible Light Exposure: An L-Band ESR Study

Although oxygen radicals are thought to play a key role in the skin injury that is caused by protoporphyria, there is no direct evidence of generation of these radicals in vivo. This study measured the generation of oxygen radicals caused by visible light non-invasively in the skin of griseofulvin-induced protoporphyria model mice, using an in vivo electron spin resonance spectrometer equipped with a surface-coil-type resonator that could detect radicals within about 0.5 mm of the skin surface. A durable nitroxyl radical was administered intravenously as a probe. Light irradiation enhanced the decay of the nitroxyl signal in griseofulvin-treated mice, whereas light irradiation did not enhance the signal decay in control mice. The enhanced signal decay was completely suppressed by intravenous administration of hydroxyl radical scavengers, superoxide dismutase or catalase, or the intraperitoneal administration of desferrioxamine. The enhanced signal decay with illumination was reversible, and quickly responded to turning the light on and off. These observations suggest that the hydroxyl radical is generated via an iron-catalyzed reaction in the skin. This paper demonstrates, for the first time, the specific generation of oxygen radicals in response to light irradiation of the skin of protoporphyria model mice.

In vivo measurement of redox status in streptozotocin-induced diabetic rat using targeted nitroxyl probes

In vivo electron paramagnetic resonance (EPR) with nitroxyl spin probes has been used for the evaluation of in vivo free radical reactions and redox status in living animals. The aim of this study was to clarify the location of free radical reactions induced by hyperglycemia in osteogenic disorder shionogi (ODS) rats using in vivo EPR spectroscopy. Diabetes was induced by intravenous injection of streptozotocin (STZ). The amount of ascorbic acid (AsA) in ODS rats was controlled by feeding AsA-containing water. Fourteen days after STZ injection, blood glucose and plasma malondialdehyde levels in STZ-treated rats significantly increased compared with untreated rats. Signal decay rates of intravenously injected 3-carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (carbamoyl-PROXYL) (less membrane permeable) and 3-carboxy-PROXYL (membrane impermeable) were enhanced in STZ-treated rats in agreement with the previous reports. The decay rate of 3-acetoxymethoxy-PROXYL (membrane permeable) was significantly enhanced by STZ treatment in AsA-depleted rats, and this enhancement was partially restored to the control value by xanthine oxidase inhibitor, although the rate in AsA-supplemented rats was not changed by STZ treatment. These results suggested that the enhancement of signal decay occurred mainly in the intravascular region in STZ-induced diabetic rats and that AsA depletion induced the enhancement of intracellular signal decay through xanthine oxidase, although it is not clear whether the enhancement of signal decay is the cause or the effect of STZ-induced diabetes.

Novel Pharmacokinetic Measurement Using Electron Paramagnetic Resonance Spectroscopy and Simulation of in Vivo Decay of Various Nitroxyl Spin Probes in Mouse Blood

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.

Magnetic resonance imaging of isolated single liposome by magnetic resonance force microscopy

Magnetic resonance imaging (MRI) is very useful spectroscopy to visualize a three-dimensional (3D) real structure inside the sample without physical destruction. The spatial resolution of the readily available MRI spectrometer is, however, limited by a few ten to hundreds of microns due to a technological boundary of generating larger magnetic field gradient and to the insensitivity inherent to the inductive signal detection. Magnetic resonance force microscopy (MRFM) is new alternative MRI spectroscopy which is anticipated to significantly surpass the conventional MRI in both resolution and sensitivity. We report two imaging experiments on our MRFM spectrometer operated at room temperature and in vacuum approximately 10(-3)Pa. One is for approximately 20 microm liposome membrane labeled entirely by a nitroxide imaging agent and the other for approximately 15 microm DPPH particles, both are nearly the same size as that of human cell. The reconstructed images at spatial resolution approximately 1 microm were in satisfactory agreement with the scanning electron microscope images. The potential capability of visualizing intrinsic radicals in the cell is suggested to investigate redox process from a microscopic point of view.

Application of in vivo ESR spectroscopy to measurement of cerebrovascular ROS generation in stroke

This study used an in vivo ESR spectroscopy/spin probe technique to measure directly the generation of reactive oxygen species (ROS) in the brain after cerebral ischemia-reperfusion. Transient middle cerebral artery occlusion (MCAO) was induced in rats by inserting a nylon thread into the internal carotid artery for 1 h. The in vivo generation of ROS and its location in the brain were analyzed from the enhanced ESR signal decay data of three intra-arterially injected spin probes with different membrane permeabilities. The ESR signal decay of the probe with intermediate permeability was significantly enhanced 30 min after reperfusion following MCAO, whereas no enhancement was observed with the other probes or in the control group. The enhanced in vivo signal decay was significantly suppressed by superoxide dismutase (SOD). Brain damage was barely discernible until 3 h of reperfusion, and was clearly suppressed with the probe of intermediate permeability. The antioxidant MCI-186 completely suppressed the enhanced in vivo signal decay after transient MCAO. These results clearly demonstrate that ROS are generated at the interface of the cerebrovascular cell membrane when reperfusion follows MCAO in rats, and that the ROS generated during the initial stages of transient MCAO cause brain injury.

In vivo electron spin resonance-computed tomography/nitroxyl probe technique for non-invasive analysis of oxidative injuries

Free radicals are widely recognized as harmful chemical species in oxidative tissue injury. However, there have been no satisfying methods to visualize free radicals in vivo non-invasively with information of their localization and amount. In vivo electron spin resonance (ESR) spectroscopy was recently developed to measure free radicals generated in rodents. Several kinds of stable nitroxyl radicals were used as spin probes to detect free radicals. ESR signal intensities reflecting the accumulation of nitroxyl probes in each organ decreases time-dependently and reduction decay rates are increased in the presence of free radicals. Such increase in signal decay rates is suppressed by prior administration of antioxidants or antioxidant enzymes. Thus, in vivo ESR techniques are useful in estimating not only in vivo free radical reactions but also the effects of antioxidants, and furthermore, in combination with other tomographic techniques, permits non-invasive localization of free radicals. Application of this technique to animal models will be described.

Non-invasive analysis of reactive oxygen species generated in NH4OH-induced gastric lesions of rats using a 300 MHz in vivo ESR technique

Free radicals are reportedly involved in mucosal injury, including NH4OH-induced gastric lesions, but the kind, location and origin of radical generation have yet to be clarified. We developed the non-invasive measurement of reactive oxygen species (ROS) in stomach, and applied to mucosal injury. NH4OH-induced gastric lesions were prepared in rats, which were then given a nitroxyl probe intragastrically or intravenously, and the spectra of the gastric region were obtained by in vivo 300 MHz electron spin resonance (ESR) spectroscopy. The spectral change of the nitroxyl probe administered intragastrically was significantly enhanced 30 min after NH4OH administration, but no change occurred when the probe was given by intravenous injection. The enhanced change was confirmed to be due to *OH generation, because it was completely suppressed by mannitol, catalase and desferrioxamine (DFO), and was not observed in neutropenic rats. NH4OH-induced neutrophil infiltration of the gastric mucosa was suppressed by intravenous injection of superoxide dismutase (SOD) or catalase, or by administration of allopurinol. The present study provided the direct evidence in NH4OH-treated living rats that *OH produced from O2*- derived from neutrophils caused gastric lesion formation, while O2*- or H2O2 derived from the xanthine oxidase system in endothelial cells was involved in neutrophil infiltration.

Confirmation of superoxide generation via xanthine oxidase in streptozotocin-induced diabetic mice

Reactive oxygen species (ROS) may play key roles in vascular inflammation and atherogenesis in patients with diabetes. In this study, xanthine oxidase (XO) system was examined as a potential source of superoxide in mice with streptozotocin (STZ)-induced experimental diabetes. Plasma XO activity increased 3-fold in diabetic mice (50 +/- 33 microU/ml) 2 weeks after the onset of diabetes, as compared with non-diabetic control mice (15 +/- 6 microU/ml). In vivo superoxide generation in diabetic mice was evaluated by an in vivo electron spin resonance (ESR)/spin probe method. Superoxide generation was significantly enhanced in diabetic mice, and the enhancement was restored by the administration of superoxide dismutase (SOD) and 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron), which was reported to scavenge superoxide. Pretreatment of diabetic mice with XO inhibitors, allopurinol and its active metabolite oxipurinol, normalized the increased superoxide generation. In addition, there was a correlation (r = 0.78) between the level of plasma XO activity and the relative degree of superoxide generation in diabetic and non-diabetic mice. Hence, the results of this study strongly suggest that superoxide should be generated through the increased XO seen in the diabetic model mice, which may be involved in the pathogenesis of diabetic vascular complications.

Kinetic study on ESR signal decay of nitroxyl radicals, potent redox probes for in vivo ESR spectroscopy, caused by reactive oxygen species

The effect of the chemical structure of nitroxyl spin probes on the rate at which ESR signals are lost in the presence of reactive oxygen species (ROS) was examined. When the spin probes were reacted with either hydroxyl radical (.OH) or superoxide anion radical (O(2)(.-)) in the presence of cysteine or NADH, the probes lost ESR signal depending on both their ring structure and substituents. Pyrrolidine nitroxyl probes were relatively resistant to the signal decay caused by O(2)(.-) with cysteine/NADH. Signal decay rates for these reactions correlated with reported redox potentials of the nitroxyl/oxoammonium couple of spin probes, suggesting that the signal decay mechanism in both cases involves the oxidation of a nitroxyl group. The apparent rate constants of the reactions between the spin probe and .OH and between the spin probe and O(2)(.-) in the presence of cysteine were estimated using mannitol and superoxide dismutase (SOD), respectively, as competitive standards. The rate constants for spin probes and .OH were in the order of 10(9) M(-1) s(-1), much higher than those for the probes and O(2)(.-) in the presence of cysteine (10(3)-10(4) M(-1) s(-1)). These basic data are useful for the measurement of .OH and O(2)(.-) in living animals by in vivo ESR spectroscopy.

[Free radicals and oxidative stress: targeted ESR measurement of free radicals]

The detection of free radicals generated within the body may contribute to clarifying the pathophysiological role of free radicals in disease processes. As an appropriate procedure to examine the generation of free radicals in a biological system, electron spin resonance (ESR) has emerged as a powerful tool for detection and identification. A method for determination of oxygen radical scavenging activity using ESR and the spin trapping technique was developed. Oxygen radicals were trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) or alpha-phenyl-N-t-butylnitrone (PBN), and the DMPO or PBN spin adduct signal was measured quantitatively by an ESR spectrometer. The spin trapping method using ESR has also been reported for not only in vitro and ex vivo measurements but also in vivo measurements. In in vivo ESR, nitroxyl radical is being used as a spin trap well. ESR signal intensities of nitroxyl radical are measured after administration to animals and the signal decay rates of nitroxyl radical have reported to be influenced by various types of oxidative stress. With this method, it is possible to specify the type of radical or the location at which the free radicals are produced. The spin trapping method by in vivo ESR is an effective procedure for giving non-invasive measurements in animals. ESR imaging in the organs of live animals can also be obtained after injection of nitroxyl radicals as an imaging agent using ESR-computed tomography. In vivo ESR imaging has been established as a powerful technique for determining the spatial distribution of free radicals in living organs and tissues.

Feasibility and assessment of non-invasive in vivo redox status using electron paramagnetic resonance imaging

PURPOSE

To test the feasibility of electron paramagnetic resonance imaging (EPRI) to provide non-invasive images of tissue redox status using redox-sensitive paramagnetic contrast agents.

MATERIAL AND METHODS

Nitroxide free radicals were used as paramagnetic agents and a custom-built 300 MHz EPR spectrometer/imager was used for all studies. A phantom was constructed consisting of four tubes containing equal concentrations of a nitroxide. Varying concentrations of hypoxanthine/xanthine oxidase were added to each tube and reduction of the nitroxide was monitored by EPR as a function of time. Tumor-bearing mice were intravenously infused with a nitroxide and the corresponding reduction rate was monitored on a pixel-by-pixel basis using 2D EPR of the tumor-bearing leg and normal leg serving as control. For animal studies, nitroxides were injected intravenously (1.25 mmol/kg) and EPR projections were collected every 3 min after injection using a magnetic field gradient of 2.5 G/cm. The reduction rates of signal intensity on a pixel-by-pixel basis were calculated and plotted as a redox map. Redox maps were also collected from the mice treated with diethylmaleate (DEM), which depletes tissue thiols and alters the global redox status.

RESULTS

Redox maps obtained from the phantoms were in agreement with the intensity change in each of the tubes where the signals were decreasing as a function of the enzymatic activity, validating the ability of EPRI to accurately access changes in nitroxide reduction. Redox imaging capability of EPR was next evaluated in vivo. EPR images of the nitroxide distribution and reduction rates in tumor-bearing leg of mice exhibited more heterogeneity than in the normal tissue. Reduction rates were found to be significantly decreased in tumors of mice treated with DEM, consistent with the depletion of thiols and the consequent alteration of the redox status.

CONCLUSION

Using redox-sensitive paramagnetic contrast agents, EPRI can non-invasively discriminate redox status differences between normal tissue and tumors.