In vivo ESR measurements of free radical reactions in living mice

Application of a Compact Magnetic Resonance Imaging System with 1.5 T Permanent Magnets to Visualize Release from and the Disintegration of Capsule Formulations in Vitro and in Vivo

Although magnetic resonance imaging (MRI) has potential in assessments of formulations, few studies have been conducted because of the size and expense of the instrument. In the present study, the processes of in vitro and in vivo release in a gelatin capsule formulation model were visualized using a compact MRI system with 1.5 T permanent magnets, which is more convenient than the superconducting MRI systems typically used for clinical and experimental purposes. A Gd-chelate of diethylenetriamine-N,N,N',N″,N″-pentaacetic acid, a contrast agent that markedly enhances proton signals via close contact with water, was incorporated into capsule formulations as a marker compound. In vitro experiments could clearly demonstrate the preparation-dependent differences in the release/disintegration of the formulations. In some preparations, the penetration of water into the formulation and generation of bubbles in the capsule were also observed prior to the disintegration of the formulation. When capsule formulations were orally administered to rats, the release of the marker into the stomach and its transit to the duodenum were visualized. These results strongly indicate that the compact MRI system is a powerful tool for pharmaceutical studies.

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.

Oxidative stress imaging in live animals with techniques based on electron paramagnetic resonance

Oxidative stress has been the object of considerable biological and biochemical investigation. Quantification has been difficult although the quantitative level of products of biological oxidations in tissues and tissue products has emerged as a widely used technique. The relationship between these products and the amount of oxidative stress is less clear. Imaging oxidative stress with electron paramagnetic resonance related magnetic resonance imaging, while not addressing the specific issue of quantification of initiating events, focuses on the anatomic specific location of the oxidative stress. Moreover, the relative quantification of oxidative stress of one location against another is possible, sharpening our understanding of oxidative stress. This promises to improve our understanding of oxidative stress and its deleterious consequences and enhance our understanding of the effectiveness of interventions to modulate oxidative stress and its consequences.

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.

Tissue Prx I in the protection against Fe-NTA and the reduction of nitroxyl radicals

Peroxiredoxin I (Prx I) is a key cytoplasmic peroxidase that reduces intracellular hydroperoxides in concert with thioredoxin. To study the role of tissue Prx I in protection from oxidative stress, we generated Prx I-/- mice by gene trapping. We then evaluated the acute-phase tissue damage caused by ferric-nitrilotriacetate (Fe-NTA). Increases in serum aspartate aminotransferase and alanine aminotransferase levels were significantly greater in Prx I-/- than wild-type mice, 4 and 12 h after the injection of Fe-NTA. Using real-time EPR imaging, we examined the reduction of the stable paramagnetic nitroxyl radical 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl in vivo, and found that the half-life of this spin probe in the liver and kidney was significantly prolonged in the Prx I-/- mice. These results demonstrate that Prx I-/- mice have less reducing activity and are more susceptible to the damage mediated by reactive oxygen species in vivo than wild-type mice.

Tumor oxygenation status as a prognostic marker

Tumor oxygenation status is an independent prognostic indicator in cancer because it influences tumor progression and treatment outcome. Its quantitative value is determined by a number of tumor vascular parameters such as microvascular density, blood flow, blood volume, blood oxygen saturation, tumor tissue pO2, and resistance to oxygen diffusion within the tumor. Over the past several years, considerable time and effort have been invested into developing techniques to effectively and reliably measure the oxygenation status of a tumor. The measurement and interpretation of data obtained with currently available methods is complicated by the heterogeneity in tumor oxygenation. Currently available techniques can be broadly classified into direct invasive methods, direct non-invasive methods, and measurement of surrogate endogenous markers of tumor oxygenation. Of these methods, the Eppendorf pO2 histograph is considered the 'gold standard' and even so has several limitations. Given the importance of tumor oxygenation status in therapy and in predicting disease progression, it is imperative that reliable, globally usable, and technically simplistic methods be developed to yield a consistent, comprehensive, and reliable profile of tumor oxygenation. Until newer more reliable techniques are developed, existing independent techniques or appropriate combinations of techniques should be optimized and validated using known endpoints in tumor oxygenation status and/or treatment outcomes.

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 detection of gastric cancer in rats by electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) enables noninvasive spatial mapping of free radical metabolism and has recently been shown to provide in vivo physiologic information regarding alterations in the redox state of tumors and neoplastic tissues. With the use of nitroxide spin probes, it has been shown that certain tumors possess a highly reduced state. To determine whether EPRI can be used for early detection and visualization of gastric carcinoma based on its altered redox metabolism, studies were performed in a rat gastric cancer model induced by 1-methyl-3-nitro-1-nitrosoguanidine. Using a specialized 750 MHz resonator and EPRI instrument, a technique was developed for imaging nitroxide radicals in the whole stomach. In vivo three-dimensional EPRI of the stomach of rats with continuous intravenous administration of nitroxide 3-carboxamido-2,2,5,5-tetramethylpyrrolidine-N-oxyl (3-carbamoyl-proxyl) [3-CP] was performed. Whereas electron paramagnetic resonance images from untreated controls provide a uniform visualization of the stomach mucosa and wall, in the treated rats with gastric cancer, holes were present in the image at the locations of tumors. With localized spectroscopy, it was confirmed that the tumor regions were devoid of signal, and this was largely due to the presence of a more reduced state with rapid reduction of nitroxide. Pharmacokinetic studies indicated that 3-CP in tumors was rapidly reduced to an undetectable level, whereas the 3-CP levels in normal stomach tissue persisted. Near-infrared reflectance measurements of indocyanine green dye uptake indicated that there were no significant differences in tumor versus normal mucosal perfusion. From these results, we concluded that gastric cancer tumors could be distinguished from normal tissue based primarily on the marked difference in their rate of radical metabolism. Because alterations in cellular redox state and radical metabolism are of critical importance in tumor biology and treatment, this methodology should provide an important new tool for the study and visualization of gastric carcinoma and may also be of use in other cancer models.

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.

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.

Effects of edaravone on experimental brain injury in view of free radical reaction

The purpose of this study is to clarify the influence of a novel free radical scavenger edaravone on experimental brain injury. Male Wistar rats were anesthetized with 1-2% halothane. Brain injury was produced using a controlled cortical impact injury device. Experimental rats were divided into 2 groups. In the edaravone group, edaravone (3 mg/kg) was twice administered intravenously for 30 minutes. In the saline group, saline solution was administered in the same way. This administration of edaravone or saline solution made it possible to evaluate the relative effects of edaravone by assessment of free radical reaction and water content. As a result, the level of oxygen free radicals at 50 minutes after brain injury was significantly lower in the edaravone group than in the saline group. The water content in the injured brain at 180 minutes was significantly lower in the edaravone group than in the saline group. Therefore, we propose that edaravone may be effective for treatment in head injury.

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.

EPR imaging of reducing activity in Nrf2 transcriptional factor-deficient mice

Mice that lack the Nrf2 (NF-E2-related factor 2) transcription factor develop a lupus-like autoimmune nephritis. The tissue-reducing activity of Nrf2-deficient mice was evaluated using a combination of real-time EPR imaging and spin probe kinetic analysis. Substantial delay in the spin probe 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (Carbamoyl-PROXYL) disappearance in the liver and kidneys of Nrf2-deficient mice was observed by EPR imaging. The half-life of the spin probe in the upper abdominal area was prolonged in both the Nrf2-deficient mice and in aged mice. The combination of Nrf2 deficiency and aging in female mice resulted in the most prolonged half-life of disappearance, which was four times longer than that of juvenile female mice with a wild-type genotype. These results indicate that the low reducing activity in these organs is brought about by both Nrf2 deficiency and the aging process, and it may play a key role in the onset of autoimmune nephritis. This combination of the EPR imaging and half-life analysis appears to be a very powerful tool in the real-time analysis of reducing activity.

Pathophysiological significance of in vivo ESR signal decay in brain damage caused by X-irradiation. Radiation effect on nitroxyl decay of a lipophilic spin probe in the head region

X-irradiation of mice decreased the decay rate of the in vivo ESR signal in the head region to 75% of the control when 3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-yloxy (MCPROXYL), a lipophilic and blood-brain barrier-permeable spin probe, was used. We attempted to identify the specific factor responsible for the decrease in the signal decay rate caused by X-irradiation. The signal decay of MCPROXYL in the head region depends on the following three factors: (1) blood concentration of MCPROXYL, (2) reduction to the corresponding hydroxylamine in the brain tissue, and (3) effusion of MCPROXYL from the brain tissue. Irradiation at 15 Gy did not significantly change the rate of decrease of blood concentration of MCPROXYL at 1 h post-irradiation. The reducing activity of the brain homogenate was not changed by the X-irradiation (15 Gy). The contents of MCPROXYL and its hydroxylamine derivative in the brain of 15 Gy-irradiated mice remained higher than in non-irradiated mice. These findings suggest that the effect of X-irradiation observed by in vivo ESR is attributable not to the redox reaction of MCPROXYL in the brain but to the change of the efflux rate of the MCPROXYL from the brain.

Noninvasive study of radiation-induced oxidative damage using in vivo electron spin resonance

Nitroxyl radicals injected into a whole body indicate the disappearance of signal intensity of in vivo electron spin resonance (ESR). The signal decay rates of nitroxyl have reported to be influenced by various types of oxidative stress. We examined the effect of X-irradiation on the signal decay rate of nitroxyl in the upper abdomen of mice using in vivo ESR. The signal decay rates increased 1 h after 15 Gy irradiation, and the enhancement was suppressed by preadministration of cysteamine, a radioprotector. These results suggest that the signal decay of nitroxyl in whole mice is enhanced by radiation-induced oxidative damage. The in vivo ESR system probing the signal decay of nitroxyl could provide a noninvasive technique for the study of oxidative stress caused by radiation in a living body.

A new nitroxyl-probe with high retention in the brain and its application for brain imaging

In order to estimate free radical reactions and image them in the brain of living animals, a nitroxyl spin-probe, carboxy-PROXYL acetoxymethyl ester (CxP-AM) was newly synthesized. CxP-AM was designed to be hydrolyzed by esterase, but not by lipase, so that it would pass through the blood-brain barrier and be retained in the cytosolic phase of parenchymal cells in the brain after intravenous injection. The pharmacokinetics of CxP-AM was compared with those of carboxy-PROXYL (CxP) and its methyl ester (CxP-M). Carboxyl esterase almost completely hydrolyzed CxP-AM within 3 min. After intravenous injection, the brain retained 1.8 times more CxP-AM than CxP-M, and retained it for more than 30 min. Electron spin resonance computed tomographic (ESR-CT) imaging of CxP-AM in the heads of mice produced marked contrast in the encephalon region, while CxP was distributed only in the extracranial region and CxP-M was distributed in both regions, confirming the pharmacokinetics of CxP-AM. The decay rate of CxP-AM determined with time-resolved ESR-CT imaging was different in the two brain regions, suggesting regional differences in the total reducing capability. CxP-AM should become a powerful probe for the investigation and diagnosis of free radical reactions and their imaging in the brain.

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

The noninvasive, real time technique of in vivo electron spin resonance (ESR) spectroscopy was used to evaluate free radical reactions catalyzed by iron in living mice. The spectra and signal decay of a nitroxyl probe, carbamoyl-PROXYL, were observed in the upper abdomen of mice. The signal decay was significantly enhanced in mice subcutaneously loaded with ferric citrate (0.2 micromol/g body wt) and the enhancement was suppressed by pre-treatment with either desferrioxamine (DF) or the chain breaking antioxidant Trolox, but only slightly suppressed by the hydroxyl radical scavenger DMSO. To determine the catalytic form of iron, DF was administered at different times with respect to iron loading: before, simultaneously, and after 20 and 50 min. The effect of DF on signal decay, liver iron content, iron excretion, and lipid peroxidation (TBARs) depended on the time of the treatment. There was a good correlation between the signal decay, iron content, and lipid peroxidation, indicating that "chelatable iron" contributed to the enhanced signal decay. The nitroxyl probe also exhibited in vivo antioxidant activity, implying that the process responsible for the signal decay of the nitroxyl probe is involved in free radical oxidative stress reactions catalyzed by iron.

Current status of electron spin resonance (ESR) for in vivo detection of free radicals

Much outstanding progress concerning the application of ESR spectroscopy/imaging in the biomedical field has been made in recent years. The literature in this field has already been specifically covered by several reviews. The aim of this article is to provide an overview of the most important findings, obtained in the last four years, in the detection and localization of different exogenous free radicals, as well as of endogenous free radicals in diverse experimental animal models.