In vivo EPR studies of the metabolic fate of nitrosobenzene in the mouse

Echo-based Single Point Imaging (ESPI): a novel pulsed EPR imaging modality for high spatial resolution and quantitative oximetry

A novel time-domain spectroscopic EPR imaging approach, that is a unique combination of already known techniques, is described. The first one is multi-gradient Single Point Imaging involving pure phase-encoding where the oximetry is based on T(2)(∗). Line width derived from T(2)(∗) is subject to susceptibility effects and therefore needs system-dependent line width calibrations. The second approach utilizes the conventional 90°-τ-180° Spin-Echo pulse sequence where the images are obtained by the filtered back-projection after FT of the echoes collected under frequency-encoding gradients. The spatially resolved oximetry information is derived from a set of T(2)-weighted images. The back-projection images suffer susceptibility artifacts with resolution determined by T(2)(∗), but the oximetry based on T(2) is quite reliable. The current approach combines Single Point Imaging and the Spin-Echo procedure to take advantage the enhanced spatial resolution associated with the former and the T(2) dependent contrast of the latter. Pairs of images are derived choosing two time points located at identical time intervals on either side of the 180° pulse. The refocusing pulse being exactly in the middle of the two points ensures that artifacts associated with susceptibility and field inhomogeneities are eliminated. In addition, the net phase accumulated by the two time points being identical results in identical field of views, thus avoiding the zoom-in effect as a function delay in regular SPI and the associated interpolation requirements employed in T(2)(∗)-weighted oximetry. The end result is superior image resolution and reliable oximetry. In spite of the fact that projection-reconstruction methods require less number of measurements compared to SPI, the enormous advantage in SNR of the SPI procedure makes the echo-based SPI equally efficient in terms of measurement time. The Fourier reconstruction, line width independent resolution and the true T(2)-weighting make this novel procedure very attractive for in vivo EPR imaging of tissue oxygen quantitatively.

Aromatic lipophilic spin traps effectively inhibit RPE65 isomerohydrolase activity

We previously showed that RPE65 does not specifically produce 11-cis-retinol only but also 13-cis-retinol, supporting a carbocation or radical cation mechanism of isomerization. The intrinsic properties of conjugated polyene chains result in facile formation of radical cations in oxidative conditions. We hypothesized that such radical intermediates, if involved in the mechanism of RPE65, could be stabilized by spin traps. We tested a variety of hydrophilic and lipophilic spin traps for their ability to inhibit RPE65 isomerohydrolase activity. We found that the aromatic lipophilic spin traps such as N-tert-butyl-α-phenylnitrone (PBN), 2,2-dimethyl-4-phenyl-2H-imidazole-1-oxide (DMPIO), and nitrosobenzene (NB) strongly inhibit RPE65 isomerohydrolase activity in vitro.

Measuring brain tissue oxygenation under oxidative stress by ESR/MR dual imaging system

The in vivo measurement of oxygen in tissues is of great interest because of oxygen's fundamental role in life. Many methods have been developed for such measurement, but all have been limited, especially with regard to repeated measurement, degree of invasiveness, and sensitivity. We describe electron spin resonance (ESR) oximetry with paramagnetic oxygen-sensing probe for in vivo measurement of oxygen in brain tissues by home-made ESR/MR dual imaging spectroscopy. Lithium 5, 9, 14, 18, 23, 27, 32, 36-octa-n-butoxy-2,3-naphthlocyanine (LiNc-BuO) radical was employed as the solid oxygen-sensing probe, and we confirmed its ability to report partial pressure of oxygen (pO(2)) in brain tissues of live animals under normal and pathological conditions for more than a month. pO(2) measurements could also be made repeatedly on the same animal and at the same location. The implantation site of LiNc-BuO in examined rats was verified by 0.5 T magnetic resonance (MR) imaging. Septic-shock rats were used to monitor tissue oxygenation during pathological state. A decline in pO(2) levels from severe hypotension during sepsis was detected, and generation of nitric oxide (NO) in brain tissues was confirmed by NO spin trapping. ESR oximetry using oxygen-sensing probe and NO spin-trapping can be used to monitor pO(2) change and NO production simultaneously and repeatedly at the same site in examined animals.

Development of an ESR/MR dual-imaging system as a tool to detect bioradicals

A system combining electron spin resonance imaging (ESRI) with another imaging modality capable of enabling visualization of the distribution of bioradicals on an anatomical map of the specimens would be a superior biomedical imaging system. We describe the development of an ESR/MR dual-imaging system with one permanent magnet and the biomedical applications of this system. The magnetic circuit developed for the ESR/MR dual-imaging system consisted of the permanent magnet made of Fe-Nd-B, pole pieces, and poke. The permanent magnet was installed on the MR side only, and the ESR side was made of pole pieces only. The magnetic field was adjusted to 0.5T at MR and to 0.042T at ESR. The overall dimensions of the magnet developed for the ESR/MR imaging system were 460 (W)x440 (D)x460 (H) mm, and it weighed 220 kg. The distance of each center for the magnet for ESR and MR imaging could be set as close as 200 mm. The entire ESR/MR imaging system can be installed in a common laboratory without magnetic shielding. MR images of plants (myoga) and small animals (mice and rats) were successfully acquired with or without ESR operation. ESR spectra of nitroxyl spin probes were also measured, even with MRI operation. ESR signals of triarylmethyl derivatives with narrow line-width (0.026 mT) were observed in living mice while MRI was operating. The ESR/MR imaging dual functions work properly with no electric or magnetic interference. The ESR/MR dual images demonstrate that this system enables visualization of the distribution of bioradicals on the anatomical map of the object.

Magnetic resonance and biochemical studies during pentylenetetrazole-kindling development: the relationship between nitric oxide, neuronal nitric oxide synthase and seizures

The major aim of this study was to elucidate the role of nitric oxide (NO) in the development of pentylenetetrazole (PTZ)-kindling as an animal model of primary generalized epilepsy. The daily administration of PTZ is associated with an increase in the amount of neuronal nitric oxide synthase (nNOS). NO generation was measured directly by in vivo and ex vivo electron paramagnetic resonance on rodents undergoing progressive convulsions. We found that primary generalized epilepsy is caused by NO induction during the persistent up-regulation of nNOS expression, but that NO induction is not associated with severe generalized seizures following long-term kindling phenomena after PTZ withdrawal. Morphological changes in the brain structure of rats were measured by magnetic resonance imaging during epileptic convulsions induced by repetitive administration of PTZ. Cerebellum volume for kindled rats decreased 20% but not in rats treated with the nNOS inhibitor, 3Br-7NI, suggesting that generation of NO in the cerebellum is related to decrease in cerebellum volume following PTZ-kindling.

Consequences of nitric oxide generation in epileptic-seizure rodent models as studied by in vivo EPR

The role of nitric oxide (NO) in epileptogenesis was studied in pentylenetetrazole (PTZ)-treated animals using in vivo and ex vivo EPR spectroscopy. NO generation was measured directly in the brain of a PTZ-induced mouse in vivo by an L-band EPR spectrometer. An elevation in NO production in the brain was observed during convulsions, and more NO was generated in the tonic seizure vs. the clonic seizure. NO content in several brain tissues (including the cerebral cortex (CR), cerebellum (CL), olfactory bulb (OB), hippocampus (HI), and hypothalamus (HT)) of PTZ-doped rats was analyzed quantitatively ex vivo by X-band EPR. To test the involvement of NO in seizure development, pharmacological analyses were performed using the NO synthase (NOS) inhibitors N(G)-nitro-L-arginine (L-NNA), N(G)-monomethyl-L-arginine (L-NMMA), and 3-bromo-7-nitroindazole (3Br-7NI). All of these inhibitors suppressed the convulsions, holding them at the clonic level, and prevented development of a tonic convulsion in rats doped with up to 80 mg/kg PTZ. 3Br-7NI completely inhibited NO production, but L-NNA and L-NMMA showed only 70% inhibition of NO production in PTZ-doped rats. In order to examine the contributions of NO in convulsions, rats were treated with anticonvulsants (phenytoin and diazepam) before PTZ treatment. Both drugs completely suppressed tonic convulsion in PTZ-doped rats at doses up to 80 mg/kg, but NO levels were similar to those detected in a clonic convulsion. These results support the notion that NO does not directly induce a clonic convulsion, but may be generated as a consequence of onset of seizure.

In vivo fate of superparamagnetic iron oxides during sepsis

The enzymatic generation of nitric oxide (NO) in vivo has been reported to be modulated by ions, such as copper and iron. Superparamagnetic iron oxide (SPIO) or ferumoxides is a liver-specific magnetic resonance contrast agent that is taken up by the Kupffer cells, where NO is generated by inducible nitric oxide synthase (iNOS). Thus, it is important to evaluate SPIO in vivo under conditions, such as infectious disease, where significant amounts of NO are generated by iNOS. In this study, we monitored the pharmacokinetics of SPIO in the liver of septic-shock mice and rats. A significant decrease in the ferric iron EPR signal was observed during NO generation in septic-shock mice compared with control mice doped with only SPIO. These results were also confirmed in a model reaction system consisting of SPIO and the NO donor, S-nitroso-N-acetyl DL penicillamine (SNAP). We compared NO generation quantitatively in the liver of the septic-shock rats, either in the presence or absence of SPIO, and found that the presence of SPIO did not affect the NO-generating activity of NOS expressed in the liver. T2-weighted MR images of an agarose gel phantom containing different SPIO to NO donor (SNAP) ratios clearly demonstrated that the contrast enhancement by SPIO decreased with increasing NO at constant SPIO levels. The reduced contrast is most probably due to the reduction of ferric to ferrous irons, resulting in a decrease in paramagnetic relaxation of water protons. These results show that SPIO can be a versatile NO-sensitive indicator, especially employing MRI as a powerful tool to 'visualize' sites of NO generation.

In vivo EPR evidence for free radical adducts of nifedipine

Nifedipine [3,5-pyridinedicarboxylic acid, 1,4-dihydro-2, 6-dimethyl-4-(2-nitrophenyl)-dimethyl ester] is a calcium channel blocker that has been widely used as a prescription drug for patients with hypertension. After illumination by ordinary light for 24 hr, nifedipine is converted completely to its nitroso analog without further photochemical degradation. Evidence for stable, nitroxyl-like free radical generation in mice was observed 15 min after intramuscular (i.m.) or intraperitoneal (i.p.) injection of illuminated nifedipine as monitored by in vivo L-band electron paramagnetic resonance (EPR) spectrometry. This was confirmed in more detail by ex vivo measurements on excised muscle and liver tissue. The nature of these radicals was surmised by comparing the reaction of illuminated nitroso-nifedipine with polyunsaturated fatty acids. Surprisingly, identical radical spectra were detected from excised liver doped with nonilluminated nifedipine, suggesting that this drug can be enzymatically converted in vivo to its nitroso analog without the requirement for illumination. This is one of the first reports of in vivo EPR evidence for a class of unsaturated fatty acid radical conjugates resulting from the normal metabolism of a common drug. Magn Reson Med 42:691-694, 1999.

High-performance liquid chromatography study of the pharmacokinetics of various spin traps for application to in vivo spin trapping

In vivo spin trapping is potentially a very useful tool to investigate the role of free radicals in physiologic processes and disease development. Unfortunately, knowledge on the stability and distribution of spin traps in living systems is limited. Therefore, in our study, we selected 11 acyclic and cyclic nitrone spin traps with diverse properties to determine their pharmacokinetics in mice. At varying times after intraperitoneal administration, we measured the concentration of the spin traps in the liver, heart, and blood. Our results showed that most spin traps were rapidly absorbed and were approximately evenly distributed throughout the mouse body. It was also found that most of the traps were relatively stable in vivo with more than half of the injected amount still available for spin trapping free radicals after an hour. Two of the 11 tested spin traps, however, decomposed after injection. These results indicate that for a successful in vivo spin trapping experiment, the stability of the spin trap is not of major concern, but the time course of distribution may be important.

Evaluation of DEPMPO as a spin trapping agent in biological systems

Cellular toxicity, pharmacokinetics, and the in vitro and in vivo stability of the SO3*- spin adduct of the spin trap, 5-diethoxyphosphoryl-5-methyl-1-pyrroline-n-oxide (DEPMPO), was investigated, and the results were compared with those of the widely used spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Similar to DMPO, DEPMPO was quickly taken up (<15 min) after intraperitoneal injection, and distributed evenly in the liver, heart, and blood of the mice. In the presence of ascorbate the in vitro stability of the adduct DEPMPO/SO3*- was 7 times better than DMPO/SO3*-. Under in vivo conditions, the spin adduct DEPMPO/SO3*- was 2-4 times more stable than DMPO/ SO3*-, depending on the route of administration of the adducts. Using a low frequency EPR spectrometer, we were able to observe the spin trapped SO3*- radical both with DMPO and DEPMPO directly in the intact mouse. DEPMPO had a detectable spin adduct signal at a concentration as low as 1 mM, as compared to 5 mM for DMPO. We conclude that DEPMPO is potentially a good candidate for trapping radicals in functioning biological systems, and represents an improvement over the commonly used trap DMPO.

Pharmaceutical applications of in vivo EPR

The aim of this article is to discuss the applications of in vivo EPR in the field of pharmacy. In addition to direct detection of free radical metabolites and measurement of oxygen, EPR can be used to characterize the mechanisms of drug release from biodegradable polymers. Unique information about drug concentration, the microenvironment (viscosity, polarity, pH) and biodistribution (by localized measurement or EPR Imaging) can be obtained.

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.

Nitric oxide: prospects and perspectives of in vivo detection by L-band EPR spectroscopy

This paper discusses, compares and evaluates various in vivo EPR methods of detection of nitric oxide (NO). In particular the various classes of agents are: Fe(II)-dithiocarbamate derivative complexes of MGD (N-methyl-D-glucamine dithiocarbamate) and DTCS [N-(dithiocarboxy)sarcosine], stable imidazolineoxy N-oxides and nitronyl N-oxides, and NO-sensitive chars. As a specific example direct, real-time, in vivo measurements of nitric oxide (NO) in mice are described with the water soluble metal chelator complex (MGD)2-Fe(II), as monitored at L-band EPR. The three-line EPR spectrum of [(MGD)2-Fe(II)-NO] was observed non-invasively in both control animals injected with the preformed product [(MGD)2-Fe(II)-NO] and from lipopolysaccharide (LPS) treated mice subsequently injected with (MGD)2-Fe(II) complex. The [(MGD)2-Fe(II)-NO] spectrum was markedly suppressed after administration of phenyl N-tert-butyl nitrone (PBN) prior to LPS injection as PBN inhibits the expression of inducible nitric oxide synthase (iNOS). When 15N-arginine was administered to LPS-treated mice, an EPR spectrum consisting of both three- and two-line EPR signals (due to (MGD)2-Fe(II)-14NO and (MGD)2-Fe(II)-15NO respectively) was observed, confirming that the trapped NO was generated through the NOS enzyme and not other chemical routes.

In vivo EPR: an effective new tool for studying pathophysiology, physiology and pharmacology

The development of spectrometers working at lower frequencies with improved resonators now permits the routine use of non-invasive EPR spectroscopy in vivo. The capabilities of EPR spectra to reflect environmental conditions, combined with the use of paramagnetic materials as selective non-toxic labels, has led to increasingly widespread and productive applications of the technique to complex problems involving physiology, pharmacology and pathophysiology. Some of the especially promising applications in which EPR techniques uniquely appear to provide valuable information are illustrated, including the measurement of oxygen and oxygen gradients, monitoring of the metabolism of xenobiotics, monitoring pharmacokinetics of drugs, measurement of perfusion, measurement of pH, recognition and labeling of receptors, and characterization of drug releasing systems.

Noninvasive measurement of the pH inside the gut by using pH-sensitive nitroxides. An in vivo EPR study

The use of pH-sensitive probes permits the measurement of the proton activity in biological systems by EPR spectroscopy. To illustrate the potential of this technique for in vivo purposes, the authors took advantage of the oral administration of nitroxides to monitor the pH value inside the stomach of mice after administration of different antacidics. The results indicate that EPR can be a valuable tool to characterize the pH in vivo in a continuous and noninvasive way.

In vivo detection of anthralin-derived free radicals in the skin of hairless mice by low-frequency electron paramagnetic resonance spectroscopy

Free radicals were directly detected in vivo in the skin of hairless mice by low-frequency electron paramagnetic resonance spectroscopy after topical application of anthralin under pertinent therapeutic conditions. The electron paramagnetic resonance signal intensity increased steadily, reaching a maximum after about 1 d and decreased slowly in the following days, probably because of desquamation of the skin. We conclude from the spectroscopic features (single line with a line width of 6 gauss; g = 2.0036) and from the pharmacokinetic pattern that the observed signal arises from the final products of anthralin metabolism (ether-insoluble polymeric structures--"anthralin brown"). Two potential antioxidants, vitamin E and the spin trap tert-butylphenylnitrone, decreased the amount of the anthralin-derived radical that was formed. Neither vitamin E radicals nor tert-butylphenylnitrone spin adducts were observed. We suggest that electron paramagnetic resonance is a valuable tool for the noninvasive and direct in vivo monitoring of drug-induced radical formation in the skin under therapeutic conditions.

Biological reduction of aromatic nitroso compounds: evidence for the involvement of superoxide anions

The in vitro formation of phenylhydronitroxide and 2-methylphenylhydronitroxide free radicals from nitrosobenzene (NB) and 2-nitrosotoluene (NT), respectively, in either red blood cells (RBC) or RBC hemolysates, was confirmed by electron spin resonance spectroscopy (ESR). Free radicals were generated nonenzymatically from reaction of the respective nitroso compounds with a number of biological reducing agents as corroborated by model studies of NB or NT with NAD(P)H. Under aerobic conditions, phenylhydronitroxide and 2-methylphenylhydronitroxide underwent a subsequent one-electron transfer to oxygen, which then resulted in the formation of superoxide anion (O2-). The latter product was confirmed by the superoxide dismutase (SOD)-inhibitable reduction of cytochrome c (cyt c). Apparently, oxygen is needed for continuous formation of the hydronitroxide radical derivatives. On the other hand, under anaerobic conditions, no phenylhydronitroxide radical was generated from NB in the presence of NADH, but the formation of phenylhydroxylamine from NB was detected by the absorption spectrometry. These results suggest that oxygen is a preferential electron acceptor for hydronitroxide radical derivatives.