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

A Review of Low-Frequency EPR Technology for the Measurement of Brain pO2 and Oxidative Stress

EPR can uniquely measure paramagnetic species. Although commercial EPR was introduced in 1950s, the early studies were mostly restricted to chemicals in solution or cellular experiments using X-band EPR equipment. Due to its limited penetration (<1 mm), experiments with living animals were almost impossible. To overcome these difficulties, Swartz group, along with several other leaders in field, pioneered the technology of low frequency EPR (e.g., L-band, 1-2 GHz). The development of low frequency EPR and the associated probes have dramatically expanded the application of EPR technology into the biomedical research field, providing answers to important scientific questions by measuring specific parameters that are impossible or very difficult to obtain by other approaches. In this review, which is aimed at highlighting the seminal contribution from Swartz group over the last several decades, we will focus on the development of EPR technology that was designed to deal with the potential challenges arising from conducting EPR spectroscopy in living animals. The second half of the review will be concentrated on the application of low frequency EPR in measuring cerebral tissue pO₂ changes and oxidative stress in various physiological and pathophysiological conditions in the brain of animal disease models.

Post-trapping derivatization of radical-derived EPR-silent adducts: application to free radical detection by HPLC/UV in chemical, biochemical, and biological systems and comparison with EPR spectroscopy

Free radicals are conventionally detected by electron paramagnetic resonance (EPR) spectroscopy after being trapped as spin adducts. Albeit this technique has demonstrated utmost efficacy in studying free radicals, its application to biological settings is intrinsically hampered by the inevitable bioreduction of radical-derived paramagnetic adducts. Herein, we describe a reliable technique to detect and quantify free radical metabolites, wherein reduced alkyl- and phenyl-5,5-dimethyl-1-pyrroline N-oxide (DMPO) adducts are converted into ultrastable N-naphthoate esters. To mimic the ubiquitous in vivo microenvironment, bioreductants, exogenous thiols, and sodium borohydride were studied. Nitroxyl reduction was confirmed using EPR and triphenyltetrazolium chloride. The formation of the N-naphthoyloxy derivatives was established by liquid chromatography/mass spectrometry (LC/MS). The derivatives were chromatographed using a binary eluent. HPLC and internal standards were synthesized using Grignard addition. The labeled DMPO adduct is (1) fluorescent, (2) stable as opposed to nitroxyl radical adducts, (3) biologically relevant, and (4) excellently chromatographed. Applications encompassed chemical, biochemical, and biological model systems generating C-centered radicals. Different levels of phenyl radicals produced in situ from whole blood were successfully determined. The method is readily applicable to the detection of hydroxyl radical. Analogously, DMPO, the spin trap, could be detected with extreme sensitivity suitable for in vivo applications. The developed method proved to be a viable alternative to EPR, where for the first time the reductive loss of paramagnetic signals of DMPO-trapped free radicals is transformed into fluorescence emission. We believe the proposed methodology could represent a valuable tool to probe free radical metabolites in vivo using DMPO, the least toxic spin trap.

In vivo evidence of free radical generation in the mouse lung after exposure to Pseudomonas aeruginosa bacterium: an ESR spin-trapping investigation

In the Pseudomonas aeruginosa-induced rodent pneumonia model, it is thought that free radicals are significantly associated with the disease pathogenesis. However, until now there has been no direct evidence of free radical generation in vivo. Here we used electron spin resonance (ESR) and in vivo spin trapping with α-(4-pyridyl-1-oxide)-N-tert-butylnitrone to investigate free radical production in a murine model. We detected and identified generation of lipid-derived free radicals in vivo (a(N) =14.86 ± 0.03 G and a(H)(β) =2.48 ± 0.09 G). To further investigate the mechanism of lipid radical production, we used modulating agents and knockout mice. We found that with GdCl(3) (phagocytic toxicant), NADPH-oxidase knockout mice (Nox2(-)/(-)), allopurinol (xanthine-oxidase inhibitor) and Desferal (metal chelator), generation of lipid radicals was decreased; histopathological and biological markers of acute lung injury were noticeably improved. Our study demonstrates that lipid-derived free radical formation is mediated by NADPH-oxidase and xanthine-oxidase activation and that metal-catalysed hydroxyl radical-like species play important roles in lung injury caused by Pseudomonas aeruginosa.

Stilbazulenyl nitrone, a second-generation azulenyl nitrone antioxidant, confers enduring neuroprotection in experimental focal cerebral ischemia in the rat: neurobehavior, histopathology, and pharmacokinetics

Stilbazulenyl nitrone (STAZN) is a potent lipophilic second-generation azulenyl nitrone antioxidant, which is highly neuroprotective in rodent models of cerebral ischemia and trauma. This study was conducted to establish whether the neuroprotection induced by STAZN persists with chronic survival and to characterize STAZN's pharmacokinetics. Physiologically regulated rats received a 2-h middle cerebral artery occlusion by intraluminal suture and were treated with either STAZN [four 0.6 mg/kg doses i.p. administered at 2 (i.e., onset of recirculation), 4, 24, and 48 h; n = 16] or dimethyl sulfoxide vehicle (n = 11). They received sequential neurobehavioral examinations followed by quantitative neuropathology at 30 days. STAZN improved neurological deficits compared with vehicle controls, beginning within <2 h of the first dose and persisting throughout a 30-day survival. Large cystic necrotic infarcts were common in vehicle-treated rats but infrequent in STAZN-treated rats, and noninfarcted forebrain tissue was increased on average by 15%. In normal rats administered 5 mg/kg STAZN i.v. in Solutol HS 15/ethanol/saline vehicle, STAZN blood levels exhibited a biexponential decline, with an initial half-life of 28 min and a subsequent slow decay with half-life of approximately 7 h. STAZN tissue levels at 2 to 3 h were, on average, 2.5% of blood levels in forebrain, 56% in myocardium, and 41% in kidney. STAZN was concentrated in liver with initial concentrations averaging 5.2-fold above blood levels and a subsequent linear decline of 40% between 24 and 72 h. These results establish that STAZN confers enduring ischemic neuroprotection, has a long circulating half-life, and penetrates well into brain and other organs-characteristics favoring its potential therapeutic utility.

In vivo lipid-derived free radical formation by NADPH oxidase in acute lung injury induced by lipopolysaccharide: a model for ARDS

Intratracheal instillation of lipopolysaccharide (LPS) activates alveolar macrophages and infiltration of neutrophils, causing lung injury/acute respiratory distress syndrome. Free radicals are a special focus as the final causative molecules in the pathogenesis of lung injury caused by LPS. Although in vitro investigation has demonstrated radical generation after exposure of cells to LPS, in vivo evidence is lacking. Using electron spin resonance (ESR) and the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN), we investigated in vivo free radical production by rats treated with intratracheal instillation of LPS. ESR spectroscopy of lipid extract from lungs exposed to LPS for 6 h gave a spectrum consistent with that of a POBN/carbon-centered radical adduct (aN=14.94+/-0.07 G and abetaH=2.42+/-0.06 G) tentatively assigned as a product of lipid peroxidation. To further investigate the mechanism of LPS-initiated free radical generation, rats were pretreated with the phagocytic toxicant GdCl3, which significantly decreased the production of radical adducts with a corresponding decrease in neutrophil infiltration. NADPH oxidase knockout mice completely blocked phagocyte-mediated, ESR-detectable radical production in this model of acute lung injury. Rats treated intratracheally with LPS generate lipid-derived free radicals via activation of NADPH oxidase.

Characterization of the initial carbon-centered pentadienyl radical and subsequent radicals in lipid peroxidation: identification via on-line high performance liquid chromatography/electron spin resonance and mass spectrometry

The previously reported combination of an on-line high-performance liquid chromatography (LC)/electron spin resonance (ESR) system with mass spectrometric analysis (MS) created a unique technique to identify a variety of lipid-derived radicals ((.)L(d)) formed from in vitro lipid peroxidation (Iwahashi et al. [20]). To improve the sensitivity, resolution, and reliability of this method for in vitro and in vivo studies, we have investigated the effects of mobile phase pH, modifiers, and columns on the chromatographic separation of linoleic acid-derived radical adducts. Using tetrahydrofuran (THF) and 0.1% glacial acetic acid (HOAc) in an H(2)O/acetonitrile (ACN) mobile phase greatly increased the resolution and retention reproducibility of lipid radical adducts in LC/ESR. In addition, these modifications allowed the elimination of an ESR tuning problem and the synchronization of UV and ESR detection of radical adducts in on-line LC/ESR, neither of which had been possible previously. Analyte purity was therefore increased, thus increasing the reliability of radical detection via on-line LC/ESR as well as radical identification via MS analysis. For the first time, POBN adducts of linoleic carbon-centered pentadienyl radicals (L(.)) were detected and identified. The optimization of chromatography in the LC/ESR and MS combination provided a reliable and sensitive way for the detection and identification of expected radical adducts in vitro and in vivo.

Free radical generation in the cochlea during combined exposure to noise and carbon monoxide: an electrophysiological and an EPR study

Ototoxicity following combined exposure to noise and carbon monoxide (CO) is known to result in more severe permanent threshold shifts than exposure to noise alone. We have previously demonstrated that such potentiation of noise-induced auditory impairment by CO can be prevented by the administration of a nitrone spin-trapping agent. Although such protection implicates injury via free radical pathways, drug-induced protection does not provide direct evidence for the presence of free radicals in the cochlea. The objective of this study was to demonstrate the actual presence of nitrone spin adducts in the cochlea following simultaneous exposure to noise and CO. Using electrophysiological end-points, the protective effects of the nitrone spin-trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) were assessed following combined exposure of adult male Long Evans hooded rats to noise and CO. In addition, an ex-vivo evaluation of POBN spin adducts was done by electron paramagnetic resonance spectroscopy (EPR). The noise used was octave band noise with center frequency 13.6 kHz at 100 dB(Lin) for a duration of 2 h. The level of CO used was 1200 ppm. Electrophysiological results demonstrate that POBN protects against combined exposure to noise plus CO. The EPR study demonstrates POBN spin adducts in the cochleae of animals exposed to noise plus CO. Therefore, this study provides evidence to the hypothesis that ototoxicity due to noise plus CO exposure is mediated via free radicals.

Diphenyleneiodonium sulfate, an NADPH oxidase inhibitor, prevents early alcohol-induced liver injury in the rat

The oxidant source in alcohol-induced liver disease remains unclear. NADPH oxidase (mainly in liver Kupffer cells and infiltrating neutrophils) could be a potential free radical source. We aimed to determine if NADPH oxidase inhibitor diphenyleneiodonium sulfate (DPI) affects nuclear factor-kappaB (NF-kappaB) activation, liver tumor necrosis factor-alpha (TNF-alpha) mRNA expression, and early alcohol-induced liver injury in rats. Male Wistar rats were fed high-fat liquid diets with or without ethanol (10-16 g. kg(-1). day(-1)) continuously for up to 4 wk, using the Tsukamoto-French intragastric enteral feeding protocol. DPI or saline vehicle was administered by subcutaneous injection for 4 wk. Mean urine ethanol concentrations were similar between the ethanol- and ethanol plus DPI-treated groups. Enteral ethanol feeding caused severe fat accumulation, mild inflammation, and necrosis in the liver (pathology score, 4.3 +/- 0.3). In contrast, DPI significantly blunted these changes (pathology score, 0.8 +/- 0.4). Enteral ethanol administration for 4 wk also significantly increased free radical adduct formation, NF-kappaB activity, and TNF-alpha expression in the liver. DPI almost completely blunted these parameters. These results indicate that DPI prevents early alcohol-induced liver injury, most likely by inhibiting free radical formation via NADPH oxidase, thereby preventing NF-kappaB activation and TNF-alpha mRNA expression in the liver.

Phthalates rapidly increase production of reactive oxygen species in vivo: role of Kupffer cells

The role of oxidants in the mechanism of tumor promotion by peroxisome proliferators remains controversial. The idea that induction of acyl-coenzyme A oxidase leads to increased production of H(2)O(2), which damages DNA, seems unlikely; still, free radicals might be important in signaling in specialized cell types such as Kupffer cells, which produce mitogens. Because hard evidence for increased oxidant production in vivo after treatment with peroxisome proliferators is lacking, the spin-trapping technique and electron spin resonance spectroscopy were used. Rats were given di(2-ethylhexyl) phthalate (DEHP) acutely. The spin trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone was also given and bile samples were collected for 4 h. Under these conditions, the intensity of the six-line radical adduct signal increased to a maximum value of 2.5-fold 2 h after administration of DEHP, before peroxisomal oxidases were induced. Furthermore, DEHP given with [(13)C(2)]dimethyl sulfoxide produced a 12-line electron spin resonance spectrum, providing evidence that DEHP stimulates (*)OH radical formation in vivo. Furthermore, when rats were pretreated with dietary glycine, which inactivates Kupffer cells, DEHP did not increase radical signals. Moreover, similar treatments were performed in knockout mice deficient in NADPH oxidase (p47(phox) subunit). Importantly, DEHP increased oxidant production in wild-type but not in NADPH oxidase-deficient mice. These data provide evidence for the hypothesis that the molecular source of free radicals induced by peroxisome proliferators is NADPH oxidase in Kupffer cells. On the contrary, radical adduct formation was not affected in peroxisome proliferator-activated receptor alpha knockout mice. These observations represent the first direct, in vivo evidence that phthalates increase free radicals in liver before peroxisomal oxidases are induced.