The effects of antioxidants on radiation-induced apoptosis pathways in TK6 cells

This study was designed to determine if radiation-mediated activation of the apoptotic pathways would be influenced by antioxidants and if a correlation would be found between radioprotection and changes in transduction pathways. Human lymphoblastoid TK6 cells, known to undergo apoptosis as a result of radiation, were irradiated (6 Gy) with and without antioxidants, and then whole-cell lysates were collected. Parallel studies were conducted to assess the survival (clonogenic assay) and apoptotic index. The impacts of two nitroxide antioxidants, tempol and CAT-1, differing in cell permeability, as well as the sulfhydryl antioxidant N-acetyl-L-cysteine (L-NAC), were estimated. Changes in apoptotic pathway proteins and p53 were assessed by Western blotting. Fraction of apoptotic cells was determined by flow cytometry. Tempol (10 mM), which readily enters cells, partially radioprotected TK6 cells against clonogenic killing, but had no effect on radiation-induced apoptotic parameters such as cleaved caspase 3 or cleaved PARP. Tempol alone did not induce cytotoxicity, yet did increase cleaved PARP levels. The radiation-induced increase in p53 protein was partly inhibited by tempol, but was unaffected by CAT-1 and L-NAC. Both CAT-1 (10 mM), which does not enter cells, and L-NAC (10 mM) had no radioprotective effect on cell survival. Although L-NAC did not protect against radiation-induced cytotoxicity, it completely inhibited radiation-induced increase in cleaved caspase 3 and cleaved PARP. Collectively, the results question the validity of using selected apoptosis pathway members as sole indicators of cytotoxicity.

Radio frequency continuous-wave and time-domain EPR imaging and Overhauser-enhanced magnetic resonance imaging of small animals: instrumental developments and comparison of relative merits for functional imaging

Electron paramagnetic resonance (EPR) imaging in the continuous wave (CW) and time-domain modes, as well as Overhauser-enhanced magnetic resonance imaging in vivo is described. The review is based mainly on the CW and time-domain EPR instrumentation at 300 MHz developed in our laboratory, and the relative merits of these methods for functional in vivo imaging of small animals to assess hypoxia and tissue redox status are described. Overhauser imaging of small animals at magnetic fields in the range 10-15 mT that is being carried out in our laboratory for tumor imaging and the evaluation of tumor hypoxia based on quantitative evaluation of Overhauser enhancement is also described. Alternate approaches to spectral-spatial imaging using the transverse decay constants to infer in situ line widths and hence in vivo pO2 using CW and time-domain EPR imaging are also discussed. The nature of the spin probes used, the quality of the images obtained in all the three methods, the achievable resolution, limitations and possible future directions in small animal functional imaging with these modalities are summarized.

Factors influencing nitroxide reduction and cytotoxicity in vitro

Nitroxides have been shown to be effective antioxidants, radiation protectors, and redox-active probes for functional electron paramagnetic resonance (EPR) imaging. More recently, the nitroxide 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-N-oxyl (Tempol) has been shown to exert differential cytotoxicity to tumor compared with normal cell counterparts. Nitroxides are readily reduced in tissues to their respective hydroxylamines, which exhibit less cytotoxicity in vitro and do not provide radiation protection or an EPR-detectable signal for imaging. In order to better understand factors that influence nitroxide reduction, the rate of reduction of Tempol in mouse and human cell lines and in primary cultures of tumor cells was measured using EPR spectroscopy. Additionally, the cytotoxicity of high concentrations of Tempol and the hydroxylamine of Tempol (Tempol-H) was evaluated in wild-type and glucose-6-phosphate dehydrogenase (G6PD)-deficient Chinese hamster ovary cells. The results show that in general Tempol was reduced at a faster rate when cells were under hypoxic compared with aerobic conditions. Neither depletion of intracellular glutathione nor treatment of cells with sodium cyanide influenced Tempol reduction rates. G6PD-deficient cells were found to reduce Tempol at a significantly slower rate than wild-type cells. Likewise, Tempol-induced cytotoxicity was markedly less for G6PD-deficient cells compared with wild-type cells. Tempol-H exhibited no cytotoxicity to either cell type. Tempol-mediated cytotoxicity was enhanced by glutathione depletion and inhibition of 6-phosphogluconate dehydrogenase in wild-type cells, but was unaltered in G6PD-deficient cells. Collectively, the results indicate that while the bioreduction of Tempol can be influenced by a number of factors, the hexose monophosphate shunt appears to be involved in both nitroxide reduction as well as cytotoxicity induced by high levels of exposure to Tempol.

Tailored sinc pulses for uniform excitation and artifact-free radio frequency time-domain EPR imaging

A method to generate shaped radiofrequency pulses for uniform excitation of electron spins in time-domain radio frequency (RF) electron paramagnetic resonance (EPR) imaging is presented. A commercial waveform generator was integrated with the transmit arm of the existing time-domain RF-EPR spectrometer to generate tailored excitation pulses with sub-nano second resolution for excitation with a 90 degrees flip-angle. A truncated sinc [sin(x)/x] pulse, tailored to compensate for the Q-profile (RF frequency response) of the resonator, was shown to yield images from phantom objects as well as in vivo images, with minimal distortion. These studies point to the advantages in using shaped sinc pulses to achieve improved uniform excitation over a relatively wide bandwidth region in time-domain RF-EPR imaging (RF-FT-EPRI).

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.

Influence of body temperature on the BOLD effect in murine SCC tumors

Changes in the blood oxygen level dependent (BOLD) enhancements in tumors (squamous cell carcinoma, (SCCVII)) implanted in mice maintained at core temperatures of 30 degrees C or 37 degrees C were measured using MRI and compared to tumor oxygen levels obtained using an oxygen-sensitive Eppendorf electrode. Tumors were implanted in a hindleg of the mice intramuscularly. Tumor-bearing mice were imaged by BOLD MRI, while first breathing air and then carbogen (95% O2, 5% CO2) for 15-min intervals at a core temperature of 30 degrees C. After an equilibration period, the identical regimen was conducted with the same animal maintained at 37 degrees C. This procedure was repeated with additional mice starting at 37 degrees C followed by imaging at 30 degrees C. Likewise, oxygen electrode measurements of the tumor were determined at core temperatures of 30 degrees C and 37 degrees C. The Eppendorf measurements showed that tumors in animals maintained at 30 degrees C were significantly more hypoxic than at 37 degrees C. MRI studies demonstrated stronger BOLD enhancement at 30 degrees C than at 37 degrees C, suggesting significant changes in hypoxia and/or blood flow in tumors at these temperatures. The findings of the study stress the importance of maintaining normal core temperature when assessing tumor oxygen status using functional imaging modalities or oxygen-sensitive electrodes.

Pharmacokinetics of a triarylmethyl-type paramagnetic spin probe used in EPR oximetry

The paramagnetic spin probe Oxo63 is used in oximetric imaging studies based on electron paramagnetic resonance (EPR) methods by monitoring the oxygen-dependent linewidth while minimizing the contributions from self-broadening seen at high probe concentrations. Therefore, it is necessary to determine a suitable dose of Oxo63 for EPR-based oxygen mapping where the self-broadening effects are minimized while signal intensity adequate for imaging can be realized. A constant tissue concentration of spin probe would be useful to image a subject and assess changes in pO2 over time; accumulation or elimination of the compound in specific anatomical regions could translate to and be mistaken for changes in local pO2, especially in OMRI-based oximetry. The in vivo pharmacokinetics of the spin probe, Oxo63, after bolus and/or continuous intravenous infusion was investigated in mice using a novel approach with X-band EPR spectroscopy. The results show that the half-life in blood was 17-21 min and the clearance by excretion was 0.033-0.040 min(-1). Continuous infusion following a bolus injection of the probe was found to be effective to obtain stable plasma concentration as well as image intensity to permit reliable pO2 estimates.

Tempol-H inhibits opacification of lenses in organ culture

Cataract is the world's leading cause of blindness and a disease for which no efficacious medical therapy is available. To screen potential anti-cataract agents, a lens organ culture model system was used. Opacification of lenses maintained in culture was induced by specific insults including H(2)O(2) or the cataractogenic sugar xylose. Potential anti-cataract agents were added to the culture medium and their ability to inhibit opacification and certain biochemical changes associated with the opacification were assessed. Among the compounds tested, Tempol-H, the hydroxylamine of the nitroxide Tempol, gave the most promising results. It significantly inhibited opacification of rat lenses in an H(2)O(2)-induced cataract system as well as opacification of rhesus monkey lenses induced by xylose. Tempol-H inhibited the loss of glutathione, the leakage of protein, and decreases in the ability of cultured lenses to accumulate (3)H-choline from the medium, all of which were associated with the development of lens opacification. The antioxidative activity of Tempol-H and its ability to re-dox cycle make it an attractive candidate as a therapeutic agent for the prevention of aging-related cataract.

An application system for automation of constant-time radio frequency electron paramagnetic resonance imaging

A Windows based application system for data collection, Fourier reconstruction and analysis of pure phase encoded constant-time radio frequency electron paramagnetic resonance (EPR) images, is described. The graphical user interface (GUI) of the system was written in MATLAB version 5.0, using its built-in GUI utilities. Design considerations of the application system included speed, flexibility and user-friendly data display and analysis. To maximize the speed of image data collection, MATLAB's built-in C interface system, MEX was not used for data collection. Instead, MATLAB programs call the C programs from the DOS prompt directly, based on the data collection parameters entered through the GUI. Computational procedures included various digital signal-processing steps such as filtering, interpolation etc. for the Fourier reconstruction of 2D, and 3D EPR images from the pure phase encoded data. Examples of 2D images illustrating the performance of the system are presented. Although the application system has been developed for the specific purpose of EPR imaging, it can easily be adapted to other areas such as magnetic resonance microscopy as well.

Application of continuous-wave EPR spectral-spatial image reconstruction techniques for in vivo oxymetry: Comparison of projection reconstruction and constant-time modalities

In this study we report the application of continuous-wave (CW) electron paramagnetic resonance (EPR) constant-time spectral spatial imaging (CTSSI) for in vivo oxymetry. 2D and 3D SSI studies of a phantom and live mice were carried out using projection reconstruction (PR) and constant-time (CT) modalities using a CW-EPR spectrometer/imager operating at 300 MHz frequency. Distortion of line shape, which is inherent in the PR method, was minimized by the CTSSI modality. It was also found that CTSSI offers improved noise reduction, restores a smoother line shape, and gives high convergence of estimated values. Spatial resolution was also improved by CTSSI, although fundamental spectral line-width broadening was observed. Although additional corrections are required for accurate estimations of spectral line width, CTSSI was able to demonstrate distinct differences in oxygen tension between a tumor and the normal legs of a C3H mouse. The PR method, on the other hand, was unable to make such a distinction unequivocally with the triarylmethyl spin probes. CTSSI promises to be a more suitable method for quantitative in vivo oxymetric studies using radiofrequency EPR imaging (EPRI).

Stochastic excitation and Hadamard correlation spectroscopy with bandwidth extension in RF FT-EPR

The application of correlation spectroscopy employing stochastic excitation and the Hadamard transform to time-domain Fourier transform electron paramagnetic resonance (FT-EPR) spectroscopy in the radiofrequency (RF) band is described. An existing, time-domain FT-EPR spectrometer system with a Larmor frequency (L(f)) of 300 MHz was used to develop this technique by incorporating a pseudo-random pulse sequence generator to output the maximum length binary sequence (MLBS, 10- and 11-bit). Software developed to control the EPR system setup, acquire the signals, and post process the data, is outlined. The software incorporates the Hadamard transform algorithm to perform the required cross-correlation of the acquired signal and the MLBS after stochastic excitation. To accommodate the EPR signals, bandwidth extension was accomplished by sampling at a rate many times faster than the RF pulse repetition rate, and subsequent digital signal processing of the data. The results of these experiments showed that there was a decrease in the total acquisition time, and an improved free induction decay (FID) signal-to-noise (S/N) ratio compared to the conventional coherent averaging approach. These techniques have the potential to reduce the RF pulse power to the levels used in continuous wave (CW) EPR while retaining the advantage of time-domain EPR methods. These methods have the potential to facilitate the progression to in vivo FT-EPR imaging of larger volumes.

Semiquinone radical intermediate in catecholic estrogen-mediated cytotoxicity and mutagenesis: chemoprevention strategies with antioxidants

Modulation of the cytotoxicity and mutagenicity of 4-hydroxyestradiol (4-OHE(2)), an oxidative metabolite of estrogen, by antioxidants was assessed in human MCF7 cells and TK-6 lymphoblast cells. The cytotoxicity of the catecholic estrogens was potentiated by depletion of intracellular glutathione and was independent of oxygen concentration. Agents such as the nitroxide Tempol can facilitate the oxidation of the semiquinone to the Q and enhanced 4-OHE(2) cytotoxicity. Conversely, reducing agents such as ascorbate, cysteine, and 1,4-dihydroxytetramethylpiperidine (THP) protected against cytotoxicity and decreased mutation induction, presumably by reducing the semiquinone to the hydroquinone. Our results support the proposition that oxidation of the semiquinone to the corresponding Q is crucial in eliciting the deleterious effects of catecholic estrogens. Furthermore, because the deleterious effects of 4-OHE(2) were abrogated by dietary and synthetic antioxidants, our results would support the chemopreventive use of diets rich in reducing substances (vitamins and added synthetic antioxidants) as a means of decreasing the risks associated with estrogen exposure and developing of breast cancer.

A low molecular weight antioxidant decreases weight and lowers tumor incidence

Stable free radical nitroxides are potent antioxidants possessing superoxide dismutase- and catalase-mimetic activity that protect cells and animals against a variety of oxidative insults. Tempol, as a representative nitroxide, was evaluated for its influence on weight maintenance and spontaneous tumor incidence in C3H mice. Tempol administered in either the drinking water or food did not show any untoward effects and prevented animals from becoming obese. Tempol-treated animals' leptin levels were reduced. Long-term treatment with Tempol significantly decreased tumorigenesis when compared to controls (10 vs. 40%, respectively). Selected tissues from Tempol-treated animals exhibited elevated levels of mitochrondrial uncoupling protein-2 (UCP-2) and HSP70. The present data suggest that nitroxides upregulate UCP-2, obviate weight gain, and decrease age-related spontaneous tumor incidence. As a class, nitroxides may provide overall health benefits by contributing to decreased obesity and tumor incidence.

Differential protection by nitroxides and hydroxylamines to radiation-induced and metal ion-catalyzed oxidative damage

Modulation of radiation- and metal ion-catalyzed oxidative-induced damage using plasmid DNA, genomic DNA, and cell survival, by three nitroxides and their corresponding hydroxylamines, were examined. The antioxidant property of each compound was independently determined by reacting supercoiled DNA with copper II/1,10-phenanthroline complex fueled by the products of hypoxanthine/xanthine oxidase (HX/XO) and noting the protective effect as assessed by agarose gel electrophoresis. The nitroxides and their corresponding hydroxylamines protected approximately to the same degree (33-47% relaxed form) when compared to 76.7% relaxed form in the absence of protectors. Likewise, protection by both the nitroxide and corresponding hydroxylamine were observed for Chinese hamster V79 cells exposed to hydrogen peroxide. In contrast, when plasmid DNA damage was induced by ionizing radiation (100 Gy), only nitroxides (10 mM) provide protection (32.4-38.5% relaxed form) when compared to radiation alone or in the presence of hydroxylamines (10 mM) (79.8% relaxed form). Nitroxide protection was concentration dependent. Radiation cell survival studies and DNA double-strand break (DBS) assessment (pulse field electrophoresis) showed that only the nitroxide protected or prevented damage, respectively. Collectively, the results show that nitroxides and hydroxylamines protect equally against the damage mediated by oxidants generated by the metal ion-catalyzed Haber-Weiss reaction, but only nitroxides protect against radiation damage, suggesting that nitroxides may more readily react with intermediate radical species produced by radiation than hydroxylamines.

In vivo measurement of regional oxygenation and imaging of redox status in RIF-1 murine tumor: effect of carbogen-breathing

The purpose of this study was to noninvasively monitor tumor oxygenation and redox status during hyperoxygenation treatment, such as carbogen-breathing, in a murine tumor model using in vivo electron paramagnetic resonance (EPR) spectroscopy and imaging techniques. The study was performed using implanted lithium phthalocyanine (LiPc) microcrystals as the oximetry probe and 3-carbamoylproxyl (3-CP) as the redox probe in RIF-1 tumors implanted in the upper hind leg of C3H mice. Repetitive measurements of pO(2) from the same tumors as a function of tumor growth (8-24 mm in size) showed that the tumors were hypoxic and that the tumor pO(2) values were decreasing with tumor growth. Carbogen-breathing mostly showed an increase in the tumor oxygenation, although there were considerable variations in the magnitude of change among the tumors. The pharmacokinetic studies with 3-CP showed a significant decrease in the overall tumor reduction status in the carbogen-breathing mice. Spatially resolved (imaging) pharmacokinetic data over the tumor volume were obtained to visualize the distribution of the redox status within the tumor. The redox images of the tumor in the air-breathing mice showed significant heterogeneity in the magnitude and spatial distribution of reducing equivalents. On carbogen-breathing the tissue reduction status decreased considerably, with a concomitant decrease in the heterogeneity of distribution of the redox status. The results suggest that 1) carbogen-breathing considerably enhances tissue oxygenation and significantly decreases the redox status in RIF-1 tumor, and 2) changes in the magnitude and distribution of the redox status within the tumor volume during carbogen-breathing are correlated with the increased tissue oxygenation.

Fluorine electron double resonance imaging for 19F MRI in low magnetic fields

This work demonstrates the feasibility of generating fluorine NMR images at a very low magnetic field of 0.015 T by making use of the Overhauser enhancement of (19)F NMR signal brought about by a stable, water-soluble, narrow-line paramagnetic contrast agent. The enhancement in the (19)F NMR images depends on the concentration of the single electron contrast agent, the pO(2), and the electron paramagnetic resonance (EPR) irradiation power. The applicability of this technique for (19)F NMR imaging is demonstrated with phantom samples, where a time resolution of 4-10 min is achieved. Proton electron double resonance imaging (PEDRI) and fluorine electron double resonance imaging (FEDRI) images were also obtained from rat kidneys ex vivo, perfused with 10 mM Oxo63 and 10 M trifluoroacetic acid. The spatial and temporal resolutions of these images are comparable to those obtained at magnetic fields 2-3 orders of magnitude larger. Constant NMR frequency (628 kHz) operation permits both FEDRI and PEDRI of identical slices without removing the object under investigation. This feasibility of coregistration of proton-based anatomical PEDRI image with physiological FEDRI image offers good potential for studying fluorine-containing tracers.

Single-point (constant-time) imaging in radiofrequency Fourier transform electron paramagnetic resonance

This study describes the use of the single-point imaging (SPI) modality, also known as constant-time imaging (CTI), in radiofrequency (RF) Fourier transform (FT) electron paramagnetic resonance (EPR). The SPI technique, commonly used for high-resolution solid-state nuclear magnetic resonance (NMR) imaging, has been successfully applied to 2D and 3D RF-FT-EPR imaging of phantoms containing narrow-line EPR spin probes. The SPI scheme is essentially a phase-encoding technique that operates by acquiring a single data point in the free induction decay (FID) after a fixed delay (phase-encoding time), following the pulsed RF excitation, in the presence of static magnetic field gradients. Since the phase-encoding time remains constant for a given image data set, the spectral information is automatically deconvolved, providing well-resolved pure spatial images. Therefore, images obtained using SPI are artifact-free and the resolution is not significantly limited by the line width, compared to the images obtained using the conventional filtered back-projection (FBP) scheme, suggesting that the SPI modality may have advantages for EPR imaging of large objects. In this work the advantages and limitations of SPI as compared to FBP are investigated by imaging suitable phantom objects. Although SPI takes longer to perform than the FBP method, optimization of the data collection scheme may increase the temporal resolution, rendering this technique suitable for in vivo studies. Spectral information can also be extracted from a series of SPI images that are generated as a function of the delay from the excitation pulse.

Kinetics and mechanism of hydroxyl radical and OH-adduct radical reactions with nitroxides and with their hydroxylamines

Stable nitroxide radicals are potent antioxidants and are among the most effective non-thiol radioprotectants, although they react with hydroxyl radicals more slowly than typical phenolic antioxidants or thiols. Surprisingly, the reduced forms of cyclic nitroxides, cyclic hydroxylamines, are better reductants yet have no radioprotective activity. To clarify the reason for this difference, we studied the kinetics and mechanisms of the reactions of nitroxides and their hydroxylamines with (*)OH radicals and with OH-adducts by using pulse radiolysis, fluorimetric determination of phenolic radiation products, and electron paramagnetic resonance spectrometric determination of nitroxide concentrations following radiolysis. Competition kinetics with phenylalanine as a reference compound in pulse radiolysis experiments yielded rate constants of (4.5 +/- 0.4) x 10(9) M(-1) s(-1) for the reaction of (*)OH radical with 2,2,6,6-tetramethylpiperidine-N-oxyl (TPO), 4-hydroxy-TPO (4-OH-TPO), and 4-oxo-TPO (4-O-TPO), (3.0 +/- 0.3) x 10(9) M(-1) s(-1) for deuterated 4-O-TPO, and (1.0 +/- 0.1) x 10(9) M(-1) s(-1) for the hydroxylamine 4-OH-TPO-H. The kinetic isotope effect suggests the occurrence of both (*)OH addition to the aminoxyl moiety of 4-O-TPO and H-atom abstraction from the 2- or 6-methyl groups or from the 3- and 5-methylene positions. This conclusion was further supported by final product analysis, which demonstrated that (*)OH partially oxidizes 4-O-TPO to the corresponding oxoammonium cation. The rate constants for the reactions of the nitroxides with the OH-adducts of phenylalanine and terephthalate have been determined to be near 4 x 10(6) M(-1) s(-1), whereas the hydroxylamine reacted at least 50 times slower, if at all. These findings indicate that the reactivity toward (*)OH does not explain the differences between the radioprotective activities of nitroxides and hydroxylamines. Instead, the radioprotective activity of nitroxides, but not of hydroxylamines, can be partially attributed to their ability to detoxify OH-derived secondary radicals.

Mechanisms underlying the cytotoxic effects of Tachpyr--a novel metal chelator

Tachpyr (N,N'N"-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane), a novel metal chelator, was previously shown to deplete intracellular iron and exert a cytotoxic effect on cultured bladder cancer cells. Tachpyr binds Fe(II) and readily reduces Fe(III). The iron(II)-Tachpyr chelate undergoes intramolecular oxidative dehydrogenation resulting in mono- and diimino Fe(II) complexes. The present study investigates the redox-activity of the Tachpyr-iron complex to better define the mechanism of Tachpyr's cytotoxicity. Tachpyr's mechanism of cytotoxicity was studied using cell-free solutions, isolated DNA, and cultured mammalian cells by employing UV-VIS spectrophotometry, oximetry, spin-trapping technique, and electron paramagnetic resonance (EPR) spectrometry. The results show that: (1) Tachpyr by itself after 24 h of incubation had a cytotoxic effect on cultured cells; (2) fully oxidized Tachpyr had no cytotoxic effects on cultured cells even after 24 h of incubation; (3) Tachpyr protected isolated DNA against H(2)O(2)-induced damage, but not against HX/XO-induced damage; and (4) Tachpyr-Fe(II) chelate slows down but does not block oxidation of Fe(II), allows O*(-)(2)-induced or Tachpyr-induced reduction of Fe(III), and consequently promotes production of *OH through the Haber-Weiss reaction cycle. The results indicate that Tachpyr can protect cells against short-term, metal-mediated damage. However, upon prolonged incubation, Tachpyr exerts cytotoxic effects. Therefore, in addition to iron depletion, low-level oxidative stress, which in part occurs because of redox cycling of the coordinated iron ion, may contribute to the cytotoxic effects of Tachpyr.

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.

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.

Acyl-protected hydroxylamines as spin label generators for EPR brain imaging

In a search for novel electron paramagnetic resonance (EPR) brain imaging agents, we have designed and synthesized the acyl-protected hydroxylamines 1-acetoxy-4-methoxycarbonyl-2,2,6,6-tetramethylpiperidine (AMCPe), 1-acetoxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (AMCPy), and 1-acetoxy-3-(acetoxymethoxy)carbonyl-2,2,5,5-tetramethylpyrrolidine (DACPy), in which both the ring size and the number of ester functions were varied. In all of them, the nitroxide was first reduced and the resultant hydroxylamine was then protected with an acetyl group. These compounds are lipophilic, which is a major prerequisite for blood-brain barrier penetration. Once in the brain, esterases and oxidants quickly convert these derivatives into ionic, water-soluble radicals and thus EPR detectable species that then reside in the central nervous system for periods of time sufficient for detection and imaging. The biological relevancy of these new compounds in mice has been assessed, and their biodistribution patterns have been compared. The five-membered ring derivative AMCPy emerged as a potent EPR brain imaging agent while the other two derivatives, AMCPe and DACPy, were quite ineffective.

Further evidence for distinct reactive intermediates from nitroxyl and peroxynitrite: effects of buffer composition on the chemistry of Angeli's salt and synthetic peroxynitrite

The nitroxyl (HNO) donor Angeli's salt (Na(2)N(2)O(3); AS) is cytotoxic in vitro, inducing double strand DNA breaks and base oxidation, yet may have pharmacological application in the treatment of cardiovascular disease. The chemical profiles of AS and synthetic peroxynitrite (ONOO(-)) in aerobic solution were recently compared, and AS was found to form a distinct reactive intermediate. However, similarities in the chemical behavior of the reactive nitrogen oxide species (RNOS) were apparent under certain conditions. Buffer composition was found to have a significant and unexpected impact on the observed chemistry of RNOS, and varied buffer conditions were utilized to further distinguish the chemical profiles elicited by the RNOS donors AS and synthetic ONOO(-). Addition of HEPES to the assay buffer significantly quenched oxidation of dihydrorhodamine (DHR), hydroxylation of benzoic acid (BA), and DNA damage by both AS and ONOO(-), and oxidation and nitration of hydroxyphenylacetic acid by ONOO(-). Additionally, H(2)O(2) was produced in a concentration-dependent manner from the interaction of HEPES with both the donor intermediates. Interestingly, clonogenic survival was not affected by HEPES, indicating that H(2)O(2) is not a contributing factor to in vitro cytotoxicity of AS. Variation in RNOS reactivity was dramatic with significantly higher relative affinity for the AS intermediate toward DHR, BA, DNA, and HEPES and increased production of H(2)O(2). Further, AS reacted to a significantly greater extent with the unprotonated amine form of HEPES while the interaction of ONOO(-) with HEPES was pH-independent. Addition of bicarbonate only altered ONOO(-) chemistry. This study emphasizes the importance of buffer composition on chemical outcome and thus on interpretation and provides further evidence that ONOO(-) is not an intermediate formed between the reaction of O(2) and HNO produced by AS.

Nitroxides as antioxidants: Tempol protects against EO9 cytotoxicity

Nitroxide free radicals have been shown to be potent antioxidants in a variety of experimental models using diverse means of insults. Among other insults, nitroxides have been shown effective in inhibiting cytotoxicity of quinone-based drugs such as streptonigrin and mitomycin C. These drugs and other chemotherapeutic agents have the potential to undergo bioreductive activation by the normal reducing enzymes within a cell. In the present work we studied the effect of the nitroxide Tempol on the cytotoxicity induced by EO9, a mitomycin C analogue, in HT29 cells under aerobic and hypoxic conditions. The study was aimed to better understand the mechanism of EO9 cytotoxicity and the molecular level of the nitroxide's mode of protection. The reactions of Tempol with activated EO9, and the reactive species formed during EO9 activation were studied in a cell-free solution, using spin-trapping, and electron paramagnetic resonance (EPR) spectrometry. Our results indicate that EO9 induced similar cytotoxicity in HT29 cells under aerobic and hypoxic conditions while Tempol provided similar and almost complete protection to both aerobic and hypoxic cells. The results indicate that EO9 cytotoxicity is due to both 1- and 2-electron reductive activation processes, with aerobic toxicity caused by back-oxidation of the hydroquinone to the semiquinone, EO9.-. Tempol serves both as a useful tool in the study of the mechanisms of quinone-mediated cytotoxicity and as a potent antioxidant against the damaging effects of redox cycling quinones and semiquinones by scavenging of EO9.- or detoxification of O2.- and H2O2.

Noninvasive in vivo oximetric imaging by radiofrequency FT EPR

A novel method, called relaxo-oximetry, for rapid spatially resolved in vivo measurements of oxygen concentration using time-domain radiofrequency (RF) electron paramagnetic resonance (EPR) is described. Time-domain data from triaryl methyl (TAM)-based single-electron contrast agents were processed by systematic deletion of the initial time points to arrive at T2*-weighted discrimination of signal amplitudes. In experiments involving phantoms, the line widths [ approximately (T2*)(-1)] increased as a function of oxygen, and the slope (line width/pO(2)) was the same for both absorption- and magnitude-mode line shapes. Line widths derived from T2* weighting and the computed pO(2) values agreed favorably with the measured ones from phantoms of known oxygen tension. In vivo relaxo-oximetry was performed on C3H mice, and it was found that the liver was more hypoxic than the kidneys. For tumors, 2D oxygen maps were generated while the animal breathed room air or Carbogen(R) (95% O(2)/5% CO(2)). Carbogen(R) enhanced oxygen concentration within the tumor, and the pO(2) histograms showed considerable heterogeneity. Clark electrode oxygen measurements on organs and tumors were in good agreement with tissue oxygen measurements done by relaxo-oximetry. Thus, from a single spatial image data set, pO(2) measurements can be done noninvasively by relaxo-oximetry, and 3D imaging can be performed in less than 3 min.

Overhauser enhanced magnetic resonance imaging for tumor oximetry: coregistration of tumor anatomy and tissue oxygen concentration

An efficient noninvasive method for in vivo imaging of tumor oxygenation by using a low-field magnetic resonance scanner and a paramagnetic contrast agent is described. The methodology is based on Overhauser enhanced magnetic resonance imaging (OMRI), a functional imaging technique. OMRI experiments were performed on tumor-bearing mice (squamous cell carcinoma) by i.v. administration of the contrast agent Oxo63 (a highly derivatized triarylmethyl radical) at nontoxic doses in the range of 2-7 mmol/kg either as a bolus or as a continuous infusion. Spatially resolved pO(2) (oxygen concentration) images from OMRI experiments of tumor-bearing mice exhibited heterogeneous oxygenation profiles and revealed regions of hypoxia in tumors (<10 mmHg; 1 mmHg = 133 Pa). Oxygenation of tumors was enhanced on carbogen (95% O(2)/5% CO(2)) inhalation. The pO(2) measurements from OMRI were found to be in agreement with those obtained by independent polarographic measurements using a pO(2) Eppendorf electrode. This work illustrates that anatomically coregistered pO(2) maps of tumors can be readily obtained by combining the good anatomical resolution of water proton-based MRI, and the superior pO(2) sensitivity of EPR. OMRI affords the opportunity to perform noninvasive and repeated pO(2) measurements of the same animal with useful spatial (approximately 1 mm) and temporal (2 min) resolution, making this method a powerful imaging modality for small animal research to understand tumor physiology and potentially for human applications.

Evaluation and comparison of pulsed and continuous wave radiofrequency electron paramagnetic resonance techniques for in vivo detection and imaging of free radicals

The performance of two electron paramagnetic resonance (EPR) spectrometers/imagers, one configured in pulsed mode and the other in continuous wave (CW) mode, at an operating frequency of 300 MHz is compared. Using the same resonator (except for altered Q-factors), identical samples and filling factors in the two techniques have been evaluated for their potentials and limitations for in vivo spectroscopic and imaging applications. The assessment is based on metrics such as sensitivity, spatial and temporal resolution, field of view, image artifacts, viable spin probes, and subjects of study. The spectrometer dead time limits the pulsed technique to samples with long phase memories (>275 ns). Nevertheless, for viable narrow-line spin probes, the pulsed technique offers better sensitivity and temporal resolution. The CW technique, on the other hand, does not restrict the choice at spin probes. In addition, the phase-sensitive narrow-band detection of the CW technique gives artifact-free images even for large objects. Selected examples illustrating the performance of the CW and pulsed techniques are presented to put the capabilities of the two techniques in perspective.

Nitroxides as radiation protectors

Selective protection of normal tissues from the damaging effects of ionizing radiation is an important objective in cancer treatment research. Likewise, radioprotective agents may be useful in protecting the human population in the event of radiation-related accidents or military conflicts. Over the past decade, we have identified stable nitroxide compounds as a unique class of antioxidants with demonstrated radioprotective properties. We have shown that nitroxides at nontoxic concentrations are effective as in vitro and in vivo antioxidants when oxidation is induced by superoxide, hydrogen peroxide, organic hydroperoxides, ionizing radiation, or specific DNA-damaging anticancer agents. Studies have shown that nitroxides protect against radiation-induced DNA damage, chromosome aberrations, mutation induction, cell killing, and lethality in mice that receive whole-body irradiation. Whether these agents provide radioprotection for low-level radiation doses remains to be determined.

Noninvasive imaging of tumor redox status and its modification by tissue glutathione levels

Therapeutic regimens such as radiation or chemotherapy attempt to exploit the physiological differences between normal and malignant tissue. Tissue redox status and pO(2) are two factors that are hypothesized to be different in normal and malignant tissues. Methods that can detect subtle differences in the above physiological parameters would greatly aid in devising appropriate treatment strategies. We have previously used in vivo electron paramagnetic resonance (EPR) spectroscopy and imaging techniques and shown that tumor tissues are highly reducing and hypoxic compared with normal tissues (P. Kuppusamy et al., Cancer Res., 58: 1562-1568, 1998). The purpose of the present study was to obtain spatially resolved redox data from normal and tumor tissues of radiation-induced fibrosarcoma (RIF-1) tumor-bearing mice and to examine the role of intracellular glutathione (GSH) on the tissue redox status. Experiments were performed using low-frequency (1.3 GHz) in vivo EPR spectroscopy and imaging techniques with a nitroxide redox probe. L-buthionine-S,R-sulfoximine (BSO), an inhibitor of GSH synthesis, was used to deplete tissue GSH levels. The results show the existence of significant heterogeneity of redox status in the tumor tissue compared with normal tissue. The tumor tissues show at least 4-fold higher concentrations of GSH levels compared with normal tissues in the tumor-bearing mice. Also BSO treatment showed a differential depletion of GSH and reducing equivalents in the tumor tissue. Thus, it appears that there is significant heterogeneity of tumor redox status and that manipulation of the tumor redox status may be important in tumor growth and therapy.

Electron paramagnetic resonance for small animal imaging applications

Magnetic resonance imaging (MRI) provides high-resolution morphological images useful in diagnostic radiology to differentiate between normal and abnormal/pathological states in tissues. More recently, emerging developments in MRI have added a functional/physiological dimension to anatomical images. Electron paramagnetic resonance (EPR), a magnetic resonance technique similar to nuclear magnetic resonance, detects paramagnetic species such as free radicals. Like MRI, EPR can be implemented as an imaging technique for small animals and potentially human applications. Because of the low abundance of naturally occurring paramagnetic species, exogenous paramagnetic species are needed for in vivo EPR imaging (EPRI). The image data from EPRI contain both spatial distribution of paramagnetic species and spectral information. Hence, spatially encoded functional information such as tissue oxygen status and redox status can be extracted and coregistered with the spatial distribution of the spin probe, to the anatomy, or both by suitable means. Ultimately, the images obtained from EPRI may be used to overlay the functional information (containing spatial tissue physiology information) onto detailed anatomical maps. With its ability to enable whole animal imaging in mice, EPRI will be a useful imaging technique that complements other techniques such as MRI and positron emission tomography in obtaining valuable functional/physiological images.

Cellular sites of H2O2-induced damage and their protection by nitroxides

While the exact mechanism of H2O2-induced cytotoxicity is unknown, there is considerable evidence implicating DNA as a primary target. A recent study showed that a cell-impermeable nitroxide protected mammalian cells from H2O2-induced cell killing and suggested that the protection was mediated through cell membrane-bound or extracellular factors. To further define the protective properties of nitroxides, Chinese hamster V79 cells were exposed to H2O2 with or without cell-permeable and impermeable nitroxides and selected metal chelators. EPR spectroscopy and paramagnetic line broadening agents were used to distinguish between intra- and extracellular nitroxide distribution. To study the effectiveness of nitroxide protection, in the absence of a cell membrane, H2O2-mediated damage to supercoiled plasmid DNA was evaluated. Both deferrioxamine and Tempol cross the cell membrane, and inhibited H2O2-mediated cell killing, whereas the cell-impermeable DTPA and nitroxide, CAT-1, failed to protect. Similar protective effects of the chelators and nitroxides were observed when L-histidine, which enhances intracellular injury, was added to H2O2. In contrast, when damage to plasmid DNA was induced (in the absence of a cell membrane), both nitroxides were protective. Collectively, these results do not support a role for membrane-bound or extracellular factors in mediating H2O2 cytotoxicity in mammalian cells.

Multifunctional antioxidant activity of HBED iron chelator

The use of N,N'-bis (2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid (HBED) for iron chelation therapy is currently being tested. Besides its affinity for iron, bioavailability, and efficacy in relieving iron overload, it is important to assess its anti- and/or pro-oxidant activity. To address these questions, the antioxidant/pro-oxidant effects of HBED in a cell-free solution and on cultured Chinese hamster V79 cells were studied using UV-VIS spectrophotometry, oximetry, spin trapping, and electron paramagnetic resonance (EPR) spectrometry. The results indicate that HBED facilitates Fe(II) oxidation but blocks O2(.-)-induced reduction of Fe(III) and consequently pre-empts production of .OH or hypervalent iron through the Haber-Weiss reaction cycle. The efficacy of HBED as a 1-electron donor (H-donation) was demonstrated by reduction of the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate)-derived nitrogen-centered radical cation (ABTS(.+)), accompanied by formation of a short-lived phenoxyl radical. HBED also provided cytoprotection against toxicity of H2O2 and t-BuOOH. Our results show that HBED can act both as a H-donating antioxidant and as an effective chelator lacking pro-oxidant capacity, thus substantiating its promising use in iron chelation therapy.

Magnetic resonance imaging for in vivo assessment of tissue oxygen concentration

Magnetic resonance imaging (MRI) provides high-resolution morphological images useful in diagnostic radiology to differentiate normal from abnormal/pathological states. More recently, emerging developments in MRI seek to add a functional/physiological dimension to the anatomic images to provide better understanding of the physiology of pathological conditions. Three MRI methods offer the promise of providing important physiologic information, such as oxygen status and redox capability of tissues, and these are discussed in the context of their potential usefulness to radiation oncology. The techniques include blood oxygen level-dependent (BOLD) MRI, Overhauser enhanced MRI (OMRI), and electron paramagnetic resonance imaging (EPRI). BOLD MRI provides information of tumor oxygen status by using the differences in MRI images from tumors obtained when breathing air or carbogen. Deoxyhemoglobin serves as an endogenous BOLD MRI contrast agent. OMRI utilizes the enhancement of proton MRI images by a nontoxic free radical contrast agent. The advantages of this technique are the very low magnetic fields used and its capability to provide quantitative information of tissue oxygen concentration. EPRI also uses free radical contrast agents and can provide redox and oxygen status differences between tumor and normal tissues. Some of the contrast agents used in EPRI have been identified as radiation protectors. The images obtained from each of the technologies may ultimately be used to overlay their respective views (containing spatial tissue physiology information) onto detailed anatomic maps.

Radiation, radicals, and images

Nitroxide stable free radicals exhibit varied chemical and biological properties. Their biological applications have been greatly expanded over the past few years. Not only have they been shown to exhibit potent antioxidant and radioprotective properties, but also they can serve as in vivo functional imaging probes that non-invasively report on the oxygen status and redox properties of tissue, which may have utility in clinical biomedical research.

Evaluation of the hydroxylamine Tempol-H as an in vivo radioprotector

Nitroxides are stable free radical compounds that protect against the toxicity of reactive oxygen species in vitro and in vivo. Tempol (Aldrich, Milwaukee, WI, USA) is a cell-permeable hydrophilic nitroxide and has been shown to be an in vitro and in vivo radioprotector. The limitations of Tempol as a systemic radioprotector are that it causes substantial reductions in arterial blood pressure when administered intravenously and is associated with seizure activity. Furthermore, Tempol is rapidly reduced to its hydroxylamine form, Tempol-H, which limits the period of time the active form of the nitroxide is available for radioprotection. Based on initial pharmacological and blood pressure experiments performed in mice, we hypothesized that the systemic administration of Tempol-H in vivo would lead to an equilibration between Tempol and Tempol-H that would limit the toxicity of the nitroxide and provide in vivo radioprotection. Tempol-H was administered in increasing doses via an intraperitoneal route to C3H mice. The maximally tolerated dose was found to be 325 mg/kg. The whole-blood pharmacology of Tempol-H was investigated with electron paramagnetic resonance spectroscopy. These studies demonstrated the appearance of Tempol in whole blood immediately after intraperitoneal injection, suggesting that rapid oxidation of Tempol-H to Tempol takes place in vivo. Although the peak concentration of Tempol in whole blood after administration of Tempol-H did not reach the same levels as those observed when Tempol is administered, the whole-blood levels of Tempol were similar by 10 min after injection. Tempol-H provided protection against the lethality of whole-body radiation in C3H mice at 30 d with a dose modification factor of 1.3, which is similar to the results obtained with Tempol. Hemodynamic measurements in C3H mice after intravenous injection showed that Tempol-H produced little effect on blood pressure or pulse compared with Tempol. Tempol-H is a systemic in vivo radioprotector of C3H mice and is associated with less hemodynamic toxicity than Tempol.

Three-dimensional whole body imaging of spin probes in mice by time-domain radiofrequency electron paramagnetic resonance

Imaging of stable paramagnetic spin probes in phantom objects and in vivo was evaluated using a RF time domain EPR spectrometer/imager operating at 300 MHz. Projections were collected using static magnetic field gradients and images were reconstructed using filtered back-projection techniques. Results from phantom objects containing approximately 10(17) spins of stable paramagnetic probes with single narrow EPR spectra provide three-dimensional spatial images with resolution better than 2 mm. When the spin probe was administered to mice, the spin probe accumulation was temporally observed in the thoracic, abdominal, and pelvic regions. A three-dimensional image (from 144 projections) from a live mouse was collected in 5 min. Using fiducial markers, the spin probe accumulation in organs such as liver, kidney, and bladder could be observed. Differences in the oxygen status between liver and kidney were observed from the EPR images from mice administered with spin probe, by treating the time-domain responses with convolution difference approach, prior to image reconstruction. The results from these studies suggest that, with the use of stable paramagnetic spin probes and time-domain RF EPR, it is possible to perform in vivo imaging on animals and also obtain important spatially resolved physiologic information.

Parallel coil resonators for time-domain radiofrequency electron paramagnetic resonance imaging of biological objects

Resonators suitable for time-domain electron paramagnetic resonance spectroscopy and imaging at a radiofrequency capable of accommodating experimental animals such as mice are described. Design considerations included B(1) field homogeneity, optimal Q, spectral bandwidth, resonator ring-down, and sensitivity. Typically, a resonator with 25-mm diameter and 25-mm length was constructed by coupling 11 single loops in parallel with a separation of 2.5 mm. To minimize the resonator ring-down time and provide the necessary spectral bandwidth for in vivo imaging experiments, the Q was reduced predominantly by overcoupling. Capacitative coupling was utilized to minimize microphonic effects. The B(1) field in the resonator was mapped both radially and axially and found to be uniform and adequate for imaging studies. Imaging studies with phantom objects containing a narrow-line spin probe as well as in vivo objects administered with the spin probe show the suitability of these resonators for valid reproduction of the spin probe distribution in three dimensions. The fabrication of such resonators is simple and can be scaled up with relative ease to accommodate larger objects as well.

Cytotoxicity, redox cycling and photodynamic action of two naturally occurring quinones

Two naturally occurring anthraquinones, barleriaquinone-I (BQ-I) and barleriaquinone-II (BQ-II), extracted from Barleria buxifolia, are tested for their cytotoxic action by aerobic incubation with human breast adenocarcinoma cells (MCF7). Cytotoxicities, measured as LD(50) (50% inhibition of colony formation) values, show BQ-II to be more active than BQ-I. Electron paramagnetic resonance studies confirm that BQ-II is reductively activated by NADH:cytochrome c reductase to superoxide anion radical. Cyclic voltammetric studies show one quasi-reversible redox couple for both BQ-I and BQ-II. Also, aerobic solutions of both BQ-I and BQ-II on visible illumination generate reactive oxygen species. Formation of O*-2 is studied by both EPR spin trapping and SOD-inhibitable cytochrome c reduction techniques. BQ-I generates more singlet oxygen as evidenced from the photobleaching of N,N-dimethyl-4-nitrosoaniline.

Hemodynamic effect of the nitroxide superoxide dismutase mimics

Reactive oxygen species play critical roles in a number of physiologic and pathologic processes. Nitroxides are stable free radical compounds that possess superoxide dismutase (SOD) mimetic activity and have been shown to protect against the toxicity of reactive oxygen species in vitro and in vivo. Tempol, a cell-permeable hydrophilic nitroxide, protects against oxidative stress and also is an in vitro and in vivo radioprotector. In the course of evaluating the pharmacology and toxicity of the nitroxides, Tempol and another nitroxide, 3-carbamoyl-PROXYL (3-CP), were administered intravenously in various concentrations to miniature swine. Tempol caused dose-related hypotension accompanied by reflex tachycardia and increased skin temperature. Invasive hemodynamic monitoring with Swan Ganz catheterization (SGC) confirmed the potent vasodilative effect of Tempol. However, 3-CP had no effect on porcine blood pressure. The hemodynamic effects of Tempol and 3-CP are discussed in the context of differential catalytic rate constants for superoxide disumation that may impact systemic nitric oxide (NO) levels and lead to vasodilation. These findings are consistent with a role for the superoxide ion in the modulation of blood pressure and have potential implications for the systemic use of nitroxides.

High-speed data acquisition system and receiver configurations for time-domain radiofrequency electron paramagnetic resonance spectroscopy and imaging

Design strategies, system configuration, and operation of a dual-channel data acquisition system for a radiofrequency (RF) time-domain electron paramagnetic resonance (EPR) spectrometer/imager operating at 300 MHz are described. This system wasconfigured to incorporate high-speed analog-to-digital conversion (ADC) and summation capabilities with both internal and external triggering via GPIB interface. The sampling rate of the ADC is programmable up to a maximum of 1 GS/s when operating in a dual-channel mode or 2 GS/s when the EPR data are collected in a single-channel mode. By using high-speed flash ADCs, a pipelined 8-bit adder, and a 24-bit accumulator, a repetition rate of 230 kHz is realized to sum FIDs of 4096 points. The record length is programmable up to a maximum of 8K points and a large number of FIDs (2(24)) can be summed without overflow before the data can be transferred to a host computer via GPIB interface for further processing. The data acquisition system can operate in a two-channel (quadrature) receiver mode for the conventional mixing to baseband. For detection using the single-channel mode, the resonance signals around the center frequency of 300 MHz were mixed with a synchronized local oscillator of appropriate frequency leading to an intermediate frequency (IF) which is sampled at a rate of 2 GS/s. Comparison of quadrature-mode and an IF-mode operation for EPR detection is presented by studying the FID signal intensity across a bandwidth of 10 MHz and as a function of transmit RF power. Imaging of large-sized phantoms accommodated in appropriately sized resonators indicates that IF-mode operation can be used to obtain distortion-free images in resonators of size 50 mm diameter and 50 mm length.

Studies of structure-activity relationship of nitroxide free radicals and their precursors as modifiers against oxidative damage

The protective effects of stable nitroxides, as well as their hydroxylamine and amine precursors, have been tested in Chinese hamster V79 cells subjected to H2O2 exposure at fixed concentration or exposure to ionizing radiation. Cytotoxicity was evaluated by monitoring the viability of the cells assessed by the clonogenic assay. The compounds tested at fixed concentration varied in terms of ring size, oxidation state, and ring substituents. Electrochemical studies were carried out to measure the redox midpoint potentials. The studies show that in the case of protection against H2O2 exposure, the protection was determined by the ring size, oxidation state, and redox midpoint potentials. In general the protection factors followed the order nitroxides > hydroxylamines > amines. Both the six-membered ring nitroxides and substituted five-membered ring nitroxides were efficient protectors. For six-membered ring nitroxides, the compounds exhibiting the lowest midpoint potentials exhibited maximal protection. In the case of X-radiation, nitroxides were the most protective though some hydroxylamines were also efficient. The amines were in some cases found to sensitize the toxicity of aerobic radiation exposure. The protection observed by the nitroxides was not dependent on the ring size. However, the ring substituents had significant influence on the protection. Compounds containing a basic side chain were found to provide enhanced protection. The results in this study suggest that these compounds are novel antioxidants which can provide cytoprotection in mammalian cells against diverse types of oxidative insult and identify structural determinants optimal for protection against individual types of damage.