Intracellular hypoxia of tumor tissue estimated by noninvasive electron paramagnetic resonance oximetry technique using paramagnetic probes

Non-invasive differentiation of hepatic steatosis and steatohepatitis in a mouse model using nitroxyl radical as an MRI-contrast agent

Drug-induced steatohepatitis is considered more serious than drug-induced hepatic steatosis, so that differentiating between the two is crucial in drug development. In addition, early detection of drug-induced steatohepatitis is considered important since recovery is possible with drug withdrawal. However, no method has been established to differentiate between the two. In the development of drug-induced steatohepatitis, reactive oxygen species (ROS) is excessively generated in the liver. It has been reported that ROS can be monitored with electron spin resonance (ESR) and dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) by using nitroxyl radicals, which are known to participate in various in vivo redox reactions. The decay/reduction rate, which is an index for monitoring nitroxyl radicals, has been reported to be increased in tissues with excessive ROS levels other than liver, but decreased in methionine choline deficient (MCD) diet-induced steatohepatitis with excess ROS. Therefore, looking to differentiate between drug-induced hepatic steatosis and steatohepatitis, we examined whether the reduction rate decreases in steatohepatitis other than the MCD-diet induced disease and whether the decrease could be detected by MRI. We used STAM™ mice in which hepatic steatosis and steatohepatitis developed sequentially under diabetic conditions. 3-carbamoyl-PROXYL (CmP), one of the nitroxyl radicals, was injected intravenously during the MRI procedure and the reduction rate was calculated. The reduction rate was significantly higher in early steatohepatitis than in hepatic steatosis and the control. Excess ROS in early steatohepatitis was detected by an immunohistochemical marker for ROS. Therefore, it was indicated that the increase or decrease in the reduction rate in steatohepatitis differs depending on the model, and early steatohepatitis could be noninvasively differentiated from hepatic steatosis using CmP in MRI. Since the change in direction of the reduction rate in steatohepatitis in clinical studies could be predicted by confirming the reduction rate in preclinical studies, the present method, which can be used consistently in clinical and preclinical studies, warrants consideration as a candidate monitoring method for differentiating between early drug-induced steatohepatitis and hepatic steatosis in drug development.

Effects of selenium deficiency on biological results of X-ray and carbon-ion beam irradiation in mice

The impact of radiation-induced hydrogen peroxide (H₂O₂) on the biological effects of X-rays and carbon-ion beams was investigated using a selenium-deficient (SeD) mouse model. Selenium is the active center of glutathione peroxidase (GSH-Px), and SeD mice lack the ability to degrade H₂O₂. Male and female SeD mice were prepared by feeding a torula yeast-based SeD diet and ultrapure water. Thirty-day survival rates after whole-body irradiation, radiation-induced leg contracture, and MRI-based redox imaging of the brain were assessed and compared between SeD and normal mice. Thirty-day lethality after whole-body 5.6 Gy irradiation with X-rays or carbon-ion beams was higher in the SeD mice than in the normal mice, while SeD did not give the notable difference between X-rays and carbon-ion beams. SeD also did not affect the maximum leg contracture level after irradiation with carbon-ion beams, but delayed the leg contraction rate. In addition, no marked effects of SeD were observed on variations in the redox status of the brain after irradiation. Collectively, the present results indicate that SeD slightly altered the biological effects of X-rays and/or carbon-ion beams. GSH-Px processes endogenous H₂O₂ generated through mitochondrial respiration, but does not have the capacity to degrade H₂O₂ produced by irradiation.

Development of a novel molecular probe for the detection of liver mitochondrial redox metabolism

Redox status influences the course of the inflammatory, metabolic, and proliferative liver diseases. Oxidative stress is thought to play a crucial and sustained role in the pathological progression of early steatosis to severe hepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma. Oxidative stress induced by reactive oxygen species which are generated in the mitochondria can lead to chronic organelle damage in hepatocytes. Currently, the diagnosis of liver disease requires liver biopsy, which is invasive and associated with complications. The present report describes the development of a novel molecular probe, EDA-PROXYL, with higher reactivity and mitochondrial selectivity than standard carboxyl-PROXYL and carbamoyl-PROXYL probes. The membrane permeability of our probe improved in aqueous environments which led to increased accumulation in the liver and interaction of EDA-PROXYL with the carnitine transporter via the amine (NH3+) group further increased accumulation. This increased mitochondrial sensitivity and enhanced accumulation highlight the potential of EDA-PROXYL as a molecular probe for determining metabolic reactions of the mitochondria. Thus, this novel probe could be a tool for the evaluation of redox status of the mitochondria to assess the degree of liver injury and, ultimately, the response to pharmacological therapy.

Effects of oxygen challenging to tissue redox and pO2 status

Nitroxide free radicals can serve as redox-sensitive MRI contrast agents useful to image the redox status of tissue of interest. In this study, the effect of oxygen content in the inspired gas on the kinetics of metabolism of three nitroxides has been evaluated in the muscle and tumor in mice. SCC tumors (approximate size of 1.0 cm³) on the right hind leg of female C3H/Hen MTV^- mice were prepared. Three nitroxides, 3-carboxy-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CxP), 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), and 4-hydroxy-tetramethylpiperidine-N-oxyl (TEMPOL), having different lipophilicities were compared using MR redox imaging. T₁-mapping of the tissues was obtained using a multi-slice multi-echo (MSME) sequence with several TRs. The three nitroxides showed differences in accumulation and metabolism/clearance in muscle and tumor. The cell impermeable nitroxide CxP displayed kinetic patterns of slow enhancement followed by a slow decline typical of clearance rather than metabolism. The cell permeable CmP on the other hand showed a relatively faster uptake and metabolism with a modestly higher rate of metabolism in the tumor than muscle. The TEMPOL on the other hand displayed a rapid uptake and reduction with a trend of significantly rapid decay rate in tumor tissue, while slightly higher maximum signal intensity and slower decay rate was observed in normal muscle. The reduction rate of TEMPOL in the tumor was significantly enhanced when the breathing gas had 100%-oxygen while it was not significantly different in the muscle. EPR oximetry studies monitoring the oxygen dependent linewidth of TEMPOL showed that the pO₂ in the healthy tissue during carbogen breathing significantly increased normal tissue pO₂ compared to air breathing whereas breathing 100%-oxygen made normal tissue slight hypoxic. Since TEMPOL is a radioprotector, our studies show that a combination of 100%-oxygen breathing and TEMPOL has a potential to enhance radioprotective effects to normal tissue.

Reactivity of redox sensitive paramagnetic nitroxyl contrast agents with reactive oxygen species

The reactivity of nitroxyl free radicals, 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), with reactive oxygen species (ROS) were compared as typical 6-membered and 5-membered ring nitroxyl compounds, respectively. The reactivity of the hydroxylamine forms of both these nitroxyl radicals (TEMPOL-H and CmP-H) was also assessed. Two free radical species of ROS, hydroxyl radical (^•OH) and superoxide (O₂^•−), were subjected to a competing reaction. ^•OH was generated by UV irradiation from an aqueous H₂O₂ solution (H₂O₂-UV system), and O₂^•− was generated by a reaction between hypoxanthine and xanthine oxidase (HX-XO system). ^•OH and O₂^•− generated by the H₂O₂-UV and HX-XO systems, respectively, were measured by electron paramagnetic resonance (EPR) spin-trapping, and the amount of spin adducts generated by each system was adjusted to be equal. The time courses of the one-electron oxidation of TEMPOL, CmP, TEMPOL-H, and CmP-H in each ROS generation system were compared. A greater amount of TEMPOL was oxidized in the HX-XO system compared with the H₂O₂-UV system, whereas the reverse was observed for CmP. Although the hydroxylamine forms of the tested nitroxyl radicals were oxidized evenly in the H₂O₂-UV and HX-XO systems, the amount of oxidized CmP-H was approximately 3 times greater compared with TEMPOL-H.

Pulsed Electron Paramagnetic Resonance Imaging: Applications in the Studies of Tumor Physiology

SIGNIFICANCE

Electron paramagnetic resonance imaging (EPRI) is capable of generating images of tissue oxygenation using exogenous paramagnetic probes such as trityl radicals or nitroxyl radicals. The spatial distribution of the paramagnetic probe can be generated using magnetic field gradients as in magnetic resonance imaging and, from its spectral features, spatial maps of oxygen can be obtained from live objects. In this review, two methods of signal acquisition and image formation/reconstruction are described. The probes used and its application to study tumor physiology and monitor treatment response with chemotherapy drugs in mouse models of human cancer are summarized. Recent Advances: By implementing phase encoding/Fourier reconstruction in EPRI in time domain mode, the frequency contribution to the spatial resolution was avoided and images with improved spatial resolution were obtained. The EPRI-generated pO₂ maps in tumor were useful to detect and evaluate the effects of various antitumor therapies on tumor physiology. Coregistration with other imaging modalities provided a better understanding of hypoxia-related alteration in physiology.

CRITICAL ISSUES

The high radiofrequency (RF) power of EPR irradiation and toxicity profile of radical probes are the main obstacles for clinical application. The improvement of RF low power pulse sequences may allow for clinical translation.

FUTURE DIRECTIONS

Pulsed EPR oximetry can be a powerful tool to research various diseases involving hypoxia such as cancer, ischemic heart diseases, stroke, and diabetes. With appropriate paramagnetic probes, it can also be applied for various other purposes such as detecting local acid-base balance or oxidative stress. Antioxid. Redox Signal. 28, 1378-1393.

In vivo EPR pharmacokinetic evaluation of the redox status and the blood brain barrier permeability in the SOD1G93A ALS rat model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder affecting the motor pathways of the central nervous system. Although a number of pathophysiological mechanisms have been described in the disease, post mortem and animal model studies indicate blood-brain barrier (BBB) disruption and elevated production of reactive oxygen species as major contributors to disease pathology. In this study, the BBB permeability and the brain tissue redox status of the SOD1^G93A ALS rat model in the presymptomatic (preALS) and symptomatic (ALS) stages of the disease were investigated by in vivo EPR spectroscopy using three aminoxyl radicals with different cell membrane and BBB permeabilities, Tempol, 3-carbamoyl proxyl (3CP), and 3-carboxy proxyl (3CxP). Additionally, the redox status of the two brain regions previously implicated in disease pathology, brainstem and hippocampus, was investigated by spectrophotometric biochemical assays. The EPR results indicated that among the three spin probes, 3CP is the most suitable for reporting the intracellular redox status changes, as Tempol was reduced in vivo within minutes (t_1/2 =2.0±0.5min), thus preventing reliable kinetic modeling, whereas 3CxP reduction kinetics gave divergent conclusions, most probably due to its membrane impermeability. It was observed that the reduction kinetics of 3CP in vivo, in the head of preALS and ALS SOD1^G93A rats was altered compared to the controls. Pharmacokinetic modeling of 3CP reduction in vivo, revealed elevated tissue distribution and tissue reduction rate constants indicating an altered brain tissue redox status, and possibly BBB disruption in these animals. The preALS and ALS brain tissue homogenates also showed increased nitrilation, superoxide production, lipid peroxidation and manganese superoxide dismutase activity, and a decreased copper-zinc superoxide dismutase activity. The present study highlights in vivo EPR spectroscopy as a reliable tool for the investigation of changes in BBB permeability and for the unprecedented in vivo monitoring of the brain tissue redox status, as early markers of ALS.

Brain contrasting ability of blood-brain-barrier-permeable nitroxyl contrast agents for magnetic resonance redox imaging

PURPOSE

The detailed in vivo T1 -weighted contrasting abilities of nitroxyl contrast agents, which have been used as redox responsive contrast agents in several magnetic resonance-based imaging modalities, in mouse brain were investigated.

METHODS

Distribution and pharmacokinetics of five types of five-membered-ring nitroxyl radical compound were compared using T1 -weighted MRI.

RESULTS

The blood-brain barrier (BBB) -impermeable 3-carboxy-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CxP) could not be distributed in the brain. The slightly lipophilic 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP) showed slight distribution only in the ventricle, but not in the medulla and cortex. The amphiphilic 3-methoxy-carbonyl-2,2,5,5-tetramethyl-pyrrolidine-N-oxyl (MCP) had good initial uniform distribution in the brain and showed typical 2-phase signal decay profiles. A brain-seeking nitroxyl probe, acetoxymethyl-2,2,5,5-tetramethyl-pyrrolidine-N-oxyl-3-carboxylate (CxP-AM), showed an accumulating phase, and then its accumulation was maintained in the medulla and ventricle regions, but not in the cortex. The lipophilic 4-(N-methyl piperidine)-2,2,5,5-tetramethylpyrroline-N-oxyl (23c) was well distributed in the cortex and medulla, but slightly in the ventricle, and showed relatively rapid linear signal decay.

CONCLUSION

Nitroxyl contrast agents equipped with a suitable lipophilic substitution group could be BBB-permeable functional contrast agents. MR redox imaging, which can estimate not only the redox characteristics but also the detailed distribution of the contrast agents, is a good candidate for a theranostic tool. Magn Reson Med 76:935-945, 2016. © 2015 Wiley Periodicals, Inc.

Redox- and hypoxia-responsive MRI contrast agents

The development of responsive or "smart" magnetic resonance imaging (MRI) contrast agents that can report specific biomarker or biological events has been the focus of MRI contrast agent research over the past 20 years. Among various biological hallmarks of interest, tissue redox and hypoxia are particularly important owing to their roles in disease states and metabolic consequences. Herein we review the development of redox-/hypoxia-sensitive T1 shortening and paramagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agents. Traditionally, the relaxivity of redox-sensitive Gd(3+) -based complexes is modulated through changes in the ligand structure or molecular rotation, while PARACEST sensors exploit the sensitivity of the metal-bound water exchange rate to electronic effects of the ligand-pendant arms and alterations in the coordination geometry. Newer designs involve complexes of redox-active metal ions in which the oxidation states have different magnetic properties. The challenges of translating redox- and hypoxia-sensitive agents in vivo are also addressed.

Clinical EPR: unique opportunities and some challenges

Electron paramagnetic resonance (EPR) spectroscopy has been well established as a viable technique for measurement of free radicals and oxygen in biological systems, from in vitro cellular systems to in vivo small animal models of disease. However, the use of EPR in human subjects in the clinical setting, although attractive for a variety of important applications such as oxygen measurement, is challenged with several factors including the need for instrumentation customized for human subjects, probe, and regulatory constraints. This article describes the rationale and development of the first clinical EPR systems for two important clinical applications, namely, measurement of tissue oxygen (oximetry) and radiation dose (dosimetry) in humans. The clinical spectrometers operate at 1.2 GHz frequency and use surface-loop resonators capable of providing topical measurements up to 1 cm depth in tissues. Tissue pO2 measurements can be carried out noninvasively and repeatedly after placement of an oxygen-sensitive paramagnetic material (currently India ink) at the site of interest. Our EPR dosimetry system is capable of measuring radiation-induced free radicals in the tooth of irradiated human subjects to determine the exposure dose. These developments offer potential opportunities for clinical dosimetry and oximetry, which include guiding therapy for individual patients with tumors or vascular disease by monitoring of tissue oxygenation. Further work is in progress to translate this unique technology to routine clinical practice.

Imaging of superoxide generation in the dopaminergic area of the brain in Parkinson's disease, using mito-TEMPO

We report a new methodology for direct visualization of superoxide production in the dopaminergic area of the brain in Parkinson's disease, based on the redox cycle of mito-TEMPO, a blood-brain barrier-, cell-, and mitochondria-penetrating nitroxide derivative with superoxide scavenging properties and T1 magnetic resonance imaging (MRI) contrast. The experiments were conducted on healthy and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice. In healthy mice, the nitroxide-enhanced MRI signal was weak and short-lived (half-life ∼ 40 s; duration ∼ 80 s). The profile of the histograms indicated a high reducing activity of normal brain tissues against mito-TEMPO. In MPTP-treated mice, the nitroxide-enhanced MRI signal was strong and long-lived (half-life > 20 min; duration > 20 min), especially in the dopaminergic area of the brain. The histograms indicated a high oxidative activity in dopaminergic tissues of MPTP-treated mice. The results show directly, on intact mammals, that superoxide is a major inducer and/or mediator of neurodegenerative damage in Parkinson's disease. The high oxidative status of brain tissue in Parkinson's disease was also confirmed on isolated tissue specimens, using total reducing capacity assay and ROS/RNS assay.

Tissue redox activity as a hallmark of carcinogenesis: from early to terminal stages of cancer

PURPOSE

The study aimed to clarify the dynamics of tissue redox activity (TRA) in cancer progression and assess the importance of this parameter for therapeutic strategies.

EXPERIMENTAL DESIGN

The experiments were carried out on brain tissues of neuroblastoma-bearing, glioma-bearing, and healthy mice. TRA was visualized in vivo by nitroxide-enhanced MRI on anesthetized animals or in vitro by electron paramagnetic resonance spectroscopy on isolated tissue specimens. Two biochemical parameters were analyzed in parallel: tissue total antioxidant capacity (TTAC) and plasma levels of matrix metalloproteinases (MMP).

RESULTS

In the early stage of cancer, the brain tissues were characterized by a shorter-lived MRI signal than that from healthy brains (indicating a higher reducing activity for the nitroxide radical), which was accompanied by an enhancement of TTAC and MMP9 plasma levels. In the terminal stage of cancer, tissues in both hemispheres were characterized by a longer-lived MRI signal than in healthy brains (indicating a high-oxidative activity) that was accompanied by a decrease in TTAC and an increase in the MMP2/MMP9 plasma levels. Cancer progression also affected the redox potential of tissues distant from the primary tumor locus (liver and lung). Their oxidative status increased in both stages of cancer.

CONCLUSIONS

The study shows that tissue redox balance is very sensitive to the progression of cancer and can be used as a diagnostic marker of carcinogenesis. The study also suggests that the noncancerous tissues of a cancer-bearing organism are susceptible to oxidative damage and should be considered a therapeutic target.

Cell-penetrating nitroxides as molecular sensors for imaging of cancer in vivo, based on tissue redox activity

The present study shows that hydrophobic and cell-penetrating piperidine-type nitroxide radicals SLENU and TEMPOL, but not hydrophilic and partially penetrating or non-penetrating pyrrolidine-type nitroxides carbamoyl-PROXYL and carboxy-PROXYL, are appropriate contrast agents for magnetic resonance imaging (MRI) of cancer, based on its functionality - tissue redox activity. The experiments were conducted on anesthetized mice: healthy and neuroblastoma-bearing in a moderate stage of cancer development. The method is based on the nitroxide redox cycle, coupled with appearance or disappearance of the MRI signal. The half-life (τ(1/2)) of a nitroxide-enhanced MRI signal in the respective tissue was used as a marker to assess tissue redox activity to the nitroxide radical. In the case of SLENU and TEMPOL, there were large differences in the histograms between control and cancer-bearing mice. All tissues (cancer and non-cancer) of cancer-bearing organisms were characterized by a long-lived MRI signal (τ(1/2) > 14 min), indicating a high oxidative activity. The tissues of healthy organisms were characterized by a short-lived MRI signal (τ(1/2) = 1-3 min), indicating a high reducing activity. In the case of carbamoyl-PROXYL and carboxy-PROXYL, there was no difference in the histograms between control and cancer-bearing mice. The data show that the penetration of nitroxide in cells and tissues is obligatory for imaging of cancer, based on its redox activity. The principle of the method is applicable also to biopsy specimens, using MRI or EPR spectroscopy. We provide direct evidence that the nitroxide redox cycle could be used as a sensing platform for functional imaging of different pathologies, based on changes in cellular and tissue redox activity, as in the case of cancer.

Tissue redox activity as a sensing platform for imaging of cancer based on nitroxide redox cycle

The experience in free radical biology and medicine shows the crucial role of redox signalling in carcinogenesis. The cells and tissues of healthy mammals are characterised by a low level of reactive oxygen species (ROS) and some constant (reference) level of reducing equivalents. Increasing of ROS above the critical level provokes genomic instability. The present study describes universal methodology for direct imaging of tissue redox activity in carcinogenesis, which allows a differentiation of cancer development from normal condition. The experiments were conducted on: neuroblastoma-bearing mice; colon cancer-bearing mice; and healthy mice. The tissue redox activity was visualised in vivo by nitroxide-enhanced magnetic resonance imaging (MRI) on anesthetised animals. The method is based on nitroxide redox cycle, coupled with appearance/disappearance of MRI signal. The half-life (τ1/2) of nitroxide-enhanced MRI signal in the respective tissue was used as a diagnostic marker. The study provides direct evidence that healthy and cancer-bearing mammalian tissues are characterised by different redox activities - a basis for cancer diagnosis. The tissues (cancer and 'normal') of cancer-bearing mammals were characterised by a long-lived MRI signal (τ1/2>14 min), indicating a high oxidative activity. The tissues of healthy organism were characterised by a short-lived MRI signal (τ1/2=1-3 min), indicating a high reducing activity. The study shows that tissue redox activity is a sensing platform for imaging of cancer using nitroxide-enhanced MRI. It also suggests that 'normal' tissues of cancer-bearing organism are susceptible to oxidative damage.

Electron paramagnetic resonance oxygen imaging in vivo

This review covers the last 15 years of the development of EPR in vivo oxygen imaging. During this time, a number of major technological and methodological advances have taken place. Narrow line width, long relaxation time, and non-toxic triaryl methyl radicals were introduced in the late 1990s. These not only improved continuous wave (CW) imaging, but also enabled the application of pulse EPR imaging to animals. Recent developments in pulse technology have brought an order of magnitude increase in image acquisition speed, enhancement of sensitivity, and considerable improvement in the precision and accuracy of oxygen measurements. Consequently, pulse methods take up a significant part of this review.