Reactive oxygen species mediate hepatotoxicity induced by the Hsp90 inhibitor geldanamycin and its analogs

Geldanamycin (GM), a benzoquinone ansamycin antibiotic, is a natural product inhibitor of Hsp90 with potent and broad anti-cancer properties. Because of its adverse effects on liver, its less toxic derivatives 17-(allylamino)-17-demethoxygeldanamycin (17-AAG) and 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG) are currently being evaluated for the treatment of cancer. Previously, it has been demonstrated that the redox cycling of GM by NADPH-cytochrome P450 reductase leads to the formation of the GM semiquinone and superoxide radicals, the latter being identified using spin-trapping. We hypothesized that the different hepatotoxicity induced by GM, 17-AAG and 17-DMAG reflects the redox active properties of the quinone moiety and possibly the extent of superoxide formation, which may stimulate cellular oxidative injury. Our data demonstrate that superoxide can be efficiently trapped during the reduction of GM, 17-AAG and 17-DMAG by NADPH-cytochrome P450 reductase, and that superoxide formation rate followed the order 17-DMAG > 17-AAG > GM. In the absence of superoxide scavengers, the rate of NADPH oxidation followed the order 17-DMAG > GM > 17-AAG. The half-wave one-electron reduction potentials (E(1/2)) of GM, 17-AAG and 17-DMAG in DMSO have been determined to be -0.37, -0.13 and -0.015V (vs. Ag/AgCl), respectively. If the same order of E(1/2) follows in neutral aqueous media, thermodynamic considerations imply that 17-DMAG is more readily reduced by the P450 reductase as well as by superoxide. The order of the drug cytotoxicity toward rat primary hepatocytes, as determined by their effect on cell viability and on intracellular oxidant level, was opposite to the order of E(1/2) of the respective quinone/semiquinone couples. These results suggest that hepatotoxicity exhibited by the Hsp90 inhibitors belonging to benzoquinone ansamycins could be attributed to superoxide. The apparent discrepancy between the order of toxicity and the orders of superoxide formation rate, which is correlated with E(1/2), is discussed.

Heterogeneity of regional redox status and relation of the redox status to oxygenation in a tumor model, evaluated using electron paramagnetic resonance imaging

It is widely accepted that redox status, along with the partial pressure of oxygen (pO(2)), determines the efficacy of some therapeutic methods applied to treat tumors, including radiation. Redox status, evaluated by the reduction of a nitroxyl probe, was reportedly heterogeneous in a mouse tumor model. However, neither variation of heterogeneity of the redox status among mice nor the relation of the redox status to pO(2) in tumors has been characterized sufficiently. In this study, the regional reduction status in a mouse radiation-induced fibrosarcoma tumor model was evaluated using sequential three-dimensional electron paramagnetic resonance (EPR) imaging after i.v. injection of a tissue-permeable nitroxyl probe, HM-PROXYL. The regional decay of HM-PROXYL signal obeyed first-order kinetics, and the amplitude of the reduction rate and extent of its heterogeneity in a tumor varied among six mice. The tissue pO(2) was measured using EPR oximetry with lithium phthalocyanine (LiPc) microcrystals implanted within the tumor. The location of LiPc was determined with EPR imaging. A sequential image was obtained following the injection of HM-PROXYL, even after LiPc implantation, by choosing an HM-PROXYL signal peak which does not overlap with the signal of LiPc. The relationship between pO(2) and the reduction rate at the region of pO(2) measurement was found to be low (r = 0.357) in 13 tumor-bearing mice, indicating that the extent of oxygenation does not necessarily affect the redox status under air-breathing conditions. The results strongly indicate the necessity of measurements of both redox status and oxygenation in every tumor to characterize tumor physiology.

Estimation of tumor microvessel density by MRI using a blood pool contrast agent

Recognition of importance of angiogenesis to tumor growth, metastasis, and treatment outcome has led to efforts to develop non-invasive methods for longitudinal monitoring of tumor microvasculature. We describe a steady-state MRI technique to determine absolute blood volume (BV) as a marker of microvascular density with improved spatial and temporal resolution using an ultra small super paramagnetic iron oxide (USPIO). A noise reduction scheme for BV imaging was also proposed based on weighting factors derived by pre-contrast signal level as an adjustable additive constant. Gradient echo sequence was used for BV imaging with MRI at 7T. Optimal imaging conditions (USPIO dose and echo time) were determined by USPIO dose-dependent studies ex vivo and in vivo. Improved analysis strategies were at first applied for cerebral BV estimation in mice, which were found in good agreement with the literature values. These methods were then used to determine tumor BV in mice. The optimal concentration of USPIO for BV estimates was found to range from 3.6 to 4.48 mM (estimated as Fe concentration) in ex vivo experiments corresponding to an in vivo dosage of 215-287 micromol/kg body weight, whereas a USPIO dose of 287 micromol/kg leads to higher cerebral BV estimate in vivo than the reported values. Application of the BV imaging method to evaluation of anti-angiogenic effect of Sunitinib in squamous cell carcinoma (SCC) tumor bearing mice revealed approximately 46% reduction in tumor BV 4 days after start of Sunitinib treatment. The results show that the MRI approach using USPIO yields high-resolution absolute BV images and the method can be conveniently applied to monitor longitudinal tumor microvessel density changes as a function of growth or in response to treatment.

The inhibitors of histone deacetylase suberoylanilide hydroxamate and trichostatin A release nitric oxide upon oxidation

Suberoylanilide hydroxamic acid (SAHA, vorinostat, Zolinza) is the lead compound of a new class of histone deacetylase (HDAC) inhibitors used as anticancer drugs that have been shown to affect multiple proteins associated with gene expression, cell proliferation, and migration. Studies have also demonstrated the essential role of the hydroxamate moiety of SAHA in HDAC inhibition. The ability of SAHA and its structural analog trichostatin A (TSA) to generate NO upon oxidation was tested directly, by spin trapping of NO using electron paramagnetic resonance spectroscopy, and also indirectly, via the determination of nitrite using the Griess assay. H2O2/metmyoglobin was used to oxidize SAHA and TSA. These studies demonstrate, for the first time, the release of NO from SAHA and its structural analog TSA. We tested the protective effects of SAHA, TSA, and valproic acid (VPA) in mammalian Chinese hamster V79 cells exposed to a bolus of H2O2 for 1 h and monitored the clonogenic cell survival. Both SAHA and TSA afforded significant cytoprotection when co-incubated with H2O2, whereas VPA was ineffective. These studies provide evidence for the release of NO by hydroxamate-containing HDAC inhibitors and their antioxidant effects. Such roles may be an added advantage of this class of HDAC agents used for epigenetic therapies in cancer.

Oxygen sensitivity and biocompatibility of an implantable paramagnetic probe for repeated measurements of tissue oxygenation

The use of oxygen-sensing water-insoluble paramagnetic probes, such as lithium octa-n-butoxynaphthalocyanine (LiNc-BuO), enables repeated measurements of pO(2) from the same location in tissue by electron paramagnetic resonance (EPR) spectroscopy. In order to facilitate direct in vivo application, and hence eventual clinical applicability, of LiNc-BuO, we encapsulated LiNc-BuO microcrystals in polydimethylsiloxane (PDMS), an oxygen-permeable and bioinert polymer, and developed an implantable chip. In vitro evaluation of the chip, performed under conditions of sterilization, high-energy irradiation, and exposure to cultured cells, revealed that it is biostable and biocompatible. Implantation of the chip in the gastrocnemius muscle tissue of mice showed that it is capable of repeated and real-time measurements of tissue oxygenation for an extended period. Functional evaluation using a murine tumor model established the suitability and applicability of the chip for monitoring tumor oxygenation. This study establishes PDMS-encapsulated LiNc-BuO as a promising choice of probe for clinical EPR oximetry.

Detection of free radical reactions in an aqueous sample induced by low linear-energy-transfer irradiation

Quantitative detection of free radical reactions induced by low linear-energy-transfer (LET) irradiation in an aqueous solution was attempted using nitroxyl radicals. The stability and reactivity of reaction mixtures containing a nitroxyl radical and a hydrogen donor, i.e., glutathione (GSH), nicotinamide adenine dinucleotide (NADH), or nicotinamide adenine dinucleotide phosphate (NADPH), were tested. X-band electron paramagnetic resonance (EPR) measurements of several reaction mixtures were performed to find a suitable preparation to quantitatively detect free radical reactions produced by low LET irradiation. The EPR signal intensity of nitroxyl radicals was decreased by low LET irradiation when a hydrogen donor coexisted in the reaction mixture. The combination of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (4-hydroxy-TEMPO, known as TEMPOL) and GSH was most preferable among other preparations tested in this paper, because of the sensitivity and irreversible reaction. The extent of the free radical reaction, i.e., formation of reactive oxygen species, in the reaction mixture depended on the radiation energy when an identical dose was given.

Pulsed EPR imaging of nitroxides in mice

Nitroxides, unlike trityl radicals, have shorter T(2)s which until now were not detectable in vivo by a time-domain pulsed Electron Paramagnetic Resonance (EPR) spectrometer at 300 MHz since their phase memory times were shorter than the spectrometer recovery times. In the current version of the time-domain EPR spectrometer with improved spectrometer recovery times, the feasibility of detecting signals from nitroxide radicals was tested. Among the nitroxides evaluated, deuterated (15)N-Tempone ((15)N-PDT) was found to have the longest T(2). The signal intensity profile as a function of concentration of these agents was evaluated and a biphasic behavior was observed; beyond a nitroxide concentration of 1.5mM, signal intensity was found to decrease as a result of self-broadening. Imaging experiments were carried out with (15)N-PDT in solutions equilibrated with 0%, 5%, 10%, and 21% oxygen using the single point imaging (SPI) modality in EPR. The image intensity in these tubes was found to depend on the oxygen concentration which in turn influences the T(2) of (15)N-PDT. In vivo experiments were demonstrated with (15)N-PDT in anesthetized mice where the distribution and metabolism of (15)N-PDT could be monitored. This study, for the first time shows the capability to image a cell-permeable nitroxide in mice using pulsed EPR in the SPI modality.

Assessment of tissue redox status using metabolic responsive contrast agents and magnetic resonance imaging

Regulation of tissue redox status is important to maintain normal physiological conditions in the living body. Disruption of redox homoeostasis may lead to oxidative stress and can induce many pathological conditions such as cancer, neurological disorders and ageing. Therefore, imaging of tissue redox status could have clinical applications. Redox imaging employing magnetic resonance imaging (MRI) with nitroxides as cell-permeable redox-sensitive contrast agents has been used for non-invasive monitoring of tissue redox status in animal models. The redox imaging applications of nitroxide electron paramagnetic resonance imaging (EPRI) and MRI are reviewed here, with a focus on application of tumour redox status monitoring. While particular emphasis has been placed on differences in the redox status in tumours compared to selected normal tissues, the technique possesses the potential to have broad applications to the study of other disease states, inflammatory processes and other circumstances where oxidative stress is implicated.

DANCING WITH THE ELECTRONS: TIME-DOMAIN AND CW IN VIVO EPR IMAGING

The progress in the development of imaging the distribution of unpaired electrons in living systems and the functional and the potential diagnostic dimensions of such an imaging process, using Electron Paramagnetic Resonance Imaging (EPRI), is traced from its origins with emphasis on our own work. The importance of EPR imaging stems from the fact that many paramagnetic probes show oxygen dependent spectral broadening. Assessment of in vivo oxygen concentration is an important factor in radiation oncology in treatment-planning and monitoring treatment-outcome. The emergence of narrow-line trairylmethyl based, bio-compatible spin probes has enabled the development of radiofrequency time-domain EPRI. Spectral information in time-domain EPRI can be achieved by generating a time sequence of T(2)* or T(2) weighted images. Progress in CW imaging has led to the use of rotating gradients, more recently rapid scan with direct detection, and a combination of all the three. Very low field MRI employing Dynamic Nuclear polarization (Overhauser effect) is also employed for monitoring tumor hypoxia, and re-oxygenation in vivo. We have also been working on the co-registration of MRI and time domain EPRI on mouse tumor models at 300 MHz using a specially designed resonator assembly. The mapping of the unpaired electron distribution and unraveling the spectral characteristics by using magnetic resonance in presence of stationary and rotating gradients in indeed 'dancing with the (unpaired) electrons', metaphorically speaking.

Pharmacologic doses of ascorbate act as a prooxidant and decrease growth of aggressive tumor xenografts in mice

Ascorbic acid is an essential nutrient commonly regarded as an antioxidant. In this study, we showed that ascorbate at pharmacologic concentrations was a prooxidant, generating hydrogen-peroxide-dependent cytotoxicity toward a variety of cancer cells in vitro without adversely affecting normal cells. To test this action in vivo, normal oral tight control was bypassed by parenteral ascorbate administration. Real-time microdialysis sampling in mice bearing glioblastoma xenografts showed that a single pharmacologic dose of ascorbate produced sustained ascorbate radical and hydrogen peroxide formation selectively within interstitial fluids of tumors but not in blood. Moreover, a regimen of daily pharmacologic ascorbate treatment significantly decreased growth rates of ovarian (P < 0.005), pancreatic (P < 0.05), and glioblastoma (P < 0.001) tumors established in mice. Similar pharmacologic concentrations were readily achieved in humans given ascorbate intravenously. These data suggest that ascorbate as a prodrug may have benefits in cancers with poor prognosis and limited therapeutic options.

Low-field paramagnetic resonance imaging of tumor oxygenation and glycolytic activity in mice

A priori knowledge of spatial and temporal changes in partial pressure of oxygen (oxygenation; pO(2)) in solid tumors, a key prognostic factor in cancer treatment outcome, could greatly improve treatment planning in radiotherapy and chemotherapy. Pulsed electron paramagnetic resonance imaging (EPRI) provides quantitative 3D maps of tissue pO(2) in living objects. In this study, we implemented an EPRI set-up that could acquire pO(2) maps in almost real time for 2D and in minutes for 3D. We also designed a combined EPRI and MRI system that enabled generation of pO(2) maps with anatomic guidance. Using EPRI and an air/carbogen (95% O(2) plus 5% CO(2)) breathing cycle, we visualized perfusion-limited hypoxia in murine tumors. The relationship between tumor blood perfusion and pO(2) status was examined, and it was found that significant hypoxia existed even in regions that exhibited blood flow. In addition, high levels of lactate were identified even in normoxic tumor regions, suggesting the predominance of aerobic glycolysis in murine tumors. This report presents a rapid, noninvasive method to obtain quantitative maps of pO(2) in tumors, reported with anatomy, with precision. In addition, this method may also be useful for studying the relationship between pO(2) status and tumor-specific phenotypes such as aerobic glycolysis.

Dynamic monitoring of localized tumor oxygenation changes using RF pulsed electron paramagnetic resonance in conscious mice

Oxygenation status is a key determinant in both tumor growth and responses to therapeutic interventions. The oxygen partial pressure (pO2) was assessed using a novel pulsed electron paramagnetic resonance (EPR) spectroscopy at 750 MHz. Crystals of lithium phthalocyanine (LiPc) implanted into either squamous cell carcinoma (SCC) tumor or femoral muscle on opposing legs of mice were tested by pulsed EPR. The results showed pO2 of SCC tumor was 2.7 +/- 0.4 mmHg, while in the femoral muscle it was 6.1 +/- 0.9 mmHg. A major advantage of pulsed EPR oximetry over conventional continuous-wave (CW) EPR oximetry is the lack of influence from subject motion, while avoiding artifacts associated with modulation or power saturation. Resonators in pulsed EPR are overcoupled to minimize recovery time. This makes changes in coupling associated with object motion minimal without influencing spectral quality. Consequently, pulsed EPR oximetry enables approximately a temporal resolution of approximately one second in pO2 monitoring in conscious subjects, avoiding significant influence of anesthetics on the physiology being studied. The pO2 in SCC tumor and muscle was found to be higher without anesthesia (3.9 +/- 0.5 mmHg for tumor, 8.8 +/- 1.2 mmHg for muscle). These results support the advantage of pulsed EPR in examining pO2 in conscious animals with LiPc chronically implanted in predetermined regions.

Brain redox imaging using blood-brain barrier-permeable nitroxide MRI contrast agent

Reactive oxygen species (ROS) and compromised antioxidant defense may contribute to brain disorders such as stroke, amyotrophic lateral sclerosis, etc. Nitroxides are redox-sensitive paramagnetic contrast agents and antioxidants. The ability of a blood-brain barrier (BBB)-permeable nitroxide, methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (MC-P), as a magnetic resonance-imaging (MRI) contrast agent for brain tissue redox imaging was tested. MC-P relaxation in rodent brain was quantified by MRI using a fast Look-Locker T(1)-mapping sequence. In the cerebral cortex and thalamus, the MRI signal intensity increased up to 50% after MC-P injection, but increased only by 2.7% when a BBB-impermeable nitroxide, 3CxP (3-carboxy-2,2,5,5,5-tetramethylpyrrolidine-1-oxyl) was used. The maximum concentrations in the thalamus and cerebral cortex after MC-P injection were calculated to be 1.9+/-0.35 and 3.0+/-0.50 mmol/L, respectively. These values were consistent with the ex vivo data of brain tissue and blood concentration obtained by electron paramagnetic resonance (EPR) spectroscopy. Also, reduction rates of MC-P were significantly decreased after reperfusion following transient MCAO (middle cerebral artery occlusion), a condition associated with changes in redox status resulting from oxidative damage. These results show the use of BBB-permeable nitroxides as MRI contrast agents and antioxidants to evaluate the role of ROS in neurologic diseases.

A novel programmable pulse generator with nanosecond resolution for pulsed electron paramagnetic resonance applications

A pulse programmer with nanosecond time resolution needed for time-domain electron paramagnetic resonance (EPR) spectroscopic applications is described. This unit uses commercially available timing and input-output port modules and control software developed in our laboratory. The pulse programmer is operated through a personal computer front panel graphic user interface (GUI) inputs to control pulse widths, delays, and the associated acquisition trigger timings. Based on these parameters, all other associated gate and trigger timings are internally generated automatically without the need to enter them explicitly. The excitation pulse widths were of nanosecond resolution while all other gate pulses can be incremented in steps of 20 ns without compromising spectrometer performance. In the current configuration, the pulse programmer permits generation of a single pulse or multiple pulse sequences for EPR imaging with minimal data entry via the front panel GUI.

Evaluation of sub-microsecond recovery resonators for in vivo electron paramagnetic resonance imaging

Time-domain (TD) electron paramagnetic resonance (EPR) imaging at 300MHz for in vivo applications requires resonators with recovery times less than 1 micros after pulsed excitation to reliably capture the rapidly decaying free induction decay (FID). In this study, we tested the suitability of the Litz foil coil resonator (LCR), commonly used in MRI, for in vivo EPR/EPRI applications in the TD mode and compared with parallel coil resonator (PCR). In TD mode, the sensitivity of LCR was lower than that of the PCR. However, in continuous wave (CW) mode, the LCR showed better sensitivity. The RF homogeneity was similar in both the resonators. The axis of the RF magnetic field is transverse to the cylindrical axis of the LCR, making the resonator and the magnet co-axial. Therefore, the loading of animals, and placing of the anesthesia nose cone and temperature monitors was more convenient in the LCR compared to the PCR whose axis is perpendicular to the magnet axis.

Monitoring redox-sensitive paramagnetic contrast agent by EPRI, OMRI and MRI

A comparative study of tissue redox-status imaging using commonly used redox sensitive nitroxides has been carried out using electron paramagnetic resonance imaging (EPRI), Overhauser magnetic resonance imaging (OMRI) and conventional T(1)-weighted magnetic resonance imaging, MRI. Imaging studies using phantoms of different nitroxides at different concentration levels showed that EPRI and OMRI sensitivities were found to be linearly dependent on line width of nitroxides up to 2 mM, and the enhancement in MRI intensity was linear up to 5 mM. The sensitivity and resolution of EPRI and OMRI images depended significantly on the line width of the nitroxides whereas the MRI images were almost independent of EPR line width. Reduction of the paramagnetic 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (3CP) by ascorbic acid (AsA) to the diamagnetic by hydroxylamine was monitored from a sequence of temporal images, acquired using the three imaging modalities. The decay rates determined by all the three modalities were found to be similar. However the results suggest that T(1)-weighted MRI can monitor the redox status, in addition to providing detailed anatomical structure in a short time. Therefore, a combination of MRI with nitroxides as metabolically responsive contrast agents can be a useful technique for the in vivo imaging probing tissue redox status.

Blocking thrombospondin-1/CD47 signaling alleviates deleterious effects of aging on tissue responses to ischemia

OBJECTIVE

Decreased blood flow secondary to peripheral vascular disease underlies a significant number of chronic diseases that account for the majority of morbidity and mortality among the elderly. Blood vessel diameter and blood flow are limited by the matricellular protein thrombospondin-1 (TSP1) through its ability to block responses to the endogenous vasodilator nitric oxide (NO). In this study we investigate the role TSP1 plays in regulating blood flow in the presence of advanced age and atherosclerotic vascular disease.

METHODS AND RESULTS

Mice lacking TSP1 or CD47 show minimal loss of their resistance to ischemic injury with age and increased preservation of tissue perfusion immediately after injury. Treatment of WT and apolipoprotein E-null mice using therapeutic agents that decrease CD47 or enhance NO levels reverses the deleterious effects of age- and diet-induced vasculopathy and results in significantly increased tissue survival in models of ischemia.

CONCLUSIONS

With increasing age and diet-induced atherosclerotic vascular disease, TSP1 and its receptor CD47 become more limiting for blood flow and tissue survival after ischemic injury. Drugs that limit TSP1/CD47 regulation of blood flow could improve outcomes from surgical interventions in the elderly and ameliorate vascular complications attendant to aging.

Therapeutic and clinical applications of nitroxide compounds

Nitroxide compounds have been used for many years as biophysical tools, but only during the past 15-20 years have the many interesting biochemical interactions been discovered and harnessed for therapeutic applications. By modifying oxidative stress and altering the redox status of tissues, nitroxides have the ability to interact with and alter many metabolic processes. This interaction can be exploited for therapeutic and research use, including protection against ionizing radiation, as probes in functional magnetic resonance imaging, cancer prevention and treatment, control of hypertension and weight, and protection from damage resulting from ischemia/reperfusion injury. Although much remains to be done, many applications have been well studied, and some are presently being tested in clinical trials. The therapeutic and research uses of nitroxides are reviewed here, with a focus on the progress from initial development to modern, state-of-the art trials.

Continuous wave EPR oximetric imaging at 300 MHz using radiofrequency power saturation effects

A novel continuous wave (CW), radiofrequency (RF), electron paramagnetic resonance (EPR) oximetric imaging technique is proposed, based on the influence of oxygen concentration on the RF power saturation of the EPR resonance. A linear relationship is demonstrated between the partial oxygen pressure (pO(2)) and the normalized signal intensity (I(N)), defined as, I(N) = (I(HP) - I(LP))/I(LP), where I(LP) and I(HP) refer to signal intensities at low (P(L)) and high (P(H)) RF power levels, respectively. A formula for the determination of pO(2), derived on the basis of the experimental results, reliably estimated various oxygen concentrations in a five-tube phantom. This new technique was time-efficient and also avoided the missing angle problem associated with conventional spectral-spatial CW EPR oximetric imaging. In vivo power saturation oximetric imaging in a tumor bearing mouse clearly depicted the hypoxic foci within the tumor.

Differential radiation protection of salivary glands versus tumor by Tempol with accompanying tissue assessment of Tempol by magnetic resonance imaging

PURPOSE

The nitroxide free radical, Tempol, was evaluated for potential differential radiation protection of salivary glands and tumor using fractionated radiation. Mechanistic information was explored by monitoring the presence and bioreduction of Tempol in both tissues noninvasively by magnetic resonance imaging (MRI).

EXPERIMENTAL DESIGN

Female C3H mice were immobilized using custom-made Lucite jigs for localized irradiation (five daily fractions) either to the oral cavity or tumor-bearing leg. Tempol (275 mg/kg) was administered (i.p.) 10 min before each radiation fraction. Salivary gland damage was assessed 8 weeks after radiation by measuring pilocarpine-mediated saliva output. Tumor growth was assessed by standard radiation regrowth methods. Dynamic T1-weighted magnetic resonance scans were acquired before and after Tempol injection using a 4.7T animal MRI instrument.

RESULTS

Tempol treatment was found to protect salivary glands significantly against radiation damage (approximately 60% improvement); whereas no tumor protection was observed. Intracellular reduction of Tempol to the nonradioprotective hydroxylamine as assessed by MRI was 2-fold faster in tumor compared with salivary glands or muscle.

CONCLUSIONS

Tempol provided salivary gland radioprotection and did not protect tumor, consistent with the hypothesis that differential radioprotection by Tempol resides in faster reduction to the nonradioprotective hydroxylamine in tumor compared with normal tissues. The unique paramagnetic properties of Tempol afforded noninvasive MRI monitoring of dynamic changes of Tempol levels in tissue to support the finding. These data support further development and consideration of Tempol for human clinical trials as a selective protector against radiation-induced salivary gland damage.

Spatially resolved biologic information from in vivo EPRI, OMRI, and MRI

EPR spectroscopy can give biologically important information, such as tissue redox status, pO2, pH, and microviscosity, based on variation of EPR spectral characteristics (i.e., intensity, linewidth, hyperfine splitting, and spectral shape of free radical probes. EPR imaging (EPRI) can obtain 1D-3D spatial distribution of such spectral components using several combinations of magnetic field gradients. Overhauser enhanced MRI (OMRI) is a double-resonance technique of electron and nuclear spins. Because the Overhauser enhancement depends on transverse relaxation rate of the electron spin, OMRI can provide pO2 information indirectly, along with a high-resolution MR image. MRI can also indirectly detect paramagnetic behaviors of free radical contrast agents. Imaging techniques and applications relating to paramagnetic species (i.e., EPRI, OMRI, and MRI) have the potential to obtain maximally 5D information (i.e., 3D spatial + 1D spectral + 1D temporal dimensions, theoretically). To obtain suitable dimensionality, several factors, such as the EPR spectral information, spatial resolution, temporal resolution, will have to be taken into account. For this review, the EPRI, OMRI, and MRI applications for the study biological systems were evaluated for researchers to apply the method of choice and the mode of measurements to specific experimental systems.

The chemistry and biology of nitroxide compounds

Cyclic nitroxides are a diverse group range of stable free radicals that have unique antioxidant properties. Because of their ability to interact with free radicals, they have been used for many years as biophysical tools. During the past 15-20 years, however, many interesting biochemical interactions have been discovered and harnessed for therapeutic applications. Biologically relevant effects of nitroxides have been described, including their ability to degrade superoxide and peroxide, inhibit Fenton reactions, and undergo radical-radical recombination. Cellular studies defined the activity of nitroxides in vitro. By modifying oxidative stress and altering the redox status of tissues, nitroxides have been found to interact with and alter many metabolic processes. These interactions can be exploited for therapeutic and research use, including protection against ionizing radiation, as probes in functional magnetic resonance imaging, cancer prevention and treatment, control of hypertension and weight, and protection from damage resulting from ischemia/reperfusion injury. Although much remains to be done, many applications have been well studied and some are currently being tested in clinical trials. The therapeutic and research uses of nitroxide compounds are reviewed here with a focus on the progress from initial development to modern trials.

A new strategy for fast radiofrequency CW EPR imaging: direct detection with rapid scan and rotating gradients

Rapid field scan on the order of T/s using high frequency sinusoidal or triangular sweep fields superimposed on the main Zeeman field, was used for direct detection of signals without low-frequency field modulation. Simultaneous application of space-encoding rotating field gradients have been employed to perform fast CW EPR imaging using direct detection that could, in principle, approach the speed of pulsed FT EPR imaging. The method takes advantage of the well-known rapid-scan strategy in CW NMR and EPR that allows arbitrarily fast field sweep and the simultaneous application of spinning gradients that allows fast spatial encoding. This leads to fast functional EPR imaging and, depending on the spin concentration, spectrometer sensitivity and detection band width, can provide improved temporal resolution that is important to interrogate dynamics of spin perfusion, pharmacokinetics, spectral spatial imaging, dynamic oxymetry, etc.

Strategies for improved temporal and spectral resolution in in vivo oximetric imaging using time-domain EPR

A radiofrequency (RF) time-domain electron paramagnetic resonance (EPR) instrument operating at 300, 600, and 750 MHz was used to image tumor hypoxia with high spatial and temporal resolution. A high-speed signal-averaging Peripheral Component Interconnect (PCI) board with flexibility in the input signal level and the number of digitized samples per free induction decay (FID) was incorporated into the receive arm of the spectrometer. This enabled effective and fast averaging of FIDs. Modification of the phase-encoding protocol, and replacement of the General Purpose Interface Bus (GPIB)-based handshake with a PCI-based D/A board for direct control of the gradient amplifier decreased the gradient settling and communication overhead times by nearly two orders of magnitude. Cyclically-ordered phase sequence (CYCLOPS) phase cycling was implemented to correct for pulse imperfections and cancel out unwanted constant signals. These upgrades considerably enhanced the performance of the imager in terms of image collection time, sensitivity, and temporal resolution. We demonstrated this by collecting a large number of 2D images successively and rapidly. The results show that it is feasible to achieve accurate, 2D pO(2) maps of tumor hypoxia with 1-mm(2) resolution and minimal artifacts using a set of multigradient images within an acceptable measuring time of about 3 s, and 3D maps can be obtained in less than 1 min.

Ascorbate in pharmacologic concentrations selectively generates ascorbate radical and hydrogen peroxide in extracellular fluid in vivo

Ascorbate (ascorbic acid, vitamin C), in pharmacologic concentrations easily achieved in humans by i.v. administration, selectively kills some cancer cells but not normal cells. We proposed that pharmacologic ascorbate is a prodrug for preferential steady-state formation of ascorbate radical (Asc(*-)) and H(2)O(2) in the extracellular space compared with blood. Here we test this hypothesis in vivo. Rats were administered parenteral (i.v. or i.p.) or oral ascorbate in typical human pharmacologic doses ( approximately 0.25-0.5 mg per gram of body weight). After i.v. injection, ascorbate baseline concentrations of 50-100 microM in blood and extracellular fluid increased to peaks of >8 mM. After i.p. injection, peaks approached 3 mM in both fluids. By gavage, the same doses produced ascorbate concentrations of <150 microM in both fluids. In blood, Asc(*-) concentrations measured by EPR were undetectable with oral administration and always <50 nM with parenteral administration, even when corresponding ascorbate concentrations were >8 mM. After parenteral dosing, Asc(*-) concentrations in extracellular fluid were 4- to 12-fold higher than those in blood, were as high as 250 nM, and were a function of ascorbate concentrations. By using the synthesized probe peroxyxanthone, H(2)O(2) in extracellular fluid was detected only after parenteral administration of ascorbate and when Asc(*-) concentrations in extracellular fluid exceeded 100 nM. The data show that pharmacologic ascorbate is a prodrug for preferential steady-state formation of Asc(*-) and H(2)O(2) in the extracellular space but not blood. These data provide a foundation for pursuing pharmacologic ascorbate as a prooxidant therapeutic agent in cancer and infections.

Dual activity of nitroxides as pro- and antioxidants: catalysis of copper-mediated DNA breakage and H2O2 dismutation

Nitroxide antioxidants can be reduced to hydroxylamines or oxidized to oxoammonium cations. Consequently, nitroxides can modify oxidative damage acting as reducing and/or as oxidizing agents, and in many cases the nitroxides are continuously recycled. They provide protection against oxidative stress via various mechanisms including SOD-mimic activity and detoxification of carbon-, oxygen-, and nitrogen-centered radicals, as well as oxidation of reduced transition metals. In contrast to the common concept, according to which the nitroxides' protective effect takes place via inhibition of the Fenton reaction, there are observations suggesting the opposite. In the present investigation, DNA breakage catalyzed by copper served as an experimental model for studying the anti- and pro-oxidative activity of nitroxides. Nitroxides provided protection in the presence of GSH, which is known to facilitate metal-catalyzed DNA damage. In the absence of a reductant, nitroxides enhanced DNA breakage under aerobic conditions with or without added H(2)O(2) and facilitated H(2)O(2) depletion. The rates of nitroxide-catalyzed DNA breakage and H(2)O(2) depletion increased as the concentrations of copper, H(2)O(2), and nitroxide increased. Although the catalytic activity of nitroxides is low, it is sufficient to induce DNA breakage. The efficacy of DNA breakage by the tested piperidine nitroxides correlated with the nitroxide-induced depletion of H(2)O(2) with the exception of the pyrrolidine nitroxide 3-carbamoylproxyl. The results suggest that the nitroxide and the copper are continuously recycled while catalyzing DNA breakage and depletion of H(2)O(2), which serves both as a source of reducing equivalents and as the electron sink.

The influence of tumor oxygenation on hypoxia imaging in murine squamous cell carcinoma using [64Cu]Cu-ATSM or [18F]Fluoromisonidazole positron emission tomography

[64Cu]Cu(II)-ATSM (64Cu-ATSM) and [18F]-Fluoromisonidazole (18F-FMiso) tumor binding as assessed by positron emisson topography (PET) was used to determine the responsiveness of each probe to modulation in tumor oxygenation levels in the SCCVII tumor model. Animals bearing the SCCVII tumor were injected with 64Cu-ATSM or 18F-FMiso followed by dynamic small animal PET imaging. Animals were imaged with both agents using different inspired oxygen mixtures (air, 10% oxygen, carbogen) which modulated tumor hypoxia as independently assessed by the hypoxia marker pimonidazole. The extent of hypoxia in the SCCVII tumor as monitored by the pimonidazole hypoxia marker was found to be in the following order: 10% oxygen>air>carbogen. Tumor uptake of 64Cu-ATSM could not be changed if the tumor was oxygenated using carbogen inhalation 90 min post-injection suggesting irreversible cellular uptake of the 64Cu-ATSM complex. A small but significant paradoxical increase in 64Cu-ATSM tumor uptake was observed for animals breathing air or carbogen compared to 10% oxygen. There was a positive trend toward 18F-FMiso tumor uptake as a function of changing hypoxia levels in agreement with the pimonidazole data. 64Cu-ATSM tumor uptake was unable to predictably detect changes in varying amounts of hypoxia when oxygenation levels in SCCVII tumors were modulated. 18F-FMiso tumor uptake was more responsive to changing levels of hypoxia. While the mechanism of nitroimidazole binding to hypoxic cells has been extensively studied, the avid binding of Cu-ATSM to tumors may involve other mechanisms independent of hypoxia that warrant further study.

A neural network approach for image reconstruction in electron magnetic resonance tomography

An object-oriented, artificial neural network (ANN) based, application system for reconstruction of two-dimensional spatial images in electron magnetic resonance (EMR) tomography is presented. The standard back propagation algorithm is utilized to train a three-layer sigmoidal feed-forward, supervised, ANN to perform the image reconstruction. The network learns the relationship between the 'ideal' images that are reconstructed using filtered back projection (FBP) technique and the corresponding projection data (sinograms). The input layer of the network is provided with a training set that contains projection data from various phantoms as well as in vivo objects, acquired from an EMR imager. Twenty five different network configurations are investigated to test the ability of the generalization of the network. The trained ANN then reconstructs two-dimensional temporal spatial images that present the distribution of free radicals in biological systems. Image reconstruction by the trained neural network shows better time complexity than the conventional iterative reconstruction algorithms such as multiplicative algebraic reconstruction technique (MART). The network is further explored for image reconstruction from 'noisy' EMR data and the results show better performance than the FBP method. The network is also tested for its ability to reconstruct from limited-angle EMR data set.

MR lymphangiography using dendrimer-based contrast agents: A comparison at 1.5T and 3.0T

Most macromolecular contrast agents (CAs) show lower r1 and higher r2 relaxivities at 3.0T than at 1.5T. MR lymphangiography in mice using a macromolecular G6 dendrimer-based CA was serially performed and compared at both 1.5T and 3.0T. The r1 and r2 relaxivities of the G6 CA were 25 and 78/s/mM at 1.5T and 17 and 82/s/mM at 3.0T, respectively. The lymph node (LN)-to-fat ratios (LN signal intensity (SI)/fat SI) of T1-weighted 3D-fast spoiled gradient-echo (3D-FSPGR) were 3.2+/-0.4 (mean+/-standard deviation (SD)) at 1.5T and 2.7+/-0.3 at 3.0T (P=0.021), and the LN-to-fat ratios of T2/T1-weighted 3D-fast imaging employing steady-state acquisition with phase cycling (3D-FIESTA-C) were 1.8+/-0.2 at 1.5T and 1.2+/-0.4 at 3.0T (P=0.003). Although 3D-FSPGR successfully delineated the LNs at both 1.5T and 3.0T, 3D-FIESTA-C at 3.0T failed to visualize the LNs.

Thrombospondin-1 limits ischemic tissue survival by inhibiting nitric oxide-mediated vascular smooth muscle relaxation

The nitric oxide (NO)/cGMP pathway, by relaxing vascular smooth muscle cells, is a major physiologic regulator of tissue perfusion. We now identify thrombospondin-1 as a potent antagonist of NO for regulating F-actin assembly and myosin light chain phosphorylation in vascular smooth muscle cells. Thrombospondin-1 prevents NO-mediated relaxation of precontracted vascular smooth muscle cells in a collagen matrix. Functional magnetic resonance imaging demonstrated that an NO-mediated increase in skeletal muscle perfusion was enhanced in thrombospondin-1-null relative to wild-type mice, implicating endogenous thrombospondin-1 as a physiologic antagonist of NO-mediated vasodilation. Using a random myocutaneous flap model for ischemic injury, tissue survival was significantly enhanced in thrombospondin-1-null mice. Improved flap survival correlated with increased recovery of oxygen levels in the ischemic tissue of thrombospondin-1-null mice as measured by electron paramagnetic resonance oximetry. These findings demonstrate an important antagonistic relation between NO/cGMP signaling and thrombospondin-1 in vascular smooth muscle cells to regulate vascular tone and tissue perfusion.

High-resolution mapping of tumor redox status by magnetic resonance imaging using nitroxides as redox-sensitive contrast agents

PURPOSE

There is considerable research directed toward the identification and development of functional contrast agents for medical imaging that superimpose tissue biochemical/molecular information with anatomical structures. Nitroxide radicals were identified as in vivo radioprotectors. Being paramagnetic, they can provide image contrast in magnetic resonance imaging (MRI) and electron paramagnetic resonance imaging (EPRI). The present study sought to determine the efficacy of nitroxide radioprotectors as functional image contrast agents.

EXPERIMENTAL DESIGN

Nitroxide radioprotectors, which act as contrast agents, were tested by EPRI and MRI to provide tissue redox status information noninvasively.

RESULTS

Phantom studies showed that the nitroxide, 3-carbamoyl-PROXYL (3CP), undergoes time-dependent reduction to the corresponding diamagnetic hydroxylamine only in the presence of reducing agents. The reduction rates of 3CP obtained by EPRI and MRI were in agreement suggesting the feasibility of using MRI to monitor nitroxide levels in tissues. The levels of 3CP were examined by EPRI and MRI for differences in reduction between muscle and tumor (squamous cell carcinoma) implanted in the hind leg of C3H mice simultaneously. In vivo experiments showed a T1-dependent image intensity enhancement afforded by 3CP which decreased in a time-dependent manner. Reduction of 3CP was found to be the dominant mechanism of contrast loss. The tumor regions exhibited a faster decay rate of the nitroxide compared to muscle (0.097 min(-1) versus 0.067 min(-1), respectively).

CONCLUSIONS

This study shows that MRI can be successfully used to co-register tissue redox status along with anatomic images, thus providing potentially valuable biochemical information from the region of interest.

Probing the Intracellular Redox Status of Tumors with Magnetic Resonance Imaging and Redox-Sensitive Contrast Agents

Nitroxide radicals are paramagnetic contrast agents, used in magnetic resonance imaging (MRI), that also exert antioxidant effects. Participating in cellular redox reactions, they lose their ability to provide contrast as a function of time after administration. In this study, the rate of contrast loss was correlated to the reducing power of the tissue or the "redox status." The preferential reduction of nitroxides in tumors compared with normal tissue was observed by MRI. The influence of the structure of the nitroxide on the reduction rate was investigated by MRI using two cell-permeable nitroxides, 4-hydroxy-2,2,6,6,-tetramethyl-1-piperidynyloxyl (Tempol) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (3CP), and one cell-impermeable nitroxide, 3-carboxy-2,2,5,5,5-tetramethylpyrrolidine-1-oxyl (3CxP). Pharmacokinetic images of these nitroxides in normal tissue, tumor, kidney, and artery regions in mice were simultaneously obtained using MRI. The decay of Tempol and 3CP in tumor tissue was significantly faster than in normal tissue. No significant change in the total nitroxide (oxidized + reduced forms) was noted from tissue extracts, suggesting that the loss in contrast as a function of time is a result of intracellular bioreduction. However, in the case of 3CxP (membrane impermeable), there was no difference in the reduction rates between normal and tumor tissue. The time course of T(1) enhancement by 3CxP and the total amount of 3CxP (oxidized + reduced) in the femoral region showed similar pharmacokinetics. These results show that the differential bioreduction of cell-permeable nitroxides in tumor and normal tissue is supported by intracellular processes and the reduction rates are a means by which the intracellular redox status can be assessed noninvasively.

In vivo spectral fluorescence imaging of submillimeter peritoneal cancer implants using a lectin-targeted optical agent

Intraperitoneal metastases commonly recur after surgery because small tumor foci escape detection within the complex anatomy of the peritoneal cavity and mesentery. Accurate localization of peritoneal implants during surgery could improve the resection of ovarian cancer and other malignancies, but few practical techniques to enhance detectability are currently available. Here, we describe a targeted molecular imaging method that employs fluorescently labeled avidin to detect submillimeter peritoneal implants of ovarian cancer in mice. After binding to surface lectins on the tumor, fluorescein-conjugated avidin enabled the spectral fluorescence imaging of disseminated peritoneal implants. High spatial resolution and high tumor-to-background ratio allowed the visualization of implants as small as 0.3 mm (with 100% sensitivity and specificity; n = 150) and the identification of even smaller lesions ex vivo. These results suggest that targeted molecular imaging with a fluorescence-labeled lectin-ligand system is a promising technique for the detection of disseminated submillimeter foci of cancer.

The influence of tumor oxygenation on (18)F-FDG (fluorine-18 deoxyglucose) uptake: a mouse study using positron emission tomography (PET)

Background

This study investigated whether changing a tumor's oxygenation would alter tumor metabolism, and thus uptake of ¹⁸F-FDG (fluorine-18 deoxyglucose), a marker for glucose metabolism using positron emission tomography (PET).

Results

Tumor-bearing mice (squamous cell carcinoma) maintained at 37°C were studied while breathing either normal air or carbogen (95% O₂, 5% CO₂), known to significantly oxygenate tumors. Tumor activity was measured within an automatically determined volume of interest (VOI). Activity was corrected for the arterial input function as estimated from image and blood-derived data. Tumor FDG uptake was initially evaluated for tumor-bearing animals breathing only air (2 animals) or only carbogen (2 animals). Subsequently, 5 animals were studied using two sequential ¹⁸F-FDG injections administered to the same tumor-bearing mouse, 60 min apart; the first injection on one gas (air or carbogen) and the second on the other gas. When examining the entire tumor VOI, there was no significant difference of ¹⁸F-FDG uptake between mice breathing either air or carbogen (i.e. air/carbogen ratio near unity). However, when only the highest ¹⁸F-FDG uptake regions of the tumor were considered (small VOIs), there was a modest (21%), but significant increase in the air/carbogen ratio suggesting that in these potentially most hypoxic regions of the tumor, ¹⁸F-FDG uptake and hence glucose metabolism, may be reduced by increasing tumor oxygenation.

Conclusion

Tumor ¹⁸F-FDG uptake may be reduced by increases in tumor oxygenation and thus may provide a means to further enhance ¹⁸F-FDG functional imaging.

Absolute oxygen tension (pO(2)) in murine fatty and muscle tissue as determined by EPR

The absolute partial pressure of oxygen (pO(2)) in the mammary gland pad and femoral muscle of female mice was measured using EPR oximetry at 700 MHz. A small quantity of lithium phthalocyanine (LiPc) crystals was implanted in both mammary and femoral muscle tissue of female C3H mice. Subsequent EPR measurements were carried out 1-30 days after implantation with or without control of core body temperature. The pO(2) values in the tissue became stable 2 weeks after implantation of LiPc crystals. The pO(2) level was found to be higher in the femoral muscle than in the mammary tissue. However, the pO(2) values showed a strong dependence on the core body temperature of the mice. The pO(2) values were responsive to carbogen (95% O(2), 5% CO(2)) breathing even 44-58 days after the implantation of LiPc. The LiPc linewidth was also sensitive to changes in the blood supply even 60 days after implantation of the crystals. This study further validates the use of LiPc crystals and EPR oximetry for long-term non-invasive assessment of pO(2) levels in tissues, underscores the importance of maintaining normal body core temperature during the measurements, and demonstrates that mammary tissue functions at a lower pO(2) level than muscle in female C3H mice.

Integration of digital signal processing technologies with pulsed electron paramagnetic resonance imaging

The integration of modern data acquisition and digital signal processing (DSP) technologies with Fourier transform electron paramagnetic resonance (FT-EPR) imaging at radiofrequencies (RF) is described. The FT-EPR system operates at a Larmor frequency (L(f)) of 300MHz to facilitate in vivo studies. This relatively low frequency L(f), in conjunction with our approximately 10MHz signal bandwidth, enables the use of direct free induction decay time-locked subsampling (TLSS). This particular technique provides advantages by eliminating the traditional analog intermediate frequency downconversion stage along with the corresponding noise sources. TLSS also results in manageable sample rates that facilitate the design of DSP-based data acquisition and image processing platforms. More specifically, we utilize a high-speed field programmable gate array (FPGA) and a DSP processor to perform advanced real-time signal and image processing. The migration to a DSP-based configuration offers the benefits of improved EPR system performance, as well as increased adaptability to various EPR system configurations (i.e., software configurable systems instead of hardware reconfigurations). The required modifications to the FT-EPR system design are described, with focus on the addition of DSP technologies including the application-specific hardware, software, and firmware developed for the FPGA and DSP processor. The first results of using real-time DSP technologies in conjunction with direct detection bandpass sampling to implement EPR imaging at RF frequencies are presented.

MR assessment of changes of tumor in response to hyperbaric oxygen treatment

Enhancement of image intensity, using the T1-weighted spoiled gradient-echo (SPGR) sequence, was measured in SCC tumor implanted in the flank of C3H mice while they were subjected to several types of oxygenation challenges inside a hyperbaric chamber designed and constructed to fit in an MRI resonator. The central portions of the tumor gave a positive enhancement, while the periphery showed signal reduction during both normobaric (NBO) and hyperbaric (HBO) oxygen challenges. In the contralateral normal leg, nearly 70% of the region showed a decrease in intensity, and the rest showed a positive enhancement. The positive signal enhancement was markedly greater under HBO compared to NBO. Calculated R1, R2, and M0 maps from multivariate fitting of images acquired by a multislice multiecho (MSME) sequence with variable TR before, during, and after HBO treatment confirm that the source of SPGR signal enhancement in the tumor is associated with shortening of T1.

Competition of nitroxyl contrast agents as an in vivo tissue redox probe: Comparison of pharmacokinetics by the bile flow monitoring (BFM) and blood circulating monitoring (BCM) methods using X-band EPR and simulation of decay profiles

Nitroxyl radicals used as tissue redox-sensitive contrast agents in electron paramagnetic resonance (EPR) and/or NMR imaging should satisfy the following two conditions: 1) the molecules disperse into tissues rapidly, and 2) paramagnetic loss occurs by simple reduction of the radical. The pharmacokinetic trends of several nitroxyl contrast agents were compared with the results obtained by bile flow monitoring (BFM) and blood circulation monitoring (BCM) methods using X-band EPR. The nitroxyl radicals (TEMPO, TEMPONE (oxo-TEMPO), and amino-TEMPO) showed additional EPR signals in the bile that were attributed to metabolites formed during transport from blood to bile through the liver. However, the highly hydrophilic CAT-1 (trimethylammonium-TEMPO), which has low membrane permeability, showed minimal concentration in the bile. Probes that have carboxyl moiety, such as carboxy-TEMPO and carboxy-PROXYL, can be transported via anion transporter into hepatic cells. The EPR signal decay profiles of the nitroxyl radicals were simulated based on the experimental data. The simulation, which we previously applied to mouse blood, was modified to simultaneously fit the experimental results of BFM and BCM obtained with rats. The simulation data showed the simplicity/complexity of the pharmacokinetic mechanisms and that carbamoyl-PROXYL and TEMPOL (hydroxy-TEMPO) are suitable contrast agents for assessing tissue redox status.

Electron paramagnetic resonance imaging of tumor hypoxia: Enhanced spatial and temporal resolution for in vivo pO2 determination

The time-domain (TD) mode of electron paramagnetic resonance (EPR) data collection offers a means of estimating the concentration of a paramagnetic probe and the oxygen-dependent linewidth (LW) to generate pO2 maps with minimal errors. A methodology for noninvasive pO2 imaging based on the application of TD-EPR using oxygen-induced LW broadening of a triarylmethyl (TAM)-based radical is presented. The decay of pixel intensities in an image is used to estimate T2*, which is inversely proportional to pO2. Factors affecting T2* in each pixel are critically analyzed to extract the contribution of dissolved oxygen to EPR line-broadening. Suitable experimental and image-processing parameters were obtained to produce pO2 maps with minimal artifacts. Image artifacts were also minimized with the use of a novel data collection strategy using multiple gradients. Results from a phantom and in vivo imaging of tumor-bearing mice validated this novel method of noninvasive oximetry. The current imaging protocols achieve a spatial resolution of approximately 1.0 mm and a temporal resolution of approximately 9 s for 2D pO2 mapping, with a reliable oxygen resolution of approximately 1 mmHg (0.12% oxygen in gas phase). This work demonstrates that in vivo oximetry can be performed with good sensitivity, accuracy, and high spatial and temporal resolution.

Pharmacologic ascorbic acid concentrations selectively kill cancer cells: action as a pro-drug to deliver hydrogen peroxide to tissues

Human pharmacokinetics data indicate that i.v. ascorbic acid (ascorbate) in pharmacologic concentrations could have an unanticipated role in cancer treatment. Our goals here were to test whether ascorbate killed cancer cells selectively, and if so, to determine mechanisms, using clinically relevant conditions. Cell death in 10 cancer and 4 normal cell types was measured by using 1-h exposures. Normal cells were unaffected by 20 mM ascorbate, whereas 5 cancer lines had EC(50) values of <4 mM, a concentration easily achievable i.v. Human lymphoma cells were studied in detail because of their sensitivity to ascorbate (EC(50) of 0.5 mM) and suitability for addressing mechanisms. Extracellular but not intracellular ascorbate mediated cell death, which occurred by apoptosis and pyknosis/necrosis. Cell death was independent of metal chelators and absolutely dependent on H(2)O(2) formation. Cell death from H(2)O(2) added to cells was identical to that found when H(2)O(2) was generated by ascorbate treatment. H(2)O(2) generation was dependent on ascorbate concentration, incubation time, and the presence of 0.5-10% serum, and displayed a linear relationship with ascorbate radical formation. Although ascorbate addition to medium generated H(2)O(2), ascorbate addition to blood generated no detectable H(2)O(2) and only trace detectable ascorbate radical. Taken together, these data indicate that ascorbate at concentrations achieved only by i.v. administration may be a pro-drug for formation of H(2)O(2), and that blood can be a delivery system of the pro-drug to tissues. These findings give plausibility to i.v. ascorbic acid in cancer treatment, and have unexpected implications for treatment of infections where H(2)O(2) may be beneficial.

Application of a macromolecular contrast agent for detection of alterations of tumor vessel permeability induced by radiation

Permeability of tumor vasculature can be a major barrier to successful drug delivery, particularly for high molecular weight agents such as monoclonal antibodies and their diagnostic or therapeutic conjugates. In this study, changes in permeability of SCCVII tumor vessels after radiation treatment were evaluated by dynamic magnetic resonance imaging as a function of time after irradiation using a generation-8 polyamidoamine dendrimer (G8-Gd-D)-based magnetic resonance imaging contrast agent shown previously to be confined to tumor blood vessels. Tumor irradiation consisted of either single doses (2-15 Gy) or various daily fractionated doses (5 days). A single radiation dose of 15 Gy resulted in significant transient image enhancement of the tumor tissue with a maximum occurring between 7 and 24 hours after radiation treatment. No observable enhancement was recorded for fractionated radiation doses. Use of dynamic magnetic resonance imaging coupled with G8-Gd-D provides an exquisite methodology capable of defining the timing of enhanced permeability of macromolecules in tumors after irradiation. Such information might be applied to optimize the efficacy of subsequent or concurrent therapies including radiolabeled antibodies or other anticancer agents in combination with external beam therapies.

Estimation of free radical formation by β-ray irradiation in rat liver

In vivo free radical reactions in rat liver as a result of exposure to low-dose beta-radiation was evaluated with electron paramagnetic resonance (EPR) spectroscopy by monitoring the reduction of the nitroxyl spin probe after intravenous administration. The EPR signal intensity of a nitroxyl probe as a function of time in bile flow was monitored by cannulating the bile duct through the cavity of an X-band EPR spectrometer. The results show that the rate of nitroxyl signal loss was higher in rats whose livers were exposed to beta-rays compared to unexposed rats. However, the rate of signal loss was lower in animals whose organs were exposed to air by opening the abdominal cavity. In vitro experiments also showed that the nitroxyl EPR signal loss was greater in an atmosphere of nitrogen than in air. Results suggest that under low levels of tissue oxygen, exposure to beta-rays results in nitroxyl signal loss, which may be mediated by free radical dependent pathways. When tissue oxygen were higher, hydrogen peroxide mediated oxidation of hydroxylamine may predominate resulting in a signal loss of smaller magnitudes. This study shows possible evidence of reactive oxygen species formation by low-dose beta-ray irradiation in a living animal.

Development of functional electron paramagnetic resonance imaging

The potential use of electron paramagnetic resonance imaging (EPRI) to obtain physiological information noninvasively is reviewed. EPR, a spectroscopic technique similar to nuclear magnetic resonance (NMR), is useful in detecting and characterizing free radical species. The ability to obtain information about tissue redox and oxygen status using nontoxic free radical spin probes is presented. The capability to encode this information spatially using magnetic field gradients, similar to magnetic resonance imaging (MRI), gives this technique the ability to overlay functional information of tissue with anatomic information. The noninvasive and quantitative nature of EPRI makes it a potentially useful technique for obtaining physiological information from tumors. The requirements for the magnetic field strengths are approximately 600 times lower than that for proton MRI at an identical frequency, making this a low-cost diagnostic tool.

Effects of hypoxia on radiation-responsive stress-activated protein kinase, p53, and caspase 3 signals in TK6 human lymphoblastoid cells

Despite significant evidence of a role of hypoxia in cellular resistance to ionizing radiation-induced toxicity, the underlying molecular mechanisms remain unclear. This study focused on the influence of hypoxia on radiation-induced signals in TK6 human lymphoblastoid cells. Hypoxic (<10 ppm oxygen) and aerobic cells were exposed to equilethal doses of ionizing radiation, radiation dose ratio, 3:1 (hypoxia:air). Hypoxia alone or radiation treatment under aerobic or hypoxic conditions led to increased levels of phospho-p44/42 mitogen-activated protein kinase. Levels of phospho-p38 mitogen-activated protein kinase did not change as a result of either hypoxia or irradiation. Hypoxia alone had no effect on expression of phospho-stress-activated protein kinase (SAPK), wild-type p53, or cleaved caspase 3. Irradiation under aerobic conditions resulted in an increase in the phospho-SAPK signal, whereas hypoxia suppressed the irradiation-induced increase in the level of phospho-SAPK. Both hypoxic and aerobic cells showed increases in p53 levels in response to radiation. Hypoxia blocked radiation-induced cleavage of caspase 3 and poly-ADP-ribose polymerase. Irradiation of aerobic and hypoxic TK6 cells using 6 and 18 Gy, respectively, resulted in a similar and significant increase in fraction of apoptotic cells within 24 hours postirradiation. In contrast, basal levels of apoptosis were observed at 24 hours postirradiation in aerobic and hypoxic NH32 cells, a p53 null derivative of TK6 cells. These results suggest that radiation-induced apoptosis under hypoxia occurs independent of phospho-SAPK and caspase 3, and the p53 response is an obligatory apoptotic signal in TK6 cells.

Effects of Hypoxia on Radiation-Responsive Stress-Activated Protein Kinase, p53, and Caspase 3 Signals in TK6 Human Lymphoblastoid Cells

Despite significant evidence of a role of hypoxia in cellular resistance to ionizing radiation–induced toxicity, the underlying molecular mechanisms remain unclear. This study focused on the influence of hypoxia on radiation-induced signals in TK6 human lymphoblastoid cells. Hypoxic (&lt;10 ppm oxygen) and aerobic cells were exposed to equilethal doses of ionizing radiation, radiation dose ratio, 3:1 (hypoxia:air). Hypoxia alone or radiation treatment under aerobic or hypoxic conditions led to increased levels of phospho-p44/42 mitogen-activated protein kinase. Levels of phospho-p38 mitogen-activated protein kinase did not change as a result of either hypoxia or irradiation. Hypoxia alone had no effect on expression of phospho-stress-activated protein kinase (SAPK), wild-type p53, or cleaved caspase 3. Irradiation under aerobic conditions resulted in an increase in the phospho-SAPK signal, whereas hypoxia suppressed the irradiation-induced increase in the level of phospho-SAPK. Both hypoxic and aerobic cells showed increases in p53 levels in response to radiation. Hypoxia blocked radiation-induced cleavage of caspase 3 and poly-ADP-ribose polymerase. Irradiation of aerobic and hypoxic TK6 cells using 6 and 18 Gy, respectively, resulted in a similar and significant increase in fraction of apoptotic cells within 24 hours postirradiation. In contrast, basal levels of apoptosis were observed at 24 hours postirradiation in aerobic and hypoxic NH32 cells, a p53 null derivative of TK6 cells. These results suggest that radiation-induced apoptosis under hypoxia occurs independent of phospho-SAPK and caspase 3, and the p53 response is an obligatory apoptotic signal in TK6 cells.

Influence of protonT1 on oxymetry using Overhauser enhanced magnetic resonance imaging

In Overhauser enhanced magnetic resonance imaging (OMRI) for in vivo measurement of oxygen partial pressure (pO2), a paramagnetic contrast agent is introduced to enhance the proton signal through dynamic nuclear polarization. A uniform proton T1 is generally assumed for the entire region of interest for the computation of pO2 using OMRI. It is demonstrated here, by both phantom and in vivo (mice) imaging, that such an assumption may cause erroneous estimate of pO2. A direct estimate of pixel-wise T1 is hampered by the poor native MR intensities, owing to the very low Zeeman field (15-20 mT) in OMRI. To circumvent this problem, a simple method for the pixel-wise mapping of proton T1 using the OMRI scanner is described. A proton T1 image of a slice through the center of an SCC tumor in a mouse clearly shows a range of T1 distribution (0.2 approximately 1.6 s). Computation of pO2 images using pixel-wise T1 values promises oximetry with minimal artifacts by OMRI.

Oxidative stress, redox, and the tumor microenvironment

Cellular metabolism is critical for the generation of energy in biological systems; however, as a result of electron transfer reactions, reactive oxygen species (ROS) are generated in aerobic cells. Although low amounts of ROS are easily tolerated by the cell, abnormally high levels of ROS induce oxidative stress. ROS are also produced after exposure to ionizing radiation, selected chemotherapeutic agents, hyperthermia, inhibition of antioxidant enzymes, or depletion of cellular reductants such as NADPH and glutathione. Oxidative stress such as ionizing radiation produces a variety of highly reactive free radicals that damage cells, initiate signal transduction pathways, and alter gene expression. Cells are capable of countering the effects of oxidative stress by virtue of a complex redox buffering system. With respect to the radiation treatment of cancer, components of the cellular redox armamentarium may be targeted to enhance cell killing in the case of tumors and/or protection in the case of normal tissues.

Integration of color and boundary information for improved region of interest identification in electron magnetic resonance images

A Windows-based, object-oriented application system for segmentation and analysis of electron magnetic resonance (EMR) images is described. The integrated system is developed for better recognition of regions of interest (ROI) in murine EMR images. The system combines the clustering method of color segmentation with boundary detection, for efficient segmentation of regions of interest in EMR images. Initially, the red/green/blue (RGB) color space is converted into spherical coordinates transform (SCT) space. Color quantization is then achieved by center split algorithm applied on the color dimensions of the SCT space. Subsequently, Laplacian boundary detection operator is used to extract the contours of the ROI from the variegated coloring information. The system is implemented in Visual C++ and tested on temporal EMR color images of mouse. The system performs well giving perceptually reasonable segmentation of tumor, kidney and bladder of the mouse image. Experimental results with extensive set of EMR color images demonstrate the efficacy of the system developed.