Development of biocompatible implants of fusinite for in vivo EPR oximetry

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

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

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

Contribution of Harold M. Swartz to In Vivo EPR and EPR Dosimetry

In 2015, we are celebrating half a century of research in the application of Electron Paramagnetic Resonance (EPR) as a biodosimetry tool to evaluate the dose received by irradiated people. During the EPR Biodose 2015 meeting, a special session was organized to acknowledge the pioneering contribution of Harold M. (Hal) Swartz in the field. The article summarizes his main contribution in physiology and medicine. Four emerging themes have been pursued continuously along his career since its beginning: (1) radiation biology; (2) oxygen and oxidation; (3) measuring physiology in vivo; and (4) application of these measurements in clinical medicine. The common feature among all these different subjects has been the use of magnetic resonance techniques, especially EPR. In this article, you will find an impressionist portrait of Hal Swartz with the description of the 'making of' this pioneer, a time-line perspective on his career with the creation of three National Institutes of Health-funded EPR centers, a topic-oriented perspective on his career with a description of his major contributions to Science, his role as a mentor and his influence on his academic children, his active role as founder of scientific societies and organizer of scientific meetings, and the well-deserved international recognition received so far.

Electron paramagnetic resonance: a powerful tool to support magnetic resonance imaging research

The purpose of this paper is to describe some of the areas where electron paramagnetic resonance (EPR) has provided unique information to MRI developments. The field of application mainly encompasses the EPR characterization of MRI paramagnetic contrast agents (gadolinium and manganese chelates, nitroxides) and superparamagnetic agents (iron oxide particles). The combined use of MRI and EPR has also been used to qualify or disqualify sources of contrast in MRI. Illustrative examples are presented with attempts to qualify oxygen sensitive contrast (i.e. T1 - and T2 *-based methods), redox status or melanin content in tissues. Other areas are likely to benefit from the combined EPR/MRI approach, namely cell tracking studies. Finally, the combination of EPR and MRI studies on the same models provides invaluable data regarding tissue oxygenation, hemodynamics and energetics. Our description will be illustrative rather than exhaustive to give to the readers a flavour of 'what EPR can do for MRI'.

In vivo study of different ointments for drug delivery into oral mucosa by EPR oximetry

The aim of the present study was to determine the rate of transport and long-term effect of a drug applied to the oral mucosa in different ointments. Three ointments with bioadhesive properties: Orabase, Carbopol 935P, and polymethyl methacrylate (PMM) and the ointment Miglyol without such properties were used. Benzyl nicotinate (BN) was used as an active ingredient that causes hyperemia. The kinetics of drug action was measured by electron paramagnetic resonance (EPR) oximetry in vivo using the paramagnetic probe (Lithium phthalocyanine) implanted beneath the epithelium of the buccal mucosa in rats. EPR spectra line-width was proportional to local changes of partial pressure of oxygen (pO(2)) in tissue and was monitored for 90 min after the application of ointments mixed with BN. The greatest increase in pO(2) and the highest efficiency of drug action was observed after the application of 2% BN in PMM (P<0.01). Additionally in PMM the drug effect increased linearly with BN concentration up to 3%, at higher concentrations (3.5 and 4% BN) no further effect was observed. The results demonstrated that the greatest and the longest effect caused by a hyperemic drug in PMM. By increasing the concentration of the drug in PMM higher pO(2) in the oral mucosa can be established but only until the saturation is reached.

In vivo electron paramagnetic resonance oximetry with particulate materials

Electron paramagnetic resonance (EPR) methods can be used to study tissue pO(2) (PtO(2)) in anesthetized or awake animals (EPR oximetry). The method takes advantage of the fact that some paramagnetic materials have an EPR linewidth that is sensitive to the pO(2) in which the material is located. This article provides an overview of the method of EPR oximetry using implanted particulate materials as the sensors of pO(2). Characteristics of these materials are described to help the reader understand the factors involved in choosing the optimum particulate material. Examples of biological studies are included that show how EPR oximetry may be used on both awake and anesthetized animals.

Microencapsulation of carbon particles used as oxygen sensors in EPR oximetry to stabilize their responsiveness to oxygen in vitro and in vivo

The electron paramagnetic resonance (EPR) spectra of some paramagnetic materials exhibit a pO2 (partial pressure of oxygen)-dependent linewidth. By recording the EPR linewidth in vivo using low-frequency EPR spectrometers, it is possible to measure the partial pressure of oxygen in tissues. It has been found, however, that some of the paramagnetic materials with optimal spectroscopic properties in vitro may lose or change their responsiveness to oxygen in tissues. The aim of this study was to microencapsulate paramagnetic particles by biopolymers in order to stabilize their responsiveness to oxygen. Carbohydrate char particles (Bubinga) were encapsulated with different biopolymers: cellulose acetate or cellulose triacetate, silicone and polyurethane. The performance of the materials was evaluated in vitro and in vivo. X-band EPR spectroscopy was used to test the variation of the calibration curve (EPR linewidth as a function of the pO2) after incubation in saline and after prolonged residence in tissues. The stability of the responsiveness to PO2 in vivo was carried out by L-band EPR spectroscopy using mice that received injection of the oxygen sensors in the muscles. After residence in saline and prolonged residence in tissues, only the calibration curve of the silicone-coated (coating weight of 0.5% (w/w)) paramagnetic materials remained unchanged, while those of oxygen sensors coated with cellulose acetate, cellulose triacetate and polyurethane changed.

Development of biocompatible oxygen-permeable films holding paramagnetic carbon particles: evaluation of their performance and stability in EPR oximetry

EPR oximetry using paramagnetic particles relies on the measurement of the EPR linewidth, which is directly related to the pO2. It was previously found that some of the paramagnetic materials with optimal EPR spectroscopic properties in vitro may lose their responsiveness to oxygen in tissues (change of the calibration curve of the EPR linewidth as a function of the pO2). We hypothesized that coating paramagnetic particle materials could improve the stability of response, as well as the biocompatibility. In this study, very thin films holding paramagnetic materials were prepared with different biopolymers (cellulose acetate, cellulose triacetate, cellulose nitrate, silicone, and polyurethane) that already are accepted for clinical applications. Their performance was evaluated in EPR oximetry by measuring the stability of the calibration curves (EPR linewidth as a function of pO2) after a prolonged period in an aqueous environment (1 week in saline) or in vivo (implantation for 3 weeks under the skin of mice). We found that one type of silicone film was able to stabilize the responsiveness of an intrinsically unstable carbon material (a wood char).

Accurate and sensitive measurements of pO(2) in vivo using low frequency EPR spectroscopy: how to confer biocompatibility to the oxygen sensors

Within the last few years, there has been a significant amount of progress using EPR oximetry, which has resulted in the availability of instrumentation and paramagnetic materials capable of measuring pO(2) in tissues with an accuracy and sensitivity comparable to or greater than that available by any other method. While the results obtained with EPR so far indicate that criteria for the measurements of pO(2)-such as accuracy, sensitivity, repeatability, and noninvasiveness-can be met, some of the paramagnetic materials with optimum spectroscopic properties (i.e., strong simple signals which are appropriately responsive to changes in pO(2)) may have some undesirable interactions with tissues, causing reactions with and/or losing responsiveness to oxygen. In this paper, several approaches are discussed, such as encapsulation procedures, which can result in the availability of oxygen-sensitive materials in a suitable configuration for long-term studies (absence of toxicity and preservation of the responsiveness to oxygen).

Carbon-centered radicals as oxygen sensors for in vivo electron paramagnetic resonance: screening for an optimal probe among commercially available charcoals

It is known that some charcoals possess paramagnetic centers with an electron paramagnetic resonance (EPR) linewidth which can be broadened by oxygen. In order to identify potential candidates as sensors for in vivo EPR oximetry, we carried out a systematic study among commercially available charcoals. A total of 34 charcoals were tested. The steps used for the screening were: (1) to check the presence of paramagnetic centers in the material; (2) to measure the EPR linewidth in nitrogen and in air on the dry material and on a aqueous suspension of particles; (3) to calibrate the oxygen sensitive materials (EPR linewidth vs. pO2); (4) to test the sensitivity and stability of the response to changes of pO2 in a simple model of hypoxia induced in mice. Seventeen charcoals contained paramagnetic centers detectable by low-frequency EPR (1.1 GHz). The EPR spectrum consist of one single line which is typical of carbon-centered radicals (g-factor approximately 2). Eight charcoals presented sufficient interesting EPR properties (linewidth in nitrogen < 0.1 mT, linewidth in air for an aqueous suspension of particles > 0.15 mT) to be further characterized in vivo. Only three charcoals presented a stable, reproducible, and sensitive response to pO2 for more than 2 months. These three coals should be considered as good candidates to be used as oxygen sensor using in vivo EPR spectroscopy.

Separation and enrichment of the active component of carbon based paramagnetic materials for use in EPR oximetry

Carbon based paramagnetic materials are frequently used for EPR oximetry, especially in vivo, but the EPR spectra of these materials often have more than one paramagnetic center and/or relatively low signal intensity. To determine whether the multi-components of carbon based materials could be separated and enriched in the active component, we used density gradient centrifugation to separate the materials into several fractions. We studied two types of coals, gloxy and Pocahontas, and found these materials to have large density distribution. The separated density fractions had very different EPR spectra and intensities. The active component from the coal material had a more homogeneous EPR signal and significantly increased EPR signal intensity, whereas for India ink, only slight changes were observed. This result can be very useful in the development of better probes for EPR oximetry.

Small particles of fusinite and carbohydrate chars coated with aqueous soluble polymers: preparation and applications for in vivo EPR oximetry

The development of oxygen-sensitive paramagnetic materials is being pursued actively because of their potential applications in in vivo EPR oximetry. Among these materials, several charcoals and carbohydrate chars are of special interest because of their desirable EPR properties: high sensitivity of the EPR linewidth to the partial pressure of oxygen, simple EPR spectra, and high spin density. Their potential use in humans, however, is limited by the need to demonstrate that they will not lead to deleterious effects. A strategy was used to optimize the biocompatibility of the oxygen-sensitive materials by decreasing the size of the particles and coating them with suspending or surfactive agents such as arabic gum, poloxamer (Pluriol 6800), and polyvinylpyrrolidone. The coated particles of a carbohydrate char and fusinite were characterized in vitro for their size, stability, and pO2 sensitivity. The feasibility of performing pO2 measurement was examined in vivo by inducing ischemia in the gastrocnemius muscle of mice. The use of arabic gum for coating the fusinite particles preserved the pO2 sensitivity in vivo, whereas the other surfactive agents led to a loss of the pO2 sensitivity in vivo. Small particles of fusinite coated by arabic gum and intravenously administered to mice accumulated in the liver, whereas the uncoated fusinite was toxic when injected intravenously due to the large size and aggregation of the particles. Histological studies performed up to 6 months after the injection in muscles of mice did not indicate any toxicity from the materials used in the present study.