Oxygen Imaging of a Rabbit Tumor Using a Human-Sized Pulse Electron Paramagnetic Resonance Imager

Reaction of polarizing agent Ox063 with pyruvic acid under standard sample preparation protocol for dissolution dynamic nuclear polarization

Dynamic nuclear polarization is a technique that significantly enhances signal intensity in nuclear magnetic resonance spectroscopy and imaging. In a DNP experiment, a sample of interest is doped with a radical, and microwaves are applied in a strong magnetic field, leading to an increase in nuclear spin polarization. Notably, the potential reactions between the sample and the polarization agent are rarely considered. Hyperpolarized 13C magnetic resonance spectroscopy (MRS) is currently used in clinical trials for various diseases. Herein, we demonstrated that with one of the mostly used DNP systems, pyruvic acid, hyperpolarized with the trityl radical Ox063, the alcohol moieties of the radical undergo esterification during sample preparation, leading to the formation of pyruvate esters on the radical, and that Ox063 has a half-life of ∼30 min in pyruvic acid at room temperature. The biological and physicochemical properties of these derivatives have not yet been studied.

Operando detection of dissolved oxygen in fluid solution using a submersible rapid scan EPR on a chip dipstick sensor

Electron paramagnetic resonance (EPR) is an accurate and efficient technique to probe unpaired electrons in many applications across materials science, chemistry, and biology. Dynamic processes are investigated using EPR; however, these applications are limited by the use of resonator-based spectrometers such that the entire process must be confined to the resonator. The EPR-on-a-chip (EPRoC) device circumvents this limitation by integrating the entire EPR spectrometer into a single microchip. In this approach, the coil of a voltage-controlled oscillator (VCO) is used as the microwave source and detector simultaneously, operating under a protective coating such that the device may be placed in the sample solution directly. Additionally, improvements in sensitivity via rapid scan EPR (RS-EPR/RS-EPRoC) increase the accessible applications where SNR per measurement time is the fundamental limit. The herein reported device combines a dipstick EPRoC sensor with the enhanced sensitivity of frequency-swept frequency modulated rapid scan to measure triarylmethyl (trityl, Ox071) oxygen-sensitive probes dissolved in aqueous solutions. EPR spectra of Ox071 solutions were recorded using the RS-EPRoC sensor while varying the oxygen concentration of the solution between normal atmosphere and after purging the solution with nitrogen gas. We demonstrate that EPRoC may be employed to monitor dissolved oxygen in fluid solution in an online fashion.

Evaluation of Mesoporous Silica Nanoparticles as Carriers of Triarylmethyl Radical Spin Probes for EPR Oximetry

In vivo measurement and mapping of oxygen levels within the tissues are crucial in understanding the physiopathological processes of numerous diseases, such as cancer, diabetes, or peripheral vascular diseases. Electron paramagnetic resonance (EPR) associated with biocompatible exogenous spin probes, such as Ox071 triarylmethyl (TAM) radical, is becoming the new gold standard for oxygen mapping in preclinical settings. However, these probes do not show tissue selectivity when injected systemically, and they are not cell permeable, reporting oxygen from the extracellular compartment only. Recently, Ox071-loaded mesoporous silica nanoparticles (MSNs) were proposed for intracellular tumor oxygen mapping in both in vitro and in vivo models. However, the EPR spectrum of the Ox071 spin probe is poorly sensitive to mobility due to the small anisotropy of its g-factor and the absence of hyperfine splitting, making it more difficult to study the mobility of the radical inside the MSNs or its location. Using 13C1 isotopologues of Ox071 and the deuterated Finland trityl (dFT) spin probes, which are highly sensitive to molecular tumbling, we showed that the loading of the probes inside homemade and commercial cationic MSNs drastically decreases their mobility while the high local concentration of the probe inside the MSNs leads to dipolar line width broadening (self-relaxation). This decrease in molecular tumbling and line broadening hampers the oxygen-sensing properties of Ox071 or dFT probes used for EPR oximetry when loaded into MSNs.

Re-examining What the Results of "a Measurement of Oxygen Level in Tissues" Really Mean

Within this special issue, many eminent investigators report on measurements of oxygen (O2) levels in tissues. Given the complexities of spatial and temporal heterogeneities of O2 in tissues and its many sources, this commentary draws attention to what such measurements do and do not actually assess regarding O2 levels in tissues. Given this limitation, it also discusses how these results can be used most effectively. To provide a convenient mechanism to discuss these issues more fully, this analysis focuses on measurements using EPR oximetry, but these considerations apply to all other techniques. The nature of the delivery of O2 to tissues and the mechanisms by which O2 is consumed necessarily result in very different levels of O2 within the volume of each voxel of a measurement. Better spatial resolution cannot fully resolve the problem because the variations include O2 gradients within each cell. Improved resolution of the time-dependent variation in O2 is also very challenging because O2 levels within tissues can have fluctuations of O2 levels in the range of milliseconds, while most methods require longer times to acquire the data from each voxel. Based on these issues, we argue that the values obtained inevitably are complex aggregates of averages of O2 levels across space and time in the tissue. These complexities arise from the complex physiology of tissues and are compounded by the limitations of the technique and its ability to acquire data. However, one often can obtain very meaningful and useful results if these complexities and limitations are taken into account. We illustrate this, using results obtained with in vivo EPR oximetry, especially utilizing its capacity to make repeated measurements to follow changes in O2 levels that occur with interventions and/or over time.