Electron paramagnetic resonance (EPR) imaging in skin: biophysical and biochemical microscopy

Accessing Properties of Molecular Compounds Involved in Cellular Metabolic Processes with Electron Paramagnetic Resonance, Raman Spectroscopy, and Differential Scanning Calorimetry

In this work, we review some physical methods of macroscopic experiments, which have been recently argued to be promising for the acquisition of valuable characteristics of biomolecular structures and interactions. The methods we focused on are electron paramagnetic resonance spectroscopy, Raman spectroscopy, and differential scanning calorimetry. They were chosen since it can be shown that they are able to provide a mutually complementary picture of the composition of cellular envelopes (with special attention paid to mycobacteria), transitions between their molecular patterning, and the response to biologically active substances (reactive oxygen species and their antagonists-antioxidants-as considered in our case study).

Nitroxyl Radical as a Theranostic Contrast Agent in Magnetic Resonance Redox Imaging

Significance:In vivo assessment of paramagnetic and diamagnetic conversions of nitroxyl radicals based on cyclic redox mechanism can be an index of tissue redox status. The redox mechanism of nitroxyl radicals, which enables their use as a normal tissue-selective radioprotector, is seen as being attractive on planning radiation therapy. Recent Advances:In vivo redox imaging using nitroxyl radicals as redox-sensitive contrast agents has been developed to assess tissue redox status. Chemical and biological behaviors depending on chemical structures of nitroxyl radical compounds have been understood in detail. Polymer types of nitroxyl radical contrast agents and/or nitroxyl radical-labeled drugs were designed for approaching theranostics. Critical Issues: Nitroxyl radicals as magnetic resonance imaging (MRI) contrast agents have several advantages compared with those used in electron paramagnetic resonance (EPR) imaging, while support by EPR spectroscopy is important to understand information from MRI. Redox-sensitive paramagnetic contrast agents having a medicinal benefit, that is, nitroxyl-labeled drug, have been developed and proposed. Future Directions: A development of suitable nitroxyl contrast agent for translational theranostic applications with high reaction specificity and low normal tissue toxicity is under progress. Nitroxyl radicals as redox-sensitive magnetic resonance contrast agents can be a useful tool to detect an abnormal tissue redox status such as disordered oxidative stress. Antioxid. Redox Signal. 36, 95-121.

Characterization of radical types, penetration profile and distribution pattern of the topically applied photosensitizer THPTS in porcine skin ex vivo

The topical photodynamic therapy (PDT) is mainly used in the treatment of dermato-oncological diseases. The distribution and functionality of the photosensitizer Tetrahydroporphyrin-Tetratosylat (THPTS) was investigated using microscopic and spectroscopic methods after topical application to excised porcine skin followed by irradiation. The distribution of THPTS was determined by two-photon tomography combined with fluorescence lifetime imaging (TPT/FLIM) and confocal Raman microspectroscopy (CRM). The radicals were quantified and characterized by electron paramagnetic resonance (EPR) spectroscopy. Results show a penetration depth of THPTS into the skin down to around 12 ± 5 µm. A penetration of THPTS through the stratum corneum was not clearly observable after 1 h penetration time, but cannot be excluded. The irradiation within the phototherapeutic window (spectral range of visible and near infrared light in the range ≈ 650-850 nm) is needed to activate THPTS. An incubation time of 10 min showed the highest radical production. A longer incubation time affected the functionality of THPTS, whereby significant less radicals were detectable. During PDT mainly reactive oxygen species (ROS) and lipid oxygen species (LOS) are produced. Overall, the irradiation dose per se influences the radical types formed in skin. While ROS are always prominent at low doses, LOS increase at high doses, independent of previous skin treatment and the irradiation wavelength used.

Oxygen Sensing with Perfluorocarbon-Loaded Ultraporous Mesostructured Silica Nanoparticles

Oxygen homeostasis is important in the regulation of biological function. Disease progression can be monitored by measuring oxygen levels, thus producing information for the design of therapeutic treatments. Noninvasive measurements of tissue oxygenation require the development of tools with minimal adverse effects and facile detection of features of interest. Fluorine magnetic resonance imaging (¹⁹F MRI) exploits the intrinsic properties of perfluorocarbon (PFC) liquids for anatomical imaging, cell tracking, and oxygen sensing. However, the highly hydrophobic and lipophobic properties of perfluorocarbons require the formation of emulsions for biological studies, though stabilizing these emulsions has been challenging. To enhance the stability and biological loading of perfluorocarbons, one option is to incorporate perfluorocarbon liquids into the internal space of biocompatible mesoporous silica nanoparticles. Here, we developed perfluorocarbon-loaded ultraporous mesostructured silica nanoparticles (PERFUMNs) as ¹⁹F MRI detectable oxygen-sensing probes. Ultraporous mesostructured silica nanoparticles (UMNs) have large internal cavities (average = 1.8 cm³ g^-1), facilitating an average 17% loading efficiency of PFCs, meeting the threshold fluorine concentrations needed for imaging studies. Perfluoro-15-crown-5-ether PERFUMNs have the highest equivalent nuclei per PFC molecule and a spin-lattice (T₁) relaxation-based oxygen sensitivity of 0.0032 mmHg^-1 s^-1 at 16.4 T. The option of loading PFCs after synthesizing UMNs, rather than traditional in situ core-shell syntheses, allows for use of a broad range of PFC liquids from a single material. The biocompatible and tunable chemistry of UMNs combined with the intrinsic properties of PFCs makes PERFUMNs a MRI sensor with potential for anatomical imaging, cell tracking, and metabolic spectroscopy with improved stability.

Non-invasive evaluation of atopic dermatitis based on redox status using in vivo dynamic nuclear polarization magnetic resonance imaging

Atopic dermatitis (AD) is a chronic inflammatory condition with complex etiology, including genetic, environmental and immunologic factors. Redox imbalance caused by excessive oxidative stress has been shown to mediate disease activity of AD. Currently, an imaging technique that can monitor the redox status of the skin in vivo has not yet been developed. Consequently, we have established such a technique that can detect and visualize the redox status of the skin using in vivo dynamic nuclear polarization magnetic resonance imaging (DNP-MRI). To evaluate this technique, we utilized an AD mouse model that was generated by repeated topical application of mite antigen in NC/Nga mice. We imaged alterations in redox balance of the resulting AD skin lesions of the mice. Using in vivo DNP-MRI and non-toxic nitroxyl radicals to visualize free radicals in vivo, we revealed that AD skin lesions demonstrated more rapid decay rates of image intensity enhancement than normal skin, indicating that our technique can monitor excessive oxidative stress occurring in AD skin lesions. Therefore, this technique has the potential to provide a novel approach for evaluating disease activity of inflammatory skin diseases, including AD, from the view point of altered redox status.

In vivo evaluation of different alterations of redox status by studying pharmacokinetics of nitroxides using magnetic resonance techniques

Free radicals, particularly reactive oxygen species (ROS), are involved in various pathologies, injuries related to radiation, ischemia-reperfusion or ageing. Unfortunately, it is virtually impossible to directly detect free radicals in vivo, but the redox status of the whole organism or particular organ can be studied in vivo by using magnetic resonance techniques (EPR and MRI) and paramagnetic stable free radicals - nitroxides. Here we review results obtained in vivo following the pharmacokinetics of nitroxides on experimental animals (and a few in humans) under various conditions. The focus was on conditions where the redox status has been altered by induced diseases or harmful agents, clearly demonstrating that various EPR/MRI/nitroxide combinations can reliably detect metabolically induced changes in the redox status of organs. These findings can improve our understanding of oxidative stress and provide a basis for studying the effectiveness of interventions aimed to modulate oxidative stress. Also, we anticipate that the in vivo EPR/MRI approach in studying the redox status can play a vital role in the clinical management of various pathologies in the years to come providing the development of adequate equipment and probes.

From the bottle to the skin: challenges in evaluating antioxidants

Endogenous production and ultraviolet-generated free radicals in the skin can lead to photoaging and even skin cancer. Topical antioxidants have been found to provide benefits against ultraviolet damage and these ingredients have been incorporated into various cosmetic products and claimed to have substantial effects. Currently, there is a lack in a standardized rating system to measure the concentration and activity levels of antioxidants in these products. As a result, it is difficult for consumers and clinicians to evaluate and select commercial products based on readily accessible evidence. In this review, we will describe four assays which have been used to measure antioxidants in various products, and the strengths and weaknesses of each test will be detailed. We will highlight key considerations for clinicians when interpreting the results of antioxidant tests when evaluating commercial products containing antioxidants.

Methods for evaluation of cosmetic antioxidant capacity

The skin as the largest part of human body is one of the main targets for ultraviolet radiation, environmental pollution, toxic chemicals and some metal ions, which share responsibility for the formation of free radicals. The resulting free radicals, both oxygen and nitrogen species are one of the main causes of aging due to impaired regulation of cell respiratory metabolism involving incomplete oxygen reduction in mitochondria and production a superoxide anion, hydroxyl radicals et al. In modern cosmetology to minimize the adverse effects of free radicals, antioxidants, which inhibit free radical reactions, mainly autoxidation processes are used. Currently, not only many cosmetic products containing antioxidants are available, but a large diversity of methods for determination of cosmetics antioxidant activity is also accessible. These methods can be divided into three main groups: in vitro, in vivo, and ex vivo as reported herein. Due to lack of standardization and validation it is necessary to use a variety of methods as well as conditions for those purposes, which are presented to the context.

Aging skin is functionally anaerobic: importance of coenzyme Q10 for anti aging skin care

The functional loss of mitochondria represents an inherent part in modern theories trying to explain the cutaneous aging process. The present study shows significant age-dependent differences in mitochondrial function of keratinocytes isolated from skin biopsies of young and old donors. Our data let us postulate that energy metabolism shifts to a predominantly non-mitochondrial pathway and is therefore functionally anaerobic with advancing age. CoQ10 positively influences the age-affected cellular metabolism and enables to combat signs of aging starting at the cellular level. As a consequence topical application of CoQ10 is beneficial for human skin as it rapidly improves mitochondrial function in skin in vivo.

In vivo proton electron double resonance imaging of the distribution and clearance of nitroxide radicals in mice

Proton electron double resonance imaging (PEDRI) is an emerging technique that utilizes the Overhauser effect to enable in vivo and in vitro imaging of free radicals in biological systems. Nitroxide spin probes enable measurement of tissue redox state based on their reduction to diamagnetic hydroxylamines. PEDRI instrumentation at 0.02 T was applied to assess the ability to image the in vivo distribution, clearance, and metabolism of nitroxide radicals in living mice. Using phantoms of 2,2,5,5-tetramethyl-3-carboxylpyrrolidine-N-oxyl (PCA) in normal saline the dependence of the enhancement on RF power and spin probe concentration was determined. Enhancements of up to -23 were obtained in phantoms with 2 mM levels. Maximum enhancement of -7 was observed in vivo. Coronal images of nitroxide-infused mice enabled visualization of the kinetics of spin probe uptake and clearance in different organs including the great vessels, heart, lungs, kidneys, and bladder with an in-plane spatial resolution of 0.6 mm. PEDRI of living mice was also performed using 3-carbamoyl-proxyl and 2,2,6,6-tetramethyl-4-oxopiperidine-N-oxyl to compare the different rate of clearance and metabolism among different nitroxide probes. PCA, due to its intravascular compartmentalization, provided the sharpest contrast for the vascular system and highest enhancement values in the PEDRI images among the three nitroxides.

In vivo measurement and mapping of skin redox stress induced by ultraviolet light exposure

Exposure of skin to UV light presents a potent oxidative stress and this could alter the skin redox state. In this context, we evaluated the ability of electron paramagnetic resonance (EPR) imaging to provide noninvasive in vivo mapping of the redox status of the skin of living rats. The redox status was measured using a topically applied nitroxyl spin probe, (15)N-PDT. The nitroxyl intensity profile obtained across the skin layers showed that the concentration of the probe was higher in the epidermis and lower in the dermis and hypodermis. Skin permeability and reduction metabolism were evaluated in the skin exposed to UVB (312 nm) radiation. Exposure of skin to UVB decreased the overall reduction rate constant of the nitroxyl probe to 25 +/- 6% of the value obtained in the untreated skin. EPR imaging data showed that after the UVB treatment, the reduction rate constant decreased to 41 +/- 1% in epidermis, 28 +/- 1% in dermis, and 21 +/- 8% in hypodermis layers. The data suggested that UVB decreased the overall reducing capability of the skin with a larger decrease in the dermis and hypodermis. In summary, in vivo EPR imaging measurements showed significant alterations in the redox state of the skin exposed to UV light.

Morphology, chemical structure and diffusion processes of root surface after Er:YAG and Nd:YAG laser irradiation

OBJECTIVES

The aim of this in vitro study was to evaluate the effects of Er:YAG and Nd:YAG lasers on morphology, chemical structure and diffusion processes of the root surface.

MATERIAL AND METHODS

60 root samples were irradiated for 1 min each either with 60 mJ/p, 80 mJ/p and 100 mJ/p using Er:YAG laser or with 0.5W, 1.0W and 1.5W using Nd:YAG laser. Scanning electron microscopy (SEM) was used to determine the morphology, infrared (IR) spectroscopy to assess the alterations in chemical structure and one dimensional electron paramagnetic resonance imaging (1-D EPRI) was used to estimate the diffusion coefficients in dental root samples.

RESULTS

Er:YAG laser treatment resulted in deep crater formation with exposed dentin. Morphological alterations of root surface after Nd:YAG laser irradiation included cracks, crater formation, meltdown of the root mineral and resolidified porous globules formation. Er:YAG laser failed to alter the intensity of Amide peaks I, II or III. In contrast, treatment with Nd:YAG laser, using the highest power setting of 1.5W, reduced the intensity of Amide peak II and III in comparison to the control. The diffusion coefficients were increased significantly in all Er:YAG and Nd:YAG treated root samples.

CONCLUSION

This study demonstrated that Er:YAG laser influences only on morphology and diffusion processes of root surfaces, while Nd:YAG laser also alters the chemical structure of root proteins.

Kinetic measurements using EPR imaging with a modulated field gradient

EPR imaging with modulated field gradient was applied for the investigation of fast diffusion processes. Three different imaging methods are possible: spectral-temporal, spatio-temporal, and spectral-spatial imaging. The time resolution is on the order of seconds and the spatial resolution is in the micrometer region. The efficiency of this imaging technique is demonstrated for the penetration of the spin probe Tempol in the skin of hairless mice biopsies. The skin is normally protected against the penetration of water soluble substances by the horny layer, a resistive thin lipophilic layer. Overcoming this horny layer for water soluble ingredients is one of the main practical problems for the topical application of pharmaceutics which could be investigated by EPR imaging. Different images represent the penetration behavior of the water soluble Tempol in the skin after treatment with the penetration enhancer DMSO (Dimethylsulfoxide) and after removing the horny layer.

In vivo EPR imaging of the distribution and metabolism of nitroxide radicals in human skin

While altered cellular free radical and redox metabolism are critical factors in many human diseases, it has not been previously possible to both measure and image these processes in humans. The development and application of electron paramagnetic resonance instrumentation capable of in vivo spectroscopy and imaging of free radicals in human skin are reported. The instrumentation uses a specially designed topical resonator and a 2.2-GHz microwave bridge. Noninvasive measurements of the distribution and metabolism of the topically applied nitroxide, (15)N-perdeuterated tempone (100 mM), in forearm skin were performed. A single broad peak due to the concentrated label at the skin surface was initially observed, followed by a sharp doublet from the diluted label that permeated the skin. The penetration of the label into the skin and its metabolic clearance were modeled using kinetic equations. It was observed that the penetration process from the skin surface into the dermis and subcutaneous regions, as well as its clearance from these regions, could be described by single exponential functions. Phantom imaging experiments using the nitroxide showed that a spatial resolution of up to 50 microm could be achieved. The skin imaging measurements showed two bands in the distribution of the label along the skin depth. The first band appeared in the outer 400 microm of the skin, the epidermis region, whereas the second band was centered at a depth of 1000 microm in the subcutaneous region with a thickness about 400 microm. These two bands decayed and merged into a single band with time. The results are important in the understanding of the permeability and metabolism of free radicals in human skin.

Cutaneous tolerance to nitroxide free radicals in human skin

No data are available on the irritant effect of nitroxide free radicals in human skin. Nitroxides are important biomedical skin probes used in Electron Paramagnetic Resonance spectroscopy and imaging. Our purpose was to study the skin irritation potential of different nitroxide free radical structures in skin of healthy human subjects. We investigated the following nitroxides: Tempo (2,2,6,6-tetramethyl-1-piperidinoxy), Doxo (2,2,5,5-tetramethyl-3-oxazolidinoxy), Proxo (2,2,5,5-tetramethyl- -dihydro-pyrrolinoxy), and Imidazo (2,2,3,4,5,5-hexamethyl-imidazoline-1-yloxyl). Cutaneous irritation was determined in human skin following a single application and after repetitive applications in comparison to the standardized irritant sodium lauryl sulfate (SLS). The response was evaluated clinically as well as by a bioengineering method analyzing transepidermal water loss (TEWL) and skin hydration (capacitance). The nitroxides were classified clinically from nonirritant (Imidazo, Proxo), to slightly irritant (Doxo, 100 mM), or moderately irritant (Tempo 100 mM) after a single application. The TEWL values were significantly increased by Doxo and Tempo, but capacitance values were not changed significantly. In the cumulative irritation test Tempo was scored as a slight irritant (10 mM). TOLH (2,2,6,6-tetramethyl-1-hydroxypiperidin), the hydroxylamine of Tempo, which is the major skin metabolite, did not cause skin irritation after a single or repetitive applications. This may indicate that a loss of cellular reducing equivalents may be involved in the inflammation process caused by Tempo. The order of nitroxide irritation potency (Tempo > Doxo >> Imidazo = Proxo) is inverse to the order of nitroxide biostability in human skin (Imidazo = Proxo >> Doxo > Tempo). In conclusion, nitroxide free radicals are classified as nonirritant to moderately irritant in human skin. Particularly, the pyrrolidine and imidazoline type nitroxides have a low potential to cause acute or subacute skin toxicity.

Cutaneous tolerance to nitroxide free radicals and nitrone spin traps in the guinea pig

The attempts to use nitroxide free radicals and nitrone spin traps topically in skin requires analysis of their potential cutaneous adverse effects. The objective of this study was to investigate the skin irritation and sensitizing potential of nitroxides and nitrones in the guinea pig. The following unsubstituted nitroxides were investigated: 2,2,6,6-tetramethyl-1-piperidinoxyl (Tempo), 2,2, 5,5-tetramethyl-3-oxazolidinoxyl (Doxo), 2,2,5,5-tetramethyl-1-dihydro-pyrrolinoxyl (Proxo), 2,2,3,4,5,5-hexamethyl-imidazoline-1-yloxyl (Imidazo) and the nitrones: 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and N-tert.-butyl-phenylnitrone (PBN). Cutaneous irritation was determined following the modified Draize protocol. The response was evaluated clinically as well as by a biophysical method analyzing transepidermal water loss (TEWL). The nitroxides and nitrones were classified clinically from non-irritant (Proxo, Imidazo, DMPO) to slightly irritant (Tempo, Doxo, PBN) according to the Draize protocol. In agreement with the clinical scoring, the TEWL values were significantly increased by Tempo, Doxo and PBN. TOLH, the hydroxylamine of Tempo and its major skin metabolite, did not cause skin irritation. The sensitizing effect was evaluated according to the Magnusson and Kligman test. The results showed no cutaneous hypersensitivity to all nitroxides and nitrones, indicating a weak sensitizing potential. That concludes that the nitroxides and nitrones tested in this study have a low potential of acute skin intolerance.

In vivo spin-label murine pharmacodynamics using low-frequency electron paramagnetic resonance imaging

A novel, very-low-frequency electron paramagnetic resonance (EPR) technique is used to image the distribution of several nitroxides with distinct pharmacologic compartment affinities in the abdomens of living mice. Image acquisition is sufficiently rapid to allow a time sequence of the distribution for each compound. The spectra and concentrations of these nitroxides are imaged with the use of spectral-spatial imaging to distinguish a single spatial dimension. Liver and bladder of the mouse anatomy are distinguished by this technique. After an intraperitoneal injection of the spin-label probes, a shift in the distribution of the compounds from the upper abdomen (primarily liver) to the lower abdomen (primarily bladder) is observed. The time dependence of the shift in regional distribution depends on the structural properties of the side chain attached to the spin label. These results indicate that this application of in vivo electron paramagnetic resonance imaging will provide a new method of magnetic resonance imaging for determination of pharmacodynamics in the body of an intact animal.

Nitroxide radical biostability in skin

Nitroxide radicals are important chemical tools in dermatologic research (e.g., for studying biophysical properties of skin lipids and epidermal membranes with the method of electron paramagnetic resonance, EPR, spectroscopy). However, nitroxides may loose their paramagnetic properties in biological tissues, which could limit their usefulness in biomedical applications. We analyzed the biostability of various chemical types of nitroxide radicals in keratinocytes, epidermis homogenate, and intact skin. EPR signal loss of imidazoline, pyrrolidine, piperidine, and oxazolidine nitroxides is attributed to their reduction to the corresponding hydroxylamine. The rate of nitroxide reduction in skin varies considerably with nitroxide ring structure and substitution. The order of nitroxide stability in isolated human keratinocytes, mouse epidermis homogenate, and intact mouse and human skin is imidazoline > pyrrolidine > di-t-butylnitroxide (DTBN) > piperidine > oxazolidine. Cationic nitroxides are reduced much faster than neutral or anionic probes, presumably due to transmembrane electron shuttle or internalization. The results indicate that imidazoline- and pyrrolidine-type nitroxides should be used when high biostability of nitroxides is needed. Piperidine-type nitroxides are versatile probes for studying one-electron transfer reactions in skin.