Imaging drug delivery to skin with stimulated Raman scattering microscopy

Biomedical applications, perspectives and tag design concepts in the cell - silent Raman window

Spectroscopic studies increasingly employ Raman tags exhibiting a signal in the cell - silent region of the Raman spectrum (1800-2800 cm-1), where bands arising from biological molecules are inherently absent. Raman tags bearing functional groups which contain a triple bond, such as alkyne and nitrile or a carbon-deuterium bond, have a distinct vibrational frequency in this region. Due to the lack of spectral background and cell-associated bands in the specific area, the implementation of those tags can help overcome the inherently poor signal-to-noise ratio and presence of overlapping Raman bands in measurements of biological samples. The cell - silent Raman tags allow for bioorthogonal imaging of biomolecules with improved chemical contrast and they have found application in analyte detection and monitoring, biomarker profiling and live cell imaging. This review focuses on the potential of the cell - silent Raman region, reporting on the tags employed for biomedical applications using variants of Raman spectroscopy.

Evaluation of chemical disposition in skin by stimulated Raman scattering microscopy

Tracking drug disposition in the skin in a non-destructive and at least semi-quantitative fashion is a relevant objective for the assessment of local (cutaneous) bioavailability. Confocal Raman spectroscopy has been shown potentially useful in this regard and, importantly, recent advances have enabled the presence of applied chemicals in the viable epidermis below the stratum corneum (SC) to be determined without ambiguity and having addressed the challenges of (a) background signals from endogenous species and noise and (b) signal attenuation due to absorption and scattering. This study aimed to confirm these observations using a different vibrational spectroscopy approach - specifically, stimulated Raman scattering (SRS) microscopy - and the more conventional in vitro skin penetration test (IVPT). SRS is a nonlinear optical imaging technique which enables more precise location of the skin surface and enhanced skin depth resolution relative to confocal Raman microscopy. The method can also probe larger areas of the sample under investigation and identify the localization of the permeating chemical in specific structural components of the skin. Here, SRS was shown capable of tracking the uptake and distribution of 4-cyanophenol (CP), the same model compound used in the recent confocal Raman investigation, at depths beyond the SC following skin treatment with different vehicles and for different times. The SRS results correlated well with those from the confocal Raman experiments, and both were consistent with independent IVPT measurements. Acquired images clearly delineated CP preference for the intercellular lipid layers of the SC relative to the corneocytes. The stage is now set to apply these and other correlative techniques to examine commercial drug products.

Determining topical product bioequivalence with stimulated Raman scattering microscopy

Generic drugs are essential for affordable medicine and improving accessibility to treatments. Bioequivalence (BE) is typically demonstrated by assessing a generic product's pharmacokinetics (PK) relative to a reference-listed drug (RLD). Accurately estimating cutaneous PK (cPK) at or near the site of action can be challenging for locally acting topical products. Certain cPK approaches are available for assessing local bioavailability (BA) in the skin. Stimulated Raman scattering (SRS) microscopy has unique capabilities enabling continuous, high spatial and temporal resolution and quantitative imaging of drugs within the skin. In this paper, we developed an approach based on SRS and a polymer-based standard reference for the evaluation of topical product BA and BE in human skin ex vivo. BE assessment of tazarotene-containing formulations was achieved using cPK parameters obtained within different skin microstructures. The establishment of BE between the RLD and an approved generic product was successfully demonstrated. Interestingly, within the constraints of the current study design the results suggest similar BA between the tested gel formulation and the reference cream formulation, despite the differences in the formulation/dosage form. Another formulation containing polyethylene glycol as the vehicle was demonstrated to be not bioequivalent to the RLD. Compared to using the SRS approach without a standard reference, the developed approach enabled more consistent and reproducible results, which is crucial in BE assessment. The abundant information from the developed approach can help to systematically identify key areas of study design that will enable a better comparison of topical products and support an assessment of BE.

Quantitative Raman chemical imaging of intracellular drug-membrane aggregates and small molecule drug precipitates in cytoplasmic organelles

Raman confocal microscopes have been used to visualize the distribution of small molecule drugs within different subcellular compartments. This visualization allows the discovery, characterization, and detailed analysis of the molecular transport phenomena underpinning the Volume of Distribution - a key parameter governing the systemic pharmacokinetics of small molecule drugs. In the specific case of lipophilic small molecules with large Volumes of Distribution, chemical imaging studies using Raman confocal microscopes have revealed how weakly basic, poorly soluble drug molecules can accumulate inside cells by forming stable, supramolecular complexes in association with cytoplasmic membranes or by precipitating out within organelles. To study the self-assembly and function of the resulting intracellular drug inclusions, Raman chemical imaging methods have been developed to measure and map the mass, concentration, and ionization state of drug molecules at a microscopic, subcellular level. Beyond the field of drug delivery, Raman chemical imaging techniques relevant to the study of microscopic drug precipitates and drug-lipid complexes which form inside cells are also being developed by researchers with seemingly unrelated scientific interests. Highlighting advances in data acquisition, calibration methods, and computational data management and analysis tools, this review will cover a decade of technological developments that enable the conversion of spectral signals obtained from Raman confocal microscopes into new discoveries and information about previously unknown, concentrative drug transport pathways driven by soluble-to-insoluble phase transitions occurring within the cytoplasmic organelles of eukaryotic cells.

Measurement of chemical penetration in skin using Stimulated Raman scattering microscopy and multivariate curve resolution - alternating least squares

The mechanistic understanding of skin penetration underpins the design, efficacy and risk assessment of many high-value products including functional personal care products, topical and transdermal drugs. Stimulated Raman scattering (SRS) microscopy, a label free chemical imaging tool, combines molecular spectroscopy with submicron spatial information to map the distribution of chemicals as they penetrate the skin. However, the quantification of penetration is hampered by significant interference from Raman signals of skin constituents. This study reports a method for disentangling exogeneous contributions and measuring their permeation profile through human skin combining SRS measurements with chemometrics. We investigated the spectral decomposition capability of multivariate curve resolution - alternating least squares (MCR-ALS) using hyperspectral SRS images of skin dosed with 4-cyanophenol. By performing MCR-ALS on the fingerprint region spectral data, the distribution of 4-cyanophenol in skin was estimated in an attempt to quantify the amount permeated at different depths. The reconstructed distribution was compared with the experimental mapping of CN, a strong vibrational peak in 4-cyanophenol where the skin is spectroscopically silent. The similarity between MCR-ALS resolved and experimental distribution in skin dosed for 4 h was 0.79 which improved to 0.91 for skin dosed for 1 h. The correlation was observed to be lower for deeper layers of skin where SRS signal intensity is low which is an indication of low sensitivity of SRS. This work is the first demonstration, to the best of our knowledge, of combining SRS imaging technique with spectral unmixing methods for direct observation and mapping of the chemical penetration and distribution in biological tissues.

Weakly Supervised Identification and Localization of Drug Fingerprints Based on Label-Free Hyperspectral CARS Microscopy

Understanding drug fingerprints in complex biological samples is essential for the development of a drug. Hyperspectral coherent anti-Stokes Raman scattering (HS-CARS) microscopy, a label-free nondestructive chemical imaging technique, can profile biological samples based on their endogenous vibrational contrast. Here, we propose a deep learning-assisted HS-CARS imaging approach for the investigation of drug fingerprints and their localization at single-cell resolution. To identify and localize drug fingerprints in complex biological systems, an attention-based deep neural network, hyperspectral attention net (HAN), was developed. By formulating the task to a multiple instance learning problem, HAN highlights informative regions through the attention mechanism when being trained on whole-image labels. Using the proposed technique, we investigated the drug fingerprints of a hepatitis B virus therapy in murine liver tissues. With the increase in drug dosage, higher classification accuracy was observed, with an average area under the curve (AUC) of 0.942 for the high-dose group. Besides, highly informative tissue structures predicted by HAN demonstrated a high degree of similarity with the drug localization shown by the in situ hybridization staining results. These results demonstrate the potential of the proposed deep learning-assisted optical imaging technique for the label-free profiling, identification, and localization of drug fingerprints in biological samples, which can be extended to nonperturbative investigations of complex biological systems under various biological conditions.

Quantification of Chemical Uptake into the Skin by Vibrational Spectroscopies and Stratum Corneum Sampling

Evaluation of the bioavailability of drugs intended to act within the skin following the application of complex topical products requires the application of multiple experimental tools, which must be quantitative, validated, and, ideally and ultimately, sufficiently minimally invasive to permit use in vivo. The objective here is to show that both infrared (IR) and Raman spectroscopies can assess the uptake of a chemical into the stratum corneum (SC) that correlates directly with its quantification by the adhesive tape-stripping method. Experiments were performed ex vivo using excised porcine skin and measured chemical disposition in the SC as functions of application time and formulation composition. The quantity of chemicals in the SC removed on each tape-strip was determined from the individually measured IR and Raman signal intensities of a specific molecular vibration at a frequency where the skin is spectroscopically silent and by a subsequent conventional extraction and chromatographic analysis. Correlations between the spectroscopic results and the chemical quantification on the tape-strips were good, and the effects of longer application times and the use of different vehicles were clearly delineated by the different measurement techniques. Based on this initial investigation, it is now possible to explore the extent to which the spectroscopic approach (and Raman in particular) may be used to interrogate chemical disposition deeper in the skin and beyond the SC.

Imaging Low-Temperature Phases of Ice with Polarization-Resolved Hyperspectral Stimulated Raman Scattering Microscopy

Water freezes into various phases of ice under different cryogenic temperatures and pressure conditions, such as ice I_h and ice XI at normal pressure. Vibrational imaging with high spectral, spatial, and polarization resolutions could provide detailed information on ice, including the phases and crystal orientations at the microscopic level. Here, we report in situ stimulated Raman scattering (SRS) imaging of ice to analyze the vibrational spectral changes of the OH stretching modes associated with the phase transition between ice I_h and ice XI. In addition, polarization-resolved measurements were performed to reveal the microcrystal orientations of the two phases of ice, with the spatial-dependent anisotropy pattern indicating the inhomogeneous distribution of their orientations. Furthermore, the angular patterns were theoretically explained by third-order nonlinear optics with the known crystal symmetries of the ice phases. Our work may provide new opportunities to investigate many intriguing physical chemistry properties of ice under low-temperature conditions.

Topical Semisolid Products-Understanding the Impact of Metamorphosis on Skin Penetration and Physicochemical Properties

Recently, the United States Food and Drug Administration published a series of product-specific guidance for the development of topical drugs, with in vitro options consisting of qualitative sameness (Q1) and quantitative sameness (Q2) assessment of formulations, physiochemical and structural characterization of formulations (Q3), and, potentially, in vitro drug release and permeation tests. In these tests, the topical semisolid product's critical quality attributes (CQAs), such as rheological properties, thermodynamic activity, particle size, globule size, and rate/extent of drug release/permeation, are evaluated to ensure the desired product quality. However, alterations in these CQAs of the drug products may occur under 'in use' conditions because of various metamorphosis events, such as evaporation that leads to supersaturation and crystallization, which may eventually result in specific failure modes of semisolid products. Under 'in use' conditions, a limited amount of formulation is applied to the skin, where physicochemical characteristics of the formulation are substantially altered from primary state to secondary and, eventually, tertiary state on the skin. There is an urgent need to understand the behavior of topical semisolid products under 'in use' conditions. In this review, we attempt to cover a series of metamorphosis events and their impact on CQAs (Q3 attributes), such as viscosity, drug activity, particle size, globule size, and drug release/permeation of topical semisolid products.

Raman Spectroscopic Tools to Probe the Skin-(Trans)dermal Formulation Interface

Medicines designed to deliver the active pharmaceutical ingredient either into or through the skin─often referred to as topicals and transdermals, respectively─are generally considered to be complex drug products. A particular challenge faced by these formulations is identifying a suitable method (ideally, in terms of specificity, accuracy, precision, and robustness) or combination of methods with which to assess the amount and rate of drug delivery to the target site. Significant research currently aims to identify and validate relevant and minimally invasive techniques that can be used to quantify both the levels of the drug attained within different parts of the skin and the kinetics with which the drug is taken up into the skin and cleared therefrom into the systemic circulation. Here, the application of confocal Raman microspectroscopy and imaging to interrogate events integral to the performance of topical and transdermal drug products at the formulation-skin interface is illustrated. Visualization, depth slicing, and profiling are used (a) to elucidate key chemical properties of both the delivery system and the skin that have impact on their interaction and the manner in which drug transfer from one to the other may occur, (b) for the transformation of a drug product from that manufactured into a residual phase post-application and inunction into the skin (including the potential for important changes in solubility of the active compound), and (c) for drug absorption into the skin and its subsequent '"clearance" into deeper layers and beyond. Overall, the Raman tools described offer both qualitative and potentially semi-quantitative insights into topical and transdermal drug product performance and provide information useful for formulation improvement and optimization.

Miniaturized Chemical Tags for Optical Imaging

Recent advances in optical bioimaging have prompted the need for minimal chemical reporters that can retain the molecular recognition properties and activity profiles of biomolecules. As a result, several methodologies to reduce the size of fluorescent and Raman labels to a few atoms (e.g., single aryl fluorophores, Raman-active triple bonds and isotopes) and embed them into building blocks (e.g., amino acids, nucleobases, sugars) to construct native-like supramolecular structures have been described. The integration of small optical reporters into biomolecules has also led to smart molecular entities that were previously inaccessible in an expedite manner. In this article, we review recent chemical approaches to synthesize miniaturized optical tags as well as some of their multiple applications in biological imaging.

Bringing Vibrational Imaging to Chemical Biology with Molecular Probes

As an emerging optical imaging modality, stimulated Raman scattering (SRS) microscopy provides invaluable opportunities for chemical biology studies using its rich chemical information. Through rapid progress over the past decade, the development of Raman probes harnessing the chemical biology toolbox has proven to play a key role in advancing SRS microscopy and expanding biological applications. In this perspective, we first discuss the development of biorthogonal SRS imaging using small tagging of triple bonds or isotopes and highlight their unique advantages for metabolic pathway analysis and microbiology investigations. Potential opportunities for chemical biology studies integrating small tagging with SRS imaging are also proposed. We next summarize the current designs of highly sensitive and super-multiplexed SRS probes, as well as provide future directions and considerations for next-generation functional probe design. These rationally designed SRS probes are envisioned to bridge the gap between SRS microscopy and chemical biology research and should benefit their mutual development.

Novel aspects of Raman spectroscopy in skin research

The analytical technology of Raman spectroscopy has an almost 100‐year history. During this period, many modifications and developments happened in the method like discovery of laser, improvements in optical elements and sensitivity of spectrometer and also more advanced light detection systems. Many types of the innovative techniques appeared (e.g. Transmittance Raman spectroscopy, Coherent Raman Scattering microscopy, Surface‐Enhanced Raman scattering and Confocal Raman spectroscopy/microscopy). This review article gives a short description about these different Raman techniques and their possible applications. Then, a short statistical part is coming about the appearance of Raman spectroscopy in the scientific literature from the beginnings to these days. The third part of the paper shows the main application options of the technique (especially confocal Raman spectroscopy) in skin research, including skin composition analysis, drug penetration monitoring and analysis, diagnostic utilizations in dermatology and cosmeto‐scientific applications. At the end, the possible role of artificial intelligence in Raman data analysis and the regulatory aspect of these techniques in dermatology are briefly summarized. For the future of Raman Spectroscopy, increasing clinical relevance and in vivo applications can be predicted with spreading of non‐destructive methods and appearance with the most advanced instruments with rapid analysis time.

Cutaneous Pharmacokinetic Approaches to Compare Bioavailability and/or Bioequivalence for Topical Drug Products

The evaluation of bioequivalence (BE) involves comparing the test product to its reference product in a study whose fundamental scientific principles allow inferring of the clinical performance of the products. Several test methods have been discussed and developed to evaluate topical bioavailability (BA) and BE. Pharmacokinetics-based approaches characterize the rate and extent to which an active ingredient becomes available at or near its site of action in the skin. Such methodologies are considered to be among the most accurate, sensitive, and reproducible approaches for determining the BA or BE of a product.

Stimulated Raman scattering microscopy in chemistry and life science - Development, innovation, perspectives

In this review, we present a summary of the basics of the Stimulated Raman Scattering (SRS) phenomenon, methods of detecting the signal, and collection of the SRS images. We demonstrate the advantages of SRS imaging, and recent developments, but also the limitations, especially in image capture speeds and spatial resolution. We also compare the use of SRS microscopy in biological system studies with other techniques such as fluorescence microscopy, second-harmonic generation (SHG)-based microscopy, coherent anti-Stokes Raman scattering (CARS), and spontaneous Raman, and we show the compatibility of SRS-based systems with other discussed methods. The review is also focused on indicating innovations in SRS microscopy, on the background of which we present the layout and performance of our homemade setup built from commercially available elements enabling for imaging of the molecular structure of single cells over the spectral range of 800-3600 cm^-1. Methods of image analysis are discussed, including machine learning methods for obtaining images of the distribution of selected molecules and for the detection of pathological lesions in tissues or malignant cells in the context of clinical diagnosis of a wide range of diseases with the use of SRS microscopy. Finally, perspectives for the development of SRS microscopy are proposed.

Direct visualization of general anesthetic propofol on neurons by stimulated Raman scattering microscopy

The consensus for the precise mechanism of action of general anesthetics is through allosteric interactions with GABA receptors in neurons. However, it has been speculated that these anesthetics may also interact with the plasma membrane on some level. Owing to the small size of anesthetics, direct visualization of these interactions is difficult to achieve. We demonstrate the ability to directly visualize a deuterated analog of propofol in living cells using stimulated Raman scattering (SRS) microscopy. Our findings support the theory that propofol is highly concentrated and interacts primarily through non-specific binding to the plasma membrane of neurons. Additionally, we show that SRS microscopy can be used to monitor the dynamics of propofol binding using real-time, live-cell imaging. The strategy used to visualize propofol can be applied to other small molecule drugs that have been previously invisible to traditional imaging techniques

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Multi-modal SRS developed for real-time biological imaging of small molecule substances

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Propofol primarily concentrates at the cell membrane of neurons

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Anesthesia dynamics can be monitored in real-time with SRS

Temporal imaging of drug dynamics in live cells using stimulated Raman scattering microscopy and a perfusion cell culture system

Multimodal imaging of drug uptake and cell viability analysis in the same live cell population is enabled using a perfusion cell culture system.

Time-course quantitative mapping of caffeine within the epidermis, using high-contrast pump-probe stimulated Raman scattering microscopy

Background

An assessment of the drug penetration and distribution profiles within the skin is essential in dermatology and cosmetology. Recent advances in label‐free imaging technologies have facilitated the direct detection of unlabeled compounds in tissues, with high resolution. However, it remains challenging to provide quantitative time‐course distribution maps of drugs within the complex skin tissue. The present study aims at acquiring the real‐time quantitative skin penetration profiles of topically applied caffeine, by means of a combination of pump–probe phase‐modulated stimulated Raman scattering (PM‐SRS) and confocal reflection microscopy. The recently developed PM‐SRS microscopy is a unique imaging tool that can minimize strong background signals through a pulse‐shaping technique, while providing high‐contrast images of small molecules in tissues.

Materials and methods

Reconstructed human skin epidermis models were used in order to analyze caffeine penetration in tissues. The penetration profiles of caffeine in an aqueous solution, an oil‐in‐water gel, and a water‐in‐oil gel were examined by combining PM‐SRS and confocal reflection microscopy.

Results

The characteristic Raman signal of caffeine was directly detected in the skin model using PM‐SRS. Integrating PM‐SRS and confocal reflection microscopy allowed real‐time concentration maps of caffeine to be obtained from formulation samples, within the skin model. Compared with the conventional Raman detection method, PM‐SRS lowered the background tissue‐oriented signals and supplied high‐contrast images of caffeine.

Conclusion

We successfully established real‐time skin penetration profiles of caffeine from different formulations. PM‐SRS microscopy proved to be a powerful, non‐invasive, and real‐time depth‐profile imaging technique for use in quantitative studies of topically applied drugs.

A decade of alkyne-tag Raman imaging (ATRI): applications in biological systems

Alkyne functional groups have Raman signatures in a region (1800 cm-1 to 2800 cm-1) that is free from interference from cell components, known as the "silent region", and alkyne signals in this region were first utilized a decade ago to visualize the nuclear localization of a thymidine analogue EdU. Since then, the strategy of Raman imaging of biological samples by using alkyne functional groups, called alkyne-tag Raman imaging (ATRI), has become widely used. This article reviews the applications of ATRI in biological samples ranging from organelles to whole animal models, and briefly discusses the prospects for this technique.

Label-free skin penetration analysis using time-resolved, phase-modulated stimulated Raman scattering microscopy

Skin penetration analysis of topically applied drugs or active compounds is essential in biomedical applications. Stimulated Raman scattering (SRS) microscopy is a promising label-free skin penetration analysis tool. However, conventional SRS microcopy suffers from limited signal contrast owing to strong background signals, which prevents its use in low-concentration drug imaging. Here, we present a skin penetration analysis method of topical agents using recently developed phase-modulated SRS (PM-SRS) microscopy. PM-SRS uses phase modulation and time-resolved signal detection to suppress both nonlinear background signals and Raman background signals from a tissue. A proof-of-concept experiment with a topically applied skin moisturizing agent (ectoine) in an in vitro skin tissue model revealed that PM-SRS with 1.7-ps probe delay yields a signal contrast 40 times higher than that of conventional amplitude-modulated SRS (AM-SRS). Skin penetration measurement of a topical therapeutic drug (loxoprofen sodium) showed that the mean drug concentration at the tissue surface layer after 240 min was 47.3 ± 4.8 mM. The proposed PM-SRS microscopy can be employed to monitor the spatial and temporal pharmacokinetics of small molecules in the millimolar concentration regime.

Targeted Delivery of Zinc Pyrithione to Skin Epithelia

Zinc pyrithione (ZnPT) is an anti-fungal drug delivered as a microparticle to skin epithelia. It is one of the most widely used ingredients worldwide in medicated shampoo for treating dandruff and seborrheic dermatitis (SD), a disorder with symptoms that include skin flaking, erythema and pruritus. SD is a multi-factorial disease driven by microbiol dysbiosis, primarily involving Malassezia yeast. Anti-fungal activity of ZnPT depends on the cutaneous availability of bioactive monomeric molecular species, occurring upon particle dissolution. The success of ZnPT as a topical therapeutic is underscored by the way it balances treatment efficacy with formulation safety. This review demonstrates how ZnPT achieves this balance, by integrating the current understanding of SD pathogenesis with an up-to-date analysis of ZnPT pharmacology, therapeutics and toxicology. ZnPT has anti-fungal activity with an average in vitro minimum inhibitory concentration of 10-15 ppm against the most abundant scalp skin Malassezia species (Malassezia globosa and Malassezia restrica). Efficacy is dependent on the targeted delivery of ZnPT to the skin sites where these yeasts reside, including the scalp surface and hair follicle infundibulum. Imaging and quantitative analysis tools have been fundamental for critically evaluating the therapeutic performance and safety of topical ZnPT formulations. Toxicologic investigations have focused on understanding the risk of local and systemic adverse effects following exposure from percutaneous penetration. Future research is expected to yield further advances in ZnPT formulations for SD and also include re-purposing towards a range of other dermatologic applications, which is likely to have significant clinical impact.

Quantification of the nonlinear susceptibility of the hydrogen and deuterium stretch vibration for biomolecules in coherent Raman micro-spectroscopy

Deuterium labelling is increasingly used in coherent Raman imaging of complex systems, such as biological cells and tissues, to improve chemical specificity. Nevertheless, quantitative coherent Raman susceptibility spectra for deuterated compounds have not been previously reported. Interestingly, it is expected theoretically that -D stretch vibrations have a Raman susceptibility lower than -H stretch vibrations, with the area of their imaginary part scaling with their wavenumber, which is shifted from around 2900 cm-1 for C-H into the silent region around 2100 cm-1 for C-D. Here, we report quantitative measurements of the nonlinear susceptibility of water, succinic acid, oleic acid, linoleic acid and deuterated isoforms. We show that the -D stretch vibration has indeed a lower area, consistent with the frequency reduction due to the doubling of atomic mass from hydrogen to deuterium. This finding elucidates an important trade-off between chemical specificity and signal strength in the adoption of deuterium labelling as an imaging strategy for coherent Raman microscopy.

DESI-MS imaging to visualize spatial distribution of xenobiotics and endogenous lipids in the skin

The cutaneous biodistribution method (CBM) yields a high-resolution quantitative profile of drug deposition as a function of skin depth. However, it provides limited details about drug spatial distribution or penetration pathways. Mass spectrometry imaging (MSI) can complement the detailed quantitative data generated by CBM studies. The objectives of this work were to use desorption electrospray ionization (DESI)-MSI to (i) investigate the spatial cutaneous distributions of a topically applied drug and excipient and relate them to skin structures and (ii) image endogenous skin components and combine these results to gain insight into drug penetration routes. Porcine skin was used to compare two bioequivalent creams of econazole nitrate (ECZ) and a micelle formulation based on D-α-tocopheryl succinate polyethylene glycol 1000 (TPGS). DESI-MSI successfully imaged the cutaneous spatial distribution of ECZ and TPGS in 40 µm-thick horizontal sections and vertical cross-sections of the skin. Interestingly, clinically bioequivalent formulations did not appear to exhibit the same molecular distribution of ECZ in XY-horizontal sections. DESI-MSI also enabled visualization of TPGS (m/z 772.4706), mainly in the upper epidermis (≤80 µm). In conclusion, through co-localization of drugs and excipients with endogenous elements of the skin, DESI-MSI could further our understanding of the cutaneous penetration pathways of xenobiotics.

Localization of Nonblinking Point Sources Using Higher-Order-Mode Detection and Optical Heterodyning: Developing a Strategy for Extending the Scope of Molecular, Super-resolution Imaging

While the stochastic, "blinking" nature of fluorescent systems has enabled the super-resolution of their localization by the fitting of their point-spread functions (PSFs), this strategy cannot be exploited for similar resolution of "nonblinking" systems, such as those that might be encountered in a coherent Raman experiment. An alternative method for subdiffraction-limited imaging lies in the exploitation of optical heterodyning. For example, if a Gaussian PSF (a TEM00 mode) of a point emitter is displaced with respect to the origin of the optical system, photons in the higher-order TEM modes carry information about that displacement. Information concerning the displacement can be extracted from photons in these higher-order modes. These photons can be collected by optical heterodyning, which exploits the large gain in a detector's response to an optical signal from an emitter coupled to a local oscillator, which is prepared in the TEM of interest, e.g., TEM10. We have generalized and developed the heterodyning technique to localize point emitters via the detection of higher-order spatial modes. We have developed a theoretical approach to find a practical estimation limit of the localization parameters using a realistic model that accounts for shot noise, background noise, and Gaussian noise. To demonstrate the applicability of the method, we designed experiments in which a laser is a surrogate for one and two point emitters. Using the Fisher information and its accompanying Cramér-Rao lower bound, we demonstrate super-resolution localization in these cases: we show that objects can be localized to roughly 2-3 orders of magnitude of their point-spread function's size for a given optical system. Finally and most importantly, it is suggested that the results will ultimately be generalizable to multiple emitters and, most importantly, to "nonblinking" molecular systems, which will be essential for broadening the scope of super-resolution measurements beyond the limits of fluorescence-based techniques.

A topical formulation containing quercetin-loaded microcapsules protects against oxidative and inflammatory skin alterations triggered by UVB irradiation: enhancement of activity by microencapsulation

Ultraviolet B (UVB) irradiation causes free radical production, increase inflammation and oxidative stress, thus, supporting the use of antioxidants by topical administration as therapeutic approaches. Quercetin (QC) is a flavonoid with antioxidant activity, however, high liposolubility makes it difficult to remain in the viable skin layer. Thus, this study evaluated whether microencapsulation of QC would enhance its activity in comparison with the same dose of free QC (non-active dose) and unloaded-microcapsules added in formulation for topical administration in a mouse model of UVB irradiation targeting the skin. Topical formulation containing Quercetin-loaded microcapsules (TFcQCMC) presents physico-chemical (colour, consistence, phase separation and pH) and functional antioxidant stability at 4 °C, room temperature and 40 °C for 6 months. TFcQCMC inhibited the UVB-triggered depletion of antioxidants observed by GSH (reduced glutathione), ability to reduce iron, ability to scavenge 2,2'-azinobis radical and catalase activity. TFcQCMC also inhibited markers of oxidation (lipid hydroperoxides and superoxide anion production). Concerning inflammation, TFcQCMC reduced the production of inflammatory cytokines, matrix metalloproteinase-9 activity, skin edoema, collagen fibre damage, myeloperoxidase activity/neutrophil recruitment, mast cell and sunburn cell counts. The pharmacological activity of TFcQCMC was not shared by the same pharmaceutical form containing the same dose of free QC or unloaded control microcapsules.

Raman microscopy for skin evaluation

Many dermatological studies have had limited success in revealing skin function because conventional histological methods are known to affect skin components. Recent progress in non-invasive optical imaging has enabled non-invasive visualization of the structure of each skin layer. However, it remains difficult to identify individual skin components. Alternatively, it is possible to obtain molecular vibrational signatures using spontaneous Raman scattering microscopy. Spontaneous Raman scattering microscopy requires long acquisition times and is rarely applied to skin imaging, especially because skin components, such as water and transepidermal agents, undergo relatively rapid changes. Consequently, non-linear Raman microscopies, such as coherent anti-Stokes Raman scattering and stimulated Raman scattering, have gradually been applied to acquire molecular imaging of skin tissue. In this review, the applications of Raman microscopies used to evaluate skin and research trends are presented. The applications of spontaneous Raman microscopy to in vivo human skin evaluation are first demonstrated with typical applications. Finally, the latest application of coherent Raman scattering microscopy to visualize 3D intracellular morphologies in the human epidermis during differentiation is described.

Coherent anti-Stokes Raman scattering imaging of microcalcifications associated with breast cancer

Chemical imaging of calcifications was demonstrated in the depth of a tissue. Using long wavelength excitation, broadband coherent anti-Stokes Raman scattering and hierarchical cluster analysis, imaging and chemical analysis were performed 2 mm below the skin level in a model system. Applications to breast cancer diagnostics and imaging are discussed together with the methods to further extend the depth and improve the spatial resolution of chemical imaging.

Coherent Raman Scattering Microscopy in Oncology Pharmacokinetic Research

The high attrition rates of anti-cancer drugs during clinical development remains a bottleneck problem in pharmaceutical industry. This is partially due to the lack of quantitative, selective, and rapid readouts of anti-cancer drug activity in situ with high resolution. Although fluorescence microscopy has been commonly used in oncology pharmacological research, fluorescent labels are often too large in size for small drug molecules, and thus may disturb the function or metabolism of these molecules. Such challenge can be overcome by coherent Raman scattering microscopy, which is capable of chemically selective, highly sensitive, high spatial resolution, and high-speed imaging, without the need of any labeling. Coherent Raman scattering microscopy has tremendously improved the understanding of pharmaceutical materials in the solid state, pharmacokinetics of anti-cancer drugs and nanocarriers in vitro and in vivo. This review focuses on the latest applications of coherent Raman scattering microscopy as a new emerging platform to facilitate oncology pharmacokinetic research.

Label-free spectral imaging to study drug distribution and metabolism in single living cells

During drug development, evaluation of drug and its metabolite is an essential process to understand drug activity, stability, toxicity and distribution. Liquid chromatography (LC) coupled with mass spectrometry (MS) has become the standard analytical tool for screening and identifying drug metabolites. Unlike LC/MS approach requiring liquifying the biological samples, we showed that spectral imaging (or spectral microscopy) could provide high-resolution images of doxorubicin (dox) and its metabolite doxorubicinol (dox’ol) in single living cells. Using this new method, we performed measurements without destroying the biological samples. We calculated the rate constant of dox translocating from extracellular moiety into the cell and the metabolism rate of dox to dox’ol in living cells. The translocation rate of dox into a single cell for spectral microscopy and LC/MS approaches was similar (~ 1.5 pM min⁻¹ cell⁻¹). When compared to spectral microscopy, the metabolism rate of dox was underestimated for about every 500 cells using LC/MS. The microscopy approach further showed that dox and dox’ol translocated to the nucleus at different rates of 0.8 and 0.3 pM min⁻¹, respectively. LC/MS is not a practical approach to determine drug translocation from cytosol to nucleus. Using various methods, we confirmed that when combined with a high-resolution imaging, spectral characteristics of a molecule could be used as a powerful approach to analyze drug metabolism. We propose that spectral microscopy is a new method to study drug localization, translocation, transformation and identification with a resolution at a single cell level, while LC/MS is more appropriate for drug screening at an organ or tissue level.

Review of Stimulated Raman Scattering Microscopy Techniques and Applications in the Biosciences

Stimulated Raman scattering (SRS) microscopy is a nonlinear optical imaging method for visualizing chemical content based on molecular vibrational bonds. Featuring high speed, high resolution, high sensitivity, high accuracy, and 3D sectioning, SRS microscopy has made tremendous progress toward biochemical information acquisition, cellular function investigation, and label-free medical diagnosis in the biosciences. In this review, the principle of SRS, system design, and data analysis are introduced, and the current innovations of the SRS system are reviewed. In particular, combined with various bio-orthogonal Raman tags, the applications of SRS microscopy in cell metabolism, tumor diagnosis, neuroscience, drug tracking, and microbial detection are briefly examined. The future prospects for SRS microscopy are also shared.

Quinine copolymer reporters promote efficient intracellular DNA delivery and illuminate a protein-induced unpackaging mechanism

Polymeric vehicles that efficiently package and controllably release nucleic acids enable the development of safer and more efficacious strategies in genetic and polynucleotide therapies. Developing delivery platforms that endogenously monitor the molecular interactions, which facilitate binding and release of nucleic acids in cells, would aid in the rational design of more effective vectors for clinical applications. Here, we report the facile synthesis of a copolymer containing quinine and 2-hydroxyethyl acrylate that effectively compacts plasmid DNA (pDNA) through electrostatic binding and intercalation. This polymer system poly(quinine-co-HEA) packages pDNA and shows exceptional cellular internalization, transgene expression, and low cytotoxicity compared to commercial controls for several human cell lines, including HeLa, HEK 293T, K562, and keratinocytes (N/TERTs). Using quinine as an endogenous reporter for pDNA intercalation, Raman imaging revealed that proteins inside cells facilitate the unpackaging of polymer-DNA complexes (polyplexes) and the release of their cargo. Our work showcases the ability of this quinine copolymer reporter to not only facilitate effective gene delivery but also enable diagnostic monitoring of polymer-pDNA binding interactions on the molecular scale via Raman imaging. The use of Raman chemical imaging in the field of gene delivery yields unprecedented insight into the unpackaging behavior of polyplexes in cells and provides a methodology to assess and design more efficient delivery vehicles for gene-based therapies.

Non-invasive drug delivery technology: development and current status of transdermal drug delivery devices, techniques and biomedical applications

Pay-load deliveries across the skin barrier to the systemic circulation have been one of the most challenging delivery options. Necessitated requirements of the skin and facilitated skin layer cross-over delivery attempts have resulted in development of different non-invasive, non-oral methods, devices and systems which have been standardized, concurrently used and are in continuous upgrade and improvements. Iontophoresis, electroporation, sonophoresis, magnetophoresis, dermal patches, nanocarriers, needled and needle-less shots, and injectors are among some of the methods of transdermal delivery. The current review covers the current state of the art, merits and shortcomings of the systems, devices and transdermal delivery patches, including drugs' and other payloads' passage facilitation techniques, permeation and absorption feasibility studies, as well as physicochemical properties affecting the delivery through different transdermal modes along with examples of drugs, vaccines, genes and other payloads.

The application of label-free imaging technologies in transdermal research for deeper mechanism revealing

The penetration behavior of topical substances in the skin not only relates to the transdermal delivery efficiency but also involves the safety and therapeutic effect of topical products, such as sunscreen and hair growth products. Researchers have tried to illustrate the transdermal process with diversified theories and technologies. Directly observing the distribution of topical substances on skin by characteristic imaging is the most convincing approach. Unfortunately, fluorescence labeling imaging, which is commonly used in biochemical research, is limited for transdermal research for most topical substances with a molecular mass less than 500 Da. Label-free imaging technologies possess the advantages of not requiring any macromolecular dyes, no tissue destruction and an extensive substance detection capability, which has enabled rapid development of such technologies in recent years and their introduction to biological tissue analysis, such as skin samples. Through the specific identification of topical substances and endogenous tissue components, label-free imaging technologies can provide abundant tissue distribution information, enrich theoretical and practical guidance for transdermal drug delivery systems. In this review, we expound the mechanisms and applications of the most popular label-free imaging technologies in transdermal research at present, compare their advantages and disadvantages, and forecast development prospects.

Label-Free Quantification of Pharmacokinetics in Skin with Stimulated Raman Scattering Microscopy and Deep Learning

The treatment of inflammatory skin conditions relies on a deep understanding of how drugs and tissue behave and interact. Although numerous methods have been developed that aim to follow and quantify topical drug pharmacokinetics, these tools can come with limitations, assumptions, and trade-offs that do not allow for real-time tracking of drug flow and flux on the cellular level in situ. We have developed a quantitative imaging toolkit that makes use of stimulated Raman scattering microscopy and deep learning-based computational image analysis to quantify the uptake of specific drug molecules in skin without the need for labels. Analysis powered by trained convolutional neural networks precisely identified features such as cells, cell junctions, and cell types within skin to enable multifactorial analysis of skin pharmacokinetics. We imaged and quantified the flow and flux of small molecule drugs through the layers and structures of ex vivo nude mouse ear skin and extracted pharmacokinetic parameters through convolutional neural network-based image processing, including relative area under the curve accumulation, time of maximum drug concentration, and in situ partition ratios. This approach, which facilitates the direct observation and quantification of pharmacokinetics, can be used to glean mechanistic insight into underlying phenomena in skin pharmacokinetics.

Profiling of drug crystallization in the skin

BACKGROUND

Drug crystallization following application of transdermal and topical formulations may potentially compromise the delivery of drugs to the skin. This phenomenon was found to be limited to the superficial layers of the stratum corneum (~7 µm) in our recent reports and tape stripping of the skin samples was necessary. It remains a significant challenge to profile drug crystallization in situ without damaging the skin samples.

METHODS

This work reports the application of an X-ray microbeam via synchrotron SAXS/WAXS analysis to monitor drug crystallization in the skin, especially in the deeper skin layers. Confocal Raman spectroscopy (CRS) was employed to examine drug distribution in the skin to complement the detection of drug crystallization using SAXS/WAXS analysis.

RESULTS

Following application of saturated drug solutions (ibuprofen, diclofenac acid, and salts), CRS depth profiles confirmed that the drugs generally were delivered to a depth of ~15 - 20 µm in the skin. This was compared with the WAXS profiles that measured drug crystal diffraction at a depth of up to ~25 µm of the skin.

CONCLUSION

This study demonstrates the potential of synchrotron SAXS/WAXS analysis for profiling of drug crystallization in situ in the deeper skin layers without pre-treatment for the skin samples. [Figure: see text].

Intradermal Delivery of an Immunomodulator for Basal Cell Carcinoma; Expanding the Mechanistic Insight into Solid Microneedle-Enhanced Delivery of Hydrophobic Molecules

Basal cell carcinoma (BCC) is the most common cutaneous malignancy in humans. One of the most efficacious drugs used in the management of BCC is the immunomodulator, imiquimod. However, imiquimod has physiochemical properties that limit its permeation to reach deeper, nodular tumor lesions. The use of microneedles may overcome such limitations and promote intradermal drug delivery. The current work evaluates the effectiveness of using an oscillating microneedle device Dermapen either as a pre- or post-treatment with 5% w/w imiquimod cream application to deliver the drug into the dermis. The effectiveness of microneedles to enhance the permeation of imiquimod was evaluated ex vivo using a Franz cell setup. After a 24-h permeation experiment, sequential tape strips and vertical cross-sections of the porcine skin were collected and analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS). In addition, respective Franz cell components were analyzed using high-performance liquid chromatography (HPLC). Analysis of porcine skin cross-sections demonstrated limited dermal permeation of 5% w/w imiquimod cream. Similarly, limited dermal permeation was also seen when 5% w/w imiquimod cream was applied to the skin that was pretreated with the Dermapen, this is known as poke-and-patch. In contrast, when the formulation was applied first to the skin prior to Dermapen application, this is known as patch-and-poke, we observed a significant increase in intradermal permeation of imiquimod. Such enhancement occurs immediately upon microneedle application, generating an intradermal depot that persists for up to 24 h. Intradermal colocalization of isostearic acid, an excipient in the cream, with imiquimod within microneedle channels was also demonstrated. However, such enhancement in intradermal delivery of imiquimod was not observed when the patch-and-poke strategy was used with a non-oscillating microneedle applicator, the Dermastamp. The current work highlights that using the patch-and-poke approach with an oscillating microneedle pen may be a viable approach to improve the current treatment in BCC patients who would prefer a less invasive intervention relative to surgery.

Enhanced Follicular Delivery of Finasteride to Human Scalp Skin Using Heat and Chemical Penetration Enhancers

PURPOSE

The aim of this work was to evaluate whether improved topical delivery of finasteride, focussed to the hair follicles of human scalp skin could be achieved with application of short durations of heat and use of specific chemical penetration enhancers.

METHODS

Franz cell experiments with human scalp skin were performed with a range of chemical penetration enhancers at 32°C and 45°C to simulate normal and heated conditions. Selected chemical penetration enhancers were taken forward for finite dose Franz cell studies which examined the effect of heat produced by a prototype external heating system that supplied either 20 or 30 min of additional heat over both a 24 h and a 1 h time period.

RESULTS

Short durations of externally applied heat significantly increased finasteride penetration into human scalp skin after 24 h. Analysis of drug distribution in the skin after 1 h and 24 h indicated that both heat and chemical penetration enhancer selection influenced drug delivery to the hair follicles.

CONCLUSION

The use of short durations of heat in combination with specific chemical penetration enhancers was able to increase the delivery of finasteride to human scalp skin and provide focussed drug delivery to the hair follicles.

Metal-Organic Framework Nanoparticles Induce Pyroptosis in Cells Controlled by the Extracellular pH

Ion homeostasis is essential for cellular survival, and elevated concentrations of specific ions are used to start distinct forms of programmed cell death. However, investigating the influence of certain ions on cells in a controlled way has been hampered due to the tight regulation of ion import by cells. Here, it is shown that lipid-coated iron-based metal-organic framework nanoparticles are able to deliver and release high amounts of iron ions into cells. While high concentrations of iron often trigger ferroptosis, here, the released iron induces pyroptosis, a form of cell death involving the immune system. The iron release occurs only in slightly acidic extracellular environments restricting cell death to cells in acidic microenvironments and allowing for external control. The release mechanism is based on endocytosis facilitated by the lipid-coating followed by degradation of the nanoparticle in the lysosome via cysteine-mediated reduction, which is enhanced in slightly acidic extracellular environment. Thus, a new functionality of hybrid nanoparticles is demonstrated, which uses their nanoarchitecture to facilitate controlled ion delivery into cells. Based on the selectivity for acidic microenvironments, the described nanoparticles may also be used for immunotherapy: the nanoparticles may directly affect the primary tumor and the induced pyroptosis activates the immune system.

Imaging and quantifying drug delivery in skin - Part 2: Fluorescence andvibrational spectroscopic imaging methods

Understanding the delivery and diffusion of topically-applied drugs on human skin is of paramount importance in both pharmaceutical and cosmetics research. This information is critical in early stages of drug development and allows the identification of the most promising ingredients delivered at optimal concentrations to their target skin compartments. Different skin imaging methods, invasive and non-invasive, are available to characterize and quantify the spatiotemporal distribution of a drug within ex vivo and in vivo human skin. The first part of this review detailed invasive imaging methods (autoradiography, MALDI and SIMS). This second part reviews non-invasive imaging methods that can be applied in vivo: i) fluorescence (conventional, confocal, and multiphoton) and second harmonic generation microscopies and ii) vibrational spectroscopic imaging methods (infrared, confocal Raman, and coherent Raman scattering microscopies). Finally, a flow chart for the selection of imaging methods is presented to guide human skin ex vivo and in vivo drug delivery studies.

Biological imaging of chemical bonds by stimulated Raman scattering microscopy

All molecules consist of chemical bonds, and much can be learned from mapping the spatiotemporal dynamics of these bonds. Since its invention a decade ago, stimulated Raman scattering (SRS) microscopy has become a powerful modality for imaging chemical bonds with high sensitivity, resolution, speed and specificity. We introduce the fundamentals of SRS microscopy and review innovations in SRS microscopes and imaging probes. We highlight examples of exciting biological applications, and share our vision for potential future breakthroughs for this technology.

Label-free stimulated Raman scattering microscopy visualizes changes in intracellular morphology during human epidermal keratinocyte differentiation

Epidermal keratinocyte (KC) differentiation, which involves the process from proliferation to cell death for shedding the outermost layer of skin, is crucial for the barrier function of skin. Therefore, in dermatology, it is important to elucidate the epidermal KC differentiation process to evaluate the symptom level of diseases and skin conditions. Previous dermatological studies used staining or labelling techniques for this purpose, but they have technological limitations for revealing the entire process of epidermal KC differentiation, especially when applied to humans. Here, we demonstrate label-free visualization of three-dimensional (3D) intracellular morphological changes of ex vivo human epidermis during epidermal KC differentiation using stimulated Raman scattering (SRS) microscopy. Specifically, we observed changes in nuclei during the initial enucleation process in which the nucleus is digested prior to flattening. Furthermore, we found holes left behind by improperly digested nuclei in the stratum corneum, suggesting abnormal differentiation. Our findings indicate the great potential of SRS microscopy for discrimination of the degree of epidermal KC differentiation.

Methods to Evaluate Skin Penetration In Vitro

Dermal and transdermal drug therapy is increasing in importance nowadays in drug development. To completely utilize the potential of this administration route, it is necessary to optimize the drug release and skin penetration measurements. This review covers the most well-known and up-to-date methods for evaluating the cutaneous penetration of drugs in vitro as a supporting tool for pharmaceutical research scientists in the early stage of drug development. The aim of this article is to present various experimental models used in dermal/transdermal research and summarize the novel knowledge about the main in vitro methods available to study skin penetration. These techniques are: Diffusion cell, skin-PAMPA, tape stripping, two-photon microscopy, confocal laser scanning microscopy, and confocal Raman microscopic method.

Vibrational spectroscopy and imaging with non-resonant coherent anti-Stokes Raman scattering: double stimulated Raman scattering scheme

We introduce a new coherent anti-Stokes Raman scattering (CARS) suppression scheme based on measuring a non-resonant CARS loss signal by three-beam (pump-Stokes-depletion) double stimulated Raman scattering (SRS) processes, which can be potentially of use for super-resolution Raman microscopy. In the converging configuration with employing both pump-depletion and Stokes-depletion SRS processes, we obtained approximately 94% suppression of non-resonant CARS signal, which is about 1.5 times more efficient than that with the parallel configuration with pump-Stokes and pump-depletion SRS processes. Such an enhanced suppression efficiency in the converging configuration results from a simultaneous loss of photons both in the pump and Stokes beams by double SRS processes, leading to an efficient suppression of the pump-Stokes-pump CARS signal. Based on the present method, we further propose two potential applications: (1) non-resonant background-free CARS imaging and (2) label-free super-resolution Raman imaging, and carry out simple numerical simulations to show their feasibility.

In vivo quantitative molecular absorption of glycerol in human skin using coherent anti-Stokes Raman scattering (CARS) and two-photon auto-fluorescence

The penetration of small molecules through the human skin is a major issue for both safety and efficacy issues in cosmetics and pharmaceutic domains. To date, the quantification of active molecular compounds in human skin following a topical application uses ex vivo skin samples mounted on Franz cell diffusion set-up together with appropriate analytical methods. Coherent anti-Stokes Raman scattering (CARS) has also been used to perform active molecule quantification on ex vivo skin samples, but no quantification has been described in human skin in vivo. Here we introduce and validate a framework for imaging and quantifying the active molecule penetration into human skin in vivo. Our approach combines nonlinear imaging microscopy modalities, such as two-photon excited auto-fluorescence (TPEF) and coherent anti-Stokes Raman scattering (CARS), together with the use of deuterated active molecules. The imaging framework was exemplified on topically applied glycerol diluted in various vehicles such as water and xanthan gel. In vivo glycerol quantitative percutaneous penetration over time was demonstrated, showing that, contrary to water, the xanthan gel vehicle acts as a film reservoir that releases glycerol continuously over time. More generally, the proposed imaging framework provides an enabling platform for establishing functional activity of topically applied products in vivo.

Depth resolved label-free multimodal optical imaging platform to study morpho-molecular composition of tissue

Multimodal imaging platforms offer a vast array of tissue information in a single image acquisition by combining complementary imaging techniques. By merging different systems, better tissue characterization can be achieved than is possible by the constituent imaging modalities alone. The combination of optical coherence tomography (OCT) with non-linear optical imaging (NLOI) techniques such as two-photon excited fluorescence (TPEF), second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) provides access to detailed information of tissue structure and molecular composition in a fast, label-free and non-invasive manner. We introduce a multimodal label-free approach for morpho-molecular imaging and spectroscopy and validate the system in mouse skin demonstrating the potential of the system for colocalized acquisition of OCT and NLOI signals.