Imaging the prodrug-to-drug transformation of a 5-fluorouracil derivative in skin by confocal Raman microscopy

Label-free Assessment of the Nascent State of Rat Non-alcoholic Fatty Liver Disease Using Spontaneous Raman Microscopy

Spontaneous Raman microscopy, which can detect molecular vibrations in cells and tissues, could be a useful tool for the label-free assessment of non-alcoholic fatty liver disease (NAFLD). However, it is unclear whether it can be used to evaluate the nascent state of NAFLD. To address this, we analyzed the Raman spectra of rat liver tissues in the nascent state of NAFLD upon excitation at 532 nm. Raman and histochemical analyses were performed of liver tissues from rats fed a high-fat, high-cholesterol diet (HFHCD). Raman microscopic imaging analysis of formalin-fixed thin tissue slices showed hepatic steatosis, as revealed by the Raman band at 2,854 cm⁻¹, whereas lipid droplets were not detectable by hematoxylin-eosin staining of images until 3 days after feeding a HFHCD. Raman signals of retinol at 1,588 cm⁻¹ emitted from hepatic stellate cells were distributed alongside hepatic cords; the retinol content rapidly decreased after feeding a HFHCD, whereas hepatic lipid content increased inversely. Raman microscopic analysis of the surface of fresh ex vivolivers enabled early detection of lipid accumulation after a 1-day feeding a HFHCD. In conclusion, spontaneous Raman microscopy can be applied to the label-free evaluation of the nascent state of NAFLD liver tissues.

Raman Imaging and Fluorescence Lifetime Imaging Microscopy for Diagnosis of Cancer State and Metabolic Monitoring

Hurdles for effective tumor therapy are delayed detection and limited effectiveness of systemic drug therapies by patient-specific multidrug resistance. Non-invasive bioimaging tools such as fluorescence lifetime imaging microscopy (FLIM) and Raman-microspectroscopy have evolved over the last decade, providing the potential to be translated into clinics for early-stage disease detection, in vitro drug screening, and drug efficacy studies in personalized medicine. Accessing tissue- and cell-specific spectral signatures, Raman microspectroscopy has emerged as a diagnostic tool to identify precancerous lesions, cancer stages, or cell malignancy. In vivo Raman measurements have been enabled by recent technological advances in Raman endoscopy and signal-enhancing setups such as coherent anti-stokes Raman spectroscopy or surface-enhanced Raman spectroscopy. FLIM enables in situ investigations of metabolic processes such as glycolysis, oxidative stress, or mitochondrial activity by using the autofluorescence of co-enzymes NADH and FAD, which are associated with intrinsic proteins as a direct measure of tumor metabolism, cell death stages and drug efficacy. The combination of non-invasive and molecular-sensitive in situ techniques and advanced 3D tumor models such as patient-derived organoids or microtumors allows the recapitulation of tumor physiology and metabolism in vitro and facilitates the screening for patient-individualized drug treatment options.

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.

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.

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.

Novel confocal Raman microscopy method to investigate hydration mechanisms in human skin

BACKGROUND

Skin hydration is essential for maintaining stratum corneum (SC) flexibility and facilitating maturation events. Moisturizers contain multiple ingredients to maintain and improve skin hydration although a complete understanding of hydration mechanisms is lacking. The ability to differentiate the source of the hydration (water from the environment or deeper skin regions) upon application of product will aid in designing more efficacious formulations.

MATERIALS AND METHODS

Novel confocal Raman microscopy (CRM) experiments allow us to investigate mechanisms and levels of hydration in the SC. Using deuterium oxide (D₂ O) as a probe permits the differentiation of endogenous water (H₂ O) from exogenous D₂ O. Following topical application of D₂ O, we first compare in vivo skin depth profiles with those obtained using ex vivo skin. Additional ex vivo experiments are conducted to quantify the kinetics of D₂ O diffusion in the epidermis by introducing D₂ O under the dermis.

RESULTS

Relative D₂ O depth profiles from in vivo and ex vivo measurements compare well considering procedural and instrumental differences. Additional in vivo experiments where D₂ O was applied following topical glycerin application increased the longevity of D₂ O in the SC. Reproducible rates of D₂ O diffusion as a function of depth have been established for experiments where D₂ O is introduced under ex vivo skin.

CONCLUSION

Unique information regarding hydration mechanisms are obtained from CRM experiments using D₂ O as a probe. The source and relative rates of hydration can be delineated using ex vivo skin with D₂ O underneath. One can envision comparing these depth-dependent rates in the presence and absence of topically applied hydrating agents to obtain mechanistic information.

Emerging imaging technologies in dermatology: Part II: Applications and limitations

Clinical examination is critical for the diagnosis and identification of response to treatment. It is fortunate that technologies are continuing to evolve, enabling augmentation of classical clinical examination with noninvasive imaging modalities. This article discusses emerging technologies with a focus on digital photographic imaging, confocal microscopy, optical coherence tomography, and high-frequency ultrasound, as well as several additional developing modalities. The most readily adopted technologies to date include total-body digital photography and dermoscopy, with some practitioners beginning to use confocal microscopy. In this article, applications and limitations are addressed. For a detailed discussion of the principles involved in these technologies, please refer to the first part of this review article.

Raman spectroscopy in pharmaceutical research and industry

In the fast-developing fields of pharmaceutical research and industry, the implementation of Raman spectroscopy and related technologies has been very well received due to the combination of chemical selectivity and the option for non-invasive analysis of samples. This chapter explores established and potential applications of Raman spectroscopy, confocal Raman microscopy and related techniques from the early stages of drug development research up to the implementation of these techniques in process analytical technology (PAT) concepts for large-scale production in the pharmaceutical industry. Within this chapter, the implementation of Raman spectroscopy in the process of selection and optimisation of active pharmaceutical ingredients (APIs) and investigation of the interaction with excipients is described. Going beyond the scope of early drug development, the reader is introduced to the use of Raman techniques for the characterization of complex drug delivery systems, highlighting the technical requirements and describing the analysis of qualitative and quantitative composition as well as spatial component distribution within these pharmaceutical systems. Further, the reader is introduced to the application of Raman techniques for performance testing of drug delivery systems addressing drug release kinetics and interactions with biological systems ranging from single cells up to complex tissues. In the last part of this chapter, the advantages and recent developments of integrating Raman technologies into PAT processes for solid drug delivery systems and biologically derived pharmaceutics are discussed, demonstrating the impact of the technique on current quality control standards in industrial production and providing good prospects for future developments in the field of quality control at the terminal part of the supply chain and various other fields like individualized medicine.

On the way from the active drug molecule (API) in the research laboratory to the marketed medicine in the pharmacy, therapeutic efficacy of the active molecule and safety of the final medicine for the patient are of utmost importance. For each step, strict regulatory requirements apply which demand for suitable analytical techniques to acquire robust data to understand and control design, manufacturing and industrial large-scale production of medicines. In this context, Raman spectroscopy has come to the fore due to the combination of chemical selectivity and the option for non-invasive analysis of samples. Following the technical advancements in Raman equipment and analysis software, Raman spectroscopy and microscopy proofed to be valuable methods with versatile applications in pharmaceutical research and industry, starting from the analysis of single drug molecules as well as complex multi-component formulations up to automatized quality control during industrial production.

Caffeine delivery in porcine skin: a confocal Raman study

Confocal Raman microscopy is a novel optical method for studies of pro-drug and drug delivery. This method is a promising technique that enables non-destructive measurement of the permeation profile through skin layers. Peaks of compounds are usually normalised to skin peaks (amino-acid and amide I) for semi-quantitative evaluation. The present study seeks to optimise a methodology for complete quantitative measurement of the amount of an active compound at different depths. Caffeine was used as a tracer to evaluate compound's skin penetration using confocal Raman microscopy. A semi-quantitative depth profile of caffeine was obtained with normalisation of the Raman intensities. These ratios of Raman intensities were correlated with the caffeine concentration using an external calibration curve. The calibration curve was carried out with porcine skin incubated in different concentrations of caffeine; afterwards, each skin sample was analysed by confocal Raman microscopy and HPLC to determine the relation between the Raman signal intensity and the caffeine concentration per skin mass and to create a depth profile. These correlation curves allow the full quantification of the caffeine in skin from Raman intensity ratios at different depths.

Skin permeation and antioxidant efficacy of topically applied resveratrol

The permeation of resveratrol was assessed by in vitro and in vivo experiments 24 h after topical administration. The in vitro profile of resveratrol was assessed by Raman spectroscopy. Human skin permeation was analysed in vivo by the tape stripping method with the progressive removal of the stratum corneum layers using adhesive tape strips. Moreover, the free radical scavenging activity of resveratrol after its topical application was determined using the DPPH assay. The Raman spectra indicated that the topically applied resveratrol penetrates deep into the skin. The results showed high amounts of resveratrol in the different stratum corneum layers close to the surface and a constant lower amount in the upper layers of the viable epidermis. The concentration of resveratrol present in the outermost stratum corneum layers was obtained by tape stripping after in vivo application. The results demonstrated that resveratrol mainly remained in the human stratum corneum layers. After topical application, resveratrol maintained its antiradical activity. The antioxidant efficacy of the compound was higher in the inner layers of the stratum corneum. As these results have demonstrated, topically applied resveratrol reinforces the antioxidant system of the stratum corneum and provides an efficient means of increasing the tissue levels of antioxidants in the human epidermis.

Imaging of intracellular behavior of polymeric nanoparticles in Staphylococcus epidermidis biofilms by slit-scanning confocal Raman microscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy

In drug delivery systems employing polymeric nanoparticles, accurate delivery of drugs to target sites such as bacterial cells, cell tissues, and organelles is essential. In particular, when designing drug delivery systems for the treatment of the biofilm infections, evaluation of the interaction between polymeric nanoparticles and biofilm or bacterial cells using a simple technique is of significant importance. Here we develop two types of novel techniques for the biological imaging of the intracellular behavior of two types of polymeric nanoparticles, biodegradable chitosan-modified poly (dl-lactide-co-glycolide) (PLGA) nanoparticles and chitosan-modified polyvinyl caprolactam - polyvinyl acetate -polyethylene glycol graft copolymer (Soluplus®, Sol) nanoparticles, within a Staphylococcus epidermidis biofilm. As the first technique, Raman imaging of unstained biological materials using slit-scanning confocal Raman microscopy (unstained Raman imaging) was performed, and as the second, field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis of biological materials labeled with quantum dots (SEM-QD imaging) was demonstrated. These analyses revealed differing localization of the respective nanoparticles within the biofilm in accordance with the specific interactions of PLGA nanoparticles and Sol nanoparticles with the biofilm. These novel techniques open the door to biological imaging and analyses with high spatial resolution, which will help to understand the efficacy of drug delivery to target materials.

Effects of permeation enhancers on flufenamic acid delivery in Ex vivo human skin by confocal Raman microscopy

For effective topical delivery, a drug must cross the stratum corneum (SC) barrier into viable tissue. The use of permeation enhancers is a widespread approach for barrier modification. In the current study, flufenamic acid (FluA), a non-steroidal anti-inflammatory drug, is a model agent for investigating the influence of hydrophobic versus hydrophilic enhancers. In separate experiments, FluA in octanol or propylene glycol/ethanol (75/25) is applied to the SC for varying times followed by confocal Raman microscopic mapping of drug and enhancer penetration and spatial distribution. Deuterated versions of the enhancers permit us to spectroscopically distinguish the exogenous chemicals from the endogenous SC lipids without affecting penetration parameters. The FluA pathway is tracked by the CC stretching mode at ∼1618cm(-1). Discrete, small inclusions of both enhancers are observed throughout the SC. High concentrations of FluA are co-localized with octanol domains which appear to provide a pathway to the viable epidermis for the drug. In contrast, FluA concentrates in the upper SC when using the hydrophilic agent and endogenous lipids appear unperturbed in regions outside the enhancer pockets. The ability to examine perturbations to endogenous ultrastructure and molecular structure in skin while tracking penetration pathways provides insight into delivery mechanisms.

Raman Spectroscopy: Incorporating the Chemical Dimension into Dermatological Diagnosis

Raman spectroscopy provides chemical analysis of tissue in vivo. By measuring the inelastic interactions of light with matter, Raman spectroscopy can determine the chemical composition of a sample. Diseases that are visually difficult to visually distinguish can be delineated based on differences in chemical composition of the affected tissue. Raman spectroscopy has successfully found spectroscopic signatures for skin cancers and differentiated those of benign skin growths. With current and on-going advances in optics and computing, inexpensive and effective Raman systems may soon be available for clinical use. Raman spectroscopy provides direct analyses of skin lesions, thereby improving both disease diagnosis and management.

ATR-FTIR and Raman spectroscopic investigation of the electroporation-mediated transdermal delivery of a nanocarrier system containing an antitumour drug

The aim of the present work was the optimization of the transdermal delivery of a lyotropic liquid crystal genistein-based formulation (LLC-GEN). LLC was chosen as medium in view of the poor solubility of GEN in water. Membrane diffusion and penetration studies were carried out with a Franz diffusion cell, through a synthetic membrane in vitro, a chick chorioallantoic membrane ex ovo, and ex vivo excised human epidermis. Thereafter, LLC-GEN was combined with electroporation (EP) to enhance the transdermal drug delivery. The synergistic effect of EP was verified by in vivo ATR-FTIR and ex vivo Raman spectroscopy on hairless mouse skin.

Raman imaging of drug delivery systems

This review article includes an introduction to the principals of Raman spectroscopy, an outline of the experimental systems used for Raman imaging and the associated important considerations and limitations of this method. Common spectral analysis methods are briefly described and examples of interesting published studies which utilised Raman imaging of pharmaceutical and biomedical devices are discussed, along with summary tables of the literature at this point in time.

Applications of Raman spectroscopy in skin research--From skin physiology and diagnosis up to risk assessment and dermal drug delivery

In the field of skin research, confocal Raman microscopy is an upcoming analytical technique. Substantial technical progress in design and performance of the individual setup components like detectors and lasers as well as the combination with confocal microscopy enables chemically selective and non-destructive sample analysis with high spatial resolution in three dimensions. Due to these advantages, the technique bears tremendous potential for diverse skin applications ranging from the analysis of physiological component distribution in skin tissue and the diagnosis of pathological states up to biopharmaceutical investigations such as drug penetration kinetics within the different tissue layers. This review provides a comprehensive introduction about the basic principles of Raman microscopy highlighting the advantages and considering the limitations of the technique for skin applications. Subsequently, an overview about skin research studies applying Raman spectroscopy is given comprising various in vitro as well as in vivo implementations. Furthermore, the future perspective and potential of Raman microscopy in the field of skin research are discussed.

Confocal Raman microscopic investigation of the effectiveness of penetration enhancers for procaine delivery to the skin

A methodology that employs confocal Raman microscopy (CRM) on ex vivo skin samples is proposed for the investigation of drug content and distribution in the skin. To this end, the influence of the penetration enhancers propylene glycol and polyoxyethylene-23-lauryl ether on the penetration and permeation of procaine as a model substance was investigated. The drug content of skin samples that had been incubated with semisolid formulations containing one of these enhancers was examined after skin segmentation. The experiments showed that propylene glycol did not affect the procaine content that was delivered to the skin, whereas polyoxyethylene-23-lauryl ether led to higher procaine contents and deeper penetration. Neither substance was found to influence the permeation rate of procaine. It is thereby shown that CRM can provide additional information on drug penetration and permeation. Furthermore, the method was found to enhance the depth from which Raman spectra can be collected and to improve the depth resolution compared to previously proposed methods.

Combining confocal Raman microscopy and freeze-drying for quantification of substance penetration into human skin

In the area of dermatological research, the knowledge of rate and extent of substance penetration into the human skin is essential not only for evaluation of therapeutics, but also for risk assessment of chemicals and cosmetic ingredients. Recently, confocal Raman microscopy emerged as a novel analytical technique for analysis of substance skin penetration. In contrast to destructive drug extraction and quantification, the technique is non-destructive and provides high spatial resolution in three dimensions. However, the generation of time-resolved concentration depth profiles is restrained by ongoing diffusion of the penetrating substance during analysis. To prevent that, substance diffusion in excised human skin can instantly be stopped at defined time points by freeze-drying the sample. Thus, combining sample preparation by freeze-drying with drug quantification by confocal Raman microscopy yields a novel analytical platform for non-invasive and quantitative in vitro analysis of substance skin penetration. This work presents the first proof-of-concept study for non-invasive quantitative substance depth profiling in freeze-dried excised human stratum corneum by confocal Raman microscopy.

Monitoring caffeine and resveratrol cutaneous permeation by confocal Raman microspectroscopy

Dynamic follow-up of exogenous molecules permeation through the skin is one among many competing applications for confocal Raman microspectroscopy. Previous studies showed the feasibility of tracking actives through the skin; the next step should be recording in vivo kinetics. Thus, we conducted a study to evaluate the possibility of detecting low concentrations of caffeine and resveratrol solutions through the skin using confocal Raman microspectroscopy. After topical application of each active on the skin surface, Raman profiles were recorded over nine hours. The challenge was to pursuit these actives respecting the concentration used in some dermatological formulations. Molecules were successfully detected and kinetic profiles were registered over time. The heterogeneity of skin structure and the complexity of molecules diffusion were reflected through the kinetic results.

Infrared spectroscopic imaging tracks lateral distribution in human stratum corneum

PURPOSE

To demonstrate the efficacy of infrared (IR) spectroscopic imaging for evaluation of lateral diffusion in stratum corneum (SC) and for elucidation of intermolecular interactions between exogenous agents and SC constituents.

METHODS

In separate experiments, acyl chain perdeuterated oleic acid (OA-d) and deuterated dimethyl sulfoxide (DMSO-d) were applied to the surface of isolated human SC. The lateral distribution of permeant concentrations was monitored using the time-dependence of IR images. Diffusion coefficients (D) were estimated from Fick's second law. Interactions between the exogenous agents and the SC were tracked from changes in CD2 and Amide I stretching frequencies.

RESULTS

Networked glyphs served as the major pathway for lateral distribution of OA-d. In glyph-poor regions, D values from 0.3-1 × 10(-8) cm(2)/s bracketed the OA-d data and apparently decreased with time. Although diffusion of DMSO-d is relatively fast compared to our experimental measurement time, the results suggest values of ~10(-7) cm(2)/s. OA-d spectral changes suggest penetration into the ordered lipids of the SC; DMSO-d penetration results in perturbation of SC keratin structure.

CONCLUSIONS

IR imaging provides concentration profiles, diffusion coefficients, and unique molecular level information about structural changes in the endogenous SC constituents and exogenous agents upon their mutual interaction. Transport along glyphs is the dominant mode of distribution for OA-d.

Quantitative analysis of microbicide concentrations in fluids, gels and tissues using confocal Raman spectroscopy

Topical vaginal anti-HIV microbicides are an important focus in female-based strategies to prevent the sexual transmission of HIV. Understanding microbicide pharmacokinetics is essential to development, characterization and implementation of efficacious microbicide drug delivery formulations. Current methods to measure drug concentrations in tissue (e.g., LC-MS/MS, liquid chromatography coupled with tandem mass spectrometry) are highly sensitive, but destructive and complex. This project explored the use of confocal Raman spectroscopy to detect microbicide drugs and to measure their local concentrations in fluids, drug delivery gels, and tissues. We evaluated three candidate microbicide drugs: tenofovir, Dapivirine and IQP-0528. Measurements were performed in freshly excised porcine buccal tissue specimens, gel vehicles and fluids using two Horiba Raman microscopes, one of which is confocal. Characteristic spectral peak calibrations for each drug were obtained using serial dilutions in the three matrices. These specific Raman bands demonstrated strong linear concentration dependences in the matrices and were characterized with respect to their unique vibrational signatures. At least one specific Raman feature was identified for each drug as a marker band for detection in tissue. Sensitivity of detection was evaluated in the three matrices. A specific peak was also identified for tenofovir diphosphate, the anti-HIV bioactive product of tenofovir after phosphorylation in host cells. Z-scans of drug concentrations vs. depth in excised tissue specimens, incubated under layers of tenofovir solution in a Transwell assay, showed decreasing concentration with depth from the surface into the tissue. Time-dependent concentration profiles were obtained from tissue samples incubated in the Transwell assay, for times ranging 30 minutes - 6 hours. Calibrations and measurements from tissue permeation studies for tenofovir showed good correlation with gold standard LC-MS/MS data. These results demonstrate that confocal Raman spectroscopy holds promise as a tool for practical, minimally invasive, label-free measurement of microbicide drug concentrations in fluids, gels and tissues.

Non-invasive depth profile imaging of the stratum corneum using confocal Raman microscopy: first insights into the method

The stratum corneum is a strong barrier that must be overcome to achieve successful transdermal delivery of a pharmaceutical agent. Many strategies have been developed to enhance the permeation through this barrier. Traditionally, drug penetration through the stratum corneum is evaluated by employing tape-stripping protocols and measuring the content of the analyte. Although effective, this method cannot provide a detailed information regarding the penetration pathways. To address this issue various microscopic techniques have been employed. Raman microscopy offers the advantage of label free imaging and provides spectral information regarding the chemical integrity of the drug as well as the tissue. In this paper we present a relatively simple method to obtain XZ-Raman profiles of human stratum corneum using confocal Raman microscopy on intact full thickness skin biopsies. The spectral datasets were analysed using a spectral unmixing algorithm. The spectral information obtained, highlights the different components of the tissue and the presence of drug. We present Raman images of untreated skin and diffusion patterns for deuterated water and beta-carotene after Franz-cell diffusion experiment.

Distribution and visualisation of chlorhexidine within the skin using ToF-SIMS: a potential platform for the design of more efficacious skin antiseptic formulations

PURPOSE

In order to increase the efficacy of a topically applied antimicrobial compound the permeation profile, localisation and mechanism of action within the skin must first be investigated.

METHODS

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to visualise the distribution of a conventional antimicrobial compound, chlorhexidine digluconate, within porcine skin without the need for laborious preparation, radio-labels or fluorescent tags.

RESULTS

High mass resolution and high spatial resolution mass spectra and chemical images were achieved when analysing chlorhexidine digluconate treated cryo-sectioned porcine skin sections by ToF-SIMS. The distribution of chlorhexidine digluconate was mapped throughout the skin sections and our studies indicate that the compound appears to be localised within the stratum corneum. In parallel, tape strips taken from chlorhexidine digluconate treated porcine skin were analysed by ToF-SIMS to support the distribution profile obtained from the skin sections.

CONCLUSIONS

ToF-SIMS can act as a powerful complementary technique to map the distribution of topically applied compounds within the skin.

Recent advances in predicting skin permeability of hydrophilic solutes

Understanding the permeation of hydrophilic molecules is of relevance to many applications including transdermal drug delivery, skin care as well as risk assessment of occupational, environmental, or consumer exposure. This paper reviews recent advances in modeling skin permeability of hydrophilic solutes, including quantitative structure-permeability relationships (QSPR) and mechanistic models. A dataset of measured human skin permeability of hydrophilic and low hydrophobic solutes has been compiled. Generally statistically derived QSPR models under-estimate skin permeability of hydrophilic solutes. On the other hand, including additional aqueous pathway is necessary for mechanistic models to improve the prediction of skin permeability of hydrophilic solutes, especially for highly hydrophilic solutes. A consensus yet has to be reached as to how the aqueous pathway should be modeled. Nevertheless it is shown that the contribution of aqueous pathway can constitute to more than 95% of the overall skin permeability. Finally, future prospects and needs in improving the prediction of skin permeability of hydrophilic solutes are discussed.

Multiscale analysis of water uptake and erosion in biodegradable polyarylates

The role of hydration in degradation and erosion of materials, especially biomaterials used in scaffolds and implants, was investigated by studying the distribution of water at length scales from 0.1 nm to 0.1 mm using Raman spectroscopy, small-angle neutron scattering (SANS), Raman confocal imaging, and scanning electron microscopy (SEM). The measurements were demonstrated using L-tyrosine derived polyarylates. Bound- and free- water were characterized using their respective signatures in the Raman spectra. In the presence of deuterium oxide (D(2)O), H-D exchange occurred at the amide carbonyl but was not detected at the ester carbonyl. Water appeared to be present in the polymer even in regions where there was little evidence for N-H to N-D exchange. SANS showed that water is not uniformly dispersed in the polymer matrix. The distribution of water can be described as mass fractals in polymers with low water content (~5 wt%), and surface fractals in polymers with larger water content (15 to 60 wt%). These fluctuations in the density of water distribution are presumed to be the precursors of the ~ 20 μm water pockets seen by Raman confocal imaging, and also give rise to 10-50 μm porous network seen in SEM. The surfaces of these polymers appeared to resist erosion while the core of the films continued to erode to form a porous structure. This could be due to differences in either the density of the polymer or the solvent environment in the bulk vs. the surface, or a combination of these two factors. There was no correlation between the rate of degradation and the amount of water uptake in these polymers, and this suggests that it is the bound-water and not the total amount of water that contributes to hydrolytic degradation.

Confocal Raman microspectroscopy for skin characterization: a comparative study between human skin and pig skin

Input of confocal Raman microspectroscopy in dermocosmetics: a spectral comparison of human skin (Transkin) and pig ear skin epidermises.

Raman mapping of pharmaceuticals

Raman spectroscopy may be implemented through a microscope to provide fine scale axial and lateral chemical maps. The molecular structure of many drugs makes Raman spectroscopy particularly well suited to the investigation of pharmaceutical systems. Chemometric methods currently used to assess bulk Raman spectroscopic data are typically applied to Raman mapping data from pharmaceuticals; few reports exist where the spatial information inherent to a mapped dataset is used for the calculation of chemical maps. Both univariate and multivariate methods have been applied to Raman mapping data to determine the distribution of active pharmaceutical ingredients (APIs) in tablets, solid dispersions for increased solubility and controlled release devices. The ability to axially (depth) profile using Raman mapping has been used in studies of API penetration through membranes, cellular uptake of drug delivery liposomes, and initial API distribution and subsequent elution from coatings of medical devices. New instrumental developments will increase the efficiency of Raman mapping and lead to greater utilisation of Raman mapping for analyses of pharmaceutical systems.

Studying the effectiveness of penetration enhancers to deliver retinol through the stratum cornum by in vivo confocal Raman spectroscopy

The purpose of this study is to monitor in vivo the effect of chemical penetration enhancers on the delivery of trans-retinol into human skin. Chemical penetration enhancers reversibly alter barrier properties of the SC by disruption of the membrane structures or maximising drug solubility with the skin. So far, most of permeation or penetration experiments are performed in vitro. Raman spectroscopy is uniquely placed to be able to measure biological processes in vivo and this paper shows for the first time that the effect of penetration enhancer on the delivery of trans-retinol can successfully be measured in vivo using this technique. Here, the volar forearm of volunteers was treated with four formulations. One formulation is a highly effective model delivery system identified from ex vivo experiments: trans-retinol in Propylene Glycol (PG)/ethanol, with PG being a well-known and efficient penetration enhancer. The other three formulations are based on 0.3% trans-retinol in Caprylic/Capric Acid Triglyceride (MYRITOL 318), an oil commonly used in skin creams but in two of them a specific penetration enhancer is added. One contains a lipid extractor, Triton X 100, whereas another formulation contains a lipid fluidiser, Oleic Acid. Solutions were applied once and measurements were performed up to 6 h after treatment. Remarkable differences in the delivery of trans-retinol between formulation with or without penetration enhancer can clearly be seen. Moreover, the type of penetration enhancer is also shown to influence the delivery. While using the Oleic Acid, which is a lipid fluidiser, a better delivery of trans-retinol in the skin can be detected. For the first time, the effect of penetration enhancer on the delivery of trans-retinol has been monitored, non invasively in vivo, with time.

Intracellular dynamics of topoisomerase I inhibitor, CPT-11, by slit-scanning confocal Raman microscopy

Most molecular imaging technologies require exogenous probes and may have some influence on the intracellular dynamics of target molecules. In contrast, Raman scattering light measurement can identify biomolecules in their innate state without application of staining methods. Our aim was to analyze intracellular dynamics of topoisomerase I inhibitor, CPT-11, by using slit-scanning confocal Raman microscopy, which can take Raman images with high temporal and spatial resolution. We could acquire images of the intracellular distribution of CPT-11 and its metabolite SN-38 within several minutes without use of any exogenous tags. Change of subcellular drug localization after treatment could be assessed by Raman imaging. We also showed intracellular conversion from CPT-11 to SN-38 using Raman spectra. The study shows the feasibility of using slit-scanning confocal Raman microscopy for the non-labeling evaluation of the intracellular dynamics of CPT-11 with high temporal and spatial resolution. We conclude that Raman spectromicroscopic imaging is useful for pharmacokinetic studies of anticancer drugs in living cells.

Application of attenuated total reflection Fourier transform infrared imaging and tape-stripping to investigate the three-dimensional distribution of exogenous chemicals and the molecular organization in Stratum corneum

Attenuated total reflection Fourier transform infrared spectroscopic imaging combined with tape-stripping is an advantageous approach to map the depth penetration and lateral distribution of topically applied chemicals in Stratum corneum (SC) and the conformational order of SC lipids. Tape-stripping progressively removes layers of SC, and chemical imaging provides spatially resolved information on the chemical composition of both the newly exposed SC surface and of the tapes used for stripping. The procedure is rapid, minimally invasive, and does not necessitate cross-sectioning of the skin. This approach offers a simple and direct way to determine the distribution of exogenous volatile and non-volatile chemicals in SC as a function of the chemical composition of the formulation and time, and the conformational order of SC lipids in native and topically treated skin. The procedure described here is well suited to address questions of relevance for the areas of drug delivery, dermatology, and skin care.

Raman characterization and chemical imaging of biocolloidal self-assemblies, drug delivery systems, and pulmonary inhalation aerosols: a review

This review presents an introduction to Raman scattering and describes the various Raman spectroscopy, Raman microscopy, and chemical imaging techniques that have demonstrated utility in biocolloidal self-assemblies, pharmaceutical drug delivery systems, and pulmonary research applications. Recent Raman applications to pharmaceutical aerosols in the context of pulmonary inhalation aerosol delivery are discussed. The "molecular fingerprint" insight that Raman applications provide includes molecular structure, drug-carrier/excipient interactions, intramolecular and intermolecular bonding, surface structure, surface and interfacial interactions, and the functional groups involved therein. The molecular, surface, and interfacial properties that Raman characterization can provide are particularly important in respirable pharmaceutical powders, as these particles possess a higher surface-area-to-volume ratio; hence, understanding the nature of these solid surfaces can enable their manipulation and tailoring for functionality at the nanometer level for targeted pulmonary delivery and deposition. Moreover, Raman mapping of aerosols at the micro- and nanometer level of resolution is achievable with new, sophisticated, commercially available Raman microspectroscopy techniques. This noninvasive, highly versatile analytical and imaging technique exhibits vast potential for in vitro and in vivo molecular investigations of pulmonary aerosol delivery, lung deposition, and pulmonary cellular drug uptake and disposition in unfixed living pulmonary cells.

Tracking the dephosphorylation of resveratrol triphosphate in skin by confocal Raman microscopy

Polyphenolic resveratrol has been identified as a potent antioxidant acting as both a free radical scavenger and an inhibitor of enzyme oxidative activity. However, the reactive propensity of resveratrol also limits its use in topical formulations. A transient derivative of resveratrol, resveratrol triphosphate, has been designed to provide a means for the delayed delivery of the active compound in skin tissue where endogenous enzymes capable of dephosphorylation reside. Confocal Raman microscopy studies of intact pigskin biopsies treated with modified resveratrol provided information about the spatial distribution and time-dependence of permeation and conversion to the native active form. Conversion to the active form was not observed when skin samples were exposed to steam, a procedure that likely inactivates endogenous skin enzymes. In addition, treatment with the triphosphate compared to the parent compound revealed a more homogeneous distribution of resveratrol throughout the stratum corneum and viable epidermis when the former was applied. Thus, the bioavailability of resveratrol in the epidermis appears to be enhanced upon application of the pro-molecule compared to resveratrol.