Time and depth resolved visualisation of the diffusion of a lipophilic dye into the hair follicle of fresh unfixed human scalp skin

Proof-of-Concept Organ-on-Chip Study: Topical Cinnamaldehyde Exposure of Reconstructed Human Skin with Integrated Neopapillae Cultured under Dynamic Flow

Pharmaceutical and personal care industries require human representative models for testing to ensure the safety of their products. A major route of penetration into our body after substance exposure is via the skin. Our aim was to generate robust culture conditions for a next generation human skin-on-chip model containing neopapillae and to establish proof-of-concept testing with the sensitizer, cinnamaldehyde. Reconstructed human skin consisting of a stratified and differentiated epidermis on a fibroblast populated hydrogel containing neopapillae spheroids (RhS-NP), were cultured air-exposed and under dynamic flow for 10 days. The robustness of three independent experiments, each with up to 21 intra-experiment replicates, was investigated. The epidermis was seen to invaginate into the hydrogel towards the neopapille spheroids. Daily measurements of lactate dehydrogenase (LDH) and glucose levels within the culture medium demonstrated high viability and stable metabolic activity throughout the culture period in all three independent experiments and in the replicates within an experiment. Topical cinnamaldehyde exposure to RhS-NP resulted in dose-dependent cytotoxicity (increased LDH release) and elevated cytokine secretion of contact sensitizer specific IL-18, pro-inflammatory IL-1β, inflammatory IL-23 and IFN-γ, as well as anti-inflammatory IL-10 and IL-12p70. This study demonstrates the robustness and feasibility of complex next generation skin models for investigating skin immunotoxicity.

Mapping hair follicle-targeted delivery by particle systems: What has science accomplished so far?

The importance of the hair follicle in the process of cutaneous drug penetration has been established since this skin appendage was recognized as an entry point for topically applied substances. A comprehensive review on the hair follicle as a target per se is here provided, exploring the current knowledge on both targeted regions and delivery systems that take advantage of this permeation route. The follicular penetration is a complex process, whose effectiveness and efficiency strongly depends on a diversity of different factors including follicular density and size, activity status of hair follicles and physicochemical properties of the topically applied substances. Nanocarriers represent a heterogeneous assembly of molecules organized into particles and they have revolutionized drug delivery in several areas of medicine, pharmacology and cosmetics. As they possess an inherent ability to use the follicular route, they are reviewed here having in perspective the hair follicle zones that they are able to reach as reported. In this way, a follicular road map for the different delivery systems was compiled to assist as a guiding tool for those that have interest in the development and/or application of such delivery systems for hair and skin treatment or care.

A niche in the spotlight: Could external factors critically disturb hair follicle homeostasis and contribute to inflammatory hair follicle diseases?

The anatomy of the hair follicle and the dynamics of its barrier provide a special space for interactions between macromolecules and the underlying tissue. Translocation across the hair follicle epithelium and immune recognition have been confirmed for proteins, nucleic acids, engineered particles, virus particles and others. Tissue responses can be modulated by pro-inflammatory stimuli as demonstrated in penetration and transcutaneous immunization studies. Even under physiological conditions, hair follicle openings are filled with exogenous material ranging from macromolecules, engineered particles to natural particles including diverse communities of microbes. The exposed position of the infundibulum suggests that local inflammatory insults could disturb the finely tuned balance and may trigger downstream responses that initiate or facilitate local outbreaks of inflammatory hair diseases typically occurring in close spatial association with the infundibulum as observed in cicatricial alopecia. The question as to how microbial colonization or deposition of contaminants on the surface of the hair follicle epithelium interact with the barrier status under the influence of individual predisposition, may help us understand local flare-ups of inflammatory hair diseases. Specifically, learning more about skin barrier alterations in the different types of inflammatory hair diseases and cross-talk with exogenous compounds could give new insights in this less explored aspect of hair follicle homeostasis. Such knowledge may not only be used to develop supportive measures to maintain a healthy scalp. It may have wider implications for our understanding on how external factors influence inflammation and immunological responses in the skin.

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.

Applications and limitations of lipid nanoparticles in dermal and transdermal drug delivery via the follicular route

Lipid nanoparticles (LN) such as solid lipid nanoparticles (SLN) and nanolipid carriers (NLC) feature several claimed benefits for topical drug therapy including biocompatible ingredients, drug release modification, adhesion to the skin, and film formation with subsequent hydration of the superficial skin layers. However, penetration and permeation into and across deeper skin layers are restricted due to the barrier function of the stratum corneum (SC). As different kinds of nanoparticles provide the potential for penetration into hair follicles (HF) LN are applicable drug delivery systems (DDS) for this route in order to enhance the dermal and transdermal bioavailability of active pharmaceutical ingredients (API). Therefore, this review addresses the HF as application site, published formulations of LN which showed follicular penetration (FP), and characterization methods in order to identify and quantify the accumulation of API delivered by the LN in the HF. Since LN are based on lipids that appear in human sebum which is the predominant medium in HF an increased localization of the colloidal carriers as well as a promoted drug release may be assumed. Therefore, sebum-like lipid material and a size of less or equal 640 nm are appropriate specifications for FP of particulate formulations.

Modeling the human skin barrier--towards a better understanding of dermal absorption

Many drugs are presently delivered through the skin from products developed for topical and transdermal applications. Underpinning these technologies are the interactions between the drug, product and skin that define drug penetration, distribution, and elimination in and through the skin. Most work has been focused on modeling transport of drugs through the stratum corneum, the outermost skin layer widely recognized as presenting the rate-determining step for the penetration of most compounds. However, a growing body of literature is dedicated to considering the influence of the rest of the skin on drug penetration and distribution. In this article we review how our understanding of skin physiology and the experimentally observed mechanisms of transdermal drug transport inform the current models of drug penetration and distribution in the skin. Our focus is on models that have been developed to describe particular phenomena observed at particular sites of the skin, reflecting the most recent directions of investigation.

Towards drug quantification in human skin with confocal Raman microscopy

Understanding the penetration behaviour of drugs into human skin is a prerequisite for the rational development and evaluation of effective dermal drug delivery. The general procedure for the acquisition of quantitative drug penetration profiles in human skin is performed by sequential segmentation and extraction. Unfortunately, this technique is destructive, laborious and lacks spatial resolution. Confocal Raman microscopy bares the potential of a chemically selective, label free and nondestructive analysis. However, the acquisition of quantitative drug depth profiles within skin by Raman microscopy is impeded by imponderable signal attenuation inside the tissue. In this study, we present a chemical semi-solid matrix system simulating the optical properties of human skin. This system serves as a skin surrogate for investigation of Raman signal attenuation under controlled conditions. Caffeine was homogeneously incorporated within the skin surrogate, and Raman intensity depth profiles were acquired. A mathematical algorithm describing the Raman signal attenuation within the surrogate was derived from these profiles. Human skin samples were incubated with caffeine, and Raman intensity depth profiles were similarly acquired. The surrogate algorithm was successfully applied to correct the drug profiles in human skin for signal attenuation. For the first time, a mathematical algorithm was established, which allows correction of Raman signal attenuation in human skin, thus facilitating reliable drug quantification in human skin by confocal Raman spectroscopy.

Human scalp permeability to the chemical warfare agent VX

The use of chemical warfare agents such as VX in terrorism act might lead to contamination of the civilian population. Human scalp decontamination may require appropriate products and procedures. Due to ethical reasons, skin decontamination studies usually involve in vitro skin models, but human scalp skin samples are uncommon and expensive. The purpose of this study was to characterize the in vitro permeability to VX of human scalp, and to compare it with (a) human abdominal skin, and (b) pig skin from two different anatomic sites: ear and skull roof, in order to design a relevant model. Based on the VX skin permeation kinetics and distribution, we demonstrated that (a) human scalp was significantly more permeable to VX than abdominal skin and (b) pig-ear skin was the most relevant model to predict the in vitro human scalp permeability. Our results indicated that the follicular pathway significantly contributed to the skin absorption of VX through human scalp. In addition, the hair follicles and the stratum corneum significantly contributed to the formation of a skin reservoir for VX.

Rational design of a topical androgen receptor antagonist for the suppression of sebum production with properties suitable for follicular delivery

A novel nonsteroidal androgen receptor antagonist, (R)-4-(1-benzyl-4,4-dimethyl-2-oxopyrrolidin-3-yloxy)-2-(trifluoromethyl)benzonitrile (1), for the topical control of sebum production is reported. This compound, which is potent, selective, and efficacious in the clinically validated golden Syrian hamster ear animal model, was designed to be delivered to the pilosebaceous unit, the site of action, preferentially by the follicular route.

Point scanning confocal microscopy facilitates 3D human hair follicle imaging in tissue sections

Efficiency is a key factor in determining whether a scientific method becomes widely accepted in practical applications. In dermatology, morphological characterisation of intact hair follicles by traditional methods can be rather inefficient. Samples are embedded, sliced, imaged and digitally reconstructed, which can be time-consuming. Confocal microscopy, on the other hand, is more efficient and readily applicable to study intact hair follicles. Modern confocal microscopes deliver and collect light very efficiently and thus allow high spatial resolution imaging of relatively thick samples. In this letter, we report that we successfully imaged entire intact human hair follicles using point scanning confocal microscopy. Light delivery and light-collection were further improved by preparing the samples in 2,2'-Thiodiethanol (TDE), thus reducing refractive index gradients. The relatively short total scan times and the high quality of the acquired 3D images make confocal microscopy a desirable method for studying intact hair follicles under normal and pathological conditions.

Follow-up of drug permeation through excised human skin with confocal Raman microspectroscopy

Skin is a multilayered organ which covers and protects the surface of human body by providing a barrier function against exogenous agents. Meanwhile, the efficacy of several topically applicated drugs is directly related to their penetration through the skin barrier. Several techniques are commonly used to evaluate the rate, the speed and the depth of penetration of these drugs, but few of them can provide real-time results. Therefore, the use of nondestructive and structurally informative techniques permits a real breakthrough in the investigations on skin penetration at a microscopic scale. Confocal Raman microspectroscopy is a nondestructive and rapid technique which allows information to be obtained from deep layers under the skin surface, giving the possibility of a real-time tracking of the drug in the skin layers. The specific Raman signature of the drug enables its identification in the skin. In this study, we try to follow the penetration of Metronidazole, a drug produced by Galderma as a therapeutic agent for Rosacea treatment, through the skin. The first step was the spectral characterization of Metronidazole in the skin. Then micro-axial profiles were conducted to follow the penetration of the drug in the superficial layers, on excised human skin specimens. For more accurate information, transverse sections were cut from the skin and spectral images were conducted, giving information down to several millimeters deep. Moreover, the collected spectra permit us to follow the structural modifications, induced by the Metronidazole on the skin, by studying the changes in the spectral signature of the skin constituents.

The influence of drug partition coefficient on follicular penetration: in vitro human skin studies

The aim of this study was to employ the novel skin sandwich system in order to quantify the influence of the octanol-water partition coefficient on follicular drug absorption in human skin. To this end, seven different drugs - estradiol, corticosterone, hydrocortisone, aldosterone, cimetidine, deoxyadenosine and adenosine - exhibiting a wide range of log octanol-water partition coefficients (logK(o/w)) but relatively similar molecular weights were selected as candidate solutes. Application of the skin sandwich technique yielded an interesting relationship between % follicular contribution and logK(o/w). The follicular contribution to total flux was small (4 and 2%) for the two most lipophilic drugs but varied between 34 and 60% for the remaining drugs of intermediate and low logK(o/w) values. Lipophilicity seems to be an important modulator of drug absorption into follicular orifices only above a critical logK(o/w) threshold. Below this critical logK(o/w) value, lipophilicity does not apparently influence the follicular contribution in an obvious way and the process is probably governed by other molecular properties. Identification of these other active properties would require performing this kind of a study on a much larger set of candidate drugs.

Follicular Penetration: Development of a Method to Block the Follicles Selectively against the Penetration of Topically Applied Substances

Investigations into the penetration pathways of topically applied substances through the skin play an important role in dermatological science. Recently, the hair follicles have increasingly been recognized as an important pathway for percutaneous penetration, although the role of the follicles has still not been clarified in detail. Therefore, the aim of the present study was to develop an in vivo method for the analysis of the follicular rate of penetration processes. In order to reach this goal, the follicles were excluded from the penetration process of topically applied substances. Different study designs were evaluated, and we were able to show that the application of nail varnish to each follicular orifice represents an effective method of blocking the follicles selectively against the penetration of topically applied substances. In dermatological science, there is new information on the physiology of skin penetration processes, thus allowing examination of the follicular rate of penetration and improvement of the pharmacokinetics of topically applied substances.

Semiquantitative Determination of the Penetration of a Fluorescent Hydrogel Formulation into the Hair Follicle with and without Follicular Closure by Microparticles by Means of Differential Stripping

Recently, hair follicles were shown to be of great importance concerning the penetration and reservoir behavior of topically applied substances. Especially microparticles are known to be efficient drug carriers into the hair follicles. It has been shown in various investigations that, depending on their size, microparticles can be utilized to enhance the penetration of substances into the hair follicles. In the present study, it was investigated whether they could also be used to block the hair follicles to stop penetration. For this purpose, the follicular penetration of a sodium fluorescein-containing hydrogel and follicular reservoir depletion were investigated by means of differential stripping in vivo. In a second study design, the skin areas were pretreated with microparticles 5 microm in diameter. Subsequently, the follicular penetration of sodium fluorescein-containing hydrogel was likewise investigated by differential stripping. The results showed that the hair follicle infundibula contained a significant fluorescent signal when sodium fluorescein in hydrogel was applied. In contrast, in the case of pretreatment with microparticles, only a very low fluorescence signal was measurable in the hair follicle infundibula. The microparticles clearly blocked the follicular orifices so that a penetration of sodium fluorescein was almost completely inhibited. This was confirmed by histological investigations. Thus, depending on their size, microparticles act as efficient drug carriers or can be utilized as follicle blockers to stop the penetration of topically applied substances. This could be very useful in skin protection to prevent the storage of harmful topically applied substances in the hair follicles, known to represent long-term reservoirs.

Transfollicular drug delivery—Is it a reality?

Once regarded as merely evolutionary remnants, the hair follicles and sebaceous glands are increasingly recognised as potentially significant elements in the percutaneous drug delivery paradigm. Interest in pilosebaceous units has been directed towards their use as depots for localised therapy, particularly for the treatment of follicle-related disorders such as acne or the alopecias. Furthermore, considerable attention has also been focused on exploiting the follicles as transport shunts for systemic drug delivery. This paper reviews various key facets of this field including; relevant aspects of pilosebaceous anatomy and physiology, the design and efficacy of follicle-targeting formulations and the emergence of quantitative modeling systems. Several novel developments in this area promise to greatly expand our understanding of this field in the near future.

On-Line Diffusion Profile of a Lipophilic Model Dye in Different Depths of a Hair Follicle in Human Scalp Skin

In skin and hair research, drug targeting to the hair follicle is of great interest in the treatment of skin diseases. The aim of this study is to visualize on-line the diffusion processes of a model fluorophore into the hair follicle at different depths using fresh human scalp skin and confocal laser scanning microscopy. Up to a depth of 500 microm in the skin, a fast increase of fluorescence is observed in the gap followed by accumulation of the dye in the hair cuticle. Penetration was also observed via the stratum corneum and the epidermis. Little label reached depths greater than 2000 microm. Fat cells accumulated the label fastest, followed by the cuticular area and the outer root sheath of the hair follicle. Sweat glands revealed very low staining, whereas the bulb at a depth of 4000 microm was visualized only by autofluorescence. From this study, we conclude that on-line visualization is a promising technique to access diffusion processes in deep skin layers even on a cellular level. Furthermore, we conclude that the gap and the cuticle play an important role in the initial diffusion period with the label in the cuticle originating from the gap.

Differential Stripping: Determination of the Amount of Topically Applied Substances Penetrated into the Hair Follicles

The determination of penetration pathways of topically applied substances into the skin is the subject of several investigations. Recently, follicular penetration has become a major focus of interest. To date, a direct, non-invasive quantification of the amount of topically applied substance penetrated into the follicles had not been possible. The development of such a method was the aim of this study. Therefore, the advantages of both stripping techniques, tape stripping and cyanoacrylate skin surface biopsy, were combined and evaluated. Tape stripping was used to remove the part of the stratum corneum that contained the topically applied dye. Subsequently, the follicular contents were ripped off by cyanoacrylate skin surface biopsy. The combined method termed "differential stripping" was evaluated in vitro and in vivo, and the amount of topically applied fluorescent dye penetrated into the hair follicles was quantified after different penetration times. After 30 min, 5% of the recovered concentration of sodium fluorescein was found in the follicular infundibula, where it was still detectable after 48 h. Altogether, the results of this investigation revealed that differential stripping is a new method that can be used to study the penetration of topically applied substances into the follicular infundibula non-invasively and selectively.