Which skin model is the most appropriate for the investigation of topically applied substances into the hair follicles?

Revisiting the Effect of Aging on the Transport of Molecules through the Skin

Both intrinsic and extrinsic aging lead to a series of morphological changes in the skin including the flattening of the dermal-epidermal junction, increased stratum corneum dryness, reduction in sebaceous gland activity and enzyme activity as well as atrophy of blood vessels. In this study, the impact of these changes on the transport of molecules through the skin was revised. The increase in the number of transdermal formulations on the market in recent decades and life expectancy represent the main reasons for an in-depth discussion of this topic. Furthermore, elderly subjects have often been excluded from clinical trials due to polypharmacy, raising concerns in terms of efficacy and safety. In this way, ex vivo and in vivo studies comparing the transport of molecules through the mature and young skin were analyzed in detail. The reduced water content in mature skin had a significant impact on the transport rate of hydrophilic molecules. The lower enzymatic activity in aged skin, in turn, would explain changes in the activation of prodrugs. Interestingly, greater deposition of nanoparticles was also found in mature skin. In vivo models should be prioritized in future experimental studies as they allow to evaluate both absorption and metabolism simultaneously, providing more realistic information.

Methodologies to Evaluate the Hair Follicle-Targeted Drug Delivery Provided by Nanoparticles

Nanotechnology has been investigated for treatments of hair follicle disorders mainly because of the natural accumulation of solid nanoparticles in the follicular openings following a topical application, which provides a drug "targeting effect". Despite the promising results regarding the therapeutic efficacy of topically applied nanoparticles, the literature has often presented controversial results regarding the targeting of hair follicle potential of nanoformulations. A closer look at the published works shows that study parameters such as the type of skin model, skin sections analyzed, employed controls, or even the extraction methodologies differ to a great extent among the studies, producing either unreliable results or precluding comparisons altogether. Hence, the present study proposes to review different skin models and methods for quantitative and qualitative analysis of follicular penetration of nano-entrapped drugs and their influence on the obtained results, as a way of providing more coherent study protocols for the intended application.

A Comparative Analysis of Biological and Synthetic Skin Models for Drug Transport Studies

Ethical restrictions as well as practical or economic issues related to use of animal and human skin has been the main reason the growth in the number of investigations with alternative models. Reconstructed skin models, for example, have been useful to evaluate the in vitro toxicity of compounds; however, these models usually overestimate the amount of drug permeated due to impaired barrier properties. In this review, the performance of synthetic and biological skin models in transport studies was compared by considering two compounds with different physicochemical properties. The advantages and limitations of each skin model are discussed in detail. Although synthetic and reconstructed skin models have shown to be useful in the formulation optimization step, they present many limitations: (1) impaired barrier properties; (2) lack of follicular transport; (3) no metabolism in synthetic membranes; (4) differences in terms of lipid organization; (5) more affected by formulation constituents. Therefore, animal and human tissues should still be prioritized in drug transport studies until new advances in alternative models are achieved. Investigations of the impact of cell-culture conditions on skin formation, in turn, bring perspectives related to the development of unhealthy/injured skin models (an aspect that still deserves attention).

Advanced Skin Antisepsis: Application of UVA-Cleavable Hydroxyethyl Starch Nanocapsules for Improved Eradication of Hair Follicle-Associated Microorganisms

Hair follicles constitute important drug delivery targets for skin antisepsis since they contain ≈25% of the skin microbiome. Nanoparticles are known to penetrate deeply into hair follicles. By massaging the skin, the follicular penetration process is enhanced based on a ratchet effect. Subsequently, an intrafollicular drug release can be initiated by various trigger mechanisms. Here, we present novel ultraviolet A (UVA)-responsive nanocapsules (NCs) with a size between 400 and 600 nm containing hydroxyethyl starch (HES) functionalized by an o-nitrobenzyl linker. A phase transfer into phosphate-buffered saline (PBS) and ethanol was carried out, during which an aggregation of the particles was observed by means of dynamic light scattering (DLS). The highest stabilization for the target medium ethanol as well as UVA-dependent release of ethanol from the HES-NCs was achieved by adding 0.1% betaine monohydrate. Furthermore, sufficient cytocompatibility of the HES-NCs was demonstrated. On ex vivo porcine ear skin, a strong UVA-induced release of the model drug sulforhodamine 101 (SR101) could be demonstrated after application of the NCs in cyclohexane using laser scanning microscopy. In a final experiment, a microbial reduction comparable to that of an ethanol control was demonstrated on ex vivo porcine ear skin using a novel UVA-LED lamp for triggering the release of ethanol from HES-NCs. Our study provides first indications that an advanced skin antisepsis based on the eradication of intrafollicular microorganisms could be achieved by the topical application of UVA-responsive NCs.

Novel Pharmaceutical Strategies for Enhancing Skin Penetration of Biomacromolecules

Skin delivery of biomacromolecules holds great advantages in the systemic and local treatment of multiple diseases. However, the densely packed stratum corneum and the tight junctions between keratinocytes stand as formidable skin barriers against the penetration of most drug molecules. The large molecular weight, high hydrophilicity, and lability nature of biomacromolecules pose further challenges to their skin penetration. Recently, novel penetration enhancers, nano vesicles, and microneedles have emerged as efficient strategies to deliver biomacromolecules deep into the skin to exert their therapeutic action. This paper reviews the potential application and mechanisms of novel skin delivery strategies with emphasis on the pharmaceutical formulations.

Multi-Modal Imaging to Assess the Follicular Delivery of Zinc Pyrithione

Zinc pyrithione (ZnPT) is a widely used antifungal, usually applied as a microparticle suspension to facilitate delivery into the hair follicles. It then dissociates into a soluble monomeric form that is bioactive against yeast and other microorganisms. In this study, we use multiphoton microscopy (MPM) and fluorescence lifetime imaging microscopy (FLIM) to characterise ZnPT formulations and map the delivery of particles into follicles within human skin. To simulate real-world conditions, it was applied using a massage or no-massage technique, while simultaneously assessing the dissolution using Zinpyr-1, a zinc labile fluorescent probe. ZnPT particles can be detected in a range of shampoo formulations using both MPM and FLIM, though FLIM is optimal for detection as it allows spectral and lifetime discrimination leading to increased selectivity and sensitivity. In aqueous suspensions, the ZnPT 7.2 µm particles could be detected up to 500 µm in the follicle. The ZnPT particles in formulations were finer (1.0–3.3 µm), resulting in rapid dissolution on the skin surface and within follicles, evidenced by a reduced particle signal at 24 h but enhanced Zinpyr-1 intensity in the follicular and surface epithelium. This study shows how MPM-FLIM multimodal imaging can be used as a useful tool to assess ZnPT delivery to skin and its subsequent dissolution.

Assessing the Dermal Penetration Efficacy of Chemical Compounds with the Ex-Vivo Porcine Ear Model

(1) Background: The ex vivo porcine ear model is often used for the determination of the dermal penetration efficacy of chemical compounds. This study investigated the influence of the post-slaughter storage time of porcine ears on the dermal penetration efficacy of chemical compounds. (2) Methods: Six different formulations (curcumin and different fluorescent dyes in different vehicles and/or nanocarriers) were tested on ears that were (i) freshly obtained, (ii) stored for 24 or 48 h at 4 °C after slaughter before use and (iii) freshly frozen and defrosted 12 h before use. (3) Results: Results showed that porcine ears undergo post-mortem changes. The changes can be linked to rigor mortis and all other well-described phenomena that occur with carcasses after slaughter. The post-mortem changes modify the skin properties of the ears and affect the penetration efficacy. The onset of rigor mortis causes a decrease in the water-holding capacity of the ears, which leads to reduced penetration of chemical compounds. The water-holding capacity increases once the rigor is released and results in an increased penetration efficacy for chemical compounds. Despite different absolute penetration values, no differences in the ranking of penetration efficacies between the different formulations were observed between the differently aged ears. (4) Conclusions: All different types of ears can be regarded to be suitable for dermal penetration testing of chemical compounds. The transepidermal water loss (TEWL) and/or skin hydration of the ears were not correlated with the ex vivo penetration efficacy because both an impaired skin barrier and rigor mortis cause elevated skin hydration and TEWL values but an opposite penetration efficacy. Other additional values (for example, pH and/or autofluorescence of the skin) should, therefore, be used to select suitable and non-suitable skin areas for ex vivo penetration testing. Finally, data from this study confirmed that smartFilms and nanostructured lipid carriers (NLC) represent superior formulation strategies for efficient dermal and transdermal delivery of curcumin.

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.

High-Payload Nanosuspension of Centella asiatica Extract for Improved Skin Delivery with No Irritation

Background

The titrated extract of Centella asiatica (CA) has received much attention as a cosmeceutical ingredient owing to its anti-wrinkle effect. However, due to the low solubility and high molecular weight of pharmacologically active constituents, including asiatic acid (AA), madecassic acid (MA), and asiaticoside (AS), it is challenging to fabricate high-payload topical preparations of CA with satisfactory skin absorption profiles.

Purpose

This study aimed to design a high-payload topical preparation of CA using nanocrystallization technique and to evaluate its skin absorption profile and local tolerability.

Methods

High-payload nanocrystal suspensions (NSs) were prepared using lab-scale bead-milling technology, by adjusting the type and amount of suspending agent, CA content, type of vehicle, and milling speed. CA-loaded NSs were characterized in terms of morphology, particle size, crystallinity, and in vitro dissolution pattern. Skin absorption of CA nanocrystals was evaluated using a vertical Franz diffusion cell mounted with porcine skin. In vivo skin irritation following topical application of high-payload NS was assessed in normal rats.

Results

The optimized NS system, composed of 10% (w/v) CA, 0.5% polyvinylpyrrolidone (PVP) K30 as steric stabilizer, and 89.5% of distilled water, was characterized as follows: spherical or elliptical in shape, 200 nm in size, with low crystallinity. The in vitro dissolution of AA or MA from NSs was markedly faster compared to raw material, under sink condition. Penetration of AA, MA, and AS in the porcine skin was markedly elevated using the high-payload NS formula, providing 5-, 4-, and 4.5-fold higher accumulation in skin layer, compared to that of the marketed cream formula (CA 1%, Madeca cream). Moreover, topical application of high-payload NS was tolerable, showing neither erythema nor oedema in normal rats.

Conclusion

The novel NS system is expected to be a virtuous approach for offering a better skin absorption of CA, without using an excess quantity of solubilizers.

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.

Improved Dermal and Transdermal Delivery of Curcumin with SmartFilms and Nanocrystals

Poor aqueous solubility of active compounds is a major issue in today's drug delivery. In this study the smartFilm-technology was exploited to improve the dermal penetration efficacy of a poorly soluble active compound (curcumin). Results were compared to the dermal penetration efficacy of curcumin from curcumin bulk suspensions and nanocrystals, respectively. The smartFilms enabled an effective dermal and transdermal penetration of curcumin, whereas curcumin bulk- and nanosuspensions were less efficient when the curcumin content was similar to the curcumin content in the smartFilms. Interestingly, it was found that increasing numbers of curcumin particles within the suspensions increased the passive dermal penetration of curcumin. The effect is caused by an aqueous meniscus that is created between particle and skin if the dispersion medium evaporates. The connecting liquid meniscus causes a local swelling of the stratum corneum and maintains a high local concentration gradient between drug particles and skin. Thus, leading to a high local passive dermal penetration of curcumin. The findings suggest a new dermal penetration mechanism for active compounds from nano-particulate drug delivery systems, which can be the base for the development of topical drug products with improved penetration efficacy in the future.

Analysis of the morphometric parameters of pig ear hair follicles

BACKGROUND

Porcine ear skin is used in studies of percutaneous penetration as a substitute for human skin. The objective of the present study was to determine the structure of the hair follicles on the dorsal area of porcine ear skin and make a morphometric comparison with the hair follicles of human skin.

MATERIALS AND METHODS

Sections of frozen biopsies were cut vertically to the skin surface in longitudinal sections using a cryotome and were investigated using microscopy. For each hair follicle, various parameters were determined.

RESULTS

The follicular density in porcine ear skin varies according to the area studied, and the length of most of the follicles was approximately 1458 ± 286 μm. The size of the follicular orifice was also determined in a total of 305 follicles. It showed a diameter of roughly 113 ± 43 μm.

CONCLUSION

The results showed a very good similarity between human and pig hair follicles. Therefore, porcine ear skin can be considered as a very suitable model of human skin in dermal and especially follicular penetration studies.

Release of the model drug SR101 from polyurethane nanocapsules in porcine hair follicles triggered by LED-derived low dose UVA light

Hair follicles (HFs) are important drug delivery targets for the therapy of miscellaneous skin diseases and for skin antisepsis. Furthermore, HFs significantly contribute to drug delivery of topically applied substances. Nanoparticulate systems are excellently suited for follicular drug delivery as they entail the opportunity of directed drug transport into HFs. Moreover, they involve the possibility of an intrafollicular drug release initiated by extrinsic or intrinsic trigger mechanisms. In this study, we present a novel preclinical model for an anatomically and temporally targeted intrafollicular drug release. In vitro release kinetics of the model drug sulforhodamine 101 (SR101) from newly synthesized ultraviolet A (UVA)-responsive polyurethane nanocapsules (NCs) were investigated by fluorescence spectroscopy. Low power density UVA radiation provided by a UVA light emitting diode (LED) induced a drug release of over 50% after 2 min. We further utilized confocal laser scanning microscopy (CLSM) to investigate follicular penetration as well as intrafollicular drug release on an ex vivo porcine ear skin model. UVA-responsive degradation of the NCs at a mean follicular penetration depth of 509 ± 104 µm ensured liberation of SR101 in the right place and at the right time. Thus, for the first time a UVA-triggered drug release from NCs within HFs was demonstrated in the present study. Cytotoxicity tests revealed that NCs synthesized with isophorone diisocyanate show sufficient biocompatibility after UVA-induced cleavage. A considerable and controllable release of various water-soluble therapeutics could be reached by means of the presented system without risking any radiation-related tissue damage. Therefore, the implementation of the presented system into clinical routine, e.g. for preoperative antisepsis of HFs, appears very promising.

Tofacitinib Loaded Squalenyl Nanoparticles for Targeted Follicular Delivery in Inflammatory Skin Diseases

Tofacitinib (TFB), a Janus kinase inhibitor, has shown excellent success off-label in treating various dermatological diseases, especially alopecia areata (AA). However, TFB's safe and targeted delivery into hair follicles (HFs) is highly desirable due to its systemic adverse effects. Nanoparticles (NPs) can enhance targeted follicular drug delivery and minimize interfollicular permeation and thereby reduce systemic drug exposure. In this study, we report a facile method to assemble the stable and uniform 240 nm TFB loaded squalenyl derivative (SqD) nanoparticles (TFB SqD NPs) in aqueous solution, which allowed an excellent loading capacity (LC) of 20%. The SqD NPs showed an enhanced TFB delivery into HFs compared to the aqueous formulations of plain drug in an ex vivo pig ear model. Furthermore, the therapeutic efficacy of the TFB SqD NPs was studied in a mouse model of allergic dermatitis by ear swelling reduction and compared to TFB dissolved in a non-aqueous mixture of acetone and DMSO (7:1 v/v). Whereas such formulation would not be acceptable for use in the clinic, the TFB SqD NPs dispersed in water illustrated a better reduction in inflammatory effects than plain TFB's aqueous formulation, implying both encouraging good in vivo efficacy and safety. These findings support the potential of TFB SqD NPs for developing a long-term topical therapy of AA.

Hair Follicle Targeting and Dermal Drug Delivery with Curcumin Drug Nanocrystals-Essential Influence of Excipients

Many active pharmaceutical ingredients (API) possess poor aqueous solubility and thus lead to poor bioavailability upon oral administration and topical application. Nanocrystals have a well-established, universal formulation approach to overcome poor solubility. Various nanocrystal-based products have entered the market for oral application. However, their use in dermal formulations is relatively novel. Previous studies confirmed that nanocrystals are a superior formulation principle to improve the dermal penetration of poorly soluble API. Other studies showed that nanocrystals can also be used to target the hair follicles where they create a drug depot, enabling long acting drug therapy with only one application. Very recent studies show that also the vehicle in which the nanocrystals are incorporated can have a tremendous influence on the pathway of the API and the nanocrystals. In order to elucidate the influence of the excipient in more detail, a systematic study was conducted to investigate the influence of excipients on the penetration efficacy of the formulated API and the pathway of nanocrystals upon dermal application. Results showed that already small quantities of excipients can strongly affect the passive dermal penetration of curcumin and the hair follicle targeting of curcumin nanocrystals. The addition of 2% ethanol promoted hair follicle targeting of nanocrystals and hampered passive diffusion into the stratum corneum of the API, whereas the addition of glycerol hampered hair follicle targeting and promoted passive diffusion. Propylene glycol was found to promote both pathways. In fact, the study proved that formulating nanocrystals to improve the bioefficacy of poorly soluble API upon dermal application is highly effective. However, this is only true, if the correct excipient is selected for the formulation of the vehicle. The study also showed that excipients can be used to allow for a targeted dermal drug delivery, which enables to control if API should be delivered via passive diffusion and/or as drug reservoir by depositing API in the hair follicles.

Hair follicle targeting with curcumin nanocrystals: Influence of the formulation properties on the penetration efficacy

Nanocrystals are a universal formulation approach for improved drug delivery of poorly water-soluble drug substances. Besides oral application, also topical application of the nanocrystals is feasible, because the increased kinetic solubility of the nanocrystals results in an increased concentration gradient, thus fostering passive, dermal penetration. Nanocrystals are also promising for targeting drug substances into the hair follicle. After penetration into the hair follicle, the nanocrystals could form a depot from which the active is released into the hair follicle. Thus, leading to a long-lasting and very efficient dermal drug delivery. The efficacy of nanocrystals to penetrate the hair follicles and the influence of the vehicle in which the nanocrystals are suspended was not yet investigated. Therefore, in this study curcumin nanocrystals with a size of about 300 nm were produced and incorporated into gels with different properties. The efficacy to penetrate the hair follicles, as well as the passive, dermal penetration, was assessed on the ex-vivo pig ear model. Nanocrystals were efficiently taken up by the hair follicles and reached the lower part of the infundibulum. This region is optimal for efficient drug delivery because the barrier of the lower infundibulum is not fully developed and thus more permeable, which results in a less hindered passive diffusion of drug substances. The penetration efficacy of the nanocrystals into the hair follicles was not affected by the different types of vehicles, which represented either oleogels or hydrogels that varied in viscosity as well as in the type and the concentration of the gelling agent. All gels possessed a shear-thinning flow behavior and it is hypothesized that all gels fluidized during the skin massage, whereby leading to similarly low viscosities than the aqueous nanosuspension and thus to similar penetration results. The passive, dermal penetration of curcumin was different for the different gels and the main driving parameter leading to good passive diffusion was caused by good skin hydrating properties of the vehicle. The best passive penetration was achieved from hydrogels that contained a humectant. However, the addition of the humectant reduced the efficacy of the nanocrystals to penetrate the hair follicle. Data so far, therefore, suggest that hair follicle targeting with nanocrystals that are suspended in water or simple, shear-thinning gels is highly effective. However, the addition of other excipients, e.g. humectants, to these vehicles might cause changes in the penetration profiles. More research in this regard is needed to understand these observations in more detail.

Ultrasound-mediated topical delivery of econazole nitrate with potential for treating Raynaud's phenomenon

The study aims to assess the ultrasound-assisted econazole nitrate (EN) permeation from topically applied formulations with potential for treating Raynaud's phenomenon. Optimization of ultrasound parameters such as the distance of the horn, application time and amplitude were performed. In vitro percutaneous absorption studies were performed using econazole formulations (F2_HPMC dispersion, F4_Lipoderm® Activemax™ Cream) across the ultrasound-treated porcine skin and were compared with the control group (skin samples without ultrasound). Histology and ATR-FTIR studies were performed on treated skin samples. A constant frequency (20 kHz) ultrasound application with 40% amplitude, 0.5 cm distance between ultrasound horn and the skin surface for 2 min was optimized. The permeation of EN was found to be higher from ultrasound-treated skin samples than the control group. Drug permeation from F2_HPMC dispersion was found to be higher as compared to the other formulations and the marketed EN cream. Histological evaluation confirmed that F2_HPMC dispersion showed no signs of toxicity. ATR-FTIR studies revealed a slight increase in the CH₂ stretching vibrations (~2920 cm^-1 and 2850 cm^-1) in ultrasound-treated skin samples as compared with the control. In conclusion, the ultrasound-assisted transdermal delivery of F2_HPMC dispersion could be further studied as a new therapy for Raynaud's phenomenon.

Imaging and quantifying drug delivery in skin - Part 1: Autoradiography and mass spectrometry imaging

In this two-part review we present an up-to-date description of different imaging methods available to map the localization of drugs on skin as a complement of established ex-vivo absorption studies. This first part deals with invasive methods which are grouped in two classes according to their underlying principles: i) methods using radioactivity such as autoradiography and ii) mass spectrometry methods such as MALDI and SIMS. For each method, a description of the principle is given along with example applications of imaging and quantifying drug delivery in human skin. Thanks to these techniques a better assessment of the fate of drugs is obtained: its localization on a particular skin structure, its potential accumulation, etc. A critical comparison in terms of capabilities, sensitivity and practical applicability is included that will help the reader to select the most appropriate technique depending on the particular problem to be solved.

Synthesis, characterization, and evaluation of novel cell-penetrating peptides based on TD-34

In this study, six N-1, N-2, or N-11 derivatives of TD-34 (a cationic cyclic cell-penetrating peptide [CPP], ACSSKKSKHCG) were designed and synthesized including both linear peptides and cyclic peptides, such as DL-1 (KWSSKKSKHCG), DLCC-1 (cyclopeptide, KWSSKKSKHCG), DL-2 (KWSSKKSKHCG-NH₂ ), DLCC-2 (cyclopeptide, KWSSKKSKHCG-NH₂ ), DL-3 (RWSSKKSKHCG), and DLCC-3 (cyclopeptide, RWSSKKSKHCG). The cyclic peptides were synthesized by disulfide bound linkages formed by N-2 and N-10 cysteine. In vitro penetration experiment was conducted to investigate the transdermal enhancement ability of these derivatives, using triptolide (TP) as model drug. The results display that at the presence of DLCC-2, the accumulative penetration amount of TP increased 1.71-fold (P < .05) within 12 hours, displaying better transdermal enhancing ability than TD-34. Meanwhile, DL-3 and DLCC-3 slightly decreased the transdermal delivery of TP, and the presence of DL-1 and DLCC-1 shows no obvious effect. In order to clarify the factors on the transdermal ability of peptides, the solubility of TP in phosphate buffer saline (PBS) at the presence of different peptides and the mechanism of transdermal delivery of CPPs was investigated. The result shows that most of these peptides have no significant effect on the solubility of TP except DLCC-3 (the solubility of TP slightly increased). And in order to investigate transdermal absorption route of DLCC-2, polyarginine linked to rhodamine b (Rh b) derivative is used. The result proved that the transdermal route of polyarginine is via hair follicle, which may change the transdermal route of its cargo molecule (TP). Our group previously proved that polyarginine and TD-34 have similar transdermal enhancing mechanism (changing the transdermal route of their cargo molecule); it is reasonably speculated that the transdermal route of DLCC-2 is the same as polyarginine and then changes the transdermal absorption route of TP. Furthermore, such results have laid a solid foundation for further investigation of CPPs and paved a way for both designing and synthesizing of new drug delivery system for therapy molecules.

Skin toxicity of topically applied nanoparticles

Nowadays, nanoproducts have found numerous applications, allowing them to enter the human body in different ways. Skin is a major body organ that acts as the first-line barrier between the internal organs and external environment. Although the inhalation and ingestion of nanoparticles is more dangerous compared with skin exposure, there are noteworthy information gaps in skin exposure to nanoparticles that need much attention. Despite the few reviews in the literature on the cytotoxic effects of nanoparticles, no research has reviewed the clinical side effects of nanoparticles following topical admonition, including skin inflammation, skin cancer and genetic toxicity.

Predicting Skin Permeability by Means of Computational Approaches: Reliability and Caveats in Pharmaceutical Studies

The skin is the main barrier between the internal body environment and the external one. The characteristics of this barrier and its properties are able to modify and affect drug delivery and chemical toxicity parameters. Therefore, it is not surprising that permeability of many different compounds has been measured through several in vitro and in vivo techniques. Moreover, many different in silico approaches have been used to identify the correlation between the structure of the permeants and their permeability, to reproduce the skin behavior, and to predict the ability of specific chemicals to permeate this barrier. A significant number of issues, like interlaboratory variability, experimental conditions, data set building rationales, and skin site of origin and hydration, still prevent us from obtaining a definitive predictive skin permeability model. This review wants to show the main advances and the principal approaches in computational methods used to predict this property, to enlighten the main issues that have arisen, and to address the challenges to develop in future research.

Temperature-Enhanced Follicular Penetration of Thermoresponsive Nanogels

Hair follicles can serve as an effective reservoir for dermal drug delivery upon the topical application of particulate substances. Here, the follicular penetration of an indodicarbocyanine-labelled thermoresponsive nanogel (189 nm) having a cloud point temperature of 34°C and linked via an acid-labile linker to the model drug indocarbocyanine was investigated. In total, 227 hair follicles of porcine ear skin were examined after topical application of the thermoresponsive nanogels at room temperature (21°C), physiological skin surface temperature (32°C) and core body temperature (37°C) for the follicular penetration depths of indodicarbocyanine and indocarbocyanine using confocal laser scanning microscopy. The results showed a significantly increased mean follicular penetration of the carrier to a depth of 298.8±85.8 μm after incubation at 37°C compared to samples incubated at 21°C and 32°C with mean follicular penetration depths of 202.7±81.7 μm and 219.4±52.9 μm, respectively (p<0.001). Possibly structural changes in the thermoresponsive nanogel induced by the increased incubation temperature led to an enhancement of follicular penetration. Therefore, thermoresponsive nanogels may be suitable for the temperature-enhanced penetration into the hair follicles under physiological conditions.

Topical Nano and Microemulsions for Skin Delivery

Nanosystems such as microemulsions (ME) and nanoemulsions (NE) offer considerable opportunities for targeted drug delivery to and via the skin. ME and NE are stable colloidal systems composed of oil and water, stabilised by a mixture of surfactants and cosurfactants, that have received particular interest as topical skin delivery systems. There is considerable scope to manipulate the formulation components and characteristics to achieve optimal bioavailability and minimal skin irritancy. This includes the incorporation of established chemical penetration enhancers to fluidize the stratum corneum lipid bilayers, thus reducing the primary skin barrier and increasing permeation. This review discusses nanosystems with utility in skin delivery and focuses on the composition and characterization of ME and NE for topical and transdermal delivery. The mechanism of skin delivery across the stratum corneum and via hair follicles is reviewed with particular focus on the influence of formulation.

Preparation and in vivo evaluation of lecithin-based microparticles for topical delivery of minoxidil

Minoxidil is widely used for treatment of androgenic alopecia. Commercial products containing minoxidil are usually in solution form. Repeated applications of minoxidil solution can lead to adverse effects such as skin irritation and horniness. The aims of this study were to prepare lecithin-based microparticle in minoxidil solution for enhancement of minoxidil topical delivery and skin protection and evaluate the ability of lecithin on in vitro delivery, in vivo hair growth, and skin trouble improvement compared to commercial minoxidil solution. In in vitro skin permeation study, minoxidil solution containing lecithin microparticle showed higher skin penetration rate and higher retention of drug inside the skin compared to minoxidil solution without lecithin. After topical application of minoxidil solutions with or without lecithin to C57BL/6 mice, minoxidil 5% solution containing lecithin microparticle showed hair re-growth as efficient as commercial product of minoxidil 5% solution. It also significantly improved skin troubles while commercial product presented horny substance and crust formation. Therefore, the lecithin-based microparticle in minoxidil 5% solution has good ability to promote hair growth without adverse effects.

Skin models for the testing of transdermal drugs

The assessment of percutaneous permeation of molecules is a key step in the evaluation of dermal or transdermal delivery systems. If the drugs are intended for delivery to humans, the most appropriate setting in which to do the assessment is the in vivo human. However, this may not be possible for ethical, practical, or economic reasons, particularly in the early phases of development. It is thus necessary to find alternative methods using accessible and reproducible surrogates for in vivo human skin. A range of models has been developed, including ex vivo human skin, usually obtained from cadavers or plastic surgery patients, ex vivo animal skin, and artificial or reconstructed skin models. Increasingly, largely driven by regulatory authorities and industry, there is a focus on developing standardized techniques and protocols. With this comes the need to demonstrate that the surrogate models produce results that correlate with those from in vivo human studies and that they can be used to show bioequivalence of different topical products. This review discusses the alternative skin models that have been developed as surrogates for normal and diseased skin and examines the concepts of using model systems for in vitro–in vivo correlation and the demonstration of bioequivalence.

Solid effervescent formulations as new approach for topical minoxidil delivery

Currently marketed minoxidil formulations present inconveniences that range from a grease hard aspect they leave on the hair to more serious adverse reactions as scalp dryness and irritation. In this paper we propose a novel approach for minoxidil sulphate (MXS) delivery based on a solid effervescent formulation. The aim was to investigate whether the particle mechanical movement triggered by effervescence would lead to higher follicle accumulation. Preformulation studies using thermal, spectroscopic and morphological analysis demonstrated the compatibility between effervescent salts and the drug. The effervescent formulation demonstrated a 2.7-fold increase on MXS accumulation into hair follicles casts compared to the MXS solution (22.0±9.7μg/cm² versus 8.3±4.0μg/cm²) and a significant drug increase (around 4-fold) in remaining skin (97.1±29.2μg/cm²) compared to the drug solution (23.5±6.1μg/cm²). The effervescent formulations demonstrated a prominent increase of drug permeation highly dependent on the effervescent mixture concentration in the formulation, confirming the hypothesis of effervescent reaction favoring drug penetration. Clinically, therapy effectiveness could be improved, increasing the administration interval, hence, patient compliance. More studies to investigate the follicular targeting potential and safety of new formulations are needed.

Penetration of topically applied nanocarriers into the hair follicles of dog and rat dorsal skin and porcine ear skin

BACKGROUND

In humans, topically applied nanocarriers penetrate effectively into the hair follicles where they can be exploited for the localized and targeted treatment of skin disorders.

OBJECTIVE

The objective of the present study was to examine the applicability of particle-based systems for follicular drug delivery in companion animals and livestock, which have a large follicular reservoir.

ANIMALS

Skin samples from 10 beagle dogs, 14 Wistar rats and four ears from freshly slaughtered cross-bred pigs were used.

METHODS

Fluoresceinamine labelled poly (L-lactide-co-glycolide) nanocarriers (256 or 430 nm) were applied on the different skin samples. After penetration, skin biopsies were removed and cryohistological cross sections prepared and investigated with regard to the follicular penetration depths (in μm ± standard deviation) of the nanocarriers using confocal laser scanning microscopy.

RESULTS

In canine, rat and porcine hair follicles, the smaller nanoparticles were detected at mean follicular penetration depths of 630.16 ± 135.75 μm, 253.55 ± 47.36 μm and 653.40 ± 94.71 μm, respectively. The larger particles were observed at average follicular depths of 604.79 ± 132.42 μm; 262.87 ± 55.25 μm and 786.81 ± 121.73 μm, respectively, in canine, rat and porcine hair follicles. Statistically significant differences (P < 0.05) in the mean follicular penetration depths of the differently sized nanocarriers could be determined for the canine and porcine skin samples.

CONCLUSION

The mean follicular penetration depths of the differently sized nanocarriers were mostly significantly different between the different species, which might be due to different species-specific follicular dimensions. This issue needs to be addressed specifically in further studies.

The effect of parathyroid hormones on hair follicle physiology: implications for treatment of chemotherapy-induced alopecia

Parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) influence hair follicles through paracrine and intracrine routes. There is significant evidence that PTH and PTHrP influence the proliferation and differentiation of hair follicle cells. The PTH/PTHrP receptor signalling plays an important role in the hair follicle cycle and may induce premature catagen-telogen transition. Transgenic mice with an overexpression or blockade (PTH/PTHrP receptor knockout mice) of PTHrP activity revealed impaired or increased hair growth, respectively. Some findings also suggest that PTHrP may additionally influence the hair cycle by inhibiting angiogenesis. Antagonists of the PTH/PTHrP receptor have been shown to stimulate proliferation of hair follicle cells and hair growth. A hair-stimulating effect of a PTH/PTHrP receptor antagonist applied topically to the skin has been observed in hairless mice, as well as in mice treated with cyclophosphamide. These data indicate that the PTH/PTHrP receptor may serve as a potential target for new (topical) hair growth-stimulating drugs, especially for chemotherapy-induced alopecia.

Triggered release of model drug from AuNP-doped BSA nanocarriers in hair follicles using IRA radiation

Recent advances in the field of dermatotherapy have resulted in research efforts focusing on the use of particle-based drug delivery systems for the stimuli-responsive release of drugs in the skin and skin appendages, i.e. hair follicles and sebaceous glands. However, effective and innocuous trigger mechanisms which result in the release of the drugs from the nanocarriers upon reaching the target structures are still lacking. For the first time, the present study demonstrated the photo-activated release of the model drug fluorescein isothiocyanate (FITC) from topically applied gold nanoparticle-doped bovine serum albumin (AuNPs-doped BSA) particles (approx. 545nm) using water-filtered infrared A (IRA) radiation in the hair follicles of an ex vivo porcine skin model. The IRA radiation-induced plasmonic heating of the AuNPs results in the partial decomposition or opening of the albumin particles and release the model drug, while control particles without AuNPs show insignificant release. The results demonstrate the feasibility of using IRA radiation to induce release of encapsulated drugs from plasmonic nanocarriers for the targeting of follicular structures. However, the risk of radiation-induced skin damage subsequent to repeated applications of high infrared dosages may be significant. Future studies should aim at determining the suitability of lower infrared A dosages, such as for medical treatment regimens which may necessitate repeated exposure to therapeutics.

STATEMENT OF SIGNIFICANCE

Follicular targeting using nanocarriers is of increasing importance in the prophylaxis and treatment of dermatological or other diseases. For the first time, the present study demonstrated the photo-activated release of the model drug fluorescein isothiocyanate (FITC) from topically applied gold nanoparticle-doped bovine serum albumin (AuNPs-doped BSA) particles using water-filtered infrared A (IRA) radiation in the hair follicles of an ex vivo porcine skin model. The results demonstrate the feasibility of using wIRA radiation to induce release of encapsulated drugs for the targeting of follicular structures, and provide a new vision on the development of optically addressable delivery systems for controlled release of drugs in the skin and skin appendages, i.e. hair follicles and sebaceous glands.

Tight junctions form a barrier in porcine hair follicles

Follicular penetration has gained increasing interest regarding (i) safety concerns about (environmentally born) xenobiotics available to the hair follicle (HF), e.g. nanomaterials or allergens which should not enter the skin, and (ii) the possibility for non-invasive follicular drug and antigen delivery. However, not much is known about barriers in the HF which have to be surpassed upon uptake and/or penetration into surrounding tissue. Thus, aim of this work was a detailed investigation of this follicular barrier function, as well as particle uptake into the HF of porcine skin which is often used as a model system for human skin for such purposes. We show that follicular tight junctions (TJs) form a continuous barrier from the infundibulum down to the suprabulbar region, complementary to the stratum corneum in the most exposed upper follicular region, but remaining as the only barrier in the less accessible lower follicular regions. In the bulbar region of the HF no TJ barrier was found, demonstrating the importance of freely supplying this hair-forming part with e.g. nutrients or hormones from the dermal microenvironment. Moreover, the dynamic character of the follicular TJ barrier was shown by modulating its permeability using EDTA. After applying polymeric model-nanoparticles (154 nm) to the skin, transmission electron microscopy revealed that the majority of the particles were localized in the upper part of the HF where the double-barrier is present. Only few penetrated deeper, reaching regions where TJs act as the only barrier, and no particles were observed in the bulbar, barrier-less region. Lastly, the equivalent expression and distribution of TJ proteins in human and porcine HF further supports the suitability of porcine skin as a predictive model to study the follicular penetration and further biological effects of dermally applied nanomaterials in humans.

Iontophoresis of minoxidil sulphate loaded microparticles, a strategy for follicular drug targeting?

The feasibility of targeting drugs to hair follicles by a combination of microencapsulation and iontophoresis has been evaluated. Minoxidil sulphate (MXS), which is used in the treatment of alopecia, was selected as a relevant drug with respect to follicular penetration. The skin permeation and disposition of MXS encapsulated in chitosan microparticles (MXS-MP) was evaluated in vitro after passive and iontophoretic delivery. Uptake of MXS was quantified at different exposure times in the stratum corneum (SC) and hair follicles. Microencapsulation resulted in increased (6-fold) drug accumulation in the hair follicles relative to delivery from a simple MXS solution. Application of iontophoresis enhanced follicular delivery for both the solution and the microparticle formulations. It appears, therefore, that microencapsulation and iontophoresis can act synergistically to enhance topical drug targeting to hair follicles.

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.

In vitro skin models as a tool in optimization of drug formulation

(Trans)dermal drug therapy is gaining increasing importance in the modern drug development. To fully utilize the potential of this route, it is important to optimize the delivery of active ingredient/drug into/through the skin. The optimal carrier/vehicle can enhance the desired outcome of the therapy therefore the optimization of skin formulations is often included in the early stages of the product development. A rational approach in designing and optimizing skin formulations requires well-defined skin models, able to identify and evaluate the intrinsic properties of the formulation. Most of the current optimization relies on the use of suitable ex vivo animal/human models. However, increasing restrictions in use and handling of animals and human skin stimulated the search for suitable artificial skin models. This review attempts to provide an unbiased overview of the most commonly used models, with emphasis on their limitations and advantages. The choice of the most applicable in vitro model for the particular purpose should be based on the interplay between the availability, easiness of the use, cost and the respective limitations.

Practical problems encountered during the cultivation of an open-source reconstructed human epidermis model on a polycarbonate membrane and protein quantification

During recent years, the importance of in vitro technology in skin research has increased significantly. A variety of skin culture models have been developed and commercialized. In this respect, the availability of reconstructed human epidermis (RHE) equivalents represents a significant improvement compared to the use of monolayer cultures. However, when an in-house RHE model is being developed, researchers might encounter some difficulties during cultivation. The scope of this paper is to report our experiences and practical problems with the development of a three-dimensional RHE model cultured on a polycarbonate membrane. Some important issues including cell density, the use of lysing enzymes, culture media, cell storage and viability, cell confluency and protein extraction are reported and optional solutions are given.

Comparative study of hair follicle morphology in eight mammalian species and humans

BACKGROUND

The objective of the present study was the investigation of hair follicle morphology in eight mammalian species in order to evaluate the species-specific contribution of hair follicles to skin penetration particularly with regard to the utilization of the different animal species as skin models for human skin.

METHODS

Cyanoacrylate skin surface biopsy method (CSSB), light microscopy and also digital photography were used for the measurements of hair follicle morphology.

RESULTS

The results revealed species-specific differences regarding the pattern of hair follicle distribution and also differences with regard to hair follicle parameters and characteristics. The results also showed that hair follicles generally possess enormous reservoir capacities, regarding the follicular volume. In all examined species, hair follicles reached at least one-fifth of stratum corneum storage capacity. The results were compared with human data obtained in a previous study.

CONCLUSION

With regard to hair follicle morphology and skin structure, the porcine skin seems to be the most appropriate skin model for human skin analog to previous investigations, whereas the skin of dog, cat, and rabbit showed the most significant differences.

Iontophoresis-targeted, follicular delivery of minoxidil sulfate for the treatment of alopecia

Although minoxidil (MX) is a drug known to stimulate hair growth, the treatment of androgenic alopecia could be improved by delivery strategies that would favor drug accumulation into the hair follicles. This work investigated in vitro the potential of iontophoresis to achieve this objective using MX sulfate (MXS), a more water-soluble derivative of MX. Passive delivery of MXS was first determined from an ethanol-water solution and from a thermosensitive gel. The latter formulation resulted in greater accumulation of MXS in the stratum corneum (skin's outermost layer) and hair follicles and an overall decrease in absorption through the skin. Anodal iontophoresis of MXS from the same gel formulation was then investigated at pH 3.5 and pH 5.5. Compared with passive delivery, iontophoresis increased the amount of drug reaching the follicular infundibula from 120 to 600 ng per follicle. In addition, drug recovery from follicular casts was threefold higher following iontophoresis at pH 5.5 compared with that at pH 3.5. Preliminary in vivo experiments in rats confirmed that iontophoretic delivery of MXS facilitated drug accumulation in hair follicles. Overall, therefore, iontophoresis successfully and significantly enhanced follicular delivery of MX suggesting a useful opportunity for the improved treatment of alopecia.