Determination of the effect of lipophilicity on the in vitro permeability and tissue reservoir characteristics of topically applied solutes in human skin layers

Simulating the Skin Permeation Process of Ionizable Molecules

It is commonly assumed that ionizable molecules, such as drugs, permeate through the skin barrier in their neutral form. By using molecular dynamics simulations of the charged and neutral states separately, we can study the dynamic protonation behavior during the permeation process. We have studied three weak acids and three weak bases and conclude that the acids are ionized to a larger extent than the bases, when passing through the headgroup region of the lipid barrier structure, at pH values close to their pKa. It can also be observed that even if these dynamic protonation simulations are informative, in the cases studied herein they are not necessary for the calculation of permeability coefficients. It is sufficient to base the calculations only on the neutral form, as is commonly done.

High mobility group box 1 cytokine targeted topical delivery of resveratrol embedded nanoemulgel for the management of atopic dermatitis

Resveratrol is a polyphenolic compound showing anti-inflammatory activity by inhibition of high mobility group box 1 cytokine responsible for the activation of nuclear factor-κB pathway in atopic dermatitis. To evaluate the efficacy of resveratrol through topical route we have developed resveratrol-loaded nanoemulgel for the effective management of atopic dermatitis in mice model. The resveratrol-loaded nanoemulsion (0.5%, 0.75% and 1% w/w) was optimized by spontaneous nano-emulsification. The optimized resveratrol-loaded nanoemulsions showed average globule size in the 180-230 nm range and found to be monodispersed. The resveratrol nanoemulgel was prepared with a SEPINEO™ P 600 gel base and propylene glycol. Ex vivo permeation and retention study resulted in significantly higher skin retention of resveratrol from resveratrol-loaded nanoemulgel than free resveratrol-loaded gel. Preclinical efficacy of resveratrol nanoemulgel displayed promising therapeutic outcomes where, western blotting of skin tissues disclosed a significant reduction in the relative expression of high mobility group box 1, the receptor for advanced glycation end products, toll-like receptor-4 and phosphorylated nuclear factor-κB. Further, real-time polymerase chain reaction also disclosed a significant reduction in pro-inflammatory cytokines such as thymic stromal lymphopoietin, interleukin-4, interleukin-13, interleukin-31, tumor necrosis factor-α and interleukin-6. The histopathological examination of skin sections showed improvement in the skin condition. Collectively, the findings from our study showcased the significant improvement in the atopic dermatitis skin condition in mice model after topical application of resveratrol loaded nanoemulgel.

Identification of dihydromyricetin as a natural DNA methylation inhibitor with rejuvenating activity in human skin

Changes in DNA methylation patterning have been reported to be a key hallmark of aged human skin. The altered DNA methylation patterns are correlated with deregulated gene expression and impaired tissue functionality, leading to the well-known skin aging phenotype. Searching for small molecules, which correct the aged methylation pattern therefore represents a novel and attractive strategy for the identification of anti-aging compounds. DNMT1 maintains epigenetic information by copying methylation patterns from the parental (methylated) strand to the newly synthesized strand after DNA replication. We hypothesized that a modest inhibition of this process promotes the restoration of the ground-state epigenetic pattern, thereby inducing rejuvenating effects. In this study, we screened a library of 1800 natural substances and 640 FDA-approved drugs and identified the well-known antioxidant and anti-inflammatory molecule dihydromyricetin (DHM) as an inhibitor of the DNA methyltransferase DNMT1. DHM is the active ingredient of several plants with medicinal use and showed robust inhibition of DNMT1 in biochemical assays. We also analyzed the effect of DHM in cultivated keratinocytes by array-based methylation profiling and observed a moderate, but significant global hypomethylation effect upon treatment. To further characterize DHM-induced methylation changes, we used published DNA methylation clocks and newly established age predictors to demonstrate that the DHM-induced methylation change is associated with a reduction in the biological age of the cells. Further studies also revealed re-activation of age-dependently hypermethylated and silenced genes in vivo and a reduction in age-dependent epidermal thinning in a 3-dimensional skin model. Our findings thus establish DHM as an epigenetic inhibitor with rejuvenating effects for aged human skin.

Methods for the characterisation of dermal uptake: Progress and perspectives for organophosphate esters

Organophosphate esters (OPEs) are a group of pollutants that are widely detected in the environment at high concentrations. They can adversely affect human health through multiple routes of exposure, including dermal uptake. Although attention has been paid to achieving an accurate and complete quantification of the dermal uptake of OPEs, existing evaluation methods and parameters have obvious weaknesses. This study reviewed two main categories of methodologies, namely the relative absorption (RA) model and the permeability coefficient (PC) model, which are widely used to assess the dermal uptake of OPEs. Although the PC model is more accurate and is increasingly used, the most important parameter in this model, the permeability coefficient (Kp), has been poorly characterised for OPEs, resulting in considerable errors in the estimation of the dermal uptake of OPEs. Thus, the detailed in vitro methods for the determination of Kp are summarised and sorted. Furthermore, the commonly used skin membranes are identified and the factors affecting Kp and corresponding mechanisms are discussed. In addition, the experimental conditions, conclusions, and available data on Kp values of the OPEs are thoroughly summarised. Finally, the corresponding knowledge gaps are proposed, and a more accurate and sophisticated experimental system and unknown Kp values for OPEs are suggested.

Microneedle-Assisted Transfersomes as a Transdermal Delivery System for Aspirin

Transdermal drug delivery systems offer several advantages over conventional oral or hypodermic administration due to the avoidance of first-pass drug metabolism and gastrointestinal degradation as well as patients' convenience due to a minimally invasive and painless approach. A novel transdermal drug delivery system, comprising a combination of transfersomes with either solid silicon or solid polycarbonate microneedles has been developed for the transdermal delivery of aspirin. Aspirin was encapsulated inside transfersomes using a "thin-film hydration sonication" technique, yielding an encapsulation efficiency of approximately 67.5%. The fabricated transfersomes have been optimised and fully characterised in terms of average size distribution and uniformity, surface charge and stability (shelf-life). Transdermal delivery, enhanced by microneedle penetration, allows the superior permeation of transfersomes into perforated porcine skin and has been extensively characterised using optical coherence tomography (OCT) and transmission electron microscopy (TEM). In vitro permeation studies revealed that transfersomes enhanced the permeability of aspirin by more than four times in comparison to the delivery of unencapsulated "free" aspirin. The microneedle-assisted delivery of transfersomes encapsulating aspirin yielded 13-fold and 10-fold increases in permeation using silicon and polycarbonate microneedles, respectively, in comparison with delivery using only transfersomes. The cytotoxicity of different dose regimens of transfersomes encapsulating aspirin showed that encapsulated aspirin became cytotoxic at concentrations of ≥100 μg/mL. The results presented demonstrate that the transfersomes could resolve the solubility issues of low-water-soluble drugs and enable their slow and controlled release. Microneedles enhance the delivery of transfersomes into deeper skin layers, providing a very effective system for the systemic delivery of drugs. This combined drug delivery system can potentially be utilised for numerous drug treatments.

Evaluation of DNA lesions and radicals generated by a 233 nm far-UVC LED in superficial ex vivo skin wounds

The application of a far-ultraviolet C (UVC) light emitting diode (LED) of 233 nm showed significant bactericidal efficacy at an applied dose between 20 and 80 mJ cm-2 as reported recently. In addition, only minor epidermal DNA lesions were observed in ex vivo human skin and in vitro epidermal models <10% of the minimal erythema dose of UVB radiation. To broaden the potential range of applications of such systems, e.g. to include postoperative application on wounds for the purpose of decontamination, we assessed how a disruption of normal anatomic skin structure and function influences the skin damage induced by light from 233 nm far-UVC LEDs. Thus, we induced superficial skin wounds by mechanical detachment of the stratum corneum in ex vivo human skin. Barrier-disruption of the skin could be successfully determined by measuring an increase in the transepidermal water loss (TEWL) and the stratum corneum loss could be determined morphologically by 2-photon microscopy (2-PM). After far-UVC irradiation of the skin, we screened the tissue for the development of cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). The abundance of DNA lesions was elevated in wound skin in comparison to intact skin after irradiation with far-UVC. However, no increase in DNA lesions was detected when artificial wound exudate consisting of cell culture medium and serum was applied to the disrupted skin surface prior to irradiation. This effect agrees with the results of ray tracing simulations of the absorption of far-UVC light incident on a superficial skin wound. Interestingly, no significant deviations in radical formation between intact skin and superficially wounded skin were detected after far-UVC irradiation as analyzed by electron paramagnetic resonance (EPR) spectroscopy. In conclusion, 233 nm LED light at a dose of 60 mJ/cm2 could be applied safely on superficial wounds for the purpose of skin antisepsis as long as the wounds are covered with wound fluid.

A regression analysis using simple descriptors for multiple dermal datasets: Going from individual membranes to the full skin

In silico methods to estimate and/or quantify skin absorption of chemicals as a function of chemistry are needed to realistically predict pharmacological, occupational, and environmental exposures. The Potts-Guy equation is a well-established approach, using multi-linear regression analysis describing skin permeability (Kp) in terms of the octanol/water partition coefficient (logP) and molecular weight (MW). In this work, we obtained regression equations for different human datasets relevant to environmental and cosmetic chemicals. Since the Potts-Guy equation was published in 1992, we explored recent datasets that include different skin layers, such as dermatomed (including dermis to a defined thickness) and full skin. Our work was consistent with others who have observed that fits to the Potts-Guy equation are stronger for experiments focused on the epidermis. Permeability estimates for dermatomed skin and full skin resulted in low regression coefficients when compared to epidermis datasets. An updated regression equation uses a combination of fitted permeability values obtained with a published 2D compartmental model previously evaluated. The resulting regression equation was: logKp = -2.55 + 0.65logP - 0.0085MW, R2  = 0.91 (applicability domain for all datasets: MW ranges from 18 to >584 g/mol and -4 to >5 for logP). This approach demonstrates the advantage of combining mechanistic with structural activity relationships in a single modeling approach. This combination approach results in an improved regression fit when compared to permeability estimates obtained using the Potts-Guy approach alone. The analysis presented in this work assumes a one-compartment skin absorption route; future modeling work will consider adding multiple compartments.

Increase of ibuprofen penetration through the skin by forming ion pairs with amino acid alkyl esters and exposure to the electromagnetic field

A method of increasing the permeability of ibuprofen through the skin using a rotating magnetic field (RMF) is presented. This study evaluated whether 50 Hz RMF modifies ibuprofen's permeability through the skin. Ibuprofen and its structural modifications in the form of ibuprofenates of isopropyl esters of L-amino acids such as L-valine, L-phenylalanine, L-proline, and L-aspartic acid were used in the research. To this end, Franz cells with skin as membrane were exposed to 50 Hz RMF with 5% ibuprofen and its derivatives in an ethanol solution for 48 h. Following the exposures, the amount of penetrated compound was analysed. Regardless of the compound tested, a significant increase in drug transport through the skin was observed. The differences in the first 30 minutes of permeation are particularly noticeable. Furthermore, it was shown that using RMF increases the permeability of ibuprofen from 4 to 244 times compared to the test without the RMF. The greatest differences were observed for unmodified ibuprofen. However, it is noteworthy that the largest amounts of the active substance were obtained with selected modifications and exposure to RMF. The RMF may be an innovative and interesting technology that increases the penetration of anti-inflammatory and anti-ache drugs through the skin.

The Science of Selecting Excipients for Dermal Self-Emulsifying Drug Delivery Systems

Self-emulsification is considered a formulation technique that has proven capacity to improve oral drug delivery of poorly soluble drugs by advancing both solubility and bioavailability. The capacity of these formulations to produce emulsions after moderate agitation and dilution by means of water phase addition provides a simplified method to improve delivery of lipophilic drugs, where prolonged drug dissolution in the aqueous environment of the gastro-intestinal (GI) tract is known as the rate-limiting step rendering decreased drug absorption. Additionally, spontaneous emulsification has been reported as an innovative topical drug delivery system that enables successful crossing of mucus membranes as well as skin. The ease of formulation generated by the spontaneous emulsification technique itself is intriguing due to the simplified production procedure and unlimited upscaling possibilities. However, spontaneous emulsification depends solely on selecting excipients that complement each other in order to create a vehicle aimed at optimizing drug delivery. If excipients are not compatible or unable to spontaneously transpire into emulsions once exposed to mild agitation, no self-emulsification will be achieved. Therefore, the generalized view of excipients as inert bystanders facilitating delivery of an active compound cannot be accepted when selecting excipients needed to produce self-emulsifying drug delivery systems (SEDDSs). Hence, this review describes the excipients needed to generate dermal SEDDSs as well as self-double-emulsifying drug delivery systems (SDEDDSs); how to consider combinations that complement the incorporated drug(s); and an overview of using natural excipients as thickening agents and skin penetration enhancers.

Quantification methods comparing in vitro and in vivo percutaneous permeation by microneedles and passive diffusion

Microneedles (MNs) are needles with a tip diameter ranging from 10 to 100 um and a length ranging up to 1 mm. The first patent for drug delivery device for percutaneous administration filed by Alza corporation dates back to 1976 (Gerstel and Place, 1976), and in between 1989 and 2021 the filed patents for MNs are more than 4500 (Banks et al., 2010). These devices can potential overcome some drawbacks of traditional needles, such as the pain generated during insertion, requirement for trained personnel to manipulate syringes, and difficulty of performing injections in elderly and obese patients. MNs and MNs arrays are emerging as a convenient method to deliver compounds and extract blood without causing any pain. A promising application is the use of MNs as alternative solution to topical creams (TC) and transdermal patches (TP) for transdermal drug delivery. The external layer of human skin, the epidermis, offers a major barrier to transdermal drug delivery, thanks to the stratum corneum (SC). Exposed to the external environment, SC ultimately protects the human body from UV light radiation, heat, water loss, bacteria, fungi and viruses, and it is the barrier that controls diffusion rate for almost all compounds. TC and TP applications are limited by the skin permeability to lipophilic compounds and small molecules, and by the slow delivery rate of some compounds. MNs have been around for more than 35 year now, and it is a general opinion that MN increase delivery compared to passive diffusion, thanks to the feature of penetrating the SC and reaching the dermis. This review recollects the existing studies that compare MNs delivery of drugs with passive diffusion of the same drugs in alive organisms, giving an overview of what are the type of MNs, the chemical delivered and the methods employed to quantify drug delivery into skin and/or in the bloodstream. The final aim is to quantify the enhancement factor of MNs with respect to passive diffusion, and establish a possible standard on how tests can be performed in order to compare different data.

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).

Water-in-oil microemulsions composed of monoolein enhanced the transdermal delivery of nicotinamide

OBJECTIVE

Nicotinamide, also known as niacinamide, is a water-soluble vitamin that is used to prevent and treat acne and pellagra. It is often found in water-based skin care cosmetics because of its high water solubility. Nicotinamide is a small molecule with a molar mass of 122.1 g/mol. However, it has a hydrophilic nature that becomes an obstacle when it penetrates through the skin. The topmost layer of the skin, the stratum corneum, acts as a strong hydrophobic barrier for such hydrophilic molecules. The oil-based formulations are expected to enhance the transdermal delivery efficiency of nicotinamide.

METHODS

We have developed oil-based microemulsion formulations composed of a squalane vehicle. Monoolein was used as an emulsifier that has a potential to enhance the nicotinamide delivery through the stratum corneum.

RESULTS

Because the mean size of the emulsions measured by dynamic light scattering was 20.9 ± 0.4 nm, the microemulsion formulation was stable under the long-term storage. Monoolein acted as a skin penetration enhancer, and it effectively enabled the penetration of nicotinamide through human abdominal skin, compared with nicotinamide in a phosphate-buffered saline solution. The flux was increased 25-fold. Microscopic imaging revealed that the hydrophilic bioactive compounds penetrated through the intercellular spaces in the epidermis.

CONCLUSION

The monoolein-based microemulsion was transparent and stable, suggesting that it is a promising formulation for a transdermal nicotinamide delivery.

Increased Therapeutic Efficacy of SLN Containing Etofenamate and Ibuprofen in Topical Treatment of Inflammation

Innovative formulations, including solid lipid nanoparticles (SLNs), have been sought to improve skin permeation of non-steroidal anti-inflammatory drugs (NSAIDs). The present study explores the use of SLNs, prepared using a fusion-emulsification method, to increase skin permeation and in vivo activity of two relevant NSAIDs: A liquid molecule (etofenamate) and a solid one (ibuprofen), formulated in a 2% hydroxypropyl methylcellulose gel through the gelation of SLN suspensions. Compritol^® 888 ATO and Tween^® 80 were used as a solid lipid and a surfactant, respectively. All production steps were up scalable, resulting in SLNs with high encapsulation efficiency (>90%), a mean particle size of <250 nm, a polydispersity index <0.2, and that were stable for 12 months. In vitro permeation, using human skin in Franz diffusion cells, showed increased permeation and similar cell viability in Df and HaCaT cell lines for SLN formulations when compared to commercial formulations of etofenamate (Reumon^® Gel 5%) and ibuprofen (Ozonol^® 5%). In vivo activity in the rat paw edema inflammation model showed that SLN hydrogels containing lower doses of etofenamate (8.3 times lower) and ibuprofen (16.6 times lower) produced similar effects compared to the commercial formulations, while decreasing edema and inflammatory cell infiltration, and causing no histological changes in the epidermis. These studies demonstrate that encapsulation in SLNs associated to a suitable hydrogel is a promising technological approach to NSAIDs dermal application.

Characterisation of Drug Delivery Efficacy Using Microstructure-Assisted Application of a Range of APIs

Polymer-based solid microstructures (MSts) have the potential to significantly increase the quantity and range of drugs that can be administered across the skin. MSt arrays are used to demonstrate their capacity to bypass the skin barrier and enhance permeability by creating microchannels through the stratum corneum, in a minimally invasive manner. This study is designed to demonstrate the ability of MSts to exceed the current boundaries for transdermal delivery of compounds with different molecular weights, partition coefficients, acid dissociation constants, melting points, and water solubilities. In vitro permeation of a range of selected molecules, including acetyl salicylic acid (aspirin), galantamine, selegiline hydrochloride (Sel-HCl), insulin, caffeine, hydrocortisone (HC), hydrocortisone 21-hemisuccinate sodium salt (HC-HS) and bovine serum albumin (BSA) has been studied across excised porcine skin with and without poke and patch application of MSts. Permeation of the molecules was monitored using Franz diffusion cells over 24 h. MSts significantly increased the permeation of all selected molecules up to 40 times, compared to topical applications of the molecules without MSts. The greatest increase in permeation was observed for caffeine with 70 ± 8% permeation and the lowest enhancement was observed for HC with a 2.4 ± 1.3% increase in permeation. The highest obtained flux was BSA (8133 ± 1365 μg/cm2/h) and the lowest flux observed for HC (11 ± 4 μg/cm2/h). BSA and HC also showed the highest (16,275 ± 3078 μg) and the lowest (73 ± 47 μg) permeation amount after 24 h respectively. MSt-treated skin exhibits greatly increased permeation. The molecule parameters (size, acid dissociation constant, partition coefficient and solubility)-traditional hurdles associated with passive diffusion through intact skin-are overcome using MSt skin treatment.

Human skin models: From healthy to disease-mimetic systems; characteristics and applications

Skin drug delivery is an emerging route in drug development, leading to an urgent need to understand the behaviour of active pharmaceutical ingredients within the skin. Given, As one of the body's first natural defences, the barrier properties of skin provide an obstacle to the successful outcome of any skin drug therapy. To elucidate the mechanisms underlying this barrier, reductionist strategies have designed several models with different levels of complexity, using non-biological and biological components. Besides the detail of information and resemblance to human skin in vivo, offered by each in vitro model, the technical and economic efforts involved must also be considered when selecting the most suitable model. This review provides an outline of the commonly used skin models, including healthy and diseased conditions, in-house developed and commercialized models, their advantages and limitations, and an overview of the new trends in skin-engineered models.

Plasma pharmacokinetics and distribution of ruxolitinib into skin following oral and topical administration in minipigs

This preclinical study compared plasma concentrations and distribution of ruxolitinib in the skin of Göttingen minipigs following twice a day oral (40 mg/kg) versus topical administration (1.5% w/w cream applied to 10% of body surface area). Following oral administration, the plasma area-under-the-curve was approximately 31-fold and maximum drug concentration was 38-fold higher than those observed following topical application. Following ruxolitinib cream application, the average plasma concentration at steady-state was 2.7 ± 1.8 nM, a concentration that is not pharmacologically relevant. The average total dermis concentration of ruxolitinib at steady-state after topical administration was 507-fold higher versus that following oral dosing, while the ratio for the total epidermal concentration following topical vs oral dosing was 1989-fold. The concentration of unbound ruxolitinib in the dermis after topical application was predicted to result in sustained and near-complete inhibition of Janus kinase/signal transducer and activator of transcription proteins (JAK/STAT) signaling in this tissue. In contrast, only partial inhibition of downstream signaling was predicted to occur after oral dosing. In conclusion, ruxolitinib cream affords an attractive disposition profile in minipigs, wherein dermis concentrations of ruxolitinib are fully effective whereas corresponding plasma concentrations are negligible. Consequently, this distribution profile should maximize the efficacy of ruxolitinib cream in the skin while minimizing the potential for deleterious systemic effects.

A deep learning approach for the blind logP prediction in SAMPL6 challenge

Water octanol partition coefficient serves as a measure for the lipophilicity of a molecule and is important in the field of drug discovery. A novel method for computational prediction of logarithm of partition coefficient (logP) has been developed using molecular fingerprints and a deep neural network. The machine learning model was trained on a dataset of 12,000 molecules and tested on 2000 molecules. In this article, we present our results for the blind prediction of logP for the SAMPL6 challenge. While the best submission achieved a RMSE of 0.41 logP units, our submission had a RMSE of 0.61 logP units. Overall, we ranked in the top quarter out of the 92 submissions that were made. Our results show that the deep learning model can be used as a fast, accurate and robust method for high throughput prediction of logP of small molecules.

Synthesis, structural characterization, in vitro DNA binding, and antitumor activity properties of Ru(II) compounds containing 2(2,6-dimethoxypyridine-3-yl)-1H-imidazo(4,5-f)[1, 10]phenanthroline

The octahedral Ru(II) complexes containing the 2(2,6-dimethoxypyridine-3-yl)-1H-imidazo(4,5-f)[1, 10]phenanthroline ligand of type [Ru(N-N)₂(L)]²⁺, where N-N = phen (1,10-phenanthroline) (1), bpy (2,2^'-bipyridine) (2), and dmb (4,4^'-dimethyl-2,2^'-bipyridine) (3); L(dmpip) = (2(2,6-dimethoxypyridine-3-yl)1Himidazo(4,5-f)[1, 10]phenanthroline), have been synthesized and characterized by UV-visible absorption, molar conductivity, elemental analysis, mass, IR, and NMR spectroscopic techniques. The physicochemical properties of the Ru(II) complexes were determined by UV-Vis absorption spectroscopy. The DNA binding studies have been explored by UV-visible absorption, fluorescence titrations, and viscosity measurements. The supercoiled pBR322 DNA cleavage efficiency of Ru(II) complexes 1-3 was investigated. The antimicrobial activity of Ru(II) complexes was done against Gram-positive and Gram-negative microorganisms. The in vitro anticancer activities of all the complexes were investigated by cell viability assay, apoptosis, cellular uptake, mitochondrial membrane potential detection, and semi-quantitative PCR on HeLa cells. The result indicates that the synthesized Ru(II) complexes probably interact with DNA through an intercalation mode of binding with complex 1 having slightly stronger DNA binding affinity and anticancer activity than 2 and 3.

Physiologically based mathematical modelling of solute transport within the epidermis and dermis

The stratum corneum is the main barrier to transdermal drug delivery which has previously resulted in mathematical modelling of solute transport in the skin being primarily directed at this skin layer. However, for topical treatment and skin toxicity studies, the concentration in the epidermis and dermis is important and needs to be modelled mathematically. Hitherto, mathematical models for viable skin layers typically simplified the clearance of solute by blood, either assuming sink condition at the top of the skin capillary loops or assuming a distributed clearance in the dermis. This paper is an attempt to develop a physiologically based mathematical model of drug transport in the viable skin. It incorporates explicit modelling of the capillary loops within the dermis and employs COMSOL Multiphysics® software to model the transport in three dimensions. Previously derived simplified models were compared to the results from this new numerical model. The results of this comparison showed that the simplified model reasonably described the average concentration in the viable skin layers when parameters of the models were chosen appropriately. When the recruitment of the capillary loops in the dermis was full and the top of capillary loops was at a depth of 100μm, the effective depth to place a sink condition in the simpler models was found to be at 150μm. However, when there was only partial recruitment of the capillaries, the effective depth increased to 180μm. The presented modelling is also essential for determining a transdermal flux when the stratum corneum barrier is compromised by such methods as microporation, application of chemical enhancers or microneedles.

Enhancing skin penetration of epigallocatechin gallate by modifying partition coefficient using reverse micelle method

Aim: (-)-Epigallocatechin gallate (EGCG) has been reported as inducing apoptosis in cervical cancer. However, EGCG demonstrates low skin permeability. In order to develop topical delivery of EGCG, the partition coefficient, log P, was modified using a reverse micelle method. Results & methodology: During this study, Tween 80 and Span 80 were added to EGCG at hydrophilic-lipophilic balance (HLB) values of 4.3, 6 and 8. The results showed that lowering the HLB value increases the log P value of EGCG and results in higher IC₅₀ values in Henrietta Lacks (HeLa) cancer cells than that of EGCG. Surfactant-modified EGCG-HLB 6 produced faster and deeper skin penetration than EGCG. Conclusion: Modification of log P value using a combination of Tween 80 and Span 80 improved cytotoxicity and topical delivery of EGCG.

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.

Effects of skin region and relative lipophilicity on percutaneous absorption in the toad Rhinella marina

Owing to the dynamic interaction between frog skin and the environment, xenobiotics in frog habitats are of particular concern, and knowledge of percutaneous absorption in frog skin is necessary for risk-mitigation purposes. Baseline transdermal kinetics in adult aquatic and arboreal frog species have recently been reported; however, there is little information regarding absorption kinetics in adult terrestrial species. The present study investigated the in vitro absorption kinetics of 3 model chemicals-caffeine, benzoic acid, and ibuprofen-through different skin regions in the terrestrial toad Rhinella marina. Caffeine flux was consistently higher than that of the other 2 chemicals (p < 0.001), whereas the fluxes of the moderately and highly lipophilic chemicals (benzoic acid and ibuprofen) were similar, regardless of skin region. When considering individual chemicals, caffeine demonstrated increased flux through the ventral pelvic skin compared with the ventral thoracic or dorsal skin regions. Flux did not differ between skin regions for either benzoic acid or ibuprofen. These findings have implications for management of environmental contamination in frog habitats, as many environmental xenobiotics are of moderate to high lipophilicity and would be expected to be equally absorbed from all skin surfaces in terrestrial toads. Environ Toxicol Chem 2019;38:361-367. © 2018 SETAC.

Nanoemulsion as a Platform for Iontophoretic Delivery of Lipophilic Drugs in Skin Tumors

Lipophilic drugs do not usually benefit from iontophoresis mainly because they do not solubilize in aqueous formulations suitable for the application of electric current. To explore the influence of iontophoresis on penetration of these drugs, a cationic nanoemulsion was developed to solubilize zinc phthalocyanine (ZnPc), a promising drug for the treatment of skin cancer. To verify the influence of particle size on iontophoresis, an emulsion of nanoemulsion-like composition was also developed. The formulations were characterized and cutaneous and tumor penetration studies were performed in vitro and in vivo, respectively. With particles of about 200 nm, the nanoemulsion solubilized 2.5-fold more ZnPc than the 13-µm emulsion. At the same concentration of ZnPc, in vitro passive penetration studies showed that the nanoemulsion increased, after 1 h of treatment, by almost 4 times the penetration of ZnPc into the viable layers of the skin when compared to the emulsion, whereas iontophoresis of nanoemulsion resulted in a 16-fold increase in ZnPc penetration in only 30 min. An in vivo study in a murine model of melanoma showed that ZnPc reached the tumor after iontophoresis of the nanoemulsion. Therefore, iontophoresis of nanoemulsions appears to be a promising strategy for the topical treatment of tumors with lipophilic drugs.

In vitro penetration through the skin layers of topically applied glucocorticoids

Corticoids are actives widely used in the treatment of skin diseases. This work aims to study the penetration of 3 corticoids (betamethasone, clobetasol, and flurandrenolide), their relationship with their Log D values and the effects of the vehicles. The 3 compounds were applied on a Franz-type diffusion cell in propylene glycol solution and their respective commercial creams and ointments. The active amounts found in the stratum corneum, epidermal, and dermal layers of the skin were investigated. Their diffusions were greatly affected by the formulation; moreover higher amounts of substance in the epidermis and dermis were detected in ointments than in creams. The enhancement effect of propylene glycol was also observed. The differences between the 3 substances could be related to their lipophilicity, molecular structure, and molecular weight. The more hydrophobic compounds (clobetasol and betamethasone) are present in higher amounts in the epidermis and dermis, while the hydrophilic compound (flurandrenolide) is mostly present in the receptor fluid.

Evaluation on the reliability of the permeability coefficient (Kp) to assess the percutaneous penetration property of chemicals on the basis of Flynn's dataset

PURPOSE

The permeability coefficient (K_p) is often used for prediction of the dermal penetration of chemicals. Mathematical models have mostly been derived on K_p data basis. However, confusing K_p values are reported, questioning the general reliability of this parameter. In this study, we tested the plausibility of K_p values expressing the dermal penetration velocity (cm h^-1) of chemicals on a larger dataset from literature.

METHODS

K_p was applied for the calculation of the time for penetration through skin membranes of defined thickness (t_CrossSkin). K_p values were obtained from Flynn's dataset (1990), containing data determined mostly under similar experimental conditions using diffusion cells. Further skin penetration parameters, e.g., times at which the chemicals were firstly measured in the receptor phase, lag times, steady-state times, and exposure duration, where available, were related to K_p values. The data congruence was tested comparing K_p values from Flynn's dataset with those reported in the EDETOX database. Variables, which could bias the results, such as different experimental protocols and research groups were also considered.

RESULTS

K_p data for 94 chemicals matched the inclusion criteria were evaluated. According to the K_p values, 21 (22%) compounds would require longer than 100 h, and 20 (21%) further compounds longer than 10 h of exposure to penetrate skin membranes of ~ 0.01-2.5 mm thickness. Obviously, erroneous K_p were found in studies of almost all research groups in Flynn's database, indicating that neither the observer nor the experimental conditions alone biased the values.

CONCLUSIONS

Our evaluation demonstrates high implausibility of K_p values to represent the dermal penetration velocity and supports general invalidity of the parameter for implementation in studies using skin membranes. The K_p should not be used to characterize the percutaneous penetration of chemicals or in risk assessment without verification.

Dermal absorption of benzo[a]pyrene into human skin from soil: Effect of artificial weathering, concentration, and exposure duration

In vitro assessments of ¹⁴C-benzo[a]pyrene (BaP) absorption through human epidermis were conducted with the sub-63-μm fraction of four test soils containing different amounts of organic and black carbon. Soils were artificially weathered for eight weeks and applied to epidermis at nominal BaP concentrations of 3 and 10 mg/kg for 8 or 24 h. Experiments were also conducted at 24 h with unweathered soils and with BaP deposited onto skin from acetone at a comparable chemical load. For the weathered soils, absorption was independent of the amount of organic or black carbon, the mass in the receptor fluid was proportional to exposure duration but independent of concentration, and the mass recovered in the skin after washing was proportional to concentration and independent of exposure time. Results from the weathered and unweathered soils were similar except for the mass recovered in the washed skin, which was lower for the weathered soil only at the higher concentration. We hypothesize that chemical concentrations exceeded the BaP sorption capacity accessible within the artificial weathering timeframe for all soils tested, and that BaP mass in the washed skin was dominated by particles that were not removed by washing. Fluxes into and through skin from soils were lower by an order of magnitude than from acetone-deposited BaP.

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.

The potential of polymeric film-forming systems as sustained delivery platforms for topical drugs

INTRODUCTION

Dosing regimens requiring multiple daily applications frequently result in poor patient compliance, especially in the treatment of chronic skin diseases. Consequently, development of sustained delivery systems for topical drugs permitting less frequent dosing is of continuing interest for dermatological therapy.

AREAS COVERED

This potential of polymeric film-forming systems (FFS), created in situ on the skin, as sustained delivery platforms for topical drug delivery is reviewed. Key formulation parameters that determine delivery efficiency are considered focussing on those that permit a drug reservoir to be established in the upper layers of the skin and/or on the skin surface from which release can be sustained over a prolonged period. The advantageous and superior cosmetic attributes of FFS (compared to conventional semi-solid formulations) that offer significantly improved patient compliance are also addressed.

EXPERT OPINION

The promise of polymeric FFS as convenient and aesthetic platforms for sustained topical drug delivery is clear. Manipulation of the formulation allows the delivery profile to be customized and optimized to take advantage of both a rapid, initial input of drug into the skin (likely due to a transient period of supersaturation) and a slower, controlled release over an extended time from the residual film created thereafter.

Effect of lipophilicity on microneedle-mediated iontophoretic transdermal delivery across human skin in vitro

The effect of lipophilicity of drug on the microneedle (MN)-mediated iontophoretic delivery across dermatomed human skin was studied. Beta blockers with similar pKa but varied log P values were selected as model drugs in this study. Iontophoresis (ITP) or MNs, when used independently, increased the transdermal flux of beta blockers as compared with passive delivery (PD). ITP across the MN-treated skin (MN + ITP) increased the permeation rate of all beta blockers as compared with PD (p < 0.001). The enhancement ratios (ER) for hydrophilic molecules (atenolol and sotalol) were 71- and 78-fold higher for ITP + MN as compared with PD. However, for lipophilic molecule such as propranolol, there was 10-fold increase in the ER as compared with PD. These observations were further substantiated by the skin retention data; an inverse relationship between the skin retention and the hydrophilicity of the drug was observed. The results in the present study point out that the lipophilicity of the molecule plays a significant role on the electrically assisted transdermal delivery of drugs across the microporated skin. Using the combination of ITP + MN, hydrophilic drugs (atenolol and sotalol) were delivered at a much higher rate as compared with lipophilic molecules (propranolol and acebutolol).

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.

Improved input parameters for diffusion models of skin absorption

To use a diffusion model for predicting skin absorption requires accurate estimates of input parameters on model geometry, affinity and transport characteristics. This review summarizes methods to obtain input parameters for diffusion models of skin absorption focusing on partition and diffusion coefficients. These include experimental methods, extrapolation approaches, and correlations that relate partition and diffusion coefficients to tabulated physico-chemical solute properties. Exhaustive databases on lipid-water and corneocyte protein-water partition coefficients are presented and analyzed to provide improved approximations to estimate lipid-water and corneocyte protein-water partition coefficients. The most commonly used estimates of lipid and corneocyte diffusion coefficients are also reviewed. In order to improve modeling of skin absorption in the future diffusion models should include the vertical stratum corneum heterogeneity, slow equilibration processes, the absorption from complex non-aqueous formulations, and an improved representation of dermal absorption processes. This will require input parameters for which no suitable estimates are yet available.

A thermodynamic study of the cyclodextrin-UC781 inclusion complex using a HPLC method

UC781, a very potent HIV-1 non-nucleoside reverse transcriptase inhibitor with extreme hydrophobicity and poor water solubility, is under development as a topical vaginal microbicide product to prevent HIV transmission. In this study, the thermodynamic behavior of the interaction between UC781 with three cyclodextrins (CDs): β-cyclodextrin (βCD), hydroxypropyl-β-cyclodextrin (HPβCD) and methyl-β-cyclodextrin (MβCD), was investigated using a reversed-phase HPLC method. A mobile phase consisting of acetonitrile: H₂O (30:70) solution containing various CD concentrations was used. The retention time at different temperatures was determined to evaluate the inclusion process. The influence of βCDs on the solubility and hydrophobicity of UC781 was characterized by retention time values. The results showed that the inclusion capacity of cyclodextrins follows the order MβCD > βCD > HPβCD. An enthalpy-entropy compensation effect was also observed. In addition, the results revealed that the change of ΔH is greater than that of ΔS. These results suggested that the complexation of UC781 with βCDs is an enthalpy driven process. The modification on β-cyclodextrin will influence the inclusion process.

Formulation and in vitro release evaluation of newly synthesized palm kernel oil esters-based nanoemulsion delivery system for 30% ethanolic dried extract derived from local Phyllanthus urinaria for skin antiaging

BACKGROUND

Recently there has been a remarkable surge of interest about natural products and their applications in the cosmetic industry. Topical delivery of antioxidants from natural sources is one of the approaches used to reverse signs of skin aging. The aim of this research was to develop a nanoemulsion cream for topical delivery of 30% ethanolic extract derived from local Phyllanthus urinaria (P. urinaria) for skin antiaging.

METHODS

Palm kernel oil esters (PKOEs)-based nanoemulsions were loaded with P. urinaria extract using a spontaneous method and characterized with respect to particle size, zeta potential, and rheological properties. The release profile of the extract was evaluated using in vitro Franz diffusion cells from an artificial membrane and the antioxidant activity of the extract released was evaluated using the 2, 2-diphenyl-1-picrylhydrazyl (DPPH) method.

RESULTS

Formulation F12 consisted of wt/wt, 0.05% P. urinaria extract, 1% cetyl alcohol, 0.5% glyceryl monostearate, 12% PKOEs, and 27% Tween 80/Span 80 (9/1) with a hydrophilic lipophilic balance of 13.9, and a 59.5% phosphate buffer system at pH 7.4. Formulation F36 was comprised of 0.05% P. urinaria extract, 1% cetyl alcohol, 1% glyceryl monostearate, 14% PKOEs, 28% Tween 80/Span 80 (9/1) with a hydrophilic lipophilic balance of 13.9, and 56% phosphate buffer system at pH 7.4 with shear thinning and thixotropy. The droplet size of F12 and F36 was 30.74 nm and 35.71 nm, respectively, and their nanosizes were confirmed by transmission electron microscopy images. Thereafter, 51.30% and 51.02% of the loaded extract was released from F12 and F36 through an artificial cellulose membrane, scavenging 29.89% and 30.05% of DPPH radical activity, respectively.

CONCLUSION

The P. urinaria extract was successfully incorporated into a PKOEs-based nanoemulsion delivery system. In vitro release of the extract from the formulations showed DPPH radical scavenging activity. These formulations can neutralize reactive oxygen species and counteract oxidative injury induced by ultraviolet radiation and thereby ameliorate skin aging.

Three-dimensional skin models as tools for transdermal drug delivery: challenges and limitations

INTRODUCTION

Transdermal drug delivery has several known advantages over the oral route and hypodermic injections. The number of drugs that can be taken up transdermally is, however, limited owing to the innate barrier function of the skin. New transdermal drug candidates need to be tested extensively before being used on humans. In this regard, in vitro permeation methods are highly important to predict in vivo permeation of drugs.

AREAS COVERED

This review illustrates how different types of reconstructed skin models are being used as alternatives to human and pig skin for in vitro permeation testing of drugs. Insights into how various factors (including the physicochemical nature of molecules and formulations) or skin properties might affect the permeability of drugs in reconstructed skin models are provided. Also, opportunities and pitfalls of reconstructed skin models are highlighted.

EXPERT OPINION

Many studies have revealed that the permeability of reconstructed skin models is much higher compared with human excised skin. This is in accordance with the incomplete barrier found in these models. Nevertheless, the reconstructed skin models available today are useful tools for estimating the rank order of percutaneous absorption of a series of compounds with different physicochemical properties. A major challenge in the further development of reconstructed skin models for drug delivery studies is to obtain a barrier function similar to in vivo skin. Whether this goal will be achieved in the near future is uncertain and will be, in the authors' opinion, a very difficult task.

Predicting skin permeability from complex chemical mixtures: dependency of quantitative structure permeation relationships on biology of skin model used

Dermal absorption of topically applied chemicals usually occurs from complex chemical mixtures; yet, most attempts to quantitate dermal permeability use data collected from single chemical exposure in aqueous solutions. The focus of this research was to develop quantitative structure permeation relationships (QSPR) for predicting chemical absorption from mixtures through skin using two levels of in vitro porcine skin biological systems. A total of 16 diverse chemicals were applied in 384 treatment mixture combinations in flow-through diffusion cells and 20 chemicals in 119 treatment combinations in isolated perfused porcine skin. Penetrating chemical flux into perfusate from diffusion cells was analyzed to estimate a normalized dermal absorptive flux, operationally an apparent permeability coefficient, and total perfusate area under the curve from perfused skin studies. These data were then fit to a modified dermal QSPR model of Abraham and Martin including a sixth term to account for mixture interactions based on physical chemical properties of the mixture components. Goodness of fit was assessed using correlation coefficients (r²), internal and external validation metrics (q²L00, q²L25%, q²EXT), and applicable chemical domain determinations. The best QSPR equations selected for each experimental biological system had r² values of 0.69-0.73, improving fits over the base equation without the mixture effects. Different mixture factors were needed for each model system. Significantly, the model of Abraham and Martin could also be reduced to four terms in each system; however, different terms could be deleted for each of the two biological systems. These findings suggest that a QSPR model for estimating percutaneous absorption as a function of chemical mixture composition is possible and that the nature of the QSPR model selected is dependent upon the biological level of the in vitro test system used, both findings having significant implications when dermal absorption data are used for in vivo risk assessments.

Determination of partition and binding properties of solutes to stratum corneum

The binding property of a number of relatively hydrophilic solutes to native and delipidized stratum corneum (SC) and their partition coefficients to extracted lipid have been measured by equilibration experiments to expand the current database which consisted of mostly hydrophobic solutes. Using the extended database, quantitative structure property relationships (QSPR) have been proposed for predicting the partition and binding coefficients of both hydrophobic and hydrophilic solutes to the SC protein, and lipid. Solute partition to the SC lipid is best fitted by PC(lip/w)=K(ow)(0.69) and solute binding to the SC protein is best described by PC(pro/w)=4.2K(ow)(0.31). The two QSPR models of solute partition to the SC lipid and binding to the SC protein have been further combined into a two-phase model to predict the overall partition coefficient of solutes to the stratum corneum (K(sc/w)). Our study not only extends the database of solute partition and binding properties of the SC to include hydrophilic solutes, but also demonstrates that the thermodynamic equilibrium properties of the SC partition and binding can be fitted with good accuracy by combining QSPR models with the multiphase and heterogeneous structures of the SC.

Metabolism of fatty acids and lipid hydroperoxides in human body monitoring with Fourier transform Infrared Spectroscopy

BACKGROUND

The metabolism of dietary fatty acids in human has been measured so far using human blood cells and stable-isotope labeled fatty acids, however, no direct data was available for human peripheral tissues and other major organs. To realize the role of dietary fatty acids in human health and diseases, it would be eager to develop convenient and suitable method to monitor fatty acid metabolism in human.

RESULTS

We have developed the measurement system in situ for human lip surface lipids using the Fourier transform infrared spectroscopy (FTIR) - attenuated total reflection (ATR) detection system with special adaptor to monitor metabolic changes of lipids in human body. As human lip surface lipids may not be much affected by skin sebum constituents and may be affected directly by the lipid constituents of diet, we could detect changes of FTIR-ATR spectra, especially at 3005 to approximately 3015 cm(-1), of lip surface polyunsaturated fatty acids in a duration time-dependent manner after intake of the docosahexaenoic acid (DHA)-containing triglyceride diet. The ingested DHA appeared on the lip surface and was detected by FTIR-ATR directly and non-invasively. It was found that the metabolic rates of DHA for male volunteer subjects with age 60s were much lower than those with age 20s. Lipid hydroperoxides were found in lip lipids which were extracted from the lip surface using a mixture of ethanol/ethylpropionate/iso-octane solvents, and were the highest in the content just before noon. The changes of lipid hydroperoxides were detected also in situ with FTIR-ATR at 968 cm(-1).

CONCLUSION

The measurements of lip surface lipids with FTIR-ATR technique may advance the investigation of human lipid metabolism in situ non-invasively.