Influence of membrane–solvent–solute interactions on solute permeation in skin

Opportunities of topical drug products in a changing dermatological landscape

Despite the prevalence and the impact on quality of life of dermatological indications, drug products to treat such conditions have rarely been blockbusters. The prevailing perception of a limited commercial potential of dermatological drug products has restricted innovation and encouraged a more conservative development approach. For example, the focus was on repurposing/reformulation of existing active pharmaceutical ingredients (APIs) specifically for the topical delivery route. However, the situation is quite different today catalyzed in part by the blockbuster success of Dupixent (dupilumab), the first monoclonal antibody treatment for atopic dermatitis which has been approved by the US Food and Drug Administration (US FDA) in 2017. Dupixent's success not only encouraged the development of other biologics but also inspired the (re-)development of new dermal drug products that can reap the many benefits of topical administration. We have also witnessed a shift toward outsourcing development efforts (and associated risks) towards small- to mid-size pharmaceutical companies which often require support of contract research and development/manufacturing organizations (CRO and CDMO). Such trends also emphasize the need of greater expertise in topical formulation design, as well as associated commercial and regulatory considerations. Today, we believe that topical drug products remain not only an essential but also commercially viable class of dermatological therapeutics. In this opinion article, we will address the challenges as well as opportunities of coherent development strategies in the current market environment, formulation innovations of topical drug products and technological advances to facilitate rational topical drug formulation development.

Enhanced degradation of aqueous caffeine via cylindrical dielectric barrier discharge plasma: Efficacy and toxicity insights

An environmentally friendly approach for caffeine degradation was explored in this study utilizing cylindrical dielectric barrier discharge (CDBD) plasma. The current-voltage characteristics and the plasma parameters of the CDBD, such as the electron temperature, electron density, density of nitrogen excited states, vibrational temperature, and rotational temperature, were assessed through electrical and optical characterization respectively. Fourier-transform infrared spectroscopy (FTIR) was employed to evaluate the reactive oxygen and nitrogen species (RONS) in the plasma-treated air. The physicochemical properties of deionized water (DW) were measured. To gain a deeper insight into the role of RONS in caffeine degradation, their concentrations in DW were analyzed. Furthermore, the effects of initial concentration, sample volume, and pH on caffeine degradation were investigated. The highest degradation of caffeine was 94% at initial concentration of 50 mg L-1, sample volume 50 mL and in neutral pH. Liquid chromatography-mass spectrometry (LC-MS) was then used to propose the degradation pathway for caffeine. The major reactive species involved in caffeine degradation was ozone. Finally, the phytotoxicity and cytotoxicity of caffeine were assessed before and after plasma treatment with plasma-treated caffeine (PTC) showing minimal toxicity to both plants and cells.

Quantitative structure permeability relationships for phenolic compounds applied to human epidermal membranes in various solvents

PURPOSE

This study examined how solvent-skin-solute interactions influenced the human epidermal permeation of three similar-sized phenolic compounds applied in a series of different solvents.

METHODS

Human epidermal permeation fluxes and lag times of three phenolic compounds were assessed in Franz cells for a range of solvents varying in molecular size and solubility parameters. In order to develop a mechanistic understanding of the determinants of the permeation findings, the solubility of the compounds in solvents and stratum corneum, the extent of solvent uptake by the stratum corneum and the impact of the solvents on skin hydration and transepidermal water loss were also measured.

RESULTS

Maximum epidermal fluxes and lag times varied greatly with the various solvent used. Markedly enhanced epidermal permeability fluxes, prolonged lag times and reduced diffusivities of the compounds were evident for many of the solvents. A solvent induced increase in stratum corneum solubility was associated with the uptake of solvent containing dissolved compound. This uptake was dependent on both the solvent molecular size and the solubility of the compounds in the solvents. The imbibed solvent acted as a reservoir in the skin, facilitating uptake and an increased thermodynamic activity that enhanced flux but, at the same time, inhibiting diffusion and prolonging lag time.

CONCLUSION

The solubility, permeation and lag times of compounds in the stratum corneum can be modulated by solvent uptake. Whilst a solvent -induced stratum corneum reservoir effect for a compound may prolong its lag time for a compound before steady state permeation is reached, it does not affect its overall steady state transport defined by diffusion of its free form.

Rational Design of Topical Semi-Solid Dosage Forms-How Far Are We?

Specific aspects of semi-solid dosage forms for topical application include the nature of the barrier to be overcome, aspects of susceptibility to physical and chemical instability, and a greater influence of sensory perception. Advances in understanding the driving forces of skin penetration as well as the design principles and inner structure of formulations, provide a good basis for the more rational design of such dosage forms, which still often follow more traditional design approaches. This review analyses the opportunities and constraints of rational formulation design approaches in the industrial development of new topical drugs. As the selection of drug candidates with favorable physicochemical properties increases the speed and probability of success, models for drug selection based on theoretical and experimental approaches are discussed. This paper reviews how progress in the scientific understanding of mechanisms and vehicle-influence of skin penetration can be used for rational formulation design. The characterization of semi-solid formulations is discussed with a special focus on modern rheological approaches and analytical methods for investigating and optimizing the chemical stability of active ingredients in consideration of applicable guidelines. In conclusion, the combination of a good understanding of scientific principles combined with early consideration of regulatory requirements for product quality are enablers for the successful development of innovative and robust semi-solid formulations for topical application.

Occupational exposure assessment with solid substances: choosing a vehicle for in vitro percutaneous absorption experiments

Percutaneous occupational exposure to industrial toxicants can be assessed in vitro on excised human or animal skins. Numerous factors can significantly influence skin permeation of chemicals and the flux determination. Among them, the vehicle used to solubilize the solid substances is a tricky key step. A "realistic surrogate" that closely matches the exposure scenario is recommended in first intention. When direct transposition of occupational exposure conditions to in vitro experiments is impossible, it is recommended that the vehicle used does not affect the skin barrier (in particular in terms of structural integrity, composition, or enzymatic activity). Indeed, any such effect could alter the percutaneous absorption of substances in a number of ways, as we will see. Potential effects are described for five monophasic vehicles, including the three most frequently used: water, ethanol, acetone; and two that are more rarely used, but are realistic: artificial sebum and artificial sweat. Finally, we discuss a number of criteria to be verified and the associated tests that should be performed when choosing the most appropriate vehicle, keeping in mind that, in the context of occupational exposure, the scientific quality of the percutaneous absorption data provided, and how they are interpreted, may have long-range consequences. From the narrative review presented, we also identify and discuss important factors to consider in future updates of the OECD guidelines for in vitro skin absorption experiments.

In Vitro-In Vivo Correlation in Dermal Delivery: The Role of Excipients

The composition of topical and transdermal formulations is known to determine the rate and the extent of drug delivery to and through the skin. However, to date, the role of excipients in these formulations on skin delivery of actives has received little attention from scientists in the field. Monitoring skin absorption of both drug and vehicle may provide insights into the mechanism by which excipients promote permeation and may facilitate the design of effective and safer products. Previously, we have investigated the use of quantitative Confocal Raman Spectroscopy (CRS) to investigate the delivery of an active to the skin, and we also reported the first fully quantitative study that compared this method with the well-established in vitro permeation test (IVPT) model. To further explore the potential of quantitative CRS in assessing topical delivery, the present work investigated the effects of commonly used excipients on the percutaneous absorption of a model drug, ibuprofen (IBU). Permeation of IBU and selected solvents following finite dose applications to human skin was determined in vitro and in vivo by Franz diffusion studies and quantitative CRS, respectively. The solvents used were propylene glycol (PG), dipropylene glycol (DPG), tripropylene glycol (TPG), and polyethylene glycol 300 (PEG 300). Overall, the cumulative amounts of IBU that permeated at 24 h in vitro were similar for PG, DPG, and TPG (p > 0.05). These three vehicles outperformed PEG 300 (p < 0.05) in terms of drug delivery. Concerning the vehicles, the rank order for in vitro skin permeation was DPG ≥ PG > TPG, while PEG 300 did not permeate the skin. A linear relationship between maximum vehicle and IBU flux in vitro was found, with a correlation coefficient (R²) of 0.95. When comparing in vitro with in vivo data, a positive in vitro–in vivo (IVIV) correlation between the cumulative permeation of IBU in vitro and the total amount of IBU that penetrated the stratum corneum (SC) in vivo was observed, with a Pearson correlation coefficient (R²) of 0.90. A strong IVIV correlation, R² = 0.82, was found following the linear regression of the cumulative number of solvents permeated in vitro and the corresponding skin uptake in vivo measured with CRS. This is the first study to correlate in vivo permeation of solvents measured by CRS with data obtained by in vitro diffusion studies. The IVIV correlations suggest that CRS is a powerful tool for profiling drug and vehicle delivery from dermal formulations. Future studies will examine additional excipients with varying physicochemical properties. Ultimately, these findings are expected to lead to new approaches for the design, evaluation, and optimization of formulations that target actives to and through the skin.

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.

Multivariate Analytical Approaches to Identify Key Molecular Properties of Vehicles, Permeants and Membranes That Affect Permeation through Membranes

There has been considerable recent interest in employing computer models to investigate the relationship between the structure of a molecule and its dermal penetration. Molecular permeation across the epidermis has previously been demonstrated to be determined by a number of physicochemical properties, for example, the lipophilicity, molecular weight and hydrogen bonding ability of the permeant. However little attention has been paid to modeling the combined effects of permeant properties in tandem with the properties of vehicles used to deliver those permeants or to whether data obtained using synthetic membranes can be correlated with those obtained using human epidermis. This work uses Principal Components Analysis (PCA) to demonstrate that, for studies of the diffusion of three model permeants (caffeine, methyl paraben and butyl paraben) through synthetic membranes, it is the properties of the oily vehicle in which they are applied that dominated the rates of permeation and flux. Simple robust and predictive descriptor-based quantitative structure-permeability relationship (QSPR) models have been developed to support these findings by utilizing physicochemical descriptors of the oily vehicles to quantify the differences in flux and permeation of the model compounds. Interestingly, PCA showed that, for the flux of co-applied model permeants through human epidermis, the permeation of the model permeants was better described by a balance between the physicochemical properties of the vehicle and the permeant rather than being dominated solely by the vehicle properties as in the case of synthetic model membranes. The important influence of permeant solubility in the vehicle along with the solvent uptake on overall permeant diffusion into the membrane was substantiated. These results confirm that care must be taken in interpreting permeation data when synthetic membranes are employed as surrogates for human epidermis; they also demonstrate the importance of considering not only the permeant properties but also those of both vehicle and membrane when arriving at any conclusions relating to permeation data.

Franz Cell Diffusion Testing and Quantitative Confocal Raman Spectroscopy: In Vitro-In Vivo Correlation

Previously, we reported the use of Confocal Raman Spectroscopy (CRS) to investigate the topical delivery of actives and excipients. We have also correlated the results from CRS with findings from in vitro diffusion studies in human skin. However, until now CRS has only been used as a semi-quantitative method of determining the skin uptake of molecules, with results expressed as arbitrary units of signal intensity. Clearly, this posed challenges for using CRS to determine skin delivery and to assess the drug bioavailability and bioequivalence of topical formulations. In the present work, the permeation of niacinamide (NIA) from various formulations in human skin was studied in vitro using conventional Franz cells and in vivo using a quantitative CRS method under finite dose conditions. The selection of NIA was based on its wide use in pharmaceutical and personal care formulations for many years. This is the first fully quantitative study to compare these methods. The vehicles investigated were neat Transcutol^® P (TC); binary combinations of propylene glycol (PG) with propylene glycol monolaurate (PGML); and ternary mixtures of PG, PGML, and isopropyl myristate (IPM). These solvents were selected to encompass a range of physicochemical properties. NIA permeation was evident from all formulations in vitro and in vivo. The vehicles PG:PGML and PG:PGML:IPM delivered comparable amounts across the skin in vitro at 24 h (100.3–106.7 µg/cm², p > 0.05) that were significantly higher compared with those of TC (1.3 µg/cm², p < 0.05). An excellent in vitro in vivo correlation (R² = 0.98) was found following the linear regression of the cumulative amounts of NIA permeated in vitro and the amounts of NIA at 2 μm in the skin measured with CRS. A very good correlation between the cumulative permeation of NIA in vitro and the total amount of NIA that penetrated the stratum corneum (SC) per unit of surface area (μg/cm²) in vivo was also observed, with a Pearson correlation coefficient (R²) of 0.94. The findings support the use of CRS for the quantitative measurement of actives delivered to the skin in vivo. Future studies will focus on exploring the reproducibility and reliability of the method by investigating the delivery of different actives from a wider range of vehicles. Additionally, quantitative CRS will be evaluated further as a method for assessing the bioequivalence of topical formulations.

Prediction of skin permeation and concentration of rhododendrol applied as finite dose from complex cosmetic vehicles

Finite dose experiments represent clinical use wherein depletion of dose, evaporation of excipients, and gradual change in vehicle composition may occur. In the present study, we attempted a mathematical approach for predicting skin permeation and concentration of a cosmetic active, rhododendrol (RD), from complex vehicle-based formulations applied in finite dose. In vitro skin permeation and concentration studies of RD were conducted from formulations containing water and polyols with concentrations ranging from 10 to 100% under infinite and finite dose conditions using vertical Franz diffusion cells. Observed data for skin permeation and the viable epidermis and dermis (VED) concentration of RD were estimated by the differential equations under Fick's second law of diffusion together with water evaporation kinetics and changes in the partition coefficient from vehicles to the stratum corneum. As a result, a goodness-of-fit was observed allowing accurate estimation of skin permeation and VED concentration of RD. This mathematical approach could become a useful tool to estimate the skin permeation and concentration of actives from topical formulation applied in finite dose conditions likened in actual use.

Preparation, Characterisation, and Topical Delivery of Terbinafine

Terbinafine (TBF) is commonly used in the management of fungal infections of the skin because of its broad spectrum of activity. Currently, formulations containing the free base and salt form are available. However, there is only limited information in the literature about the physicochemical properties of this drug and its uptake by the skin. In this work, we conducted a comprehensive characterisation of TBF, and we also examined its percutaneous absorption in vitro in porcine skin. TBF-free base was synthesised from the hydrochloride salt by a simple proton displacement reaction. Both the free base and salt form were further analysed using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Delivery of TBF-free base in excised porcine skin was investigated from the following solvents: Isopropyl myristate (IPM), propylene glycol monolaurate (PGML), Transcutol^® (TC), propylene glycol (PG), polyethylene glycol 200 (PEG 200), oleic acid (OL), ethanol (EtOH), and isopropyl alcohol (IPA). Permeation and mass balance studies confirmed that PG and TC were the most efficacious vehicles, delivering higher amounts of TBF-free base to the skin compared with a commercial gel (p < 0.05). These preliminary results are promising and will inform the development of more complex formulations in future work.

3-O-ethyl-l-ascorbic acid: Characterisation and investigation of single solvent systems for delivery to the skin

l-ascorbic acid (AA), commonly known as vitamin C, has been widely used in topical formulations for many years as an antioxidant and anti-aging ingredient. However, the physicochemical properties of AA are not optimal for skin uptake and the molecule is also unstable, readily undergoing oxidation on exposure to air. The compound 3-o-ethyl-l-ascorbic acid (EA) has been developed as a stable vitamin C derivative and has been used in topical products. The aims of this work were to conduct a comprehensive characterisation of physicochemical properties of EA as well as to investigate the influence of various neat solvents on EA skin delivery. Nuclear magnetic resonance (NMR), mass spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterise the molecule. The pK_a of the compound and the partition coefficient logP_(o/w) were experimentally determined. A new HPLC method for analysis of the molecule was also developed and validated. A number of solvents for topical preparations were selected based on their wide use as excipients in topical formulations, their potential to act as skin penetration enhancers and their favourable safety profiles. The solubility and stability of EA was examined. Skin permeation of the molecule in full thickness porcine skin in vitro was investigated using Franz-type diffusion cells. The melting point, log P_(o/w) value and pK_a value of EA were determined to be 114.39 ± 0.5 °C, −1.07 ± 0.03 and 7.72 ± 0.01 respectively. Skin penetration of EA was evident for the following vehicles 1,2 hexanediol (HEX), glycerol (GLY), propylene glycol (PG), 1,2 pentanediol (1-2P), isopropyl alcohol (IPA), propylene glycol monolaurate (PGML) and propylene glycol monocaprylate (PGMC). Skin uptake but no permeation through the skin was observed for Transcutol® (TC) and dipropylene glycol (DiPG), while no penetration was observed for the solvents 1,5 pentanediol (1-5P) and tripropylene glycol (TriPG). The findings of the permeation experiments confirm the potential of simple formulations to deliver EA to the skin. Studies are ongoing to identify complex vehicles for synergistic enhancement of EA skin penetration. To our knowledge this is the first study to conduct a comprehensive characterization of EA and examine its skin uptake and permeation properties in porcine skin.

Comparison of normal versus imiquimod-induced psoriatic skin in mice for penetration of drugs and nanoparticles

Background

As an immune-mediated skin disease, psoriasis encounters therapeutic challenges on topical drug development due to the unclear mechanism, and complicated morphological and physiological changes in the skin.

Methods

In this study, imiquimod-induced psoriatic mouse skin (IMQ-psoriatic skin) was chosen as the in vitro pathological model to explore the penetration behaviors of drugs and nanoparticles (NPs).

Results

Compared with normal skin, significantly higher penetration and skin accumulation were observed in IMQ-psoriatic skin for all the three model drugs. When poorly water-soluble curcumin was formulated as NPs that were subsequently loaded in gel, the drug’s penetration and accumulation in both normal and IMQ-psoriatic skins were significantly improved, in comparison with that of the curcumin suspension. Interestingly, the NPs’ size effect, in terms of their penetration and accumulation behaviors, was more pronounced for IMQ-psoriatic skin. Furthermore, by taking three sized FluoSpheres^® as model NPs, confocal laser scanning microscopy demonstrated that the penetration pathways of NPs no longer followed the hair follicles channels, instead they were more widely distributed in the IMQ-psoriatic skin.

Conclusion

In conclusion, the alternation of the IMQ-psoriatic skin structure will lead to the enhanced penetration of drug and NPs, and should be considered in topical drug formulation and further clinical practice for psoriasis therapy.

Evaluation of Transdermal Drug Permeation as Modulated by Lipoderm and Pluronic Lecithin Organogel

The transdermal delivery of 2 fluorescent probes with similar molecular weight but different lipophilicity, into and through the skin from 2 commercially available transdermal bases, pluronic lecithin organogel, and Lipoderm^® has been evaluated. First, in vitro penetration of fluorescein sodium and fluorescein (free acid) through porcine skin was evaluated. Retention and depth distribution profiles in skin were obtained by tape stripping and then followed by optical sectioning using multiphoton microscopy. The results showed that Lipoderm^® led to an enhanced penetration of the hydrophilic compound, fluorescein sodium. For the lipophilic compound fluorescein (free acid), Lipoderm^® performed similar to pluronic lecithin organogel base, where minimal drug was detected in either receptor phase. The skin retention and depth distribution results also showed that the hydrophilic fluorescein sodium had high skin retention with Lipoderm^®, whereas fluorescein (free acid) had very low penetration and retention with increasing skin depth. Moreover, optical sectioning by multiphoton microscopy revealed an uneven distribution of probes across the skin in the x-y plane for both transdermal bases. This work showed that a hydrophilic compound has significantly increased skin penetration and retention when formulated with Lipoderm^®, and the skin retention of the probe was the main determinant of its skin flux.

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

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

An explanation for the difference in the percutaneous penetration behavior of tamsulosin induced by two different O-acylmenthol derivatives

Using tamsulosin (TAL) as a model drug, the aim of this study was to investigate and compare the percutaneous permeation behavior of two menthol derivatives, 2-isopropyl-5-methylcyclohexyl heptanoate (M-HEP) and 2-isopropyl-5-methylcyclohexyl decanoate (M-DEC). In vitro transdermal permeation study was carried out using porcine skin. The residual amount of enhancers in the skin after permeation experiment was determined by gas chromatographic (GC) method. The penetration depths of fluorescein were visualized by two-photon confocal laser scanning microscopy (2P-LSM) after the skin being treated with different enhancers. Furthermore, changes in the stretching frequency of functional group of ceramide were investigated by using attenuated total reflectance Fourier transform infrared (ATR-FTIR) technique. After M-HEP addition, the cumulative amount of TAL permeated in 8 h (Q8) reached 20.57±0.54 μg/cm2 and the depth of fluorescein was 40 μm; the CH2 of ceramide symmetric stretching frequency was 4 cm−1 blue shifted. However, M-DEC has an opposite effect on TAL permeation compared with that of M-HEP. TAL is a crucial factor affecting permeation procedure, and microenvironment of lipid region determines promotion capability of the enhancers.

Flux through silicone and human skin fitted to a series/parallel model

Background: Recent reports of the good correlation between maximum flux through human skin in vitro from water, JMHAQ, and maximum flux through silicone from water, JMPAQ, demand that the mechanism of maximum flux across these two apparently quite different membranes be compared to understand the bases of the correlation. Results/discussion: A n = 70 log JMPAQ database and a matched n = 55 log JMHAQ database of molecules were found to fit well to a series/parallel model where three parallel solubility dependent pathways existed: a lipid pathway, an aqueous pathway, and a series pathway of alternating lipid and aqueous phases. Conclusion: The results of this analysis surprisingly suggest that the architecture of the two membranes present similar solubility based pathways through which drugs diffuse.

Concentration dependency in nicotine skin penetration flux from aqueous solutions reflects vehicle induced changes in nicotine stratum corneum retention

PURPOSE

This study sought to understand the mechanism by which the steady state flux of nicotine across the human skin from aqueous solutions is markedly decreased at higher nicotine concentrations.

METHODS

Nicotine's steady state flux through human epidermis and its amount in the stratum corneum for a range of aqueous nicotine solutions was determined using Franz diffusion cells, with the nicotine analysed by high performance liquid chromatography (HPLC). Nicotine's thermodynamic activity in the various solutions was estimated from its partial vapour pressure and stratum corneum hydration was determined using a corneometer. The amount of nicotine retained in the stratum corneum was estimated from the nicotine amount found in individual stratum corneum tape strips and a D-Squame determined weight for each strip.

RESULTS

The observed steady state flux of nicotine across human epidermis was found to show a parabolic dependence on nicotine concentration, with the flux proportional to its thermodynamic activity up to a concentration of 48% w/w. The nicotine retention in the stratum corneum showed a similar dependency on concentration whereas the diffusivity of nicotine in the stratum corneum appeared to be concentration independent. This retention, in turn, could be estimated from the extent of stratum corneum hydration and the nicotine concentration in the applied solution and volume of water in the skin.

CONCLUSIONS

Nonlinear dependency of nicotine skin flux on its concentration results from a dehydration induced decrease in its stratum corneum retention at higher concentration and not dehydration induced changes nicotine diffusivity in the stratum corneum.

In vitro-in vivo correlation in skin permeation

PURPOSE

In vitro skin permeation studies have been used extensively in the development and optimisation of delivery of actives in vivo. However, there are few reported correlations of such in vitro studies with in vivo data. The aim of this study was to investigate the skin permeation of a model active, niacinamide, both in vitro and in vivo.

METHODS

Conventional diffusion cell studies were conducted in human skin to determine niacinamide permeation from a range of vehicles which included dimethyl isosorbide (DMI), propylene glycol (PG), propylene glycol monolaurate (PGML), N-methyl 2-pyrrolidone (NMP), Miglyol 812N® (MG), and mineral oil (MO). Single, binary or ternary systems were examined. The same vehicles were subsequently examined to investigate niacinamide delivery in vivo. For this proof-of-concept study one donor was used for the in vitro studies and one volunteer for the in vivo investigations to minimise biovariability. Analysis of in vitro samples was conducted using HPLC and in vivo uptake of niacinamide was evaluated using Confocal Raman spectroscopy (CRS).

RESULTS

The amount of niacinamide permeated through skin in vitro was linearly proportional to the intensity of the niacinamide signal determined in the stratum corneum in vivo. A good correlation was observed between the signal intensities of selected vehicles and niacinamide signal intensity.

CONCLUSIONS

The findings provide further support for the use of CRS to monitor drug delivery into and across the skin. In addition, the results highlight the critical role of the vehicle and its disposition in skin for effective dermal delivery.

A surrogate for topical delivery in human skin: silicone membranes

We have identified, for any surrogate membrane and human skin in vitro, the maximum flux through the membrane (output) should be measured if a correlation between the two is to be obtained. We also identified from an analysis of the passive permeation process that molecular weight, lipid and aqueous solubilities (which are easily measured) constitute the physicochemical properties of the active (input), upon which prediction of flux through the surrogate membrane and through skin in vitro should be based. Besides providing the bases for predicting flux, changes in these physicochemical properties can be easily implemented by those wishing to optimize new cosmetics or topical products. Maximum flux values through silicone membrane (n = 70) and through human skin in vitro (n = 52) have been collected and a good correlation between the flux through human skin in vitro and flux through silicone membrane (for the same molecules) was found.

Breaching the skin barrier--insights from molecular simulation of model membranes

Breaching the skin's barrier function by design is an important strategy for delivering drugs and vaccines to the body. However, while there are many proposed approaches for reversibly breaching the skin barrier, our understanding of the molecular processes involved is still rudimentary. Molecular simulation offers an unprecedented molecular-level resolution with an ability to reproduce molecular and bulk level properties. We review the basis of the molecular simulation methodology and give applications of relevance to the skin lipid barrier, focusing on permeation of molecules and chemical approaches for breaching the lipid barrier by design. The bulk kinetic model based on Fick's Law describing absorption of a drug through skin has been reconciled with statistical mechanical quantities such as the local excess chemical potential and local diffusion coefficient within the membrane structure. Applications of molecular simulation reviewed include investigations of the structure and dynamics of simple models of skin lipids, calculation of the permeability of molecules in simple model membranes, and mechanisms of action of the penetration enhancers, DMSO, ethanol and oleic acid. The studies reviewed illustrate the power and potential of molecular simulation to yield important physical insights, inform and rationalize experimental studies, and to predict structural changes, and kinetic and thermodynamic quantities.

How membrane permeation is affected by donor delivery solvent

We investigate theoretically and experimentally how the rate and extent of membrane permeation is affected by switching the donor delivery solvent from water to squalane for different permeants and membranes. In a model based on rate-limiting membrane diffusion, we derive explicit equations showing how the permeation extent and rate depend mainly on the membrane-donor and membrane-receiver partition coefficients of the permeant. Permeation results for systems containing all combinations of hydrophilic or hydrophobic donor solvents (aqueous solution or squalane), permeants (caffeine or testosterone) and polymer membranes (cellulose or polydimethylsiloxane) have been measured using a cell with stirred donor and re-circulating receiver compartments and continuous monitoring of the permeant concentration in the receiver phase. Relevant partition coefficients are also determined. Quantitative comparison of model and experimental results for the widely-differing permeation systems successfully enables the systematic elucidation of all possible donor solvent effects in membrane permeation. For the experimental conditions used here, most of the permeation systems are in agreement with the model, demonstrating that the model assumptions are valid. In these cases, the dominant donor solvent effects arise from changes in the relative affinities of the permeant for the donor and receiver solvents and the membrane and are quantitatively predicted using the separately measured partition coefficients. We also show how additional donor solvent effects can arise when switching the donor solvent causes one or more of the model assumptions to be invalid. These effects include a change in rate-limiting step, permeant solution non-ideality and others.

The effect of solute-membrane interaction on solute permeation under supersaturated conditions

The purpose of this work was to investigate the effect of solute-membrane interaction under supersaturated conditions on the transport of model solute (salicylic acid) across poly(dimethylsiloxane) (PDMS) membrane. Supersaturated systems with a degree of saturation (DS) up to 8 were prepared using a molecular form technique with water as the vehicle to minimize the vehicle-membrane interaction. The spectroscopic and thermal analysis revealed the presence of both hydrogen bonding and nonpolar interaction between the solute and PDMS. Upon treatment by supersaturated solutions the degree of solute-membrane interaction increased with increasing DS. This enhanced the barrier property of PDMS and thus led to the flux attenuation compared to that calculated by Higuchi equation. This work highlighted the importance of solute-membrane interaction under supersaturation in the flux reduction, which should be considered when designing, and optimizing supersaturated topical and transdermal drug delivery systems.

Estimation of the solubility parameters of model plant surfaces and agrochemicals: a valuable tool for understanding plant surface interactions

Background

Most aerial plant parts are covered with a hydrophobic lipid-rich cuticle, which is the interface between the plant organs and the surrounding environment. Plant surfaces may have a high degree of hydrophobicity because of the combined effects of surface chemistry and roughness. The physical and chemical complexity of the plant cuticle limits the development of models that explain its internal structure and interactions with surface-applied agrochemicals. In this article we introduce a thermodynamic method for estimating the solubilities of model plant surface constituents and relating them to the effects of agrochemicals.

Results

Following the van Krevelen and Hoftyzer method, we calculated the solubility parameters of three model plant species and eight compounds that differ in hydrophobicity and polarity. In addition, intact tissues were examined by scanning electron microscopy and the surface free energy, polarity, solubility parameter and work of adhesion of each were calculated from contact angle measurements of three liquids with different polarities. By comparing the affinities between plant surface constituents and agrochemicals derived from (a) theoretical calculations and (b) contact angle measurements we were able to distinguish the physical effect of surface roughness from the effect of the chemical nature of the epicuticular waxes. A solubility parameter model for plant surfaces is proposed on the basis of an increasing gradient from the cuticular surface towards the underlying cell wall.

Conclusions

The procedure enabled us to predict the interactions among agrochemicals, plant surfaces, and cuticular and cell wall components, and promises to be a useful tool for improving our understanding of biological surface interactions.

Toxicological implications of the delivery of fentanyl from gel extracted from a commercial transdermal reservoir patch

Fentanyl in a rate controlling membrane (RCM) transdermal patch form has been available since the early 1990s for outpatient management of chronic pain. Fatalities associated with misuse or overuse of fentanyl patches have been reported. Concerns have also been raised about the possibility that defects in such patches may result in leaking of the reservoir of the patch onto patients' skin and consequent overdose. In order to investigate the possibility of fentanyl toxicity arising from leaking of patches, the permeation of fentanyl from the reservoir gel of a commercially available fentanyl transdermal patch was examined in vitro. Finite doses of the formulation were applied to human skin and permeation was monitored, at 32°C under non-occluded conditions, for 48 h. Similar levels of skin permeation of fentanyl from the 1% gel formulation were obtained for the two skin donor samples tested. After 48 h, the dose of fentanyl that had permeated was 7.4 (± 3.6)% and 7.7 (± 1.9)% of the respective total amounts applied. At the end of the experiment, most of the drug was found in the residual formulation at the skin surface (i.e. 63-66%). For both the skin samples, a relatively small amount of the fentanyl applied (2-3%) was present in the skin at 48 h after application. The maximum flux from the data generated was between 6 and 24h over which time frame it was 0.3 μg/cm(2)/h. Assuming spreading of leaked gel over an area of 100 cm(2), this would result in a plasma level of 0.6 ng/mL. The anticipated plasma levels from a 100 μg/h patch are known to be approximately 2.5 ng/ml. Thus, the maximum increase in the plasma levels from a patch which leaks gel is calculated to be, at most, about 25%.

Membrane permeation of testosterone from either solutions, particle dispersions, or particle-stabilized emulsions

We derive a unified model that accounts for the variation in extent and rate of membrane permeation by a permeating species with the type of donor compartment formulation (aqueous and oil solutions, particle dispersions, and oil-in-water and water-in-oil emulsions stabilized by particles) initially containing the permeant. The model is also applicable to either closed-loop or open-flow configurations of the receiver compartment of the permeation cell. Predictions of the model are compared with measured extents and rates of permeation of testosterone across an 80 μm thick polydimethylsiloxane (PDMS) membrane from donor compartments initially containing testosterone dissolved in either aqueous or isopropylmyristate (IPM) solutions, aqueous or IPM dispersions of silica nanoparticles or IPM-in-water or water-in-IPM emulsions stabilized by silica nanoparticles. Using a single set of input parameters, the model successfully accounts for the wide variations in permeation behavior observed for the different donor formulation types with either closed-loop or open flow configurations of the permeation cell receiver compartment.

Lack of a primary physicochemical determinant in the direct transport of drugs to the brain after nasal administration in rats: potential involvement of transporters in the pathway

The objectives of this study were to evaluate the relative contribution of the direct pathway in overall brain transport for 17 model drugs with different physicochemical properties after nasal administrations and to identify factors that govern the fraction of the dose transported to the brain via the direct pathway (F(a, direct)). When the model drugs were nasally administered to rats, 5 of the 17 model drugs were delivered to a significant extent to the brain via the direct pathway. Multiple linear regression analyses showed that the correlation between various physicochemical properties and F(a, direct) was not statistically significant, indicative of a lack of primary physicochemical determinants in the direct transport pathway. Transporters such as rOAT3 and rOCT2 were expressed at significant levels in rat olfactory epithelia, and uptakes of standard substrates were significantly decreased in HEK293 cells expressing rOAT3 and rOCT2 in the presence of the five model drugs that were delivered to appreciable extents to the brain via the direct pathway. Therefore, these observations indicate that carrier-mediated transport may play a role in the brain delivery of drugs from the nose via the direct transport pathway.

In vitro enhancement of lactate esters on the percutaneous penetration of drugs with different lipophilicity

Lactate esters are widely used as food additives, perfume materials, medicine additives, and personal care products. The objective of this work was to investigate the effect of a series of lactate esters as penetration enhancers on the in vitro skin permeation of four drugs with different physicochemical properties, including ibuprofen, salicylic acid, dexamethasone and 5-fluorouracil. The saturated donor solutions of the evaluated drugs in propylene glycol were used in order to keep a constant driving force with maximum thermodynamic activity. The permeability coefficient (K(p)), skin concentration of drugs (SC), and lag time (T), as well as the enhancement ratios for K(p) and SC were recorded. All results indicated that lactate esters can exert a significant influence on the transdermal delivery of the model drugs and there is a structure-activity relationship between the tested lactate esters and their enhancement effects. The results also suggested that the lactate esters with the chain length of fatty alcohol moieties of 10-12 are more effective enhancers. Furthermore, the enhancement effect of lactate esters increases with a decrease of the drug lipophilicity, which suggests that they may be more efficient at enhancing the penetration of hydrophilic drugs than lipophilic drugs. The influence of the concentration of lactate esters was evaluated and the optimal concentration is in the range of 5-10 wt.%. In sum, lactate esters as a penetration enhancer for some drugs are of interest for transdermal administration when the safety of penetration enhancers is a prime consideration.

Alcohol enhanced permeation in model membranes. Part I. Thermodynamic and kinetic analyses of membrane permeation

While it is well recognised that formulation components influence drug permeation, few studies have addressed the influence of vehicles on drug transport in artificial or biological membranes Previously we have investigated the effects of temperature on the uptake of model vehicles (i.e. alcohols) into silicone membrane. The present study evaluates the permeation of the model drug methyl paraben in the presence of butanol or heptanol. Drug permeation through silicone membranes was studied at different temperatures for each vehicle. Thermodynamic and kinetic analyses of the permeation data were conducted to elucidate the possible mechanisms of drug transport. Independent examination of the partition and diffusion coefficients estimated for the permeation studies at different temperatures showed a break point occurring near 20 degrees C for butanol, but not heptanol. This transition temperature separated two different mechanisms of solute diffusion and partitioning, which may be associated with a change in the properties of the solvent. This was not observed from an analysis of flux data, owing to compensatory influences on the diffusion and partition behaviour of the drug. The study underlines the importance of appropriate temperature control when studying drug permeation.