Prediction of steady-state skin permeabilities of polar and nonpolar permeants across excised pig skin based on measurements of transient diffusion: characterization of hydration effects on the skin porous pathway

PAMAM dendrimers as mediators of dermal and transdermal drug delivery: a review

OBJECTIVES

Poly(amidoamine) dendrimers have been widely investigated as potential nanomaterials that can enhance the skin permeation of topically applied drugs. This article reviews the studies that have used dendrimers as penetration enhancers and examines the mechanisms by which enhancement is claimed.

KEY FINDINGS

A wide range of studies have demonstrated that, in certain circumstances and for certain drugs, the incorporation of dendrimers into a topically applied formulation can significantly increase the amount of drug passing into and through the skin. In some cases, dendrimers offered little or no enhancement of skin permeation, suggesting that the drug-dendrimer interaction and the selection of a specific dendrimer were central to ensuring optimal enhancement of skin permeation. Significant interactions between dendrimers and other formulation components were also reported in some cases.

SUMMARY

Dendrimers offer substantial potential for enhancing drug delivery into and across the skin, putatively by mechanisms that include occlusion and changes to surface tension. However, most of these studies are conducted in vitro and limited progress has been made beyond such laboratory studies, some of which are conducted using membranes of limited relevance to humans, such as rodent skin. Thus, the outcomes and claims of such studies should be treated with caution.

Dose-dependent effect on skin permeation of polar and non-polar compounds

The study of the relationship between the amount of drug applied to the skin and fraction of drug absorbed can improve our understanding of finite-dose percutaneous absorption in the development of topical products and risk assessment of hazardous chemical exposure. It has been previously shown that an increase in the dose applied to the skin leads to a decrease in the fraction of drug permeated the skin (dose-dependent effect). The objective of this research was to examine the dose-dependent effect using permeants of varying physiochemical properties. The dose-dependent effect was studied using human epidermal membrane under finite dose conditions in Franz diffusion cell with model permeants at doses ranging from 0.1 to 200 μg. The dose-dependent effect was evident with model permeants caffeine, corticosterone, dexamethasone, and estradiol, consistent with the relationship of decreasing fraction of dose permeated the skin at increasing the applied dose. However, no significant dose-dependent effect was observed for the polar model permeants urea, mannitol, tetraethyl ammonium, and ethylene glycol, suggesting different transport mechanisms for these permeants. It was also found that, at relatively high doses, estradiol, dexamethasone, and corticosterone could increase the permeation of polar and lipophilic permeants, which could counter the dose-dependent effect under the conditions studied.

Human epidermal in vitro permeation test (IVPT) analyses of alcohols and steroids

This study examined a number of factors that can impact the outcomes of in vitro human epidermal permeation coefficients for aliphatic alcohols and steroids, including receptor phase composition and study conditions. We determined experimentally the solubilities and IVPT permeation of a homologous series of ¹⁴C labeled aliphatic alcohols (ethanol, propanol, pentanol, heptanol, octanol and decanol) in different receptor fluids as recommended by Organisation Economic Co-operation and Development (OECD). We used human epidermal membranes at 25°C and phosphate-buffered saline (PBS), 2% w/v bovine serum albumin (2%w/v BSA), 50% v/v ethanol and 0.1, 2 and 6% w/v Oleth-20 receptor phases. We also explored and confirmed the discrepancies between in vitro human epidermal permeability coefficients (k_p) and diffusion lag times for steroids from Scheuplein's group with our own work and that of others. The main reason for the observed differences is not clear but is likely to be multifactorial, including the effects of diffusion cell design, receptor phase solubility, unstirred receptor phase effects, epidermal membrane hydration, diffusion cell configuration, transport through appendageal pathways and steroid lipophilicity. We conclude with a summary of experimental conditions that should be considered in undertaking IVPT studies.

Dermal absorption of gallium antimonide in vitro and pro-inflammatory effects on human dermal fibroblasts

Gallium antimonide (GaSb) is a group III-V compound semiconductor with a comparatively narrow band gap energy (0.73 eV at 300 K) that allows efficient operation in the near-infrared region. This property may be useful in developing new biomedical instruments such as epidermal optoelectronic devices. The present study investigated the absorption of GaSb in pig skin in vitro for 24 h using Franz cells. A donor solution was prepared by soaking GaSb thin films in synthetic sweat. The results showed that both gallium and antimony penetrated the skin, and permeation and resorption occurred for gallium. Histopathological findings showed no inflammatory responses in pig skin exposed to GaSb for 24 h. Cytotoxicity was significantly elevated after 3 and 7 days, and pro-inflammatory cytokines and IL-8 levels were low after 1 and 3 days but elevated 7 days following the direct culturing of human dermal fibroblasts (HDF) on GaSb thin films. These results demonstrate that the short-term cytotoxicity and pro-inflammatory effect of GaSb on HDF were relatively low.

Bio-Integrated Wearable Systems: A Comprehensive Review

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.

Salt water and skin interactions: new lines of evidence

In Health Resort Medicine, both balneotherapy and thalassotherapy, salt waters and their peloids, or mud products are mainly used to treat rheumatic and skin disorders. These therapeutic agents act jointly via numerous mechanical, thermal, and chemical mechanisms. In this review, we examine a new mechanism of action specific to saline waters. When topically administered, this water rich in sodium and chloride penetrates the skin where it is able to modify cellular osmotic pressure and stimulate nerve receptors in the skin via cell membrane ion channels known as "Piezo" proteins. We describe several models of cutaneous adsorption/desorption and penetration of dissolved ions in mineral waters through the skin (osmosis and cell volume mechanisms in keratinocytes) and examine the role of these resources in stimulating cutaneous nerve receptors. The actions of salt mineral waters are mediated by a mechanism conditioned by the concentration and quality of their salts involving cellular osmosis-mediated activation/inhibition of cell apoptotic or necrotic processes. In turn, this osmotic mechanism modulates the recently described mechanosensitive piezoelectric channels.

How Sensitive Are Transdermal Transport Predictions by Microscopic Stratum Corneum Models to Geometric and Transport Parameter Input?

While predictive models of transdermal transport have the potential to reduce human and animal testing, microscopic stratum corneum (SC) model output is highly dependent on idealized SC geometry, transport pathway (transcellular vs. intercellular), and penetrant transport parameters (e.g., compound diffusivity in lipids). Most microscopic models are limited to a simple rectangular brick-and-mortar SC geometry and do not account for variability across delivery sites, hydration levels, and populations. In addition, these models rely on transport parameters obtained from pure theory, parameter fitting to match in vivo experiments, and time-intensive diffusion experiments for each compound. In this work, we develop a microscopic finite element model that allows us to probe model sensitivity to variations in geometry, transport pathway, and hydration level. Given the dearth of experimentally-validated transport data and the wide range in theoretically-predicted transport parameters, we examine the model's response to a variety of transport parameters reported in the literature. Results show that model predictions are strongly dependent on all aforementioned variations, resulting in order-of-magnitude differences in lag times and permeabilities for distinct structure, hydration, and parameter combinations. This work demonstrates that universally predictive models cannot fully succeed without employing experimentally verified transport parameters and individualized SC structures.

Multimodal epidermal devices for hydration monitoring

Precise, quantitative in vivo monitoring of hydration levels in the near surface regions of the skin can be useful in preventing skin-based pathologies, and regulating external appearance. Here we introduce multimodal sensors with important capabilities in this context, rendered in soft, ultrathin, 'skin-like' formats with numerous advantages over alternative technologies, including the ability to establish intimate, conformal contact without applied pressure, and to provide spatiotemporally resolved data on both electrical and thermal transport properties from sensitive regions of the skin. Systematic in vitro studies and computational models establish the underlying measurement principles and associated approaches for determination of temperature, thermal conductivity, thermal diffusivity, volumetric heat capacity, and electrical impedance using simple analysis algorithms. Clinical studies on 20 patients subjected to a variety of external stimuli validate the device operation and allow quantitative comparisons of measurement capabilities to those of existing state-of-the-art tools.

Analysis of steady state and non-steady state corneal permeation of diclofenac

The present study was undertaken for characterization of the steady state and non steady state corneal permeation kinetics of diclofenac potassium (DCP) using statistical moment theory.

Potential and problems in ultrasound-responsive drug delivery systems

Ultrasound is an important local stimulus for triggering drug release at the target tissue. Ultrasound-responsive drug delivery systems (URDDS) have become an important research focus in targeted therapy. URDDS include many different formulations, such as microbubbles, nanobubbles, nanodroplets, liposomes, emulsions, and micelles. Drugs that can be loaded into URDDS include small molecules, biomacromolecules, and inorganic substances. Fields of clinical application include anticancer therapy, treatment of ischemic myocardium, induction of an immune response, cartilage tissue engineering, transdermal drug delivery, treatment of Huntington’s disease, thrombolysis, and disruption of the blood–brain barrier. This review focuses on recent advances in URDDS, and discusses their formulations, clinical application, and problems, as well as a perspective on their potential use in the future.

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.

Measuring the stratum corneum reservoir: desorption kinetics from keratin

High keratin binding and slow desorption kinetics are assumed to be responsible for the formation of the stratum corneum (SC) reservoir. We measured equilibrium binding coefficients (K(b)) and desorption rate constants (k(off)) with bovine hoof/horn keratin and six solutes with similar molecular weight (180-288 Da) and varying lipophilicities [expressed as octanol-water distribution coefficient, i.e., a partition coefficient corrected for pH (log K(pH))-0.13 to 3.8]. Two ionizable solutes within this set were tested at different pH values as degree of ionization and lipophilicity were expected to influence equilibrium binding and desorption kinetics. The unbound fraction at equilibrium varied between 18% and 93%. All solutes exhibited linear binding isotherms within the investigated concentration range. Equilibrium binding and the rate of desorption are both functions of solute lipophilicity [log K(b) = 1.23 + 0.32 log K(pH); log k(off) = 1/(25.75 + 8.35 K(pH) (0.34))]. Our results prove that slow desorption from keratin may be a major contributor to the SC reservoir. Also, they prove that reservoir formation is relevant for lipophilic solutes independent of drug class, thus allowing new options for topical pharmacotherapy.

Ultrasound-mediated transdermal drug delivery: mechanisms, scope, and emerging trends

The use of ultrasound for the delivery of drugs to, or through, the skin is commonly known as sonophoresis or phonophoresis. The use of therapeutic and high frequencies of ultrasound (≥0.7MHz) for sonophoresis (HFS) dates back to as early as the 1950s, while low-frequency sonophoresis (LFS, 20-100kHz) has only been investigated significantly during the past two decades. Although HFS and LFS are similar because they both utilize ultrasound to increase the skin penetration of permeants, the mechanisms associated with each physical enhancer are different. Specifically, the location of cavitation and the extent to which each process can increase skin permeability are quite dissimilar. Although the applications of both technologies are different, they each have strengths that could allow them to improve current methods of local, regional, and systemic drug delivery. In this review, we will discuss the mechanisms associated with both HFS and LFS, specifically concentrating on the key mechanistic differences between these two skin treatment methods. Background on the relevant physics associated with ultrasound transmitted through aqueous media will also be discussed, along with implications of these phenomena on sonophoresis. Finally, a thorough review of the literature is included, dating back to the first published reports of sonophoresis, including a discussion of emerging trends in the field.

Effects of ultrasound and sodium lauryl sulfate on the transdermal delivery of hydrophilic permeants: Comparative in vitro studies with full-thickness and split-thickness pig and human skin

The simultaneous application of ultrasound and the surfactant sodium lauryl sulfate (referred to as US/SLS) to skin enhances transdermal drug delivery (TDD) in a synergistic mechanical and chemical manner. Since full-thickness skin (FTS) and split-thickness skin (STS) differ in mechanical strength, US/SLS treatment may have different effects on their transdermal transport pathways. Therefore, we evaluated STS as an alternative to the well-established US/SLS-treated FTS model for TDD studies of hydrophilic permeants. We utilized the aqueous porous pathway model to compare the effects of US/SLS treatment on the skin permeability and the pore radius of pig and human FTS and STS over a range of skin electrical resistivity values. Our findings indicate that the US/SLS-treated pig skin models exhibit similar permeabilities and pore radii, but the human skin models do not. Furthermore, the US/SLS-enhanced delivery of gold nanoparticles and quantum dots (two model hydrophilic macromolecules) is greater through pig STS than through pig FTS, due to the presence of less dermis that acts as an artificial barrier to macromolecules. In spite of greater variability in correlations between STS permeability and resistivity, our findings strongly suggest the use of 700microm-thick pig STS to investigate the in vitro US/SLS-enhanced delivery of hydrophilic macromolecules.

Solid emulsion gel as a novel construct for topical applications: synthesis, morphology and mechanical properties

A series of the solid emulsion gels with the oil volume fraction in the range of 0-50% were synthesized through a polycondensation reaction between activated p-nitrophenyl carbonate poly(ethylene glycol) and protein-stabilized oil-in-water emulsions. The resultant structures were investigated in terms of swelling behavior, composition, morphology, mechanical and skin hydration properties. Solid emulsions gels share the properties of both hydrogel and emulsion. Similar to the classical hydrogel, the SEG swells in water up to equilibrium swelling degree, which decreases as the oil volume fraction increases, and comprises immobilized drops of protein-stabilized oil. The impregnation of the oil phase is found to reduce tensile stiffness of the material, but improves material's extensibility. The mechanical properties of the constructs (Young moduli in the range of 9-15 kPa and the elongation at break of 120-220%) are interpreted according to the "rule of elasticity mixture" that considers the elasticity of the composite material to be a sum of the contributions from individual components, i.e. hydrogel and dispersed oil drops. An idealized model that takes into account the history of the material preparation has been proposed to explain the improved extensibility of the constructs. The results of the mechanical tests, equilibrium swelling, and the skin hydration effect of the solid emulsion gels in vivo are discussed from the perspective of the biomedical applications of the solid emulsion gels, in particular, for the transdermal delivery of hydrophilic and lipophilic drugs.

Reconstructed epidermis and full-thickness skin for absorption testing: influence of the vehicles used on steroid permeation

A protocol for percutaneous absorption studies has been validated, based on the use of reconstructed human epidermis (RHE) and aqueous solutions of test substances. However, it is often the case that it is more-complex formulations of drugs or chemicals which will make contact with the skin surface. To investigate whether RHE and the reconstructed full-thickness skin model (FT-model) can be used to predict uptake from formulations, we compared the permeation of hydrocortisone and testosterone when applied in emulsion form and as a solution containing the penetration enhancer, ethanol. Human and pig skin and a non-cornified alveolar model served as references. The results were compared with steroid release from the formulations. The permeation rates of the steroids were ranked as: alveolar model >> RHE > FT-model, pig skin > human skin. In accordance with the rapid hydrocortisone release from the formulations, the permeation rates of this steroid exceeded those of testosterone. Only minor differences were observed when comparing the testosterone formulations, in terms of release and permeation. However, the ranking of the permeation of the hydrocortisone formulations was: solution > w/o emulsion > o/w emulsion, which permitted the elucidation of penetration enhancing effects, which is not possible with drug release studies. Differences in penetration were most obvious with native skin and reconstructed tissues, which exhibited a well-developed penetration barrier. In conclusion, RHE and skin preparations may be useful in the development of topical dermatics, and in the framework of hazard analysis of toxic compounds and their various formulations.

The use of reconstructed human epidermis for skin absorption testing: Results of the validation study

A formal validation study was performed, in order to investigate whether the commercially-available reconstructed human epidermis (RHE) models, EPISKIN, EpiDerm and SkinEthic, are suitable for in vitro skin absorption testing. The skin types currently recommended in the OECD Test Guideline 428, namely, ex vivo human epidermis and pig skin, were used as references. Based on the promising outcome of the prevalidation study, the panel of test substances was enlarged to nine substances, covering a wider spectrum of physicochemical properties. The substances were tested under both infinite-dose and finite-dose conditions, in ten laboratories, under strictly controlled conditions. The data were subjected to independent statistical analyses. Intra-laboratory and inter-laboratory variability contributed almost equally to the total variability, which was in the same range as that in preceding studies. In general, permeation of the RHE models exceeded that of human epidermis and pig skin (the SkinEthic RHE was found to be the most permeable), yet the ranking of substance permeation through the three tested RHE models and the pig skin reflected the permeation through human epidermis. In addition, both infinite-dose and finite-dose experiments are feasible with RHE models. The RHE models did not show the expected significantly better reproducibility, as compared to excised skin, despite a tendency toward lower variability of the data. Importantly, however, the permeation data showed a sufficient correlation between all the preparations examined. Thus, the RHE models, EPISKIN, EpiDerm and SkinEthic, are appropriate alternatives to human and pig skin, for the in vitro assessment of the permeation and penetration of substances when applied as aqueous solutions.

Evaluation of the porosity, the tortuosity, and the hindrance factor for the transdermal delivery of hydrophilic permeants in the context of the aqueous pore pathway hypothesis using dual-radiolabeled permeability experiments

The aqueous pore pathway hypothesis has been modified to include both transient and steady-state domains of diffusive transport to evaluate the porosity, the tortuosity, and the hindrance factor of the skin aqueous pore channels from an individual dual-radiolabeled permeability experiment. Using these theoretical and experimental methods, the porosity (epsilon), the tortuosity (tau), and the hindrance factor (H) of the skin aqueous pore channels were evaluated as a function of: (i) the radius of the selected model hydrophilic permeants (urea, mannitol, raffinose, and inulin), and (ii) the extent of skin perturbation present in untreated skin, skin pretreated at a low dose, and a high dose, with a simultaneous application of 20 kHz ultrasound and the surfactant sodium lauryl sulfate (SLS), and the dermis. The results of this investigation revealed that the tortuosity decreased, and only the hindrance factor for inulin was significantly less than 1, over the range of permeant radii examined. Furthermore, only the porosity increased over the range of skin perturbation examined (over 100-fold), suggesting that a surface-related phenomenon is primarily responsible for the observed enhancement in the transdermal permeability of hydrophilic permeants induced by the simultaneous application of ultrasound and SLS.

Transcellular route of diffusion through stratum corneum: results from finite element models

Insight into the stratum corneum (SC) permeation pathway for hydrophilic compounds is gained by comparing experimental measurements of permeability and lag time (tlag) with the predictions of a finite element (FE) model. A database of permeability and lag time measurements (n=27) of hydrophilic compounds was compiled from the literature. Transcellular and lateral lipid diffusion pathways were modeled within a brick-and-mortar geometry representing fully hydrated human SC. Modeled tlag's for the lipid pathway are too brief to account for the experimental quantities, whereas the transcellular pathway with preferential corneocyte partitioning does account for them. Measured tlag's are highly correlated (p<0.0001) with the compound's octanol-water partition coefficient, supporting the hypothesis of an aqueous-lipid partition mechanism in the permeation of hydrophilic compounds. The importance of the lag time for identifying the diffusion pathway is demonstrated.

Effect of application sites and multiple doses on nicotine pharmacokinetics in healthy male Japanese smokers following application of the transdermal nicotine patch

The transdermal nicotine patch, which contains 25 mg nicotine per 30 cm(2), is designed to deliver approximately 15 mg nicotine to the blood circulation in 16 hours of application for the treatment of smoking cessation. It was applied to 3 different skin sites (upper arm, abdomen, and back) to examine regional variations in percutaneous nicotine absorption in a single-dose, 3-period, crossover study involving 9 healthy male Japanese smokers. Nicotine pharmacokinetics during once-daily application of the transdermal nicotine patch for 5 days was also investigated in 10 healthy smokers. There were statistically significant effects of application sites on percutaneous nicotine absorption. The ratios (90% confidence intervals) of AUC and C(max) for comparison to the upper arm were 102% (88, 117%) and 106% (95, 119%) for the back and 75% (65, 87%) and 75% (66, 84%) for the abdomen, respectively. These suggest that systemic exposure after application to the upper arm was greater compared with the abdomen but equivalent to the back. Following multiple doses, linear pharmacokinetics and no significant accumulation of nicotine concentrations were observed, and steady state was reached by day 2. Only mild itching and erythema were observed at the application sites. The transdermal nicotine patch was well tolerated in both studies.

In vitro delivery of novel, highly potent anti-varicella zoster virus nucleoside analogues to their target site in the skin

PURPOSE

To determine the in-vitro dermal delivery of a new class of lipophilic, highly potent and uniquely selective anti-VZV nucleoside analogues in comparison with aciclovir.

METHODS

Three test compounds (Cf1698, Cf1743, Cf1712) and aciclovir were formulated into propylene glycol/aqueous cream BP formulations and finite doses applied to full-thickness pig ear skin for 48 hours in vertical Franz-type diffusion cells. Receptor phase samples were taken at specific intervals to determine permeation, and depth profiles were constructed following tape stripping and membrane separation.

RESULTS

All three test compounds reached the target basal epidermis in concentrations suggesting they would be highly efficacious in reducing viral load. Furthermore, the data showed that each of the test compounds would perform in a far superior manner to aciclovir, the current treatment of choice.

CONCLUSIONS

The dermatomal site of viral replication during secondary infection--the basal epidermis--was successfully targeted. Topical delivery of these compounds is highly promising as a new first line treatment of VZV infections. By attacking the virus at the first sign of reactivation, it is proposed that the extent of damage caused by the virus would be significantly lowered, thereby limiting the extent and severity of post-herpetic neuralgia.

Experimental demonstration of the existence of highly permeable localized transport regions in low‐frequency sonophoresis

Recent advances in low-frequency sonophoresis have focused on the existence of hypothesized localized transport regions (LTRs). However, there has been no actual experimental demonstration that the hypothesized LTRs are, in fact, localized regions of high permeability. Through a series of low-frequency sonophoresis experiments conducted with full-thickness pig skin, in the presence of the surfactant sodium lauryl sulfate (SLS), in which we have separately measured the transport of calcein through the LTRs, which have areas ranging from 10 to 40 mm(2), and the surrounding regions of the skin (the non-LTRs) by means of a novel masking technique, we demonstrate that the calcein permeability through the LTRs is approximately 80-fold higher than the calcein permeability through the non-LTRs, suggesting that the LTRs are structurally perturbed to a greater extent than the non-LTRs from the exposure to the ultrasound/SLS system. In addition, we propose basic models to predict the total skin transdermal permeability from the transdermal permeabilities of the LTRs and the non-LTRs, and then compare the predictions to the experimental data obtained from the masking experiments. We also demonstrate that both the LTRs and the non-LTRs exhibit significant decreases in skin electrical resistivity relative to untreated skin ( approximately 5000-fold and approximately 170-fold, respectively), suggesting the existence of two levels of significant skin structural perturbation due to ultrasound exposure in the presence of SLS. Finally, an analysis of the porosity/tortuosity ratio (epsilon/tau) values suggests that trans-cellular transdermal transport pathways are present within the highly permeable, and highly structurally perturbed, LTRs.

Characterization of Transport of Chlorhexidine-Loaded Nanocapsules through Hairless and Wistar Rat Skin

Nanocapsules appear a promising approach as a drug system for topical application. However, the transport mechanism of nanocapsule-associated drug through the skin is still being questioned. In the present study, the transport of chlorhexidine-loaded poly(epsilon-caprolactone) nanocapsules through full-thickness and stripped hairless rat skin was investigated in static-diffusion cell. The chlorhexidine permeation profiles fitting the Fickian diffusion model showed that the drug encapsulation reduced the percutaneous drug absorption through stripped skin. Possible nanocapsule transport within skin conducts was suggested from the analysis of permeation parameters and confirmed by confocal laser microscopy studies. Furthermore, the chlorhexidine permeation and drug release data were highly correlated, suggesting that the magnitude of percutaneous absorption was controlled by the diffusion across the polymeric carrier. The behavior of nanocapsules at the skin interface was investigated by contact angle and surface tension measurements. The small 'wetting' of the nanocapsule on the stratum corneum surface preserved the mechanical integrity of the carrier characterized by a high specific surface at the skin interface. The flexibility of the nanocapsules assured a satisfying bioadhesion to the skin, whereas the rigidity of the carrier limited the molecular 'spill' into the skin and controlled the drug delivery to the skin.

Mechanistic and empirical modeling of skin permeation of drugs

The skin forms a barrier to the external environment, maintaining body fluids within our system and excluding harmful substances, while the skin is a site of administration of drugs for topical and systemic chemotherapy. It is an important issue to predict the rate at which drugs or other xenobiotics penetrate the skin. In this article, we review modeling approaches for predicting skin permeation of compounds, including both mechanistic and empirical approaches. Mechanistic approaches can give us much information on understanding of skin permeation of the compounds, such as structure-permeability relationship, contribution of each barrier step, mechanism of penetration enhancers, and in vivo-in vitro relationship. On the other hand, empirical modeling can overcome any inaccuracies of mechanistic models caused by the existence of uncertainties and, therefore, give us better predictions from the practical point of view. Artificial neural networks are being available for empirical modeling of complex skin transport phenomenon.

Transdermal delivery of zidovudine: effect of vehicles on permeation across rat skin and their mechanism of action

The purpose of this study was to investigate the effects of various solvent systems containing water, ethanol, propylene glycol (PG), and their binary combinations on the ex vivo permeation of zidovudine (AZT) across Sprague-Dawley rat skin using Franz diffusion cells at 37 degrees C. Further, saturation solubility and epidermis/vehicle partition coefficient of AZT in the solvent systems, and their effect on percentage hydration of epidermis using thermogravimetric analysis were determined to understand the mechanisms by which these solvent systems change drug permeability properties. All binary combinations of PG, ethanol and water significantly increased saturation solubility of AZT. Maximum AZT flux was observed with 66.6% ethanol among ethanol-water solvents, with 33.3% PG in PG-water solvents and with 100% ethanol among PG-ethanol combinations. PG-water and PG-ethanol solvents neither reduced the lag time nor increased AZT flux across rat skin. In addition, high concentrations of PG in both water and ethanol reduced steady state flux of AZT. Further, thermogravimetric studies revealed that solvents containing high PG concentrations dehydrate epidermis. Among all the solvent combinations, highest flux and short lag time were achieved with ethanol at 66.6% in water and hence is a suitable vehicle for transdermal delivery of AZT.