Liposomes and skin: From drug delivery to model membranes

Predicting skin permeability from complex vehicles

It is now widely accepted that vehicle and formulation components influence the rate and extent of passive chemical absorption through skin. Significant progress, over the last decades, has been made in predicting dermal absorption from a single vehicle; however the effect of a complex, realistic mixture has not received its due attention. Recent studies have aimed to bridge this gap by extending the use of quantitative structure-permeation relationship (QSPR) models based on linear free energy relationships (LFER) to predict dermal absorption from complex mixtures with the inclusion of significant molecular descriptors such as a mixture factor that accounts for the physicochemical properties of the vehicle/mixture components. These models have been compiled and statistically validated using the data generated from in vitro or ex vivo experimental techniques. This review highlights the progress made in predicting skin permeability from complex vehicles.

Deformable liposomes by reverse-phase evaporation method for an enhanced skin delivery of (+)-catechin

BACKGROUND

(+)-catechin, as the most common catechin isomer, is recognized to be an antioxidant which benefits the skin in many ways. The purpose of the present study was to prepare and evaluate a suitable liposomal delivery systems for (+)-catechin topical application.

METHODS

In this study, catechin-loaded conventional liposomal delivery system, deformable conventional liposomal delivery system and deformable liposomes prepared by reverse-phase evaporation (REV) method were compared. The three systems were characterized for liposome particle size, zeta-potential, entrapment efficiency, drug release, permeability across porcine skin and catechin deposition in the skin.

RESULTS

It was revealed that the size of deformable conventional liposomes before freeze-drying and deformable REV liposomes after freeze-drying range from 335.6 ± 71.7 nm to 551.1 ± 53.4 nm, respectively, which were considered to be suitable for skin delivery. The deformable REV liposomes had a higher aqueous volume and thus were able to entrap greater amounts of hydrophilic (+)-catechin (50.0 ± 5.9%) compared to conventional (30.0 ± 3.8%) and deformable conventional liposomes (36.1 ± 4.6%). All liposomal formulations exhibited a prolonged catechin release. Compared to deformable liposomes, the REV deformable liposomes showed a significantly better deposition of (+)-catechin while catechin solution did not permeate into the porcine ear skin.

CONCLUSION

Among all formulations studied, deformable REV liposomes were considered to be favorable for catechin topical delivery.

In vivo microdialysis for the evaluation of transfersomes as a novel transdermal delivery vehicle for cinnamic acid

In this study, cinnamic acid-loaded transfersomes were prepared and dermal microdialysis sampling was used in Sprague-Dawley rats to compare the amount of drug released into the skin using transfersomes as transdermal carriers with that released on using conventional liposomes. The formulation of cinnamic acid-loaded transfersomes was optimized by a uniform design through in vitro transdermal permeation studies. Hydration time was confirmed as a significant factor influencing the entrapment efficiency of transfersomes, further affecting their transdermal flux in vitro. The fluxes of cinnamic acid from transfersomes were all higher than those from conventional liposomes, and the flux from the optimal transfersome formulation was 3.01-fold higher than that from the conventional liposomes (p < 0.05). An in vivo microdialysis sampling method revealed that the dermal drug concentrations from transfersomes applied on various skin regions were much lower than those required with conventional liposomes. After the administration of drug-containing transfersomes and liposomes on abdominal skin regions of rats for a period of 10 h, the Cmax of cinnamic acid from the compared liposomes was 3.21 ± 0.25 μg/mL and that from the transfersomes was merely 0.59 ± 0.02 μg/mL. The results suggest that transfersomes can be used as carriers to enhance the transdermal delivery of cinnamic acid, and that these vehicles may penetrate the skin in the complete form, given their significant deformability.

Colloidal, in vitro and in vivo anti-leishmanial properties of transfersomes containing paromomycin sulfate in susceptible BALB/c mice

The aim of this study was to develop transfersomal formulation with respect to dermal delivery of paromomycin sulfate (PM) for possible topical therapy of cutaneous leishmaniasis (CL). PM transfersomal formulations (PMTFs) with different percent of soy phosphatidylcholine, sodium cholate (Na-Ch) and ethanol were prepared and characterized for the size, zeta potential and encapsulation efficiency. The results showed that the most stable formulations with suitable colloidal properties were obtained by 2% Na-Ch which had average size of around 200 nm. The in vitro permeation study using Franz diffusion cells fitted with mouse skin at 37°C for 24h showed that almost 23% of the PMTFs applied penetrated the mouse skin, and the amount retained in the skin was about 67% for both formulations; however, the percent of penetration and retention for PM conventional cream was 49 and 13, respectively. The 50% effective doses of PMTFs against Leishmania major promastigotes and amastigotes in culture were significantly less than cream and/or solution of PM. Selected PMTFs and empty transfersomes showed no cytotoxicity in J774 A.1 mouse macrophage cell line. Selected PMTFs was used topically twice a day for 4 weeks to treat L. major lesions on BALB/c mice, and the results showed a significantly (P<0.05) smaller lesion size in the mice in the treated groups than in the mice in the control groups, which received either empty transfersomes or phosphate-buffered saline (PBS) and also PM cream. The spleen parasite burden was significantly (P<0.01) lower in mice treated with selected PMTFs than in mice treated with PBS or control transfersomes, and PM cream. The results of this study showed that PMTFs prepared with 2% of Na-Ch with and without 5% ethanol might be useful as a candidate for the topical treatment of CL.

Development and evaluation of antimicrobial herbal formulations containing the methanolic extract of Samadera indica for skin diseases

Samadera indica Gaetrn (Simaroubaceae) is claimed to possess various pharmacological activities like antioxidant, antifungal, antitumor, antiviral, and so on, but its taste is bitter. The aim of the present study is to investigate the toxicity of the methanolic extract and to develop suitable herbal formulations of the methanolic extract of Samadera indica, having efficient antimicrobial activity. The methanolic extract prepared from the dried leaves of Samadera indica by continuous hot percolation, were used to examine the toxicity, according to the OECD 423 guidelines, in Swiss Albino mice. Topical formulations were prepared by incorporating Samadera indica (5% w / w) in an emulsifying ointment and a carbopol gel base and evaluated for physical parameters and in-vitro antimicrobial activity (S. aureus, P. aeruginosa and C. albicans). The study reveals that no animals under the study showed any clinical signs of toxicity or mortality when administered a dose of 5 - 2000 mg / kg body weight. Therefore, the maximum tolerated dose of the methanolic extract of Samadera indica was above 2000 mg / kg body weight. The formulated ointment and gel had acceptable physical parameters that showed that they were compatible with the skin, and in addition to this, these formulations passed the short-term stability studies. The in-vitro antimicrobial activity studies showed that the formulated ointment showed significantly strong (p < 0.05) activity against S. aureus, P. aeruginosa and C. albicans than the formulated gel. Thus, the present study concludes that the formulated ointment and gel are safe and efficient antimicrobial formulations for the topical delivery of the methanolic extract of Samadera indica.

Drug delivery systems for the topical treatment of cutaneous leishmaniasis

INTRODUCTION

The parenteral administration of pentavalent antimonials for the treatment of all forms of leishmaniasis, including cutaneous leishamniasis (CL), has several limitations. Therapy is long, requiring repeated doses and the adverse reactions are frequent. Topical treatment is an attractive alternative for CL, offering significant advantages over systemic therapy: fewer adverse effects, ease of administration, and lower costs.

AREAS COVERED

This review covers, from 1984 to the present, the progress achieved for the development of topical treatment for CL, using different drugs such as paromomycin (PA), imiquimod, amphotericin B (AmB), miltefosine, and buparvaquone. PA is the most commonly studied drug, followed by AmB and Imiquimod. These drugs were incorporated in conventional dosage forms or loaded in lipid nanocarries, which have been used mainly for improved skin delivery and antileishmanial activity.

EXPERT OPINION

Developing an effective topical treatment for CL using these antileishmanial drugs still remains a great challenge. Insights into the most promising delivery strategies to improve treatment of CL with PA and AmB using conventional dosage forms, lipid nanocarriers, and combined therapy are presented and discussed. The results obtained with combined therapy and alternative delivery systems are promising perspectives for improving topical treatment of CL.

Hybrid QM/MM Molecular Dynamics Study of Benzocaine in a Membrane Environment: How Does a Quantum Mechanical Treatment of Both Anesthetic and Lipids Affect Their Interaction

Biomolecular dynamics studies using a QM/MM approach have been largely used especially to study enzymatic reactions. However, to the best of our knowledge, the very same approach has not been used to study the water/membrane interface using a quantum mechanical treatment for the lipids. Since a plethora of biochemical processes take place in this region, we believe that it is of primary importance to understand, at the level of molecular orbitals, the behavior of a drug in such an odd environment. In this work, we take advantage of an integration of the CPMD and the GROMACS code, using the Car-Parrinello method, to treat the benzocaine local anesthetic as well as two of the membrane lipids and the GROMOS force field to treat the remaining lipids and the water molecules.

Ultradeformable liposomes with terpenes for delivery of hydrophilic compound

Ultradeformable liposomes containing penetration enhancers were created to deliver NaFl. Vesicles were investigated for their particle size, zeta potential, NaFl entrapment efficiency (%EE), loading efficiency, and in vitro skin penetration. The vesicles obtained were spherical in shape, with a particle size of less than 100 nm and a negative surface charge (-6 to -11 mV). The %EE of NaFl loaded in vesicles ranged from 37 to 48%. Ultradeformable liposomes with monoterpenes (d-limonene, 1,8-cineole and geraniol) significantly improved NaFl penetration through the skin. Confocal laser scanning microscopy analysis confirmed skin-penetration results and was used to evaluate the behavior of hydrophilic compounds penetrating through the skin.

Altered stratum corneum barrier and enhanced percutaneous immune responses in filaggrin-null mice

BACKGROUND

Loss-of-function mutations in filaggrin are major predisposing factors for atopic dermatitis. Although various reports suggest a critical role for filaggrin in stratum corneum (SC) barrier formation, the lack of filaggrin-null (Flg(-/-)) mice has hampered detailed in vivo analysis of filaggrin's functions.

OBJECTIVE

We sought to generate Flg(-/-) mice and to assess the effect of filaggrin loss on SC barrier function and percutaneous immune responses.

METHODS

We generated Flg(-/-) mice using gene targeting and assessed the morphology, hydration, mechanical strength, and antigen permeability of their SC. Percutaneous immune responses were evaluated through irritant- and hapten-induced contact hypersensitivity studies and by measuring humoral responses to epicutaneous sensitization with protein antigen.

RESULTS

Newborn Flg(-/-) mice exhibited dry scaly skin. Despite marked decreases in natural moisturizing factor levels, which are filaggrin degradation products, SC hydration and transepidermal water loss were normal. Microscopic analyses suggested premature shedding of SC layers, and indeed, increased desquamation under mechanical stress was demonstrated. Loss of keratin patterns, which are critical for corneocyte stabilization, is likely attributable to fragility in the Flg(-/-) SC. Antigens penetrated the Flg(-/-) SC more efficiently, leading to enhanced responses in hapten-induced contact hypersensitivity and higher serum levels of anti-ovalbumin IgG(1) and IgE.

CONCLUSION

Complete filaggrin deficiency led to altered barrier integrity and enhanced sensitization, which are important factors in early-phase atopic dermatitis. Flg(-/-) mice should provide a valuable tool to further explore additional factors the dysfunction of which leads to uncontrolled inflammation in patients with atopic diseases.

Liposomal photosensitizers: potential platforms for anticancer photodynamic therapy

Photodynamic therapy is a well-established and clinically approved treatment for several types of cancer. Antineoplastic photodynamic therapy is based on photosensitizers, i.e., drugs that absorb photons translating light energy into a chemical potential that damages tumor tissues. Despite the encouraging clinical results with the approved photosensitizers available today, the prolonged skin phototoxicity, poor selectivity for diseased tissues, hydrophobic nature, and extended retention in the host organism shown by these drugs have stimulated researchers to develop new formulations for photodynamic therapy. In this context, due to their amphiphilic characteristic (compatibility with both hydrophobic and hydrophilic substances), liposomes have proven to be suitable carriers for photosensitizers, improving the photophysical properties of the photosensitizers. Moreover, as nanostructured drug delivery systems, liposomes improve the efficiency and safety of antineoplastic photodynamic therapy, mainly by the classical phenomenon of extended permeation and retention. Therefore, the association of photosensitizers with liposomes has been extensively studied. In this review, both current knowledge and future perspectives on liposomal carriers for antineoplastic photodynamic therapy are critically discussed.

Transdermal drug delivery: from micro to nano

Delivery across skin offers many advantages compared to oral or intravenous routes of drug administration. Skin however is highly impermeable to most molecules on the basis of size, hydrophilicity, lipophilicity and charge. For this reason it is often necessary to temporarily alter the barrier properties of skin for effective administration. This can be done by applying chemical enhancers, which alter the lipid structure of the top layer of skin (the stratum corneum, SC), by applying external forces such as electric currents and ultrasounds, by bypassing the stratum corneum via minimally invasive microneedles or by using nano-delivery vehicles that can cross and deliver their payload to the deeper layers of skin. Here we present a critical summary of the latest technologies used to increase transdermal delivery.

An update on the application of physical technologies to enhance intradermal and transdermal drug delivery

A large number of biopharmaceuticals and other macromolecules are being developed for therapeutic applications. Conventional oral delivery is not always possible due to first-pass metabolism and degradation in the GI tract. Parenteral delivery is invasive and has poor patient compliance. Transdermal delivery provides one attractive route of administration. Transdermal administration can achieve the continuous and non-invasive delivery of drugs. However, passive transdermal delivery is restricted to small lipophilic molecules. Active physical-enhancement technologies are being investigated to increase the scope of transdermal delivery to hydrophilic molecules and macromolecules. Recent developments in transdermal technologies, such as microporation, iontophoresis and sonophoresis can enable therapeutic delivery of many drug molecules, biopharmaceuticals, cosmeceuticals and vaccines. This review provides an update of recent developments in transdermal delivery focusing on physical-enhancement technologies.

Cluster TOF-SIMS imaging of human skin remains: analysis of a South-Andean mummy sample

A skin sample from a South-Andean mummy dating back from the XI(th) century was analyzed using time-of-flight secondary ion mass spectrometry imaging using cluster primary ion beams (cluster-TOF-SIMS). For the first time on a mummy, skin dermis and epidermis could be chemically differentiated using mass spectrometry imaging. Differences in amino-acid composition between keratin and collagen, the two major proteins of skin tissue, could indeed be exploited. A surprising lipid composition of hypodermis was also revealed and seems to result from fatty acids damage by bacteria. Using cluster-TOF-SIMS imaging skills, traces of bio-mineralization could be identified at the micrometer scale, especially formation of calcium phosphate at the skin surface. Mineral deposits at the surface were characterized using both scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy and mass spectrometry imaging. The stratigraphy of such a sample was revealed for the first time using this technique. More precise molecular maps were also recorded at higher spatial resolution, below 1 µm. This was achieved using a non-bunched mode of the primary ion source, while keeping intact the mass resolution thanks to a delayed extraction of the secondary ions. Details from biological structure as can be seen on SEM images are observable on chemical maps at this sub-micrometer scale. Thus, this work illustrates the interesting possibilities of chemical imaging by cluster-TOF-SIMS concerning ancient biological tissues.

Liquid-crystalline membrane permeation ability for selected nitro hair dyes

AIM

A comparison of permeation ability of selected semi-permanent hair dyes and an attempt to estimate the influence of fundamental physicochemical parameters on dyes' epidermal penetration rate.

METHODS

Dyes' permeation ability through liposome membrane (as a model of stratum corneum) with side-by-side cells was assessed.

RESULTS

It has been shown that the chosen dyes are capable of permeating the membrane. High penetration coefficients (Kp) were obtained for a simple nitrophenylenediamines and nitroaminophenols. Their N-, O-hydroxyalkyl substitution significantly limits penetration. H-bonding capability has a major impact on the investigated dyes' permeation ability. Substituents with H-bonding properties can significantly limit dyes' penetration, even in the case of lipophilic structures. Special attention should be placed into compounds with strong intramolecular H-bonding properties, which improve transmembrane transport.

CONCLUSION

Substitution patterns have an influence on selected nitro dyes' permeation through a model stratum corneum. Permeation is limited by dyes diffusive properties (mostly by its H-bonding properties). Hydroxyalkylation results in hindered dyes permeation: purple, violet and blue nitrophenylenediamine or nitroaminophenol derivatives are less permeable than its not substituted analogues.

A novel murine model for the in vivo study of transdermal drug penetration

Enhancement of the transdermal penetration of different active agents is an important research goal. Our aim was to establish a novel in vivo experimental model which provides a possibility for exact measurement of the quantity of penetrated drug. The experiments were performed on SKH-1 hairless mice. A skin fold in the dorsal region was fixed with two fenestrated titanium plates. A circular wound was made on one side of the skin fold. A metal cylinder with phosphate buffer was fixed into the window of the titanium plate. The concentration of penetrated drug was measured in the buffer. The skin fold was morphologically intact and had a healthy microcirculation. The drug appeared in the acceptor buffer after 30 min, and its concentration exhibited a continuous increase. The presence of ibuprofen was also detected in the plasma. In conclusion, this model allows an exact in vivo study of drug penetration and absorption.

Variation assessment for in vitro permeabilities through Yucatan micropig skin

The purpose of this study was to evaluate the variations in the in vitro Yucatan micropig (YMP) skin permeabilities of drugs and to clarify whether YMP skin can be used to predict human skin permeability. In vitro permeation studies of the three model drugs, nicorandil, isosorbide dinitrate and flurbiprofen, through YMP skin were performed using Franz-type diffusion cells. The permeation rates of the three model drugs were determined, and their variations were evaluated. The inter-individual variations in YMP skin permeability for the three model drugs were smaller than that in human skin permeability, and the permeation rates of the three model drugs through the YMP skin were approximately half that through human skin. In addition, the intra-individual variations in YMP skin permeability for nicorandil and flurbiprofen were much smaller than the inter-individual variations in YMP skin. The inter- and intra-regional variations in YMP skin permeability were very small. The markedly smaller variation in the permeability through YMP skin as compared with that through human skin indicated that in vitro permeation studies using YMP skin would be particularly useful for evaluating differences in the skin permeability of the three model drugs as well as for predicting human skin permeability.

New formulations of local anaesthetics-part I

Part 1 comments on the types of local anaesthetics (LAs); it provides a better understanding of the mechanisms of action of LAs, and their pharmacokinetics and toxicity. It reviews the newer LAs such as levobupivacaine, ropivacaine, and articaine, and examines the newer structurally different LAs. The addition of adjuvants such as adrenaline, bicarbonate, clonidine, and corticosteroids is explored. Comment is made on the delivery of topical LAs via bioadhesive plasters and gels and controlled-release local anaesthetic matrices. Encapulation matrices such as liposomes, microemulsions, microspheres and nanospheres, hydrogels and liquid polymers are discussed as well. New innovations pertaining to LA formulations have indeed led to prolonged action and to novel delivery approaches.

Nanosized ethosomes bearing ketoprofen for improved transdermal delivery

The potential of ethosomes for delivering ketoprofen via skin was evaluated. The ethosomes were prepared, optimized and characterized. Vesicular shape, size and entrapment efficiency were determined by transmission electron microscopy, dynamic light scattering and minicolumn centrifugation technique, respectively. Vesicle sizes varied from 120.3±6.1 to 410.2±21.8 nm depending on the concentrations of soya phosphatidyl choline (SPC) and ethanol. Entrapment efficiency increased with concentrations of SPC and ethanol. The formulations exhibited entrapment efficiencies of 42-78%. In vitro release through cellophane membrane showed sustained release of drug from ethosomal formulations in contrast to hydroalcoholic drug solution (HA), which released most of the drug within 2-3 h. In vitro drug permeation across human skin revealed improved drug permeation and higher transdermal flux with ethosomal formulations compared to hydroethanolic drug solution. Kinetics of in vitro skin permeation showed zero order drug release from formulations. Based on in vitro transdermal flux, the estimated steady state in vivo plasma concentration from ethosomes attained therapeutic drug levels whereas hydroalcoholic drug solution exhibited sub therapeutic drug concentration with a patch size of 50 cm(2). Skin permeation of ethosomal formulations assessed by confocal microscopy revealed enhanced permeation of Rhodamine 123 loaded formulation in comparison to the hydroalcoholic solution.

Novel gel formulations with catanionic aggregates enable prolonged drug release and reduced skin permeation

Objectives

The aim of this study was to investigate skin permeation rates of a drug substance when applied in novel gel formulations with catanionic aggregates.

Methods

Reference gel without catanionic aggregates was compared with formulations with catanionic aggregates composed of tetracaine and either sodium dodecyl sulphate (SDS) or capric acid. Carbomer and SoftCAT were used to compare the effect of different gel types to elucidate if physically cross-linked, ‘self-destructing’ systems had benefits compared with classical, covalently cross-linked, gels.

Key findings

The rheological investigation showed that the interactions between the SoftCAT polymer and tetracaine/SDS aggregates were stronger than when the tetracaine/capric acid aggregates were used. The skin permeation was measured ex vivo in horizontal Ussing chambers and the permeation of tetracaine was significantly lower when formulations with tetracaine/SDS aggregates were applied (P &lt; 0.001), but not statistically different from the reference when capric acid was used.

Conclusions

No morphological differences could be distinguished between the skin samples exposed to the different formulations or the reference. Skin permeation was compared with silicone sheet permeation and the results indicated that silicone sheets could be used as a model of skin when using these formulations.

Preparation and evaluation of quercetin-loaded lecithin-chitosan nanoparticles for topical delivery

BACKGROUND

The purpose of this study was to investigate lecithin-chitosan nanoparticles as a topical delivery system for quercetin.

METHODS

Tocopheryl propylene glycol succinate was chosen to be the surfactant for the nanosystem. The mean particle size of the nanoparticles was 95.3 nm, and the entrapment efficiency and drug loading for quercetin were 48.5% and 2.45%, respectively. Topical delivery in vitro and in vivo of the quercetin-loaded nanoparticles was evaluated using quercetin propylene glycol solution as the control.

RESULTS

Compared with quercetin solution, the quercetin-loaded nanoparticles showed higher permeation ability, and significantly increased accumulation of quercetin in the skin, especially in the epidermis. Microstructure observation of the skin surface after administration indicated that the interaction between ingredients of the nanoparticles and the skin surface markedly changed the morphology of the stratum corneum and disrupted the corneocyte layers, thus facilitating the permeation and accumulation of quercetin in skin.

CONCLUSION

Lecithin-chitosan nanoparticles are a promising carrier for topical delivery of quercetin.

Differential scanning calorimetry: An invaluable tool for a detailed thermodynamic characterization of macromolecules and their interactions

Differential Scanning Calorimetry (DSC) is a highly sensitive technique to study the thermotropic properties of many different biological macromolecules and extracts. Since its early development, DSC has been applied to the pharmaceutical field with excipient studies and DNA drugs. In recent times, more attention has been applied to lipid-based drug delivery systems and drug interactions with biomimetic membranes. Highly reproducible phase transitions have been used to determine values, such as, the type of binding interaction, purity, stability, and release from a drug delivery mechanism. This review focuses on the use of DSC for biochemical and pharmaceutical applications.

Main approaches for delivering antioxidant vitamins through the skin to prevent skin ageing

INTRODUCTION

One of the major contributions to skin photoageing and diseases is oxidative stress, caused by UV radiation inducing reactive oxygen and nitrogen species. Successful prophylaxis and therapy would necessitate control of the oxidant/antioxidant balance at the affected site, which can be achieved through the external supply of endogenous antioxidants.

AREAS COVERED

This review discusses possible strategies for dermal delivery of the antioxidant vitamins E and C, as oral supplementation has proved insufficient. These antioxidants have low skin bioavailability, owing to their poor solubility, inefficient skin permeability, or instability during storage. These drawbacks can be overcome by various approaches, such as chemical modification of the vitamins and the use of new colloidal drug delivery systems. New knowledge is included about the importance of: enhancing the endogenous skin antioxidant defense through external supply; the balance between various skin antioxidants; factors that can improve the skin bioavailability of antioxidants; and new delivery systems, such as microemulsions, used to deliver vitamins C and E into the skin simultaneously.

EXPERT OPINION

A promising strategy for enhancing skin protection from oxidative stress is to support the endogenous antioxidant system, with antioxidants containing products that are normally present in the skin.

Drug delivery through the skin: molecular simulations of barrier lipids to design more effective noninvasive dermal and transdermal delivery systems for small molecules, biologics, and cosmetics

The delivery of drugs through the skin provides a convenient route of administration that is often preferable to injection because it is noninvasive and can typically be self-administered. These two factors alone result in a significant reduction of medical complications and improvement in patient compliance. Unfortunately, a significant obstacle to dermal and transdermal drug delivery alike is the resilient barrier that the epidermal layers of the skin, primarily the stratum corneum, presents for the diffusion of exogenous chemical agents. Further advancement of transdermal drug delivery requires the development of novel delivery systems that are suitable for modern, macromolecular protein and nucleotide therapeutic agents. Significant effort has already been devoted to obtain a functional understanding of the physical barrier properties imparted by the epidermis, specifically the membrane structures of the stratum corneum. However, structural observations of membrane systems are often hindered by low resolutions, making it difficult to resolve the molecular mechanisms related to interactions between lipids found within the stratum corneum. Several models describing the molecular diffusion of drug molecules through the stratum corneum have now been postulated, where chemical permeation enhancers are thought to disrupt the underlying lipid structure, resulting in enhanced permeability. Recent investigations using biphasic vesicles also suggested a possibility for novel mechanisms involving the formation of complex polymorphic lipid phases. In this review, we discuss the advantages and limitations of permeation-enhancing strategies and how computational simulations, at the atomic scale, coupled with physical observations can provide insight into the mechanisms of diffusion through the stratum corneum.

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.

Topical and mucosal liposomes for vaccine delivery

Mucosal (and in minor extent transcutanous) stimulation can induce local or distant mucosa secretory IgA. Liposomes and other vesicles as mucosal and transcutaneous adjuvants are attractive alternatives to parenteral vaccination. Liposomes can be massively produced under good manufacturing practices and stored for long periods, at high antigen/vesicle mass ratios. However, their uptake by antigen-presenting cells (APC) at the inductive sites remains as a major challenge. As neurotoxicity is a major concern in intranasal delivery, complexes between archaeosomes and calcium as well as cationic liposomes complexed with plasmids encoding for antigenic proteins could safely elicit secretory and systemic antigen-specific immune responses. Oral bilosomes generate intense immune responses that remain to be tested against challenge, but the admixing with toxins or derivatives is mandatory to reduce the amount of antigen. Most of the current experimental designs, however, underestimate the mucus blanket 100- to 1000-fold thicker than a 100-nm diameter liposome, which has first to be penetrated to access the underlying M cells. Overall, designing mucoadhesive chemoenzymatic resistant liposomes, or selectively targeted to M cells, has produced less relevant results than tailoring the liposomes to make them mucus penetrating. Opposing, the nearly 10 µm thickness stratum corneum interposed between liposomes and underlying APC can be surpassed by ultradeformable liposomes (UDL), with lipid matrices that penetrate up to the limit with the viable epidermis. UDL made of phospholipids and detergents, proved to be better transfection agents than conventional liposomes and niosomes, without the toxicity of ethosomes, in the absence of classical immunomodulators.

Effect of Edge Activator on Characteristic and in Vitro Skin Permeation of Meloxicam Loaded in Elastic Liposomes

The aim of this study was to prepare and investigate the potential use of liposomes in the transdermal drug delivery of meloxicam (MX). The vesicles containing a constant amount of MX, phosphatidylcholine (PC), cholesterol (Chol) and cetylpyridinium chloride (CPC) (1:5:1:1 MX/PC/Chol/CPC molar ratio) to obtain liposomes. MX loaded liposomes were investigated for particle size, zeta potential, entrapment efficiency (%EE) and in vitro skin permeation. The results indicated that the liposomes were spherical in structure, 77 to 100 nm in size and charged. The %EE of MX in the vesicles ranged from 55 to 56%. The elastic liposomes consisting of MX/PC/Chol/CPC provided a significantly higher skin permeation of MX compared to the other formulations. Fourier Transform Infrared Spectroscopy (FT-IR) and Differential Scanning Calorimetry (DSC) analysis indicated that the application of liposomes may disrupt the stratum corneum lipid. Our research suggests that MX loaded elastic liposomes can be potentially used as a transdermal drug delivery system.