A propagating heat wave model of skin electroporation

The main barrier to transdermal drug delivery in human skin is the stratum corneum. Pulsed electric fields (PEFs) of sufficient amplitude can create new aqueous pathways across this barrier and enhance drug delivery through the skin. Here, we describe a model of pore formation between adjacent corneocytes that predicts the following sequence of events: (1) the PEF rapidly charges the stratum corneum near the electrode until the transepidermal potential difference is large enough to drive water into a small region of the stratum corneum, creating new aqueous pathways. (2) PEFs then drive a high current density through this newly created electropore to generate Joule heating that warms the pore perimeter. (3) This temperature rise at the perimeter increases the probability of further electroporation there as the local sphingolipids reach their phase transition temperature. (4) This heat-generated wave of further electroporation propagates outward until the surface area of the pore becomes so large that the reduced current density no longer generates sufficient heat to reach the phase transition temperature of the sphingolipids. (5) Cooling and partial recovery occurs after the field pulse.This process yields large, high permeability regions in the stratum corneum at which molecules can more readily cross this skin barrier. We present a model for this process that predicts that the initial radius of the first aqueous pathway is approximately 5 nm for a transdermal voltage of 60 V at room temperature.     

Transdermal drug delivery

Transdermal drug delivery has made an important contribution to medical practice, but has yet to fully achieve its potential as an alternative to oral delivery and hypodermic injections. First-generation transdermal delivery systems have continued their steady increase in clinical use for delivery of small, lipophilic, low-dose drugs. Second-generation delivery systems using chemical enhancers, noncavitational ultrasound and iontophoresis have also resulted in clinical products; the ability of iontophoresis to control delivery rates in real time provides added functionality. Third-generation delivery systems target their effects to skin's barrier layer of stratum corneum using microneedles, thermal ablation, microdermabrasion, electroporation and cavitational ultrasound. Microneedles and thermal ablation are currently progressing through clinical trials for delivery of macromolecules and vaccines, such as insulin, parathyroid hormone and influenza vaccine. Using these novel second- and third-generation enhancement strategies, transdermal delivery is poised to significantly increase its impact on medicine.     

Elastic Vesicles as a Tool for Dermal and Transdermal Delivery

The main problem in delivery of drugs across the skin is the barrier function of the skin, which is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid layers, the so-called lipid lamellae. When applying drugs onto the skin, the major penetration pathway is the tortuous intercellular route along the lipid lamellae. In order to increase the number of drugs administered via the transdermal route, novel drug delivery systems have to be designed. Among these systems are iontophoresis, electroporation, microneedles, and vesicular systems.     

Elastic vesicles as a tool for dermal and transdermal delivery

The main problem in delivery of drugs across the skin is the barrier function of the skin, which is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid layers, the so-called lipid lamellae. When applying drugs onto the skin, the major penetration pathway is the tortuous intercellular route along the lipid lamellae. In order to increase the number of drugs administered via the transdermal route, novel drug delivery systems have to be designed. Among these systems are iontophoresis, electroporation, microneedles, and vesicular systems.     

Synergistic Transdermal Delivery of Biomacromolecules Using Sonophoresis after Microneedle Treatment

Transdermal drug delivery systems have been studied as an attractive alternative to conventional delivery routes. However, the outermost layer of the skin, the stratum corneum, acts as a primary barrier to drug delivery. A synergistic combination of microneedles (MNs) and low-frequency ultrasound (U) was used to enhance the penetration of siRNA and ovalbumin. The specific gene knockdown caused by siRNAs through the RNA interference pathway is more stable when delivered via the transdermal route. Ovalbumin, a representative adjuvant, causes a more efficient immune response in the skin because of the numerous immune cells in the skin. The synergistic transdermal delivery resulted in approximately 7 times and 15 times greater penetration of siRNA and ovalbumin respectively than in their respective negative controls, and histological analysis showed minimal invasion. Thus, as the synergistic transdermal delivery enhanced the penetration of biomacromolecules into the skin, this technique is expected to yield a promising technology for a transdermal drug delivery system.     

综述——纳米材料与药物输递：经皮给药中纳米制剂与皮肤的质构效关系

皮肤是人体最大的器官,也为药物的递送提供了重要途径。经皮给药是药物以皮肤为媒介,透过皮肤吸收的途径。因此,皮肤角质层是经皮给药的最大限速障碍。纳米经皮给药系统,具有提高透皮效率、缓释性、避免药物肝首过效应、减少副作用等优点,是通过纳米制剂与皮肤组织之间的相互作用实现的。其中,纳米制剂的结构和组分与其发挥皮肤促渗效用密切相关。对纳米制剂与皮肤质构效关系深入透彻的了解,有助于新型透皮纳米制剂的设计,并利用综合手段构建安全、高效、实用的经皮给药系统。     

Using the method of homogenization to calculate the effective diffusivity of the stratum corneum with permeable corneocytes

The stratum corneum is the outermost layer of the skin, which acts as a barrier membrane against the penetration of molecules into and out of the body. It has a biphasic structure consisting of keratinized cells (corneocytes) that are embedded in a lipid matrix. The macroscopic transport properties of the stratum corneum are functions of its microstructure and the transport properties of the corneocytes and the lipid matrix, and are of considerable interest in the context of transdermal drug delivery and quantifying exposure to toxins, as well as for determining the relation of skin disorders to disruption of the stratum corneum barrier. Due to the complexity of the tissue and the difference in length scales involved in its microstructure, a direct analysis of the mass transport properties of the stratum corneum is not feasible. In this study, we undertake an approach where the macroscopic diffusion tensor of the stratum corneum is obtained through homogenization using the method of asymptotic expansions. The biphasic structure of the stratum corneum is fully accounted for by allowing the corneocytes to be permeable and considering the partitioning between the corneocytes and the lipid phases. By systematically exploring the effect of permeable corneocytes on the macroscopic transport properties of the stratum corneum, we show that solute properties such as lipophilicity and relative permeabilities in the two phases have large effects on its transdermal diffusion behavior.     

Comparison of rat epidermal keratinocyte organotypic culture (ROC) with intact human skin: Lipid composition and thermal phase behavior of the stratum corneum

The present report is a part of our continuing efforts to explore the utility of the rat epidermal keratinocyte organotypic culture (ROC) as an alternative model to human skin in transdermal drug delivery and skin irritation studies of new chemical entities and formulations. The aim of the present study was to compare the stratum corneum lipid content of ROC with the corresponding material from human skin. The lipid composition was determined by thin-layer chromatography (TLC) and mass-spectrometry, and the thermal phase transitions of stratum corneum were studied by differential scanning calorimetry (DSC). All major lipid classes of the stratum corneum were present in ROC in a similar ratio as found in human stratum corneum. Compared to human skin, the level of non-hydroxyacid-sphingosine ceramide (NS) was increased in ROC, while α-hydroxyacid-phytosphingosine ceramide (AP) and non-hydroxyacid-phytosphingosine ceramides (NP) were absent. Also some alterations in fatty acid profiles of ROC ceramides were noted, e.g., esterified ω-hydroxyacid-sphingosine contained increased levels of oleic acid instead of linoleic acid. The fraction of lipids covalently bound to corneocyte proteins was distinctly lower in ROC compared to human skin, in agreement with the results from DSC. ROC underwent a lipid lamellar order to disorder transition (T₂) at a slightly lower temperature (68 °C) than human skin (74 °C). These differences in stratum corneum lipid composition and the thermal phase transitions may explain the minor differences previously observed in drug permeation between ROC and human skin.     

The nanoscopic molecular pathway through human skin

BackgroundKnowledge regarding the barrier properties of human skin is important for understanding skin pathology, developing of transdermal drug delivery systems and computational skin absorption models; however, the molecular pathways through human skin remains to be fully investigated on a nanoscopic level. In particular the nanoscopic pathway of molecules passing the intercellular lipid bilayers separating the corneocytes in the stratum corneum (SC) is not fully elucidated.MethodsUsing stimulated emission depletion microscopy (STED) and Förster resonance energy transfer (FRET) the molecular pathways through the SC, the main barrier of the skin, are determined for lipophilic and water-soluble molecules at a nanoscopic resolution.ResultsUsing STED and confocal microscopy, water-soluble dyes, were observed to be present in both the corneocytes and in the intercellular lipid matrix, whereas the lipophilic dyes were predominately in the intercellular lipid bilayers. FRET was observed in the SC between the lipophilic and water-soluble dyes, the existence of a minimum possible distance between acceptor and donor molecules of 4.0 ± 0.1 nm was found.ConclusionsThe results indicate that lipophilic molecules penetrate the stratum corneum via the intercellular lipids bilayers separating the corneocytes in the SC, while the more water-soluble molecules penetrate the stratum corneum via the transcellular route through the corneocytes and intercellular lipid bilayers via the polar head groups of lipid molecules in the bilayers.General significanceKnowledge of the nanoscopic molecular pathways through human skin will help understand the skin barrier function and will be of use for computational skin absorption models and transdermal drug delivery strategies.     

Interactions of inertial cavitation bubbles with stratum corneum lipid bilayers during low-frequency sonophoresis

Interactions of acoustic cavitation bubbles with biological tissues play an important role in biomedical applications of ultrasound. Acoustic cavitation plays a particularly important role in enhancing transdermal transport of macromolecules, thereby offering a noninvasive mode of drug delivery (sonophoresis). Ultrasound-enhanced transdermal transport is mediated by inertial cavitation, where collapses of cavitation bubbles microscopically disrupt the lipid bilayers of the stratum corneum. In this study, we describe a theoretical analysis of the interactions of cavitation bubbles with the stratum corneum lipid bilayers. Three modes of bubble-stratum corneum interactions including shock wave emission, microjet penetration into the stratum corneum, and impact of microjet on the stratum corneum are considered. By relating the mechanical effects of these events on the stratum corneum structure, the relationship between the number of cavitation events and collapse pressures with experimentally measured increase in skin permeability was established. Theoretical predictions were compared to experimentally measured parameters of cavitation events.     

Development and characterization of a novel controlled release drug delivery system based on nanostructured lipid carriers gel for meloxicam

Aim

The main objective of the current investigation was to develop nanostructured lipid carriers (NLC) based gel for the enhancement of transdermal absorption of meloxicam (MLX) to achieve local as well as systemic drug action without concurrent gastrointestinal toxicity.

Main methods

NLC gel containing MLX was prepared and characterized for particle size, polydispersity, zeta potential, pH, rheology, entrapment efficiency, occlusion factor, and thermal behavior. In vitro drug release, in vitro skin permeation and deposition studies were carried out using Franz diffusion cells. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) of MLX-NLC gel treated stratum corneum (SC) were undertaken to get an insight into the skin permeation enhancement mechanism of MLX-NLC gel. Toxicity potential of the developed gel formulation was assessed by in vitro hemolysis and histopathological examinations. The rat paw edema test was performed to evaluate the anti-inflammatory activity of MLX-NLC gel.

Key findings

MLX-NLC gel demonstrated sustained release and enhanced the skin permeation and deposition of meloxicam especially into the dermis in comparison to meloxicam gel (control). MLX-NLC had an impact on the barrier properties of the skin and acted via protein and lipid modifications in the stratum corneum. MLX-NLC gel turned out to be hemocompatible, non-irritant, and non toxic with significant anti-inflammatory activity.

Significance

The results suggest that NLC gel could be a promising carrier for the transdermal delivery of meloxicam.     

Nanoemulsion based gel for transdermal delivery of meloxicam: Physico-chemical,mechanistic investigation

AimsThe aim of the present investigation was to develop a nanoemulsion (NE) gel formulation for the transdermal delivery of meloxicam (MLX) in order to ensure maximum controlled and sustained drug release capacity.Main methodsThe MLX containing NE gel was prepared and characterized for particle size, zeta potential, pH, rheology, in vitro drug release, in vitro skin permeation, and in vitro hemolysis. Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) of MLX-NE gel treated rat skin was performed to investigate the skin permeation mechanism of meloxicam from NE gel. Skin permeation potential of the developed gel formulation was assessed using confocal laser scanning microscopy (CLSM). The in vivo toxicity of MLX-NE gel was assessed by histopathological examination in rat. The rat paw edema test was performed to evaluate the anti-inflammatory activity of MLX-NE gel.Key findingsPercutaneous absorption studies demonstrated a higher permeation of meloxicam from NE gel, than the drug solution. FTIR and DSC studies supported stratum corneum lipid extraction as a possible penetration enhancer mechanism for MLX-NE gel. CLSM studies confirmed the permeation of the NE gel formulation to the deeper layers of the skin (up to 130 μm). MLX-NE gel turned out to be non-irritant, biocompatible, and provided maximum inhibition of paw edema in rats over 24 h in contrast to MLX solution.SignificanceThe nanoemulsion gel formulation may hold promise as an effective alternative for the transdermal delivery of meloxicam.     

Fluidity enhancement: a critical factor for performance of liposomal transdermal drug delivery system

Liposomes are well known lipid carriers for drug delivery of bioactive molecules encapsulated inside their membrane. Liposomes as skin drug delivery systems were initially promoted primarily for localized effects with minimal systemic delivery. Subsequently, a novel vesicular system, transferosomes was reported for transdermal delivery with efficiency similar to subcutaneous injection. The multiple bilayered organizations of lipids applied in these vesicles structure are somewhat similar to complex nature of stratum corneal intercellular lipids domains. The incorporation of novel agents into these lipid vesicles results in the loss of entrapped markers but it is similar to fluidization of stratum corneum lipids on treatment with a penetration enhancer. This approach generated the utility of penetration enhancers/fluidizing agents in lipids vesicular systems for skin delivery. For the transdermal and topical applications of liposomes, fluidity of bilayer lipid membrane is rate limiting which governs the permeation. This article critically reviews the relevance of using different types of vesicles as a model for skin in permeation enhancement studies. This study has also been designed to encompass all enhancement measurements and analytical tools for characterization of permeability in liposomal vesicular system.     

Characterization of electric-pulse-induced permeabilization of porcine skin using surface electrodes

We measured the transient and long-term changes of permeability of full-thickness porcine skin after the application of a single or a train of electric pulses, as the basis for optimization of the electrical parameters for enhancing transdermal drug or gene delivery by electroporation. Two electrodes were attached to the stratum corneum of excised skin for transdermal electric pulse delivery and impedance measurement. Both transient and long-term permeabilization were found to be dependent on the electrical exposure dose, i.e., the product of pulse voltage and cumulative pulsing (exposure) time. Skin resistance dropped to about 20% of its prepulsing value when pulsed beyond a critical dosage of 0.4 V-s (with 20-40 V across each skin path), but recovered rapidly within seconds after the pulse. Long-term permeabilization of the skin required repeated pulsing with a minimum potential of 160 V (80 V across each skin path). The maximum long-term resistance drop, to 35% of the initial value, required a dose greater than 200 V-s, recovering slowly and seldom completely in tens of minutes to hours. The decrease and recovery of the resistance were dependent on the frequency and pulse length only for low-dose electrical exposure.     

Development of triptolide-nanoemulsion gels for percutaneous administration: physicochemical,transport, pharmacokinetic and pharmacodynamic characteristics

Background

This work aimed to provide useful information on the use of nanoemulsions for the percutaneous administration of triptolide. Lipid nanosystems have great potential for transdermal drug delivery. Nanoemulsions and nanoemulsion gels were prepared to enhance percutaneous permeation. Microstructure and in vitro/in vivo percutaneous delivery characteristics of triptolide (TPL)-nanoemulsions and TPL-nanoemulsion gels were compared. The integrity of the nanoemulsions and nanoemulsion gels during transdermal delivery and its effects on the surface of skin were also investigated. The penetration mechanisms of nanoemulsions and nanoemulsion gels were investigated by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). The transport characteristics of fluorescence-labelled nanoemulsions were probed using laser scanning confocal microscopy. A chronic dermatitis/eczema model in mice ears and the pharmacodynamic of the TPL-nanoemulsion gels were also investigated.

Results

Compared to TPL gels, significantly greater cumulative amounts of TPL-nanoemulsion gels and TPL-nanoemulsions penetrated rat skin in vitro. The in vivo microdialysis showed the concentration–time curve AUC_0–t for TPL-NPs is bigger than the TPL-gels. At the same time, TPL-NPs had a larger effect on the surface of skin. By hydrating keratin and changing the structure of both the stratum corneum lipids and keratin, nanoemulsions and nanoemulsion gels influence skin to promote percutaneous drug penetration. Both hairfollicles and the stratum corneum are also important in this transdermal drug delivery system. Moderate and high dosages of the TPL-nanoemulsion gels can significantly improve the symptoms of dermatitis/eczema inflammation and edema erythematic in mice ears and can reduce the expression of IFN-γ and IL-4. Moreover, the TPL-nanoemulsion gels cause less gastrointestinal damage than that of the Tripterygium wilfordii oral tablet does.

Conclusions

Nanoemulsions could be suitable for transdermal stably releasing drugs and maintaining the effective drug concentration. The TPL-nanoemulsion gels provided higher percutaneous amounts than other carriers did. These findings suggest that nanoemulsion gels could be promising percutaneous carriers for TPL. The TPL-nanoemulsion gels have a significant treatment effect on dermatitis/eczema in the mice model and is expected to provide a new, low-toxicity and long-term preparation for the clinical treatment of dermatitis/eczema in transdermal drug delivery systems.

     

Local temperature rises influence in vivo electroporation pore development: a numerical stratum corneum lipid phase transition model

Electroporation is an approach used to enhance transdermal transport of large molecules in which the skin is exposed to a series of electric pulses. Electroporation temporarily destabilizes the structure of the outer skin layer, the stratum corneum, by creating microscopic pores through which agents, ordinarily unable to pass into the skin, are able to pass through this outer barrier. Long duration electroporation pulses can cause localized temperature rises, which result in thermotropic phase transitions within the lipid bilayer matrix of the stratum corneum. This paper focuses on electroporation pore development resulting from localized Joule heating. This study presents a theoretical model of electroporation, which incorporates stratum corneum lipid melting with electrical and thermal energy equations. A transient finite volume model is developed representing electroporation of in vivo human skin, in which stratum corneum lipid phase transitions are modeled as a series of melting processes. The results confirm that applied voltage to the skin results in high current densities within the less resistive regions of the stratum corneum. The model captures highly localized Joule heating within the stratum corneum and subsequent temperature rises, which propagate radially outward. Electroporation pore development resulting from the decrease in resistance associated with lipid melting is captured by the lipid phase transition model. As the effective pore radius grows, current density and subsequent Joule heating values decrease.     

Fabrication,Physicochemical Characterization,and Performance Evaluation of Biodegradable Polymeric Microneedle Patch System for Enhanced Transcutaneous Flux of High Molecular Weight Therapeutics

A revolutionary paradigm shift is being observed currently, towards the use of therapeutic biologics for disease management. The present research was focused on designing an efficient dosage form for transdermal delivery of α-choriogonadotropin (high molecular weight biologic), through biodegradable polymeric microneedles. Polyvinylpyrrolidone-based biodegradable microneedle arrays loaded with high molecular weight polypeptide, α-choriogonadotropin, were fabricated for its systemic delivery via transdermal route. Varied process and formulation parameters were optimized for fabricating microneedle array, which in turn was expected to temporally rupture the stratum corneum layer of the skin, acting as a major barrier to drug delivery through transdermal route. The developed polymeric microneedles were optimized on the basis of quality attributes like mechanical strength, axial strength, insertion ratio, and insertion force analysis. The optimized polymeric microneedle arrays were characterized for in vitro drug release studies, ex vivo drug permeation studies, skin resealing studies, and in vivo pharmacokinetic studies. Results depicted that fabricated polymeric microneedle arrays with mechanical strength of above 5 N and good insertion ratio exhibited similar systemic bioavailability of α-choriogonadotropin in comparison to marketed subcutaneous injection formulation of α-choriogonadotropin. Thus, it was ultimately concluded that the designed drug delivery system can serve as an efficient tool for systemic delivery of therapeutic biologics, with an added benefit of overcoming the limitations of parenteral delivery, achieving better patient acceptability and compliance.     

Evaluation of alternative strategies to optimize ketorolac transdermal delivery

In the present study, 2 alternative strategies to optimize ketorolac transdermal delivery, namely, prodrugs (polyoxyethylene glycol ester derivatives, I–IV) and nanostructured lipid carriers (NLC) were investigated. The synthesized prodrugs were chemically stable and easily degraded to the parent drug in human plasma. Ketorolac-loaded NLC with high drug content could be successfully prepared. The obtained products formulated into gels showed a different trend of drug permeation through human stratum corneum and epidermis. Particularly, skin permeation of ester prodrugs was significantly enhanced, apart from ester IV, compared with ketorolac, while the results of drug release from NLC outlined that these carriers were ineffective in increasing ketorolac percutaneous absorption owing to a higher degree of mutual interaction between the drug and carrier lipid matrix. Polyoxyethylene glycol esterification confirmed to be a suitable approach to enhance ketorolac transdermal delivery, while NLC seemed more appropriate for sustained release owing to the possible formation of a drug reservoir into the skin. Published: August 4, 2006     

Transdermal Delivery of Proteins

Transdermal delivery of peptides and proteins avoids the disadvantages associated with the invasive parenteral route of administration and other alternative routes such as the pulmonary and nasal routes. Since proteins have a large size and are hydrophilic in nature, they cannot permeate passively across the skin due to the stratum corneum which allows the transport of only small lipophilic drug molecules. Enhancement techniques such as chemical enhancers, iontophoresis, microneedles, electroporation, sonophoresis, thermal ablation, laser ablation, radiofrequency ablation and noninvasive jet injectors aid in the delivery of proteins by overcoming the skin barrier in different ways. In this review, these enhancement techniques that can enable the transdermal delivery of proteins are discussed, including a discussion of mechanisms, sterility requirements, and commercial development of products. Combination of enhancement techniques may result in a synergistic effect allowing increased protein delivery and these are also discussed.     

The skin barrier in healthy and diseased state

The primary function of the skin is to protect the body for unwanted influences from the environment. The main barrier of the skin is located in the outermost layer of the skin, the stratum corneum. The stratum corneum consists of corneocytes surrounded by lipid regions. As most drugs applied onto the skin permeate along the lipid domains, the lipid organization is considered to be very important for the skin barrier function. It is for this reason that the lipid organization has been investigated quite extensively. Due to the exceptional stratum corneum lipid composition, with long chain ceramides, free fatty acids and cholesterol as main lipid classes, the lipid organization is different from that of other biological membranes. In stratum corneum, two lamellar phases are present with repeat distances of approximately 6 and 13 nm. Moreover the lipids in the lamellar phases form predominantly crystalline lateral phases, but most probably a subpopulation of lipids forms a liquid phase. Diseased skin is often characterized by a reduced barrier function and an altered lipid composition and organization. In order to understand the aberrant lipid organization in diseased skin, information on the relation between lipid composition and organization is crucial. However, due to its complexity and inter-individual variability, the use of native stratum corneum does not allow detailed systematic studies. To circumvent this problem, mixtures prepared with stratum corneum lipids can be used. In this paper first the lipid organization in stratum corneum of normal and diseased skin is described. Then the role the various lipid classes play in stratum corneum lipid organization and barrier function has been discussed. Finally, the information on the role various lipid classes play in lipid phase behavior has been used to interpret the changes in lipid organization and barrier properties of diseased skin.