Transdermal Drug Permeation Research Articles

Contribution of chemical permeation enhancers to the process of transdermal drug delivery: Adsorption, microscopic interactions, and mechanism

Transdermal drug delivery (TDD) has attracted widespread attention because of the advantage of its non-invasive nature, easy self-administration, and low side effects. The key to this pathway of drug delivery is how to overcome the barrier of the lipid matrix in the stratum corneum (SC). In this work, molecular dynamics (MD) were employed to investigate the adsorption of thyrotropin-releasing hormone (TRH) on the SC, and the effects of three different chemical permeation enhancers (ethanol (ETOH), carveol (CAV), and borneol (BOR)) on the SC were analyzed. The results showed that ETOH hardly altered the order of lipids in the SC, while CAV and BOR disrupted the morphology of the SC. The primary target of CAV was the CHOL in SC, which not only disrupted the ordered arrangement of CHOL, but also “extracted” CHOL from SC. The thickness distribution of SC became more inhomogeneous in the presence of CAV and BOR, which facilitated the penetration of drug molecules. Compared to no chemical permeation enhancers, the free energy of permeation in the presence of chemical permeation enhancers was less than 4–10 kcal mol−1, which suggested that chemical permeation enhancers were more favorable for the permeation of drugs from viewpoints of thermodynamics. All the results provided theoretical insights into the effect of chemical permeation enhancers on the transdermal permeation of drugs.

Investigating the efficacy of mirtazapine-embedded invasomal gel nanocarriers via I-optimal design for management of atopic dermatitis

The objective of the present study was to optimize a novel invasomal gel (inva-gel) formulation enriched with terpene for upgrading the therapeutic action of anti-depressant mirtazapine on atopic dermatitis as an off-label use. Various factors were studied by I-optimal design to propose an optimized invasome with desirable characteristics. Its incorporation into different hydrogel bases was targeted to suggest a desirable inva-gel formulation, inspect its ex-vivo performance, and conduct in-vivo pre- and post-sacrifice assessments on atopic dermatitis-induced rats. Results revealed positive ethanol action on increasing entrapment efficiency (up to 77.88%) and decreasing size of invasomal vesicles (from 390.30 nm to 190.60 nm). Increased terpene amounts provided more space for mirtazapine incorporation with enhanced solubility and encapsulation inside invasomes. Terpene's molecular weight and lipophilicity significantly affected physicochemical characteristics of invasomes. More cross-linkages would occur by raising gel:liquid invasome ratio (from 0.5:1 to 2.5:1) and using carbopol other than hydroxylpropyl methyl cellulose (HPMC), therefore increasing rheological properties of inva-gels. Synergistic effects of phospholipid, ethanol, limonene, and carbopol to enhance mirtazapine permeation through skin were observed. Throughout the pre-sacrifice assessments, optimized inva-gel demonstrated a significant decrease in dermatitis severity score (from 8.33 to 3.00) and scratching behavior (from 184.32 s to 37.42 s) in disease-induced rats, hence showing favorable skin profile. Concerning the post-sacrifice assessments, optimized inva-gel significantly alleviated inflammation and pruritus symptoms by minimizing epidermal thickening (from 92.32 μm to 50.17 μm), down-regulating various inflammatory mediators, and decreasing infiltration of inflammatory mast cells. Besides, the ear swelling and spleen index values were markedly decreased (from 77.37 mg to 22.76 mg and from 0.56% to 0.35%, respectively). In conclusion, novel anti-depressant mirtazapine-embedded inva-gel displayed successful transdermal drug permeation offering valuable merits in atopic dermatitis management.

Innovative insight into the mechanism of enantioselective skin permeation for chiral drugs

ABSTRACT Objectives A profound comprehension of the molecular mechanisms underpinning the enantioselective transdermal permeation of chiral drugs is critical in the design and assessment of transdermal preparations. The primary objective of this study is to investigate the distinct skin permeation behaviors exhibited by enantiomers of non-steroidal anti-inflammatory drugs (NSAIDs) and elucidate the intricate molecular mechanism at play. Methods In vitro and in vivo transdermal permeation studies of chiral NSAIDs were performed using transdermal patch and solution system. Chiral interaction between NSAIDs enantiomers and synthesized chiral ceramide present in the skin was characterized to clarify the different transdermal behaviors. Results The S-enantiomers of NSAIDs exhibited higher permeability through the skin than R-enantiomer in vitro (1.5-fold) and in vivo (2.0-fold), which was attributed to a stronger interaction between S-enantiomer and ceramide caused by more favorable spatial conformations. S-enantiomer required lower activation energy (24.4 kJ/mol) and Gibbs energy (43.3 kJ/mol), which was favorable in forming the H-bond with ceramide in the skin, resulting in more permeation. Conclusion This research furnished an innovative comprehension of the molecular underpinnings governing the enantioselective permeation of drug enantiomers through the skin, fostering the minimization of undesired enantiomer ingestion (distomers) and amplifying therapeutic efficiency.

Realizing zero-order controlled transdermal drug permeation through competing doubly ionic H-bond in patch

Transdermal drug delivery system (TDDS) was an effective way to realize controlled drug delivery. However, realizing zero-order controlled drug skin delivery was still challenging in the drug-in-adhesive patch. This study provided a strategy to accomplish this delivery form by stabilizing the drug concentration in adhesive through concentration-dependent competitive interaction. Clonidine (CLO) and Granisetron (GRA) were chosen as the model drugs which were of high skin permeability, and polydimethylaminoethyl acrylate (EA) as an excipient to interact with hydroxyphenyl adhesive (HP). Drug release, permeation and pharmacokinetic study were conducted to evaluate the controlled effect of HP-EA. The molecular interaction was characterized by FT-IR, 1H NMR and XPS. Dynamic simulation and molecular docking further clarified the competitive interaction involved in the release process. Both the drug skin permeation study of CLO and GRA patch based on the HP-EA adhesive showed good zero-order fitting with r of 0.994 and 0.998, compared with non-functional adhesive (0-PSA). Furthermore, the pharmacokinetic study of the CLO patch showed a plateau phase for around 52 h without influencing the area under concentration–time curve (AUC), indicating that the HP-EA could realize zero-order drug skin delivery. The mechanism study revealed that EA serving as a ‘buffer component’ promoted the conversion of the ionic CLO to the neutrals the as the neutrals released, which stabilized ‘1% neutrals CLO concentration’. In conclusion, the drug delivery system based on the concentration-dependent competitive interaction broadened our understanding of the molecular mechanisms involved in zero-order controlled release in transdermal patches which would promote the development of zero-order drug delivery in TDDS.

Polymeric Microarray Patches for Enhanced Transdermal Delivery of the Poorly Soluble Drug Olanzapine.

Transdermal drug delivery is an alternative route of administration that offers avoidance of the associated drawbacks of orally and parenterally administered hydrophobics. However, owing to the extremely specific set of physicochemical characteristics required for passive transdermal drug permeation, the development of marketed transdermal products containing poorly soluble drugs has been severely limited. Microarray patches (MAPs) are a type of transdermal patch that differ from the traditional patch design due to the presence of tiny, micron-sized needles that permit enhanced drug permeation on their application surface. To date, MAPs have predominantly been used to deliver hydrophilic compounds. However, this work challenges this trend and focuses on the use of MAPs, in combination with commonly utilized solubility-enhancing techniques, to deliver the hydrophobic drug olanzapine (OLP) across the skin. Specifically, cyclodextrin (CD) complexation and particle size reduction were employed in tandem with hydrogel-forming and dissolving MAPs, respectively. In vivo experimentation using a female Sprague-Dawley rat model confirmed the successful delivery of OLP from hydrogel-forming MAPs (Cmax = 611.13 ± 153.34 ng/mL, Tmax = 2 h) and dissolving MAPs (Cmax = 690.56 ± 161.33 ng/mL, Tmax = 2 h) in a manner similar to that of oral therapy in terms of the rate and extent of drug absorption, as well as overall drug exposure and bioavailability. This work is the first reported use of polymeric MAPs in combination with the solubility-enhancing techniques of CD complexation and particle size reduction to successfully deliver the poorly soluble drug OLP via the transdermal route. Accordingly, this paper provides significant evidence to support an expansion of the library of molecules amenable to MAP-mediated drug delivery to include those that exhibit poor aqueous solubility.

Dermaplaning for Transdermal Drug Permeation Enhancement: A Qualitative and Quantitative Assessment.

In this study, we sought to investigate the effect of dermaplaning, a popular cosmeceutical skin rejuvenation technique on the permeation of drugs. Baclofen and diclofenac were used as hydrophilic and hydrophobic model drugs, respectively. A specific area of skin was treated with 4 strokes of a dermaplane device. Interindividual variability was assessed by having multiple users operate the device for the study. Dermaplaned skin was histologically evaluated and characterized for resistance drop and the depletion of the stratum corneum (SC). The effect of dermaplaning on drug permeation was investigated via in vitro permeation studies. Histology studies depicted the removal of SC and some parts of viable epidermis by dermaplaning. A significant drop in electrical resistance post skin dermaplaning was observed for all treatment groups, signifying the depletion of barrier properties of SC (p < 0.05). Consequently, significant drug flux and permeation were observed over 24h for the model drugs across dermaplaned skin. However, varied absorption profile was observed in vitro for both drugs across dermaplaned skin. Dermaplaning displayed a better suitability for significantly enhancing the permeation of the hydrophilic drug, baclofen. Evidence of variation in results post dermaplaning was observed amidst multiple users as well (p < 0.05).

Mechanism and Application of Chitosan and Its Derivatives in Promoting Permeation in Transdermal Drug Delivery Systems: A Review.

The mechanisms and applications of chitosan and its derivatives in transdermal drug delivery to promote drug permeation were reviewed in this paper. Specifically, we summarized the permeation-promoting mechanisms of chitosan and several of its derivatives, including changing the structure of stratum corneum proteins, acting on the tight junction of granular layers, affecting intercellular lipids, and increasing the water content of stratum corneum. These mechanisms are the reason why chitosan and its derivatives can increase the transdermal permeation of drugs. In addition, various transdermal preparations containing chitosan and its derivatives were summarized, and their respective advantages were expounded, including nanoparticles, emulsions, transdermal microneedles, nanocapsules, transdermal patches, transdermal membranes, hydrogels, liposomes, and nano-stents. The purpose of this review is to provide a theoretical basis for the further and wider application of chitosan in transdermal drug delivery systems. In the future, research results of chitosan and its derivatives in transdermal drug delivery need more support from in vivo experiments, as well as good correlation between in vitro and in vivo experiments. In conclusion, the excellent permeability-promoting property, good biocompatibility, and biodegradability of chitosan and its derivatives make them ideal materials for local transdermal drug delivery.

Nanostructured lipid carrier for transdermal gliclazide delivery: development and optimization by Box-Behnken design

The aim of current study was to prepare and optimize the nanostructured lipid carrier (NLC) for transdermal gliclazide (GCZ) delivery. The melt emulsification followed by ultrasonication technique was utilized to prepare GCZ–loaded-NLC. The optimization of NLC was achieved by Box-Behnken design. The surfactant (Tween 20) % (X1), total lipid (glyceryl monostearate + Lauroglycol™ 90) % (X2), and sonication time (min) (X3) were chosen as independent variables, while particles size (Y1), polydispersity index (Y2), and entrapment efficiency (Y3) were selected as response. Further, the prepared NLC was estimated for in vitro drug release study, ex vivo skin permeation and confocal laser scanning microscopy (CLSM) studies. Results of present study demonstrated that the optimized GCZ loaded NLC formulation presented the particles size, polydispersity index, zeta potential, and entrapment efficiency of 120.4 nm, 0.316, −5.58 mV, and 87.32% respectively. The in vitro drug release study indicated drug release of 68.27 ± 2.98% and 45.87 ± 2.85% for GCZ-NLC and conventional GCZ dispersion respectively. The GCZ-NLC-Gel formulation exhibited transdermal drug permeation of 76.89 ± 2.52% as compared to conventional gel (45.65 ± 2.79%). The CLSM of rat skin treated with NLC gel exhibited a deeper permeation (35 μm) in comparison to the conventional gel (15 μm). In conclusion, GCZ loaded NLC was successfully optimized using Box-Behnken design that contributed many advantages over conventional formulation and thus demonstrating NLC as propitious carriers for the transdermal GCZ delivery.

Transdermal Enhancement Strategy of Lappaconitine: Alteration of Keratin Configuration by Counter-Ion.

The objective of this study was to develop a lappaconitine (LA) transdermal patch with counter-ion to increase the transdermal permeability of the drug, and a theory of counter-ion altering the conformation of the skin keratin was put forward based on the in vitro skin permeation study and physicochemical properties of ion-pairs. Formulation factors including pressure sensitive adhesives (PSAs), drug-loading, counter-ions and molar ratios of counter-ion were screened by in vitro skin permeation study. The optimized formulation was composed of 7% LA, 1.5 mole cinnamic acid and AAOH(PSAcontaining hydroxyl group synthesized byour laboratory) as an adhesive matrix. The optimized patch was evaluated by the pharmacokinetic and analgesic pharmacodynamic studies. AUC0-t and pain inhibition ratio of the optimized patch were 2450.40 ± 848.52 h ng/mL and 81.18%, which showed good absorption into the skin and excellent analgesic effect. The mechanism of facilitated transdermal drug permeation by counter-ion was investigated by ATR-FTIR, thermal analysis, FTIR, XPS and molecular docking. The results indicated that after the formation of ion-pairs, the excess counter-ions would alter the conformation of the skin keratin, thus increasing the transdermal penetration of LA. In conclusion, the LA patch was successfully optimized, and the effect of counter-ions on the skin was clarified at the molecular level. These findings provided additional references for the application of counter-ion in the transdermal drug delivery system.

EFFECT OF ESSENTIAL OILS ON TRANSDERMAL PERMEATION OF METOPROLOL SUCCINATE

The objective of the present study was to prepare a matrix-type transdermal drug delivery system containing metoprolol succinate and to evaluate the effect of essential oils on transdermal permeation of drug. The solvent evaporation method was employed to fabricate transdermal patches of drug using HPMC K15M and Acrycoat L100 as polymers and polyethylene glycol 400 as a plasticizer. Prepared patches were subjected for evaluation of physical as well as physicochemical properties, moisture characteristics, and ex-vivo drug permeation studies. The results of physical characterization showed uniform casting and good appearance of patches. Satisfactory mechanical strength was revealed in the results of tensile strength and folding endurance. Ex-vivo permeation study indicated enhancing effect of essential oils on drug permeation through the skin. The maximum flux observed was 94 μg/cm2.hr achieved by formulation M3 containing lemongrass oil (20%). The results suggested that lemongrass oil was effective at higher concentration (20%) after 14 hrs of permeation while peppermint oil and eucalyptus oil were effective at low concentration (10%). Release kinetic suggested diffusion-controlled release of drug which followed the Korsmeyer-Peppas model. In conclusion, essential oils can be utilized as a safe and effective alternative for the permeation enhancement of drugs through transdermal delivery.

Investigation of muscone as transdermal penetration enhancer: Enhancing activity and molecular mechanisms

The aim of this present study was to investigate the enhancing activities of animal-derived muscone on the transdermal permeation of drugs with different lipophilicities and shed light on its possible mechanisms of action. The epidermal keratinocytes and dermal fibroblast cell lines were employed to evaluate the cytotoxicity of muscone using MTT assay. A series of model drugs with a wide range of lipophilicity were tested using in vitro permeation studies in which Franz diffusion cells and rat skin were used. The saturation solubilities and SC/vehicle partition coefficients of model drugs were measured to monitor the effect of muscone on drug thermodynamic activities and partition of drug into SC layer, respectively. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and Raman spectroscopy were used to understand the effect of muscone on molecular organization of the stratum corneum (SC). It was found that muscone displayed lower cytotoxicity than the commonly-used and standard penetration enhancer Azone. Meanwhile, muscone could significantly promote the percutaneous absorption of all of five model drugs. The molecular mechanism studies revealed that muscone could enhance the skin permeability predominantly by interacting with the SC lipids, especially the disorder of the lipid bilayer packing. These results suggested that muscone could be a safe and efficient penetration enhancer in the external use.

Dense nanolipid fluid dispersions comprising ibuprofen: Single step extrusion process and drug properties

Dense nanolipid fluid (DNLF) dispersions are highly concentrated aqueous dispersions of lipid nanocarriers (LNCs) with more than 1015 lipid particles per cubic centimeter. Descriptions of dense nanolipid fluid dispersions in the scientific literature are rare, and they have not been used to encapsulate drugs. In this paper we describe the synthesis of DNLF dispersions comprising ibuprofen using a recently described twin-screw extrusion process. We report that such dispersions are stable, bind ibuprofen tightly and yet provide high transdermal drug permeation. Ibuprofen DNLF dispersions prepared according to the present study provide up to five times greater flux of the pharmacologically active S-ibuprofen isomer through human skin than a commercially available racemic ibuprofen emulsion product. We demonstrate scaling up the twin-screw extrusion method to pilot production for a stable, highly permeating ibuprofen DNLF composition based on excipients approved by the US FDA for use in topical products as a key step towards development of a commercially viable, FDA approvable topical ibuprofen medicine to treat osteoarthritis, which has never before been accomplished.

EFFECT OF ESSENTIAL OILS ON TRANSDERMAL PERMEATION OF METOPROLOL SUCCINATE

The objective of the present study was to prepare a matrix-type transdermal drug delivery system containing metoprolol succinate and to evaluate the effect of essential oils on transdermal permeation of drug. The solvent evaporation method was employed to fabricate transdermal patches of drug using HPMC K15M and Acrycoat L100 as polymers and polyethylene glycol 400 as a plasticizer. Prepared patches were subjected for evaluation of physical as well as physicochemical properties, moisture characteristics, and ex-vivo drug permeation studies. The results of physical characterization showed uniform casting and good appearance of patches. Satisfactory mechanical strength was revealed in the results of tensile strength and folding endurance. Ex-vivo permeation study indicated enhancing effect of essential oils on drug permeation through the skin. The maximum flux observed was 94 μg/cm2 hr achieved by formulation M3 containing lemongrass oil (20 %). The results suggested that lemongrass oil was effective at higher concentration (20%) after 14 hours of permeation while peppermint oil and eucalyptus oil were effective at low concentration (10%). Release kinetic suggested diffusion-controlled release of drug which followed the Korsmeyer-Peppas model. In conclusion, essential oils can be utilized as a safe and effective alternative for the permeation enhancement of drugs through transdermal delivery.

Applications of cold atmospheric plasma for transdermal drug delivery: a review.

Although transdermal drug delivery would be very useful for the treatment of many diseases, in practice it is difficult to accomplish for the obstruction of the stratum corneum. The application of cold atmospheric plasma (CAP) as a pretreatment to the skin surface helps to enhance the delivery of topically applied drugs into the skin and the systemic circulation. CAP can change the skin properties to improve drug penetration by various different effects based on its multiple components. This review first introduces the skin barrier properties and some traditional transdermal drug delivery strategies. Next what is known about the application of CAP in transdermal drug delivery has been summarized, including the mechanisms and possible side effects. We believe that CAP could be developed as a non-invasive and efficient pretreatment to improve the transdermal permeation of drugs in clinical practice, although more research needs to be done to overcome the challenges. Graphical Abstract.

Multiscale study on the enhancing effect and mechanism of borneolum on transdermal permeation of drugs with different log P values and molecular sizes.

D-borneolum is commonly used as a permeation enhancer in Traditional Chinese Medicine (TCM) formulas for transdermal application. Additionally, two other sources of borneolums were recorded in the 2015 edition of the Chinese Pharmacopoeia (ChP), including L-borneolum and borneolum syntheticum. To guide the selection and application of borneolum, the safety and enhancing effect of three sources of borneolums were investigated on transdermal permeation of compounds with different octanol-water partition coefficient (log P) values and molecular weights (MWs). Both the results of cellular cytotoxicity and in vitro transdermal permeation experiments showed that all three sources of borneolums could be applied in TDDS as permeation enhancers. Moreover, all three sources of borneolums achieved optimal permeation-enhancing performances on transdermal drugs with lower log P values as well as higher MWs. Further study was carried out to elucidate the potential molecular mechanism of borneolum enhancing transdermal drug delivery via transmission electron microscope (TEM) and coarse-grained molecular dynamic (CG-MD) simulation. Borneolum significantly promoted transdermal delivery of drugs via changing the dense morphology of the stratum corneum (SC), disturbing the ordered arrangement of ceramide (CER) and free fatty acid (FFA) molecules in lipid layers, and further increasing the diffusion rate of drugs in the lipid layers.

Drug/Vehicle Impacts and Formulation Centered Stratagems for Enhanced Transdermal Drug Permeation, Controlled Release and Safety: Unparalleled Past and Recent Innovations-An Overview

:Transdermal drug delivery systems (TDDS) are one of the fascinating unconventional drug delivery systems offering plentiful advantages of which patient compliance is of paramount importance. However, as a matter of fact, the transdermal delivery of drug molecules is absolutely a tedious job which is precisely influenced by a number of factors including penetration barrier properties of the skin, drug characteristics formulation allied issues, etc. Over the years, innumerable tremendous efforts have been made in transporting the drugs through the skin into the systemic circulation by noteworthy tactics. This paper discusses such revolutionary formulation based techniques that have been endeavored in achieving the enhanced skin permeation of drugs, controlled release, and safety.

Mechanistic insights into high permeation vesicle-mediated synergistic enhancement of transdermal drug permeation.

Aim: To design a nanocarrier platform for enhanced transdermal drug permeation. Materials& methods: Gel-based high permeation vesicles (HPVs) were developed and their performance in terms of transdermal flux improvement, in vitro release and skin irritancy was assessed. The mechanistic insights of permeation enhancement were explored using confocal laser scanning microscopy, ATR-FTIR, DSC and P31 NMR. Results: HPVs exhibited as vesicles with uniform size (∼150nm), extended drug-release profile (∼48h) and improved transdermal flux. HPVs were also nontoxic and nonirritant to skin. Enhanced vesicle deformability, improved vesicle membrane fluidity and synergistic permeation enhancement action of synergistic combination of permeation enhancer components was found to be responsible for HPV-mediated permeation enhancement. Conclusion: Overall, the study established that HPVs demonstrate a promising therapeutic advantage over conventional transdermal drug carriers.

Utility of Three-Dimensional Skin From Human-Induced Pluripotent Stem Cells as a Tool to Evaluate Transdermal Drug Permeation

Transdermal drug delivery is an attractive route for administration of drugs, and it offers several advantages such as painless administration. To accurately predict the rate of human skin permeation for new transdermal drug formulations, we developed a novel assessment system using induced pluripotent stem cells (iPSCs). Skin was generated from iPSC-derived keratinocytes and fibroblasts. In the histological and immunohistochemical examination, cellular markers (keratin 14 and keratin 10) for the epidermal basal and suprabasal layers were clearly detected within the multilayer structures produced in the human iPSC-based three-dimensional skin model. The results from our permeation study indicate that an initial lag time exists during permeation of 5(6)-carboxyfluorescein and fluorescein isothiocyanate dextran 4000. Furthermore, the permeation for these model drugs in human iPSC-based skin was inversely proportional to the molecular weight of the drugs. These results of the present iPSC-based skin are useful basic information as a first step for developing a new assessment system to predict the efficacy of drug permeation in human skin by using iPSC-based skin.

Mechanisms of white mustard seed (Sinapis alba L.) volatile oils as transdermal penetration enhancers

We investigated the transdermal drug permeation enhancement properties and associated mechanisms of white mustard (Sinapis alba L.) seed volatile oil (SVO). Using gas chromatography–mass spectrometry, we showed that SVO was composed primarily of allylisothiocyanate and isothiocyanatocyclopropane. Compared with azone, SVO had better penetration-enhancing effects on three model drugs (5-Fluorouracil, Osthole, and Paeonol), with each having different oil-water partition coefficients. Histopathology showed that SVO did not induce skin irritation when the concentration was lower than 2% (v/v), and it induced less irritation than azone. According to attenuated total reflection-Fourier transform infrared spectroscopy and transmission electron microscopy, SVO induced skin lipid structural disorder and increased the distance between the stratum corneum, which is beneficial to the penetration of drugs. Cellular experiments showed that SVO inhibited Ca2+-ATPase activity, increased intracellular Ca2+ concentration, and changed the membrane potential in HaCaT cells, which promoted drug transfer into the skin. Our findings reveal that SVO is a safe and efficient natural product that has great potential as skin penetration enhancer.

Effect of the Dispersion States of Azone in Hydroalcoholic Gels on Its Transdermal Permeation Enhancement Efficacy

The objective of this study was to investigate the effect of dispersion states of azone in gels on the transdermal permeation of levamisole hydrochloride (LH). LH hydroalcoholic gels containing azone of different dispersion states were prepared by varying the contents of azone and Tween 80, and the in vitro transdermal permeation of LH across excised rat skin was evaluated. Depending on the content of azone, mixed solvents, and solubilizer used, azone presented as dissolved molecules, solubilized in micelles, and fine or coarse emulsion droplets in gels. Dramatically increased transdermal permeation of LH within the azone contents between 0.25% and 0.75% indicated high transdermal enhancement efficiency of the molecular or micellar azone, and extra azone that existed as oil droplets did not fully exert transdermal penetration enhancement of LH. Although solubilizer (Tween 80) can greatly increase the solubility of azone, only small amount of Tween 80 (0.5%) in the gel significantly increased the steady-state flux of LH. Addition of extra amount of Tween 80 (>0.5%) reduced the amount of azone distributed in the skin, and thus decreased the transdermal drug permeation. The results partly elucidated the versatile effects of the dispersion states of azone on the transdermal permeation of hydrophilic drug from semisolid gels.