Effect of fatty acid diesters on permeation of anti-inflammatory drugs through rat skin

Carbon dioxide foam bubbles enhance skin penetration through the stratum corneum layer with mechanical mechanism

Topical skin formulations often include penetration enhancers that interact with the outer stratum corneum (SC) layer to chemically enhance diffusion. Alternatively, penetration can be mechanically enhanced with simple rubbing in the presence of solid particles sometimes included to exfoliate the top layers of the SC. Our goal was to evaluate micron-sized carbon dioxide bubbles included in a foamed moisturizing formulation as a mechanical penetration enhancement strategy. We show that moisturizing foam bubbles cause an increase in SC formulation penetration using both mechanical and spectroscopic characterization. Our results suggest viscous liquid film drainage between coalescing gaseous bubbles creates local regions of increased hydrodynamic pressure in the foam liquid layer adjacent to the SC surface that enhances treatment penetration. An SC molecular diffusion model is used to rationalize the observed behavior. The findings indicate marked increased levels of treatment concentration in the SC at 2 h and that persists to 18 h after exposure, far exceeding non-foamed treatments. The study suggests an alternate strategy for increasing formulation penetration with a non-chemical mechanism.

Optimization and Evaluation of the In Vitro Permeation Parameters of Topical Products with Non-Steroidal Anti-Inflammatory Drugs through Strat-M® Membrane

Pharmaceutical products containing non-steroidal anti-inflammatory drugs (NSAIDs) are among the most prescribed topical formulations used for analgesic and antirheumatic properties. These drugs must overcome the skin barrier to cause a therapeutic effect. Human skin has been widely used as a model to study in vitro drug diffusion and permeation, however, it suffers from many limitations. Therefore, to perform in vitro permeation test (IVPT), we used a Strat-M® membrane with diffusion characteristics well-correlated to human skin. This study's objective was to optimize the IVPT conditions using Plackett-Burman experimental design for bio-predictive evaluation of the in vitro permeation rates of five non-steroidal anti-inflammatory drugs (diclofenac, etofenamate, ibuprofen, ketoprofen, naproxen) across Strat-M® membrane from commercial topical formulations. The Plackett-Burman factorial design was used to screen the effect of seven factors in eight runs with one additional center point. This tool allowed us to set the sensitive and discriminative IVPT final conditions that can appropriately characterize the NSAIDs formulations. The permeation rate of etofenamate (ETF) across the Strat-M® membrane was 1.7-14.8 times faster than other NSAIDs from selected semisolids but 1.6 times slower than the ETF spray formulation.

Emollient structure and chemical functionality effects on the biomechanical function of human stratum corneum

OBJECTIVE

Cosmetic emollients are widely used in skincare formulations due to their ability to 'soften' the skin and modulate formulation spreadability. Though emollients are commonly used, little is known about their effects on the biomechanical barrier properties of human stratum corneum (SC), which play a critical role in consumer perception of formulation efficacy. Accordingly, our objective was to provide new insights with a study involving fourteen cosmetic emollient molecules with widely varying structures, molecular weights, SC diffusivities, topological polar surface areas (TPSAs), viscosities and chemical functionalities.

METHODS

Mechanical stress in the SC was measured in vitro using a substrate curvature measurement technique. Stress development due to SC drying was measured before and after topical treatment with cosmetic emollients. Emollient diffusivity and alterations to lipid content in SC after treatment were measured via ATR-FTIR spectroscopy. The maximum penetration volume of emollient in SC was characterized to elucidate mechanisms underlying emollient effects on stress.

RESULTS

The application of all cosmetic emollients caused a reduction in SC mechanical stress under dehydrating conditions, and a linear correlation was discovered between emollient penetration volume and the degree of stress reduction. These molecules also induced increases in stress equilibration rate, signalling changes to SC transport kinetics. Stress equilibration rate increases linearly correlated with decreasing intensity of the νCH2 band, indicating a previously unknown interaction between cosmetic emollients and SC lipids. Stress and penetration volume results were rationalized in terms of a multi-parameter model including emollient molecular weight, diffusivity, TPSA and viscosity.

CONCLUSION

We provide a new rational basis for understanding the effects of cosmetic emollient choice on biomechanical properties affecting SC barrier function and consumer perception. We demonstrate for the first time that emollients very likely reduce SC mechanical stress through their ability to take up volume when penetrating the SC, and how molecular weight, SC diffusivity, TPSA and viscosity are predictive of this ability. As cosmetic formulations continue to evolve to meet the needs of customers, emollient molecules can be selected that not only contribute to formulation texture and/or spreadability but that also leverage this novel connection between emollient penetration and SC biomechanics.

Investigation of the Compatibility of the Skin PAMPA Model with Topical Formulation and Acceptor Media Additives Using Different Assay Setups

The Skin Parallel Artificial Membrane Permeability Assay (PAMPA) is a 96-well plate-based skin model with an artificial membrane containing free fatty acid, cholesterol, and synthetic ceramide analogs to mimic the stratum corneum (SC) barrier. The current study evaluates the compatibility of lipophilic solvents/penetration enhancer, topical emulsions containing different emulsifier systems, and organic acceptor media additives with the artificial membrane of the assay. Additionally, different assay setups (standard setup: donor in bottom plate versus modified setup: donor in top plate) were compared. Methylparaben (MP), ethylparaben (EP), and propylparaben (PP) were used as model permeants and internal standards for proper assay execution. The permeation order of the parabens (MP > EP > PP) remained the same with different lipophilic solvents, and the ranking of lipophilic solvents was comparable under standard and modified conditions (isopropyl myristate, IPM > dimethyl isosorbide, DMI ≥ propylene glycol, PG > diisopropyl adipate, DIPA). Pre-incubation of the Skin PAMPA plates with IPM, DIPA, and DMI, as well as with formulations that contain non-ionic emulsifiers, and acceptor solutions containing DMSO or EtOH (≤ 50%) for 4 h did not increase the percentage of permeated parabens in the main experiment, suggesting that those compounds do not make the artificial membrane more permeable. High-resolution mass spectrometry confirmed that acceptor solutions with ≤ 50% DMSO or EtOH do not extract stearic acid, cholesterol, and certramides at standard assay conditions. Hence, if certain constraints are considered, the Skin PAMPA model can be used as a pre-screening tool for topical formulation selection.

Novel mono, di and tri-fatty acid esters bearing secondary amino acid ester head groups as transdermal permeation enhancers

Mono-oleate derivative (MOAPE) enhancing in vitro transdermal permeation of tenofovir.

Photo-co-carcinogenesis of Topically Applied Retinyl Palmitate in SKH-1 Hairless Mice

Cosmetic products that contain retinyl palmitate are popular as antiaging skin treatments; however, recent studies suggest a risk for enhanced skin tumor development with topical retinyl palmitate applications and exposure to solar ultraviolet radiation (UVR). In this study, we investigated the potential of retinyl palmitate to enhance UVR-induced photo-co-carcinogenesis. Groups of 36 male and 36 female SKH-1 hairless mice were exposed to simulated solar light (SSL) and treated with the control cream or creams containing retinyl palmitate, 5 days per week for 40 weeks. Other groups of mice were exposed to SSL and received no cream treatment or received cream treatments and were exposed to ultraviolet-A or ultraviolet-B. Mice were monitored for the development of skin tumors, and the incidences and multiplicities of squamous cell neoplasia were determined by histopathology. In both the absence and presence of SSL, mice administered the control cream developed skin tumors earlier and had higher incidences and multiplicities of skin squamous cell neoplasms than mice that received no cream treatment. Compared to the control cream groups, mice exposed to SSL and administered the retinyl palmitate creams demonstrated earlier onsets of skin tumors and had increased incidences and multiplicities of squamous cell skin neoplasms.

Effect of Esters on the Permeation of Chemicals with Different Polarities through Synthetic Artificial Membranes Using a High-Throughput Diffusion Cell Array

This study investigated the effects of 25 kinds of esters that are used in cosmetics on the permeation of four model compounds with different polarities (caffeine [CF], aminopyrine [AMP], benzoic acid [BA], and flurbiprofen [FP]). The amount of each model compound that permeated through two types of artificial membrane (silicone and Strat-M^®) was measured and correlated with the physicochemical properties of the esters, including their solubility, viscosity, wettability, surface tension, and uptake. The amount of each model compound that permeated through the silicone membrane was not significantly correlated with the solubility of the esters but was significantly correlated with all other measured physical properties of the esters. Similar correlations were observed for the amounts of AMP, BA, and FP that passed through the Strat-M^® membrane. However, the amount of CF that permeated through the Strat-M^® membrane also correlated with the solubility of the esters. There was a highly significant correlation between the amount permeating through the silicone and Strat-M^® membranes because the model compounds had high lipophilicity. These findings demonstrated that to control the permeation of various chemicals through artificial membranes, it is important to consider the uptake of the esters and that the solubility of the esters is also an important consideration when using a more complex membrane.

Topical Nonsteroidal Anti-Inflammatory Drugs: The Importance of Drug, Delivery, and Therapeutic Outcome

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely used in the treatment of pain associated with a variety of indications, including arthritic conditions, but their usefulness is often limited by dose-dependent adverse events (AEs), such as gastrointestinal disturbances, cardiovascular events, and renal toxicity. The risk of such effects could be reduced by the use of topical formulations, which offer the potential to deliver analgesic concentrations locally, at the site of inflammation, while minimizing systemic concentrations. The topical preparations currently approved in the United States are diclofenac sodium 1.5% topical solution (containing dimethyl sulfoxide as a penetration enhancer), diclofenac sodium gel 1%, and a diclofenac hydroxyethylpyrrolidine 1.3% patch. Each of these topical NSAIDs provide drug delivery to subcutaneous tissues for the management of pain associated with osteoarthritis or soft-tissue injuries. Furthermore, these formulations are not significantly associated with the systemic AEs associated with oral NSAIDs; the most common AEs associated with topical formulations are local skin reactions, which are usually mild and self-limiting. Other topical NSAID preparations approved in the European Union include ibuprofen creams and gels, ketoprofen gel, felbinac gel and cutaneous foam, and piroxicam gel. Meta-analyses have confirmed the efficacy and safety of these preparations. However, it is important to recognize that pharmacokinetic absorption from topical formulations can vary markedly, even between different formulations of the same drug, depending on the agent, the underlying disorder, and the site of application. It is therefore essential to consider the patient, the drug, and the drug delivery mechanism when selecting a topical NSAID preparation.

Adjuvant Effect of an Alternative Plasticizer, Diisopropyl Adipate, on a Contact Hypersensitivity Mouse Model: Link with Sensory Ion Channel TRPA1 Activation

Due to health concerns about phthalate esters, the use of alternative plasticizers is being considered. Phthalate esters enhance skin sensitization to fluorescein isothiocyanate (FITC) in mouse models. We have demonstrated that phthalate esters stimulate transient receptor potential ankyrin 1 (TRPA1) cation channels expressed on sensory neurons. We also found a correlation between TRPA1 activation and the enhancing effect on FITC-induced contact hypersensitivity (CHS) when testing various types of phthalate esters. Here we investigated the effects of an alternative plasticizer, diisopropyl adipate (DIA). Activation of TRPA1 by DIA was demonstrated by calcium mobilization using Chinese hamster ovary cells expressing TRPA1 in vitro. The effect of DIA was inhibited by a TRPA1-specific antagonist, HC-030031. The presence of DIA or dibutyl phthalate (DBP; positive control) during skin sensitization of BALB/c mice to FITC augmented the CHS response, as revealed by the level of ear-swelling. The enhancing effect of DIA was inhibited by in vivo pretreatment with HC-030031. FITC-presenting CD11c(+) dendritic cell (DC)-trafficking to draining lymph nodes was facilitated both by DIA and by DBP. DBP and DIA were similarly active in the enhancement of interferon-γ production by draining lymph nodes, but the effect on interleukin-4 production was weaker with DIA. Overall, DIA activated TRPA1 and enhanced FITC-induced CHS, as DBP did. The adjuvant effects of adipate esters may need to be considered because they are used as ingredients in cosmetics and drug formulations topically applied to the skin.

Novel dendritic derivatives of unsaturated fatty acids as promising transdermal permeation enhancers for tenofovir

Novel dendritic ester derivatives of unsaturated fatty acids as potential transdermal permeation enhancers.

Novel oleic acid derivatives enhance buccal permeation of didanosine

The aim of this study was to explore the potential of novel oleic acid (OA) derivatives as buccal permeation enhancers for the delivery of didanosine (ddI). The OA derivatives, i.e. ester derivative (OA1E), the dicarboxylic acid derivative (OA1A) and the bicephalous dianionic surfactant (OA1ANa) were synthesized and their effects were compared to the parent OA. OA, OA1E, OA1A and OA1ANa at 1% w/w all showed potential for enhancing the buccal permeability of ddI with enhancement ratio (ER) of 1.29, 1.33, 1.01 and 1.72, respectively. OA1ANa at 1% w/w demonstrated the highest flux (80.30 ± 10.37 µg cm(-2 )h), permeability coefficient (4.01 ± 0.57 × 10(-3) cm h(-1)) and ER (1.72). The highest flux for ddI (144.00 ± 53.54 µg cm(-2 )h) was reported with OA1ANa 2% w/w, which displayed an ER of 3.09 more than that with ddI alone. At equivalent concentrations, OA1ANa (ER = 3.09) had a significantly higher permeation-enhancing effect than its parent OA (ER = 1.54). Histomorphological studies confirmed that OA1ANa at all concentrations (0.5, 2.0 and 6.0% w/w) had no adverse effects on the mucosae. Morphological changes such as vacuoles formation and increased intercellular spaces were attributed to the buccal permeation-enhancing effect of OA1ANa. This study demonstrated the potential of novel OA derivatives as buccal permeation enhancers. OA1ANa at 2% w/w was also identified as the optimal novel OA derivative to widen the pool of fatty acid derivatives as chemical permeation enhancers for buccal drug delivery.

Machine learning study for the prediction of transdermal peptide

In order to develop a computational method to rapidly evaluate transdermal peptides, we report approaches for predicting the transdermal activity of peptides on the basis of peptide sequence information using Artificial Neural Network (ANN), Partial Least Squares (PLS) and Support Vector Machine (SVM). We identified 269 transdermal peptides by the phage display technique and use them as the positive controls to develop and test machine learning models. Combinations of three descriptors with neural network architectures, the number of latent variables and the kernel functions are tried in training to make appropriate predictions. The capacity of models is evaluated by means of statistical indicators including sensitivity, specificity, and the area under the receiver operating characteristic curve (ROC score). In the ROC score-based comparison, three methods proved capable of providing a reasonable prediction of transdermal peptide. The best result is obtained by SVM model with a radial basis function and VHSE descriptors. The results indicate that it is possible to discriminate between transdermal peptides and random sequences using our models. We anticipate that our models will be applicable to prediction of transdermal peptide for large peptide database for facilitating efficient transdermal drug delivery through intact skin.

Emollient molecule effects on the drying stresses in human stratum corneum

BACKGROUND

Emollient molecules are widely used in skin care formulations to improve skin sensory properties and to alleviate dry skin but little is understood regarding their effects on skin biomechanical properties.

OBJECTIVES

To investigate the effects of emollient molecules on drying stresses in human stratum corneum (SC) and how these stresses are related to SC components and moisture content.

METHODS

The substrate curvature method was used to measure the drying stresses in isolated SC following exposure to selected emollient molecules. While SC stresses measured using this method have the same biaxial in vivo stress state and moisture exchange with the environment, a limitation of the method is that moisture cannot be replenished by the underlying skin layers. This provides an opportunity to study the direct effects of emollient treatments on the moisture content and the components of the SC. Attenuated total reflectance Fourier transform infrared spectroscopy was used to determine the effects of emollient molecules on SC lipid extraction and conformation. Results Emollient molecules resulted in a complex SC drying stress profile where stresses increased rapidly to peak values and then gradually decreased to significantly lower values compared with the control. The partially occlusive treatments also penetrated into the SC where they caused extraction and changes in lipid conformation. These effects together with their effects on SC moisture content are used to rationalize the drying stress profiles.

CONCLUSIONS

Emollient molecules have dramatic effects on SC drying stresses that are related to their effects on intercellular lipids and SC moisture content.

Percutaneous penetration modifiers and formulation effects: thermal and spectral analyses

The study investigated the formulation effects of laurocapram and iminosulfurane derived penetration modifiers on human stratum corneum using thermal and spectral analyses. Firstly, formulations of penetration modifiers were assessed as enhancers/retardants using the model permeant, diethyl-m-toluamide followed by investigation of their mechanisms of action using differential scanning calorimetry (DSC) and attenuated total reflectance Fourier-transform infra-red spectroscopy. The penetration modifiers investigated were laurocapram, 3-dodecanoyloxazolidin-2-one (N-0915), S,S-dimethyl-N-(4-bromobenzoyl) iminosulfurane (DMBIS), S,S-dimethyl-N-(2-methoxycarbonylbenzenesulfonyl) iminosulfurane (DMMCBI) and tert-butyl 1-dodecyl-2-oxoazepan-3-yl-carbamate (TBDOC) that were formulated in either water, propylene glycol (PG), ethanol or polyethylene glycol 400 (PEG 400). The results explain the mechanism for the first time why an enhancer can become a retardant or vice versa depending upon the vehicle in which it is applied to the skin. DSC indicated that penetration modifier formulations enhanced permeation of active mainly by disruption and fluidization of the stratum corneum lipid bilayers while IR data indicated characteristic blue shifts with decreases in peak intensity. On the other hand, DSC of penetration modifier formulations showing retardation depicted elevated T (m2) with a strengthening of lipid-protein complex while IR results indicated formation of multiple peaks around 1,738 cm(-1) transition in stratum corneum spectra suggesting retardation may be caused by organization of SC lipids by increased H-bonding.

Hydration effects on skin microstructure as probed by high-resolution cryo-scanning electron microscopy and mechanistic implications to enhanced transcutaneous delivery of biomacromolecules

Although hydration is long known to improve the permeability of skin, penetration of macromolecules such as proteins is limited and the understanding of enhanced transport is based on empirical observations. This study uses high-resolution cryo-scanning electron microscopy to visualize microstructural changes in the stratum corneum (SC) and enable a mechanistic interpretation of biomacromolecule penetration through highly hydrated porcine skin. Swollen corneocytes, separation of lipid bilayers in the SC intercellular space to form cisternae, and networks of spherical particulates are observed in porcine skin tissue hydrated for a period of 4-10 h. This is explained through compaction of skin lipids when hydrated, a reversal in the conformational transition from unilamellar liposomes in lamellar granules to lamellae between keratinocytes when the SC skin barrier is initially established. Confocal microscopy studies show distinct enhancement in penetration of fluorescein isothiocyanate-bovine serum albumin (FITC-BSA) through skin hydrated for 4-10 h, and limited penetration of FITC-BSA once skin is restored to its natively hydrated structure when exposed to the environment for 2-3 h. These results demonstrate the effectiveness of a 4-10 h hydration period to enhance transcutaneous penetration of large biomacromolecules without permanently damaging the skin.

Efficient mucosal delivery of optical contrast agents using imidazole-modified chitosan

The clinical applicability of antibodies and plasmonic nanosensors as topically applied, molecule-specific optical diagnostic agents for noninvasive early detection of cancer and precancer is severely limited by our inability to efficiently deliver macromolecules and nanoparticles through mucosal tissues. We have developed an imidazole-functionalized conjugate of the polysaccharide chitosan (chitosan-IAA) to enhance topical delivery of contrast agents, ranging from small molecules and antibodies to gold nanoparticles up to 44 nm in average diameter. Contrast agent uptake and localization in freshly resected mucosal tissues was monitored using confocal microscopy. Chitosan-IAA was found to reversibly enhance mucosal permeability in a rapid, reproducible manner, facilitating transepithelial delivery of optical contrast agents. Permeation enhancement occurred through an active process, resulting in the delivery of contrast agents via a paracellular or a combined paracellular/transcellular route depending on size. Coadministration of epidermal growth factor receptor-targeted antibodies with chitosan-IAA facilitated specific labeling and discrimination between paired normal and malignant human oral biopsies. Together, these data suggest that chitosan-IAA is a promising topical permeation enhancer for mucosal delivery of optical contrast agents.

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

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

In vitro skin absorption and drug release – A comparison of six commercial prednicarbate preparations for topical use

Reconstructed human epidermis is a useful tool for in vitro skin absorption studies of chemical compounds. If this may hold true also for topical dermatics, we investigated the glucocorticoid prednicarbate applied by two sets (innovator and generic) of cream, ointment and fatty ointment using the commercially available EpiDerm model. Moreover, stability and local tolerability of the preparations as well as drug release were studied, to estimate an influence on prednicarbate absorption and metabolism. While release ranked in the order cream<fatty ointment<ointment for both sets of preparations, prednicarbate penetration and permeation of the EpiDerm model did not. Less PC uptake observed with the generic ointment and fatty ointment appeared to be linked to impaired enzymatic ester cleavage within the tissue. Thus with drugs subject to skin metabolism, cutaneous uptake is not to be derived from drug release studies, yet has to be studied experimentally with viable skin or reconstructed human epidermis.

A Novel Solid-in-oil Nanosuspension for Transdermal Delivery of Diclofenac Sodium

PURPOSE

We formulated a solid-in-oil nanosuspension (SONS) as a novel transdermal delivery carrier for diclofenac sodium (DFNa). The basic transdermal characteristics of the SONS were evaluated using a Yucatan micropig (YMP) skin model.

METHODS

DFNa-sucrose erucate (i.e. surfactant) complexes were prepared via the formation of a water-in-oil emulsion. The complexes were suspended in isopropyl myristate (IPM) to form a SONS. The basic transdermal characteristics of the SONS were examined using full-thickness YMP dorsal skin in a Franz-type diffusion cell. DFNa powder suspended in IPM without complex formation was used as a control. The effect of the weight ratio of surfactant to DFNa on DFNa penetration of the skin was evaluated.

RESULTS

DFNa was successfully dispersed into IPM as a nanosized suspension via complex formation with sucrose erucate. The resultant SONS increased the permeability flux of DFNa across the YMP skin by up to 3.8-fold compared with the control. The size of the SONS depended on the weight ratio of the surfactant to DFNa. The optimal weight ratio for the highest DFNa permeation was 8.8, at which point the mean diameter of the SONS was 14.4 nm.

CONCLUSION

The SONS formulation can enhance the percutaneous absorption of DFNa.

Transdermal Delivery of Ketoprofen: The Influence of Drug–Dioleylphosphatidylcholine Interactions

PURPOSE

Considering that most inflammatory diseases occur locally and near the body surface, transdermal delivery of non-steroidal anti-inflammatory drugs (NSAIDs) may be an interesting strategy for delivering these drugs directly to the diseased site. To optimize ketoprofen (KP) transdermal delivery we investigated the influence of dioleylphosphatidylcholine (DOPC) on skin permeation.

MATERIALS AND METHODS

The formulations studied were: i) a physical mixture of KP and DOPC and ii) DOPC and KP complex, in a molar ratio of 1:3, obtained by dissolution of the components in chloroform followed by drying under a N2 atmosphere. Both systems were dispersed in mineral oil and the in vitro percutaneous was assayed by absorption using a flow through diffusion cell. Differential Scanning Calorimetry (DSC) and 1H NMR studies were carried out to characterize KP and DOPC interactions. Geometry optimizations using Density Functional Theory and semiempirical methods, as well as a flexible docking procedure were carried out to obtain a binding model for KP with DOPC. KP solubility and partition studies in the formulations, as well as skin irritation and hypersensitivity assays were also carried out.

RESULTS

DSC determinations in the complex showed enthalpy and temperature depressions, indicating KP and DOPC interaction. In addition, dipole-dipole interactions between the KP carboxylic acid and OH groups in phospholipids were shown by 1H NMR studies. Based on the NMR studies, a KP-DOPC binding model is proposed, in which KP is involved by the two long aliphatic chains of the phospholipid. Solubility studies indicated that DOPC improved drug solubility. KP permeation was enhanced by both formulations tested, but the complex also increased its skin uptake. Such behavior could be attributed to the solubilizing, melting and enhancing effects of DOPC. Skin irritation and hypersensitivity were not significantly changed compared to control, suggesting that the formulation may be therapeutically explored for KP transdermal delivery.

Penetration enhancers

One long-standing approach for improving transdermal drug delivery uses penetration enhancers (also called sorption promoters or accelerants) which penetrate into skin to reversibly decrease the barrier resistance. Numerous compounds have been evaluated for penetration enhancing activity, including sulphoxides (such as dimethylsulphoxide, DMSO), Azones (e.g. laurocapram), pyrrolidones (for example 2-pyrrolidone, 2P), alcohols and alkanols (ethanol, or decanol), glycols (for example propylene glycol, PG, a common excipient in topically applied dosage forms), surfactants (also common in dosage forms) and terpenes. Many potential sites and modes of action have been identified for skin penetration enhancers; the intercellular lipid matrix in which the accelerants may disrupt the packing motif, the intracellular keratin domains or through increasing drug partitioning into the tissue by acting as a solvent for the permeant within the membrane. Further potential mechanisms of action, for example with the enhancers acting on desmosomal connections between corneocytes or altering metabolic activity within the skin, or exerting an influence on the thermodynamic activity/solubility of the drug in its vehicle are also feasible, and are also considered in this review.