Effect of sodium lauryl sulfate-induced skin irritation on in vivo percutaneous penetration of four drugs

Polyaphron Dispersion Technology, A Novel Topical Formulation and Delivery System Combining Drug Penetration, Local Tolerability and Convenience of Application

Topical formulation and delivery technologies for pharmaceutical application should simultaneously address efficacy, safety and convenience of therapy. This has historically proven to be challenging, since formulation features that drive efficacy often have undesirable consequences for safety and convenience and vice versa. Polyaphron dispersion (PAD) technology is a novel topical formulation and drug delivery system developed with the purpose of preserving these key attributes. PAD formulations are typically oil-in-water dispersions consisting of oil droplets encapsulated in a multi-molecular shell structure. This shell structure protects potentially unstable active molecules solubilized in the oil from hydrolytic degradation. Example data are presented of enhanced drug penetration from PAD formulations, including dermal delivery of calcipotriene, betamethasone dipropionate and tacrolimus as well as ocular delivery of ciclosporin A. Local tolerability is an important safety parameter for topical formulations, where high levels of surfactants can cause skin irritation. In this regard, a key benefit of PAD formulations is the inherent reduced requirement for surfactants to generate stable formulations compared to conventional emulsion systems. Patients with chronic diseases with topical manifestations such as psoriasis or atopic dermatitis have been reported to miss up to 70% of planned topical applications, mainly due to a lack of satisfaction with their therapy. Patients generally prefer light, moisturizing, non-greasy and quickly absorbed vehicles that are simple to use on all body parts. PAD formulations can generally be designed to meet these criteria. In conclusion, PAD technology provides high flexibility in topical drug design and can be applied to several body locations without compromising efficacy, safety or convenience of therapy.

Clinical Trial Register: Clinicaltrials.gov: NCT03802344.

Partition and Solubilization of Phospholipid Vesicles by Noncovalently Constructed Oligomeric-like Surfactants

This work has investigated the interaction of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles with oligomeric surfactants noncovalently formed by sodium dodecyl sulfate (SDS) and a series of polyamines, 1,3-diaminopropane (PDA), triamine, spermidine, and spermine. The partition coefficients (P) of these surfactants between lipid bilayers and the aqueous phase are measured by isothermal titration microcalorimetry (ITC), showing that the P value increases and the Gibbs free energy of the partition becomes more negative with increasing oligomerization degree of the surfactants. This changing trend is similar to that of synthetic oligomeric surfactants regardless of the charge properties, suggesting that the polyamine and SDS molecules interact with the DOPC bilayer simultaneously. Meanwhile, the DOPC solubilization by these surfactants is evaluated by the effective surfactant-to-lipid molar ratios for the onset (R_e^sat) and end (R_e^sol) of the solubilization process, which are determined from the phase boundaries obtained by ITC, turbidity, and dynamic light scattering measurements. With the increment of oligomerization degree, the R_e^sat and R_e^sol values increase anomalously and are much larger than those of the synthetic surfactants with the same oligomerization degree, suggesting that noncovalently constructed oligomeric surfactants exhibit lower solubilization ability to phospholipid vesicles than the corresponding covalent oligomeric surfactants. Therefore, the noncovalently constructed oligomeric-like surfactants facilitate strong partition but weak solubilization to phospholipid vesicles, which may provide a useful strategy to mildly adjust the permeation and fluidity of phospholipid vesicles with solubilization delay.

Partition of Glutamic Acid-Based Single-Chain and Gemini Amphiphiles into Phospholipid Membranes

Understanding the interactions of amphiphile molecules with biological membranes is very important to many practical applications. Amino acid amphiphiles are a kind of mild surfactants and have many unique performances. However, their interactions with phospholipid membranes have scarcely been studied. This work has studied the interactions of glutamic acid-based gemini amphiphile C₁₂(Glu)₂C₁₂ and single-chain amphiphile C₁₂Glu with the model biomembrane formed by the phospholipid 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC). The partition coefficients of C₁₂(Glu)₂C₁₂ and C₁₂Glu into the DOPC vesicles were derived from the observed enthalpy curves obtained by isothermal titration calorimetry at temperatures of 25.0 and 37.0 °C, and pHs of 5.6 and 7.4, corresponding to the skin surface and human physiological conditions. The results from cryogenic transmission electron microscopy, dynamic light scattering, and zeta potential measurements show that the amphiphile molecules form different aggregates, which make the amphiphile molecules exhibit different partition abilities to the DOPC vesicles. For C₁₂Glu, the molecules form shorter wormlike micelles with a lower surface charge at all the pHs and temperatures used, and the partition coefficient of C₁₂Glu into the DOPC vesicles does not change with temperature and pH. Differently, the C₁₂(Glu)₂C₁₂ molecules form fibers with a larger negative charge and belts with a smaller negative charge at pHs 7.4 and 5.6, respectively, no matter what temperature is used. As a result, the partitions of C₁₂(Glu)₂C₁₂ into the DOPC vesicles are markedly different at these two pH values, and the belts at pH 7.4 exhibit a stronger partition ability than the fibrils at pH 5.6. Moreover, at any temperature and pH, C₁₂(Glu)₂C₁₂ shows a stronger partition ability than C₁₂Glu. This work can help to understand the relationship between the molecular structure and aggregate structure of amino acid amphiphiles and their partition abilities into the biomembranes.

Skin and Eye Irritation Assessment of Oil Palm (Elaeis guineensis) Leaf Extract for Topical Application

Many types of phytochemicals have been found to be present in oil palm leaf and could potentially be used as functional ingredients for skincare product. However, as of today, there is no published report on hazard identification and safety assessment of oil palm ( Elaeis guineensis) leaf extract (OPLE), particularly on skin and eye irritation. In this study, potential hazard of OPLE on skin and eye irritation was evaluated as an initial step to the safety assessment of OPLE. In vitro cell viability study of OPLE on normal human dermal fibroblasts showed that OPLE was nontoxic to the cells with percentage viability more than 90% after 24 and 48 hours of incubation. Skin irritation potential of OPLE was evaluated using in vitro SkinEthic reconstructed human epidermis (RHE) model (Organization for Economic Cooperation and Development [OECD] Test Guideline 439, 2015), while eye irritation potential was evaluated using in vitro SkinEthic Human corneal epithelium (HCE) model (OECD test guideline 492, 2017). Hazard identification results showed that OPLE at 1%, 5%, and 10% (wt/wt) was classified as nonirritant to the skin and eye where mean tissue viabilities of SkinEthic RHE and SkinEthic HCE were more than 50% and 60%, respectively. Therefore, we recommend a further safety assessment, such as human patch testing, to confirm the nonirritant of OPLE.

Interactions of Cationic/Anionic Mixed Surfactant Aggregates with Phospholipid Vesicles and Their Skin Penetration Ability

This work studied the interactions of an oppositely charged surfactant mixture of oleyl bis(2-hydroxyethyl)methyl ammonium bromide (OHAB) and sodium dodecyl sulfate (SDS) with 1,2-di-(9Z-octadecenoyl)-sn-glycero-3-phosphocholine (DOPC) vesicles as well as the penetration of the OHAB/SDS mixture through model skin, aimed at understanding the relationship between the ability of different surfactant aggregates in solubilizing phospholipid vesicles and their potential in irritating skin. By changing the molar fraction of OHAB (X_OHAB), five kinds of aggregates are constructed: OHAB and SDS separately form cationic and anionic small micelles, whereas the OHAB/SDS mixtures form cationic and anionic vesicles at X_OHAB = 0.30 and 0.70, respectively, and weakly charged vesicles at X_OHAB = 0.50. The mixtures have much lower critical micellar concentrations (CMCs) and much larger aggregates than either OHAB or SDS alone, and the CMC and the size of the OHAB/SDS vesicles decrease with the increase in X_OHAB. The phase diagrams indicate that the OHAB/SDS mixtures show much stronger ability in solubilizing the DOPC vesicles than individual OHAB and SDS and decrease in the order of X_OHAB = 0.30 > 0.50 > 0.70 ≫ 1.00 > 0. However, the ability of the surfactants in penetrating the model skin decreases reversely, and the penetration of the surfactants are significantly reduced by mixing. These results indicate that the surfactant mixture with a larger aggregate size and a smaller CMC value displays much stronger ability in solubilizing the DOPC vesicles but much weaker ability in penetrating the skin.

Phytosphingosine, sphingosine and dihydrosphingosine ceramides in model skin lipid membranes: permeability and biophysics

Ceramides based on phytosphingosine, sphingosine and dihydrosphingosine are essential constituents of the skin lipid barrier that protects the body from excessive water loss. The roles of the individual ceramide subclasses in regulating skin permeability and the reasons for C4-hydroxylation of these sphingolipids are not completely understood. We investigated the chain length-dependent effects of dihydroceramides, sphingosine ceramides (with C4-unsaturation) and phytoceramides (with C4-hydroxyl) on the permeability, lipid organization and thermotropic behavior of model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesteryl sulfate. Phytoceramides with very long C24 acyl chains increased the permeability of the model lipid membranes compared to dihydroceramides or sphingosine ceramides with the same chain lengths. Either unsaturation or C4-hydroxylation of dihydroceramides induced chain length-dependent increases in membrane permeability. Infrared spectroscopy showed that C4-hydroxylation of the sphingoid base decreased the relative ratio of orthorhombic chain packing in the membrane and lowered the miscibility of C24 phytoceramide with lignoceric acid. The phase separation in phytoceramide membranes was confirmed by X-ray diffraction. In contrast, phytoceramides formed strong hydrogen bonds and highly thermostable domains. Thus, the large heterogeneity in ceramide structures and in their aggregation mechanisms may confer resistance towards the heterogeneous external stressors that are constantly faced by the skin barrier.

Percutaneous absorption in diseased skin: an overview

The stratum corneum's (SC) functions include protection from external hazardous environments, prevention of water loss and regulation of body temperature. While intact skin absorption studies are abundant, studies on compromised skin permeability are less common, although products are often used to treat affected skin. We reviewed literature on percutaneous absorption through abnormal skin models. Tape stripping is used to disrupt water barrier function. Studies demonstrated that physicochemical properties influence the stripping effect: water-soluble drugs are more affected. Abrasion did not affect absorption as much. Freezing is commonly used to preserve skin. It does not seem to modify water absorption, but still increases the penetration of compounds. Comparatively, heating the skin consistently increased percutaneous absorption. Removing SC lipids may increase percutaneous absorption of drugs. Many organic solvents are employed to delipidize. Delipidization with chloroform-methanol increased hydrophilic compound permeability, but not lipophilic. Acetone pre-treatment enhanced hydrophilic compound penetration. More data is needed to determine influence on highly lipophilic compound penetration. Sodium lauryl sulfate (SLS) induces irritant dermatitis and is frequently used as a model. Studies revealed that SLS increases hydrophilic compound absorption, but not lipophilic. However, skin irritation with other chemicals increases lipophilic penetration as much as hydrophilic. Animal studies show that UV exposure increases percutaneous absorption whereas human studies do not. Human studies show increased penetration in psoriatic and atopic dermatitis skin. The data summarized here begin to characterize flux alteration associated with damaged skin. Understanding the degree of alteration requires interpretation of involved conditions and the enlarging of our database to a more complete physicochemical spectrum.

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 < 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.

Damage/recovery by additive on lipid membrane as a mimicry of human stratum corneum

The effects of sodium dodecyl sulfate (SDS) on the model lipid membrane of human stratum corneum, composed of three main lipids of ceramide III, palmitic acid, and cholesterol, have been examined as a function of exposure period. Cholesterol first got to elute, palmitic acid followed it late, and the remaining solid was mainly ceramide III. The removal of lipids influenced the configurational structure of remaining lipid and the intralayer structure of lamellae. Monitoring of structural reorganization in the damaged membrane was carried out on the recovering procedure of palmitic acid and cholesterol. Both lipids were penetrated in the damaged membrane and recovered mostly the configurational lipid structure and the lamellar structure. Especially, it can be noted that cholesterol is more effective than palmitic acid on recovery.

Surfactant effects on skin absorption of model organic chemicals: implications for dermal risk assessment studies

Occupational and environmental exposures to chemicals are major potential routes of exposure for direct skin toxicity and for systemic absorption. The majority of these exposures are to complex mixtures, yet most experimental studies to assess topical chemical absorption are conducted neat or in simple aqueous vehicles. A component of many industrial mixtures is surfactants that solubilize ingredients and stabilize mixtures of oily components when present in aqueous vehicles. The purpose of this series of experiments was to use two well-developed experimental techniques to assess how solution interactions present in a pure nonbiological in vitro system (membrane coated fibers, MCF) compare to those seen in a viable ex vivo biological preparation (isolated perfused porcine skin flap, IPPSF). Two widely encountered anionic surfactants, sodium lauryl sulfate (SLS) and linear alkylbenzene sulfonate (LAS), were studied in 10% solutions. The rank orders of absorption were: water: pentachlorophenol (PCP) > 4-nitrophenol (PNP) > parathion > fenthion > simazine > propazine; SLS: PNP > PCP > parathion > simazine > fenthion > propazine; and LAS: PNP > PCP > simazine > parathion > fenthion > propazine. For all penetrants, absorption was greater in SLS compared to LAS mixtures, a finding consistent with smaller micelle sizes seen with SLS. For these low-water-solubility compounds, absorption was greater from aqueous solutions in nearly every case. The inert three-fiber MCF array predicted absorptive fluxes seen in the ex vivo IPPSF, suggesting lack of any biological effects of the surfactants on skin.

In vitro skin diffusion study of pure forskolin versus a forskolin-containing Plectranthus barbatus root extract

An in vitro skin diffusion study of pure forskolin (1) versus a 1-containing Plectranthus barbatus root extract (P. barbatus extract) in hairless guinea pig skin and human skin in a flow-through diffusion cell system was conducted and is being reported for the first time. Both topical agents were formulated in a solution of 70% ethanol and 30% propylene glycol (v/v). The results showed that forskolin can be delivered through the stratum corneum and that the flux of this compound was enhanced when 1 was delivered as a constituent of the P. barbatus extract as compared to an equivalent amount in pure form. These results suggest that the P. barbatus extract used contains permeation enhancement activity from other compound(s) contained in the crude root extract. It is possible that P. barbatus root extract may be used as an economical source of 1 to perform topical chemical manipulation of pigmentation in high-risk populations.

Absorption of chemicals through compromised skin

Skin is an important route of entry for many chemicals in the work place. To assess systemic uptake of a chemical in contact with the skin, quantitative information on dermal absorption rates of chemicals is needed. Absorption rates are mainly obtained from studies performed with intact, healthy skin. At the work place, however, a compromised skin barrier, although not necessarily visible is common, e.g. due to physical and chemical damage. As reviewed in this article, there are several lines of evidence that reduced integrity of the skin barrier may increase dermal absorption of chemicals in the occupational setting. An impaired skin barrier might lead not only to enhanced absorption of a specific chemical, but also to entrance of larger molecules such as proteins and nanoparticles which normally are not able to penetrate intact skin. In addition to environmental influences, there is increasing evidence that some individuals have an intrinsically affected skin barrier which will facilitate entrance of chemicals into and through the skin making these persons more susceptible for local as well for systemic toxicity. This review addresses mechanisms of barrier alteration caused by the most common skin-damaging factors in the occupational settings and the consequences for dermal absorption of chemicals. Furthermore, this review emphasizes the importance of maintained barrier properties of the skin.

Controlled release in transdermal pressure sensitive adhesives using organosilicate nanocomposites

Polydimethyl siloxane (PDMS) based pressure sensitive adhesives (PSA) incorporating organo-clays at different loadings were fabricated via solution casting. Partially exfoliated nanocomposites were obtained for the hydroxyl terminated PDMS in ethyl acetate solvent as determined by X-ray diffraction and atomic force microscopy. Drug release studies showed that the initial burst release was substantially reduced and the drug release could be controlled by the addition of organo-clay. Shear strength and shear adhesion failure temperature (SAFT) measurements indicated substantial improvement in adhesive properties of the PSA nanocomposite adhesives. Shear strength showed more than 200% improvement at the lower clay loadings and the SAFT increased by about 21% due to the reinforcement provided by the nano-dispersed clay platelets. It was found that by optimizing the level of the organosilicate additive to the polymer matrix, superior control over drug release kinetics and simultaneous improvements in adhesive properties could be attained for a transdermal PSA formulation.

Pimecrolimus permeates less than tacrolimus through normal, inflamed, or corticosteroid-pretreated skin

The permeabilities of normal human and normal, inflamed, or corticosteroid (CS) pretreated skin of young domestic pigs for pimecrolimus and tacrolimus were compared in vitro, using Franz-type diffusion cells. The test articles were either used as 1.0% solutions or as the marketed formulations (Elidel 1% cream, Protopic 0.1%, and 0.03% ointment). In normal human skin, the permeation rate of pimecrolimus from the 1% cream was about sixfold lower than that of tacrolimus from 0.1% ointment and by a factor of 4.3 lower compared with tacrolimus from Protopic 0.03%. In pigs, sodium laurylsulfate-induced irritant contact dermatitis resulted in significantly faster skin permeation of both drugs from applied solutions. The permeation rate for pimecrolimus was lower than that for tacrolimus. Thus, at 24 h, pimecrolimus concentrations in the receptor fluid were 2.8-fold lower than the tacrolimus levels. Compared with normal porcine skin, permeation of drugs through hydrocortisone (1.0%)-, mometasone (0.1%)-, or clobetasol-17-butyrate (0.05%)-pretreated skin was increased by factors of 3.6 (pimecrolimus, applied as 1% cream) and 1.7 (tacrolimus, applied as 0.1% ointment). In normal pig skin, the permeation rate of tacrolimus was found to be 11.2 times higher than that of pimecrolimus and 3.5- to 7.1-fold higher in CS-pretreated skin, independent of the potency of the CSs. The present in vitro data suggest that in patients with acute skin inflammation or after therapy with topical CSs, percutaneous absorption and, as a consequence, systemic drug exposure will be lower with Elidel 1% cream as compared with Protopic 0.1% and 0.03% ointment.

Simultaneous sodium lauryl sulphate testing improves the diagnostic validity of allergic patch tests. Results from a prospective multicentre study of the German Contact Dermatitis Research Group (Deutsche Kontaktallergie-Gruppe, DKG)

BACKGROUND

There is evidence that a higher skin susceptibility may induce nonspecific erythematous or weak positive reactions to contact allergens in patch testing.

OBJECTIVES

To evaluate whether simultaneous application of sodium lauryl sulphate (SLS) along with diagnostic patch tests with contact allergens can provide information regarding skin irritability which may help to discriminate allergic from nonspecific irritant reactions to contact allergens.

METHODS

Between July 2001 and June 2003, this prospective study collected patch test data of 5971 patients from 19 centres in Germany and Austria in the Information Network of Departments of Dermatology (IVDK). In addition to contact allergens (standard series and eight known 'problematic' allergens with a low reaction index and a high positivity ratio: 1,3-diphenylguanidine, amerchol L-101, benzalkonium chloride, benzoyl peroxide, cocamidopropyl betaine, octyl gallate, phenyl mercuric acetate and propylene glycol), patches with SLS 0.5% and 0.25% aq. were applied. Reactions to the allergens and to SLS were analysed at the IVDK data centre. The association between an erythematous or positive reaction to a certain allergen and an irritant reaction to SLS was assessed with logistic regression analysis, at the same time controlling for the influence of age and sex.

RESULTS

Of the 29 allergens of the standard series, 23 and 21 gave a higher percentage of nonspecific erythematous reactions in patients with an irritant reaction to 0.25% and 0.5% SLS, respectively, in comparison with SLS-negative patients. All eight 'problematic' allergens gave an increased percentage of nonspecific erythematous reactions. Similarly, 22 and 21 allergens of the standard series gave a higher percentage of positive allergic reactions in patients with an irritant reaction to 0.25% and 0.5% SLS, respectively, and seven of the eight 'problematic' allergens gave a higher percentage of positive allergic reactions (exception: octyl gallate). For most allergens, the markers of skin reaction (reaction index and positivity ratio) were worse in SLS-positive patients. Differences were more pronounced when testing with SLS 0.25% than with SLS 0.5%.

CONCLUSIONS

Because there is a convincing association between skin irritability (evaluated by SLS test) and the degree of skin reaction to contact allergens, the SLS test may help in deciding whether a doubtful erythematous or weakly 'positive' skin reaction should be interpreted as allergic or irritant.

Stratum corneum adhesive tape stripping: influence of anatomical site, application pressure, duration and removal

BACKGROUND

Tape stripping is a common method for investigating stratum corneum (SC) physiology as well as bioavailability and bioequivalence of topical drugs.

OBJECTIVES

To investigate the influence of procedures (anatomical site, pressure, pressure duration, tape removal rate) inherent in each stripping protocol on changes in skin physiology.

METHODS

Tape stripping was performed using tapes on the forearm, forehead and back. On the forearm different pressures (165 and 330 g cm(-2)), durations of pressure (2 and 10 s), and removal rate (slow and rapid removal) were used. Changes in skin physiology were evaluated by measurement of transepidermal water loss (TEWL) and hydration.

RESULTS

A significant influence of all parameters on the TEWL increase as a function of tape strip number was observed. The fastest increase was demonstrated on the forehead, followed by the back and, lastly, the forearm. Rapid removal produced a protracted increase in comparison with slow removal. Pressure for 10 s induced a faster increase in TEWL than 2 s pressure. Likewise, pressure at 330 g cm(-2) induced an earlier increase than pressure at 165 g cm(-2). Skin hydration was not influenced by the variables tested.

CONCLUSIONS

Tape stripping results are influenced dramatically by all investigated parameters. A dynamic SC stress test to investigate SC cohesion more closely is proposed based on the present observations.

Mixture additives inhibit the dermal permeation of the fatty acid, ricinoleic acid

Ricinoleic acid (RA) like many of the ingredients in machine cutting fluids and other industrial formulations are potential dermal irritants, yet very little is known about its permeability in skin. 3H-ricinoleic acid mixtures were formulated with three commonly used cutting fluid additives; namely, triazine (TRI), linear alkylbenzene sulfonate (LAS), and triethanolamine (TEA) and topically applied to inert silastic membranes and porcine skin in vitro as aqueous mineral oil (MO) or polyethylene glycol (PEG) mixtures. These additives significantly decreased ricinoleic acid partitioning from the formulation into the stratum corneum (SC) in PEG-based mixtures. Except for LAS, all other additives produced a more basic formulation (pH = 9.3-10.3). In silastic membranes and porcine skin, individual additives or combination of additives significantly reduced ricinoleic permeability. This trend in ricinoleic acid disposition in both membranes suggests that the mixture interaction is more physicochemical in nature and probably not related to the chemical-induced changes in the biological membrane as may be assumed with topical exposures to potentially irritant formulations.

Chronic UVB exposure enhances in vitro percutaneous penetration of 5-aminulevulinic acid in hairless mouse skin

BACKGROUND AND OBJECTIVES

(Pre)cancerous skin lesions accumulate more protoporphyrin IX (PpIX) upon topical application of 5-aminolevulinic acid (ALA) than the surrounding normal skin. This might be the result of a higher percutaneous penetration of ALA into (pre)cancerous skin.

STUDY DESIGN/MATERIALS AND METHODS

ALA penetration through (1) healthy skin with intact stratum corneum, (2) healthy skin with reduced stratum corneum (i.e. tape stripped skin) and (3) diseased skin with dysplastic and thickened epidermis (chronically UVB-exposed skin) was determined in an in vitro model with hairless mouse skin.

RESULTS

More ALA had penetrated through chronically UVB-exposed skin than through normal non-exposed skin after 8 hours ALA application. The amount of ALA penetrated through chronically UVB-exposed skin was smaller than through tape stripped skin.

CONCLUSIONS

The stratum corneum barrier function is less effective in chronically UVB-exposed skin than in normal non-exposed skin, but more effective than in tape stripped skin. A higher penetration rate of ALA into (pre)cancerous lesions may be (partly) responsible for the greater accumulation of PpIX in such lesions.

Influence of the temperature in the adsorption of sodium dodecyl sulfate on phosphatidylcholine liposomes

The influence of the temperature on the adsorption of monomeric and micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS) on phosphatidylcholine (PC) liposomes was investigated using the fluorescent probe 2-(p-toluidinyl)-naphthalene-6-sodium sulfonate (TNS). The number of adsorbed molecules was quantified by measuring changes in the electrostatic potential (Psi(o)) of the liposomes/probe during an incubation with SDS at varying temperatures. At low surfactant concentrations (from 0.05 to 0.25 mM), the increase in temperature reduced the number of surfactant molecules incorporated per vesicle regardless of the incubation time, whereas at high surfactant concentrations (from 0.50 to 1.0 mM) the incubation time has an opposite effect on this process. Thus, after 10s, the surfactant adsorption decreased with temperature, yet it increased progressively with time. The adsorption was linear with temperature below critical micellar concentration (CMC) of SDS and this linear tendency did not change above CMC. This suggests an adsorption of SDS monomers regardless of the surfactant concentration.

Assembly properties and applications of a new exopolymeric compound excreted by Pseudoalteromonas antarctica NF3

The self assembly properties and applications of an exopolymeric compound (EC) of a glycoprotein character excreted by a new gram-negative species, Pseudoalteromonas antarctica NF3, have been reviewed. This compound exhibited surface-active properties in water, with a concentration of 0.20 mg ml(-1) being the key value associated with its physicochemical properties. Unsonicated EC aqueous dispersions showed the coexistence of concentric multilamellar and small unilamellar aggregates by transmission electron microscopy (TEM). Sonication of these dispersions revealed that each lamellae of the initial multilamellar structures were made up of various subunits coiled coils. As for the ability of this exopolymeric biomaterial to coat phosphatidylcholine (PC) liposomes and to protect these vesicles against different surfactants, freeze-fracture TEM micrographs of liposome/EC aggregates revealed that the addition of the EC to liposomes led to the formation of a film (polymer adsorbed onto the bilayers) that coated very well the PC bilayers. The complete coating was already achieved at a PC:EC weight ratio of about 9:1. An increasing resistance of PC liposomes to surfactants (in particular sodium dodecyl sulfate) occurred as the proportion of EC in the system rose, although this effect was more effective at low EC proportions (PC:EC weight ratios from 9:1 to 8:2). Although a direct dependence was found between the growth of the enveloping structure and the resistance of the coated liposomes to be affected by the surfactants, the best protection occurred when this structure was a thin film of about 20-25 nm formed by nine to ten layers of about 2-3 nm.

Skin sensitisation testing--new perspectives and recommendations

Various methodological aspects of skin sensitisation testing have been explored, particularly in the context of animal welfare considerations and reliability and sensitivity of test methods. Recommendations are made for the conduct of current and proposed OECD skin sensitisation tests with respect to appropriate test configurations for the purposes of hazard identification and labelling, and the requirement for positive controls. Specifically, the following aspects of guinea pig sensitisation test methods have been addressed: (1) the number of test and control animals required; (2) the option of using joint positive controls between independent laboratories; (3) the choice of positive control chemicals; (4) the optimal conduct and interpretation of rechallenge; and (5) the requirement for pretreatment with sodium lauryl sulfate. In addition, the use of the murine local lymph node assay (LLNA) has been considered. A number of conclusions have been drawn and recommendations made as follows: In many instances, particularly with the conduct of the guinea pig maximisation test, it is acceptable to halve the number of test and control animals used. An optional scheme for the conduct of joint positive control studies within a co-ordinated group of laboratories is appropriate. Only one positive control chemical (alpha-hexyl cinnamic aldehyde) is necessary for the routine assessment of assay sensitivity. The proper conduct and interpretation of rechallenge can provide valuable information and confirmation of results in guinea pig sensitisation tests. Sodium lauryl sulfate should no longer be used as a pretreatment in the guinea pig maximisation test. The LLNA is a viable and complete alternative to traditional guinea pig test methods for the purposes of skin sensitisation hazard identification. These recommendations provide the opportunity for both animal welfare benefits and improved hazard identification.

Microdialysis for the evaluation of penetration through the human skin barrier - a promising tool for future research?

The direct measurement of local drug concentration levels at discreet skin locations with minor trauma has recently become possible with the introduction of cutaneous microdialysis. Cutaneous microdialysis is an in vivo sampling technique for measuring solutes in the extracellular fluid of the dermis. When used in combination with other experimental approaches, for example with a variety of non-invasive techniques to describe the functional status of the skin (bioengineering methods), it may help investigators to gain new insights into the fields of skin diseases, metabolism and drug absorption/penetration. An important parameter to describe the efficacy of microdialysis is the relative recovery. This is the ratio between the concentration of a substance in the dialysate and the true extracellular concentration. Several methods are in common use to describe the relative recovery (no-net-flux method or retrodialysis). Parameters such as probe design, depth of the probe in the dermis, physico-chemical properties of the compound of interest, and analytical aspects are important factors influencing microdialysis. Microdialysis has been used to investigate the influence of penetration enhancers, vehicles or iontophoresis on percutaneous absorption, performed by in vivo studies in rats. In human volunteers, most of the experiments have been performed to study the kinetics of fast penetrating substances, e.g. nicotine, non-steroidal antiinflammatory drugs, local anaesthetics, or solvents. Problems have been encountered in the detection of lipophilic and highly protein-bound substances. Further, dermal metabolism and the influence of barrier perturbation on percutaneous absorption have been analyzed. Investigations suggest that microdialysis, in combination with traditional techniques, might give valuable information regarding the assessment of the penetration of drugs and other exogenous agents through the skin. In spite of the clearly defined and accepted advantages of microdialysis technology for studies of transdermal drug delivery, to date no standardized test procedure exists nor has the reproducibility of the results been evaluated. In the future, these problems have to be solved to enable this method to find its place in standard research.

Different stratum corneum lipid liposomes as models to evaluate the effect of the sodium dodecyl sulfate

The stability of stratum corneum (SC) liposomes against the action of surfactants has been revised. To this end, two types of vesicles were used; vesicles formed with the lipid and protein material extracted from SC, and lipid mixtures approximating the SC composition. In this case, the proportion of ceramides (Cer) and cholesteryl sulfate (Chol-sulf) was varied and the relative proportion of the other lipids remained constant. The increasing presence of these two lipids increased the resistance of liposomes against the action of the anionic surfactant sodium dodecyl sulfate (SDS). The rise in the cell-to-cell cohesion that occurred in recessive X-linked ichthyosis due to the accumulation of Chol-sulf could be associated in part to the enhanced stability of (Chol-sulf)-enriched bilayers. It is noteworthy that the surfactant partitioning between bilayers and the aqueous phase increased and decreased, respectively, as the proportion of Cer and Chol-sulf increased. This effect may be attributed to the variations in both the electrostatic interactions lipid-surfactant (electrostatic repulsion between the sulfate groups of both Chol-sulf and SDS), and the hydrophilic lipophilic balance of the lipid mixtures, in which Cer is replaced by the major polar lipid of the mixture (Chol-sulf). The fact that the free surfactant concentration was always smaller than its critical micelle concentration indicates that the permeability alterations were mainly ruled by the action of surfactant monomers, in agreement with the results reported for sublytic interactions of this surfactant with PC liposomes.

Alterations in stratum corneum lipid liposomes due to the action of triton X-100. Influence of the level of ceramides on this process

The role played by the ceramides (Cer) in the interaction of Triton X-100 (T(X-100)) with liposomes modeling the stratum corneum (SC) lipid composition was studied. The surfactant/lipid molar ratios (Re) and the bilayer/aqueous phase partition coefficients (K) were determined at sublytic level by monitoring the changes in the fluorescence intensity of liposomes due to the 5(6)-carboxyfluorescein (CF) released from the interior of vesicles. Higher and lower Cer proportions than those existing in the SC (40%) led respectively to a fall and to a rise in the surfactant ability to alter these liposomes. However, the surfactant partitioning between bilayers and water (bilayer affinity with vesicles) exhibited a maximum for 40% Cer. Thus, at low Cer proportions the ability of T(X-100) molecules to alter these bilayers was maximum despite their reduced partitioning into bilayers, in line with the reported interaction of the anionic surfactant model sodium dodecyl sulfate with these vesicles. These findings underline the fragility of these bilayers as an effective barrier and could explain in part the reported dependencies of low level of Cer in skin lipids and function barrier abnormalities. The fact that the free surfactant concentration needed to achieve the two interaction levels investigated was lower than the surfactant CMC indicates that permeability alterations were mainly ruled by the action of surfactant monomers, regardless of Cer proportion in bilayers.

Transdermal delivery of drugs for the treatment of bone diseases

The current status of transdermal drug delivery for the treatment of bone diseases is described in this review. The structure, physiology and function of skin and their importance in determining delivery into and across skin are discussed. Special emphasis has been devoted to a description of the major pathways of transport across the skin and the quite continuing controversy over the importance of the transfollicular route. An overview of anatomic site-dependent drug absorption is also provided and is particularly relevant to determination of transdermal patch location. Brief descriptions of the criteria for selection of transdermal drug candidate, transdermal patch designs and currently marketed transdermal products are also included. Transdermal estradiol delivery systems are examined in more detail for their clinical and biological effects. Finally, the feasibility of delivering drugs such as bisphosphonates across skin is discussed.

Roles for tumor necrosis factor receptor p55 and sphingomyelinase in repairing the cutaneous permeability barrier

Epidermal TNF expression increases in response to cutaneous permeability barrier disruption and wound healing. TNF signaling is mediated by acid and neutral sphingomyelinases (A- and N-SMase), which generate ceramide, an important regulator of proliferation, differentiation, and apoptosis. In the epidermis, ceramide is known to be an integral part of the extracellular stratum corneum (SC) lipid bilayers that constitute the permeability barrier of the skin. We show here that topical application of TNF after experimental injury to the SC of hairless mice (hr(-/-)) enhances barrier repair. In TNF receptor p55-deficient (TNF-R55-deficient) mice (hr(+/+)), cutaneous barrier repair was delayed compared with wild-type (hr(+/+)) or TNF-R75-deficient (hr(+/+)) animals. After barrier disruption in hairless (hr(-/-)) and wild-type (hr(+/+)), but not in TNF-R55-deficient (hr(+/+)) mice, the enzymatic activities of both A-SMase and N-SMase were significantly enhanced. Stimulation of SMase activities was accompanied by an increase in C(24)-ceramide levels. Most A-SMase activity in hairless mice (hr(-/-)) was found in the outer epidermal cell layers and colocalized in the lamellar bodies with A-SMase and sphingomyelin. Reduction of epidermal A-SMase activity by the inhibitor imipramine resulted in delayed permeability barrier repair after SC injury. Together, these results suggest that TNF-R55 signaling pathways contribute to cutaneous permeability barrier repair through SMase-mediated generation of ceramide.

Influence of ceramides in the solubilization of stratum corneum lipid liposomes by C(12)-betaine/sodium dodecyl sulfate mixtures

The solubilization of liposomes modeling the stratum corneum (SC) lipid composition and those obtained varying the proportion of ceramides by means of dodecyl betaine (C(12)-Bet)/sodium dodecyl sulfate (SDS) mixtures was studied. The surfactant/lipid molar ratios (Re) and the bilayer/aqueous phase partition coefficients (K) were determined by monitoring the changes in the static light scattering of the system during solubilization. The fact that the free surfactant concentration was always similar to its critical micelle concentration (CMC) indicates that the liposome solubilization was mainly ruled by the formation of mixed micelles. The mole fraction of the zwitterionic component (X(zwitter)) of 0.4 showed the lowest ability to saturate or solubilize liposomes, although exhibiting the highest degree of partitioning into liposomes. This X(zwitter) corresponded to the highest derivation of the CMCs of these mixtures (negative synergism) and to the highest reduction in the skin irritation with respect to the anionic component. Higher and lower proportion of ceramides in the mixture led to a fall and to a rise in both the activity and the partitioning of a specific surfactant mixture (X(zwitter)=0.4). This finding could be related to the recently reported dependences of the level of ceramides in skin and function barrier abnormalities. Comparison of the present Re and K values with those reported for phosphatidylcholine (PC) liposomes shows that, although SC liposomes were more resistant to the action of surfactant mixtures, the surfactant partitioning into SC bilayers was similar to that reported for PC ones in all cases.

Effect of barrier perturbation on cutaneous salicylic acid penetration in human skin: in vivo pharmacokinetics using microdialysis and non-invasive quantification of barrier function

We have used microdialysis in the dermis for assessing penetration kinetics of salicylic acid (SA) in healthy volunteers (n = 18), following application on the volar aspect of the left forearm. Penetration was monitored at four locations: in normal (unmodified) skin and in skin with perturbed barrier function from (i) repeated tape stripping (ii) irritant dermatitis from 1 or 2% sodium lauryl sulphate (SLS) for 24 h and (iii) delipidization by acetone. The order of the treatments was randomized according to a latin square design. Epidermal barrier function and skin irritation were assessed in each location using evaporimetry and colorimetry. Transepidermal water loss (TEWL) values confirmed that both mild (acetone), moderate (1% SLS) and severe barrier damage (tape stripping and 2% SLS) had occurred. Microdialysis sampling with two parallel probes in the dermis was performed in each of the four treatment areas for every subject. SA (5% in ethanol) was applied in a chamber glued to the skin overlying the microdialysis probes and sampling was continued for 4 h. SA was detectable in all samples and measurable in all samples from penetration through perturbed skin. Comparing the SA penetration in barrier-perturbed skin with the penetration in unmodified skin in the same subject, the mean SA penetration increase was 2.2-fold in acetone-treated skin (P = 0.012), 46-fold in mild dermatitis and 146- and 157-fold in severe dermatitis and tape stripped skin, respectively (P < 0.001). The penetration of SA significantly correlated with the measurements of barrier perturbation by TEWL (P = 0.01) and erythema (P = 0.02) for each individual. Microdialysis sampling of SA penetration was more sensitive than non-invasive measuring techniques in detecting significant barrier perturbation in acetone-treated skin. A positive dose-response relationship for the percutaneous penetration of SA in response to increasing SLS pretreatment concentrations and thus the degree of irritant dermatitis was found. When analysing data by location on the forearm, a tendency towards an intraregional variation in the reactivity to barrier damage was found, with the most proximal location displaying higher reactivity scores than the most distal location in response to the same barrier perturbation procedures. The penetration of SA was not significantly different between locations. In conclusion, using microdialysis in the dermis to obtain real-time dermal pharmacokinetics in the target organ, this study demonstrates highly increased and differentiated cutaneous penetration of SA in barrier-perturbed skin. The measured drug penetration was demonstrated to correlate with non-invasive quantification of barrier damage.

The effect of a sodium lauryl sulfate-free dentifrice on patients with recurrent oral ulceration

OBJECTIVE

Sodium lauryl sulphate (SLS) is the most commonly used detergent in dentifrices. Recent reports have suggested that it may exacerbate conditions with compromised epithelial integrity. The aim of this study was to compare the effect of an SLS-free dentifrice and an SLS-containing dentifrice on recurrent oral ulceration (ROU).

DESIGN

A double-blind crossover clinical trial was carried out during which subjects used an SLS dentifrice for 8 weeks and an SLS-free dentifrice for 8 weeks. Each phase was preceded by a 2-week washout period.

SETTING

A UK dental teaching hospital.

SUBJECTS AND METHODS

Forty-seven subjects completed the trial. They were all in the age range 10-62 years, had regularly recurrent oral ulceration reporting at least one to two ulcers per month and had normal levels of vitamin B12, ferritin and folate.

MAIN OUTCOME MEASURES

The trial phases were compared for the following ulcer parameters--number of ulcer days, total pain scores, number of ulcer episodes, and number of ulcers. Additional parameters compared were the number of ulcers per episode, ulcer duration, total pain per episode and ulcer size.

RESULTS

None of the ulcer parameters measured was significantly affected by the use of the SLS-free dentifrice as compared with the SLS dentifrice.

CONCLUSION

SLS-free dentifrice had no significant effect on ulcer pattern in the ROU study group.

Influence of the level of ceramides in the permeability of stratum corneum lipid liposomes caused by sodium dodecyl sulfate

The role played by the ceramides in the sublytic interactions of sodium dodecyl sulfate (SDS) with liposomes modeling the stratum corneum (SC) lipid composition was studied. The surfactant/lipid molar ratios (Re) and the bilayer/aqueous phase partition coefficients (k) were determined by monitoring the changes in the fluorescence intensity of liposomes due to the 5(6) carboxyfluorescein (CF) released from the interior of vesicles. The presence in liposomes of higher and lower ceramide proportions than that existing in the SC lipids led to a fall and to a rise in the sublytic activity of SDS on these structures. However, the SDS partitioning into liposomes (or affinity with these bilayer structures) increased as the proportion of Cer increased up to achieve almost a constant value for a Cer proportion similar to that in the SC lipids (about 40%). Thus, at low Cer proportions the ability of SDS molecules to alter these bilayer structures was higher than that for liposomes approximating the SC lipid composition despite their reduced partitioning into liposomes. These findings are in agreement with the recently reported dependencies of the level of ceramides in skin lipids and function barrier abnormalities and could explain in part these dependencies. The fact that the free surfactant concentration needed to achieve the two interaction levels investigated was lower than the surfactant critical micellar concentration (CMC) indicates that permeability alterations were mainly ruled by the action of surfactant monomers, regardless of the liposome lipid composition.

Transdermal drug delivery. Clinical aspects

The delivery of drugs into and through the skin is a recognized and effective means of therapy for dermatologic, regional, and systemic disease. The selection of drug candidates and the rational design of suitable formulations depends upon the biological make-up of the skin's barrier, and the physiochemical interactions between the membrane, the delivery system technology, and the active agent. This article summarizes the state of the art and examines more recent developments that are the subject of considerable research at this time. In addition, the potential to use the skin as a portal for noninvasive clinical chemistry (e.g., for glucose monitoring in diabetics) is discussed.

Transmission electron microscopy and light scattering studies on the interaction of a nonionic/anionic surfactant mixture with phosphatidylcholine liposomes

The interaction of an equimolecular mixture of nonylphenol polyethoxylated [NP(EO)10] and sodium dodecyl sulfate (SDS) surfactants with phosphatidylcholine (PC) liposomes was studied by means of transmission electron microscopy (TEM) and changes in the mean particle size (quasielastic light scattering; QELS) and in the static light scattering (SLS) of the system during liposome solubilization. A good correlation was found between the TEM diameter of particles and the mean hydrodynamic diameter (HD) determined by QELS. The aggregates resulting in this interaction were dependent on the surfactant concentration in the system. Thus, an initial vesicle growth occurred when the surfactant concentration was 15.98 mol%, together with the formation of a very small percentage of smaller particles. Additional surfactant amounts (28.32 mol%) led first to the formation of largest vesicles (HD 418 nm) and second to a fall in the vesicle diameter and in the SLS of the system. Thus, for 38.27 mol%, the TEM picture still showed the presence of vesicles, albeit with signs of disintegration. When additional amounts of surfactant were added to the system, the size curve started to show a bimodal distribution. Thus, for 51.81 mol% surfactant concentration, a sharp curve appeared at 51 nm, corresponding to the formation of small particles and TEM pictures clearly showed vesicle disintegration with formation of tubular structures. It is noteworthy that additional surfactant amounts (from 52 to 60 mol%) led to the formation of unclosed multilayered structures together with smaller aggregates. The gradual increase in the proportion of these smaller aggregates (mixed micelles) led to the complete solubilization of liposomes.

Changes in phosphatidylcholine liposomes caused by a mixture of Triton X-100 and sodium dodecyl sulfate

The mechanisms governing the interaction of equimolecular mixtures of Triton X-100 (Tx-100) and sodium dodecyl sulfate (SDS) with phosphatidylcholine liposomes were investigated. Permeability alterations were determined as a change in 5(6)-carboxyfluorescein released from the interior of vesicles and bilayer solubilization as a decrease in the static light-scattered by liposome suspensions. At subsolubilizing level, a maximum bilayer/water partitioning of surfactant mixture was reached at 30% CF release, which correlated with the increased presence of SDS in the bilayers. However, transition stages between 70% CF release and 100% light-scattering corresponded to the increased presence of Tx-100 in these structures. These findings may be correlated with the reduced deleterious effects caused by this mixture in different tissues versus pure SDS, given that the presence of Tx-100 may modulate the level of SDS partitioning in the human stratum corneum. At subsolubilizing level, the mixture showed higher affinity with bilayers than those reported for single components, whereas at solubilizing level this affinity was slightly lower and higher than those reported for Tx-100 and SDS respectively. A direct relationship was established in the initial interaction steps between the growth of vesicles, the leakage of entrapped CF and the effective molar ratio of surfactant to phospholipid in bilayers (Re). This dependence was also detected during solubilization, where the decrease in the vesicle size and in the scattered light of the system depended on the Re parameter and hence on the bilayer composition. The fact that the free surfactant concentration at subsolubilizing and solubilizing levels showed respectively lower and similar values than the critical micelle concentration (c.m.c.) of the surfactant mixture indicates that permeability alterations and solubilization were determined respectively by the action of surfactant monomer and by the formation of mixed micelles. This finding supports the generally admitted assumption, for single surfactants, that the concentration of free surfactant must reach the c.m.c. for solubilization to occur and highlights the influence of the negative synergism of this surfactant mixture on the free surfactant concentration needed to saturate or solubilize liposomes.