Molecular weight dependence of polyethylene glycol penetration across acetone-disrupted permeability barrier

Occupational exposure assessment with solid substances: choosing a vehicle for in vitro percutaneous absorption experiments

Percutaneous occupational exposure to industrial toxicants can be assessed in vitro on excised human or animal skins. Numerous factors can significantly influence skin permeation of chemicals and the flux determination. Among them, the vehicle used to solubilize the solid substances is a tricky key step. A "realistic surrogate" that closely matches the exposure scenario is recommended in first intention. When direct transposition of occupational exposure conditions to in vitro experiments is impossible, it is recommended that the vehicle used does not affect the skin barrier (in particular in terms of structural integrity, composition, or enzymatic activity). Indeed, any such effect could alter the percutaneous absorption of substances in a number of ways, as we will see. Potential effects are described for five monophasic vehicles, including the three most frequently used: water, ethanol, acetone; and two that are more rarely used, but are realistic: artificial sebum and artificial sweat. Finally, we discuss a number of criteria to be verified and the associated tests that should be performed when choosing the most appropriate vehicle, keeping in mind that, in the context of occupational exposure, the scientific quality of the percutaneous absorption data provided, and how they are interpreted, may have long-range consequences. From the narrative review presented, we also identify and discuss important factors to consider in future updates of the OECD guidelines for in vitro skin absorption experiments.

Chromium-induced skin damage among Taiwanese cement workers

Little research has been done on the relationships between chromium exposure, skin barrier function, and other hygienic habits in cement workers. Our purpose was to investigate chromium-induced skin barrier disruption due to cement exposure among cement workers. One hundred and eight cement workers were recruited in this study. Urinary chromium concentration was used to characterize exposure levels. The biological exposure index was used to separate high and low chromium exposure. Transepidermal water loss (TEWL) was used to assess the skin barrier function. TEWL was significantly increased in workers with high chromium exposure levels than those with low chromium exposure levels ( p = 0.048). A positive correlation was also found between urinary chromium concentration and TEWL ( R = 0.28, p = 0.004). After adjusting for smoking status and glove use, a significant correlation between urinary chromium concentrations and TEWL remained. Moreover, workers who smoked and had a high chromium exposure had significantly increased TEWL compared to nonsmokers with low chromium exposure ( p = 0.01). Skin barrier function of cement workers may have been disrupted by chromium in cement, and smoking might significantly enhance such skin barrier perturbation with chromium exposure. Decreased chromium skin exposure and smoking cessation should be encouraged at work.

Skin membrane electrical impedance properties under the influence of a varying water gradient

The stratum corneum (SC) is an effective permeability barrier. One strategy to increase drug delivery across skin is to increase the hydration. A detailed description of how hydration affects skin permeability requires characterization of both macroscopic and molecular properties and how they respond to hydration. We explore this issue by performing impedance experiments on excised skin membranes in the frequency range 1 Hz to 0.2 MHz under the influence of a varying gradient in water activity (aw). Hydration/dehydration induces reversible changes of membrane resistance and effective capacitance. On average, the membrane resistance is 14 times lower and the effective capacitance is 1.5 times higher when the outermost SC membrane is exposed to hydrating conditions (aw = 0.992), as compared to the case of more dehydrating conditions (aw = 0.826). Molecular insight into the hydration effects on the SC components is provided by natural-abundance (13)C polarization transfer solid-state NMR and x-ray diffraction under similar hydration conditions. Hydration has a significant effect on the dynamics of the keratin filament terminals and increases the interchain spacing of the filaments. The SC lipids are organized into lamellar structures with ∼ 12.6 nm spacing and hexagonal hydrocarbon chain packing with mainly all-trans configuration of the acyl chains, irrespective of hydration state. Subtle changes in the dynamics of the lipids due to mobilization and incorporation of cholesterol and long-chain lipid species into the fluid lipid fraction is suggested to occur upon hydration, which can explain the changes of the impedance response. The results presented here provide information that is useful in explaining the effect of hydration on skin permeability.

Glycerol and urea can be used to increase skin permeability in reduced hydration conditions

The natural moisturizing factor (NMF) is a group of hygroscopic molecules that is naturally present in skin and protects from severe drying. Glycerol and urea are two examples of NMF components that are also used in skin care applications. In the present study, we investigate the influence of glycerol and urea on the permeability of a model drug (metronidazole, Mz) across excised pig skin membranes at different hydrating conditions. The degree of skin hydration is regulated by the gradient in water activity across the membrane, which in turn depends on the water activity of the formulation in contact with the skin membrane. Here, we determine the water activity of all formulations employed using an isothermal calorimetric method. Thus, the gradient in water activity is controlled by a novel experimental set-up with well-defined boundary conditions on both sides of the skin membrane. The results demonstrate that glycerol and urea can retain high steady state flux of Mz across skin membranes at dehydrating conditions, which otherwise would decrease the permeability due to dehydration. X-ray diffraction measurements are performed to give insight into the effects of glycerol and urea on SC molecular organization. The novel steady state flux results can be related to the observation that water, glycerol, and urea all affect the structural features of the SC molecular components in a similar manner.

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.

Size of bicelle defects probed via diffusion nuclear magnetic resonance of PEG

Diffusion of various poly(ethylene glycol) (PEG) tracers of well-defined molecular weight and narrow polydispersity confined within the aqueous interstices between positively magnetically aligned bicelles was measured using pulsed-field-gradient (1)H nuclear magnetic resonance. The bicelles consisted of mixtures of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG), and dihexanoylphosphatidylcholine (DHPC) in the molar ratios q = [100 DMPC +5 DMPG]/[DHPC] = 3.5, 4.5, and 5.5, to which Yb(3+) had been added in the ratio 1:75 Yb(3+)/phospholipid. The field gradients were applied such that diffusion was measured in the direction parallel to the normal to the bicelles' planar regions, thereby rendering the experiment sensitive to the ability of PEG to traverse lamellar defects within the bicelles. The pulsed-field-gradient nuclear magnetic resonance diffusive intensity decays were diffusion-time-independent in all cases, with diffusive displacements corresponding to many hundreds of bicellar lamellae. This permitted a description of such diffusive decays in terms of a mean behavior involving a combination of straight obstruction effects common to all PEG, with hindrance to diffusion proportional to the relative size of a given PEG with respect to the size of the lamellar defects. Across the range of PEG molecular weights (200-4600) and bicelle compositions examined, the apparent radial dimension of the lamellar defects decreased from 165 A with q = 3.5 to 125 A with q = 5.5. This is opposite to the trend predicted from static geometric models of either bicelle disks or perforated lamellae. Qualitatively, the observed trend suggests that mobility of the obstructions to diffusion will need to be considered to reconcile these differences.

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.

Altered Penetration of Polyethylene Glycols into Uninvolved Skin of Atopic Dermatitis Patients

Involved regions of the skin in atopic dermatitis (AD) patients have an altered barrier function. Whether uninvolved skin also has a diminished barrier is controversial. To assess the barrier function of uninvolved skin in AD patients, the percutaneous penetration of polyethylene glycols (PEGs) of various molecular sizes was determined in vivo in AD patients and control subjects using tape stripping of the stratum corneum (SC). The diffusion and partition coefficients were determined using Fick's second law of diffusion. The SC thickness was similar in both groups; however, the trans-epidermal water loss was higher in atopic skin. The apparent diffusion coefficient of PEGs through atopic skin was twice as high as through normal skin, and decreased with increasing molecular weight (MW) in both groups. The partition coefficient in the skin of AD patients was half of that for normal skin but as for normal skin, there was no MW dependency. Although atopic skin exhibited altered barrier with respect to diffusion and partitioning, the permeability coefficients were nearly the same for atopic and normal skin. The results support the assumption of altered skin barrier of AD patients even in the skin that is visibly unaffected by disease.

Increased permeability for polyethylene glycols through skin compromised by sodium lauryl sulphate

In this in vivo human study we assessed the influence of skin damage by sodium lauryl sulphate (SLS) on percutaneous penetration of polyethylene glycols (PEGs) of different molecular weights (MW). Percutaneous penetration of PEGs was determined using tape stripping of the stratum corneum (SC). The forearm skin of volunteers was pretreated with 5% w/w SLS for 4 h, and 24 h later patches with PEGs were applied for 6 h. The penetration parameters were deduced by data regression to Fick's law for unsteady-state diffusion. The trans-epidermal water loss (TEWL) increased after SLS treatment from 6.3 +/- 2.1 to 17.9 +/- 8.7 g/m(2)/h. The diffusion coefficient for all PEGs was increased in the SLS-damaged skin. The increase was smaller for higher MW. In addition, the partition coefficient of PEGs between SC and water was larger in the SLS-compromised skin and showed a tendency to increase with MW. The permeability coefficient decreased gradually with increasing MW of PEGs in both control and SLS-compromised skin. SLS caused a threefold increase in the permeability coefficient for all MWs ranging in control skin from 0.34 to 0.70 x 10(-5) cm/h and in the SLS-compromised skin from 1.20 to 2.09 x 10(-5) cm/h for MW of 590-282 Da. The results of this study show the deleterious effect of SLS on the skin barrier for hydrophilic PEGs. A defective skin barrier will facilitate absorption of other chemicals and local skin effects.

Effect of occupational exposure to rayon manufacturing chemicals on skin barrier to evaporative water loss

To evaluate the effects of the occupational exposure to rayon manufacturing chemicals (RMC, containing predominantly carbon disulfide (CS(2)) and minor sulfuric acid) in a rayon factory on the basal transepidermal water loss (TEWL), barrier integrity (BI), and sequential increasing TEWL profiles. Six Thais and five Chinese workers in the spinning department of a rayon manufacturing plant and five healthy unexposed controls were recruited as the test subjects. An area of 4.5 x 5.5 cm on the mid-side of the volar forearm on the right hand was stripped by means of moderate pressure with commercially available adhesive tape by the same technician throughout the experiment. The skin was progressively stripped until glistening. TEWL was measured at every three and five tape strips on the right hand. The corresponding site on the left hand was measured parallel as the self-control. We found significant differences in basal TEWL and in BI between Chinese workers and Chinese controls, and between Thai workers and Chinese workers, respectively. Two-stage patterns of progressive TEWL profiles were found in such a chronic and repeated occupational exposure to RMC containing CS(2). The occupational exposure to RMC could result in the perturbation of the skin barrier function. Basal TEWL might be more sensitive to chronic skin irritant exposure. The TEWL profile achieved to the glistening stage might be necessary to avoid erroneous pattern estimation. Due to the lack of Thais control in this study, the racial difference in response to the RMC warrants further study.

Amphiphilic star-like macromolecules as novel carriers for topical delivery of nonsteroidal anti-inflammatory drugs

The objective of this study was to evaluate amphiphilic star-like macromolecules (ASMs) as a topical drug delivery system. Indomethacin, piroxicam, and ketoprofen were individually encapsulated into the ASMs using coprecipitation. The effects of the ASMs on percutaneous permeation of nonsteroidal anti-inflammatory drugs (NSAIDs) across full thickness, hairless mouse skin were evaluated in vitro using modified Franz diffusion cells. In addition, solubility and in vitro release experiments were performed to characterize ASMs behavior in aqueous media. Poly(ethylene glycol) (PEG) and Pluronic P-85 were used as polymer controls to compare the role of PEG and amphiphilic behavior in the ASMs. In vitro release experiments indicated that ASMs can delay drug release (P <.05), whereas solubility measurements showed that ASMs can increase NSAIDs aqueous solubility (P <.05). Percutaneous permeation studies revealed that ASMs decreased both flux and Q24 of drugs compared with the control (P <.10). Skin pretreatment studies with ASM-containing solution before drug application demonstrated that pretreatment similarly influenced NSAID percutaneous permeation. In conclusion, ASMs likely slow drug permeation through 2 mechanisms, delayed drug diffusion from its core and skin dehydration by its shell. Thus, ASMs may be useful for delayed dermal delivery or prevention of compound permeation through the skin (eg, sunscreens, N,N-diethyl-m-toluamide [DEET]) from aqueous formulations.

Modeling skin permeability to hydrophilic and hydrophobic solutes based on four permeation pathways

Barrier properties of skin originate from low permeability of stratum corneum. The objective of this paper is to compile fundamentally-based analytical expressions that can be used to predict skin permeability to hydrophilic as well as hydrophobic solutes. Solute permeation through four possible routes in stratum corneum including free-volume diffusion through lipid bilayers, lateral diffusion along lipid bilayers, diffusion through pores, and diffusion through shunts was analyzed. Contribution of free-volume diffusion through lipid bilayers was determined using Scaled Particle Theory. This theory relates solute partition and diffusion coefficients to the work required to create cavities in a lipid bilayers to allow solute incorporation and motion. Contribution of lateral lipid diffusion was determined from the literature data. Contribution of pores was estimated using hindered transport theory. This theory assumes that hydrophilic solutes permeate across the skin through imperfections in the lipid bilayers modeled as pores. Finally, contribution of shunts was determined using a simple diffusion model. The model yielded a series of equations to predict skin permeability based on solute radius and octanol-water partition coefficient. Predictions of the model compare well with the experimental data.

In vitro/in vivo correlations between transdermal delivery of 5-aminolaevulinic acid and cutaneous protoporphyrin IX accumulation and effect of formulation

BACKGROUND

Photodynamic therapy (PDT) using topical application of 5-aminolaevulinic acid (ALA) has been widely reported for the treatment of a variety of neoplastic and non-neoplastic cutaneous diseases. Although different formulations containing variable amounts of ALA have been applied in PDT, the dose-response relationships between transdermal ALA delivery and cutaneous protoporphyrin IX (PpIX) accumulation have not been studied.

OBJECTIVES AND METHODS

The objectives of this study were to investigate the effect of permeability barrier function, ALA concentration and formulation on the in vitro penetration of ALA through nude mouse skin and cutaneous PpIX formation at 2 h following a 2-h application of ALA to nude mouse skin in vivo, and to delineate the relationships in between.

RESULTS

Results demonstrated that variations in barrier integrity, in addition to ALA concentration, profoundly influenced ALA delivery to generate PpIX. Saturable correlations were found to exist between PpIX concentrations in both the epidermis and dermis in vivo and its transdermal flux in vitro, and the relationships were well described by the Emax model. The established correlations based on pure aqueous solutions were applicable to different formulations containing hydroxypropylmethylcellulose as the gelling agent and ethylenediamine tetraacetic acid as the iron chelator. Moreover, incorporation of desferrioxamine, another iron chelator, in the formulation prolonged cutaneous PpIX accumulation in the skin in comparison with 3% ALA aqueous solution, but the peak PpIX levels were not increased. Application of a liposomal formulation resulted in similar prolongation in ALA-induced PpIX accumulation, as well as better epidermal targeting.

CONCLUSIONS

Knowledge of the dose-response relationships and the effect of formulation is important for designing optimal formulations and treatment schedules for topical ALA-PDT.