The prediction of percutaneous absorption: I. Influence of the dermis on in vitro permeation models

In percutaneous penetration in vitro techniques, excised full-thickness skin with its stratum corneum, viable epidermis and dermis is often used. Since penetrants can be absorbed in vivo immediately below the viable epidermis, the dermal layer could act as an additional barrier in the in vitro experiments relative to the actual in vivo process. In the present paper, in vitro penetration studies through excised hairless rat skin devoid of its dermal layer are reported and compared with those previously carried out with the excised full-thickness skin of the same animal. A homologous series of 4-alkylanilines was used in all the studies, and the correlations found between permeability coefficients and n-octanol partition coefficients were analyzed. Correlations are bilinear in nature in both cases, but in the absence of the dermal layer the correlation line seems to tend to hyperbolicity, as assessed by a significant increment in permeability coefficients for the highly lipophilic compounds of the series and by a displacement of the optimal lipophilicity value (vertex of the correlation line) to a higher partition coefficient. It can be concluded that the heterogeneous nature of the skin, as far as absorption is concerned, may be due to the presence of the two anatomical hydrophilic layers, dermis and viable epidermis, rather than to the stratum corneum itself. A critical review of the results reported in the literature showed good agreement with these conclusions. The biophysical penetration model was identical and the optimal lipophilicity values very similar, so it may be that these features are independent of the type of epidermis used (rat, mouse or man) and also of the chemical composition of the penetrants.     

In vitro skin penetration of acetyl hexapeptide-8 from a cosmetic formulation

There is a concern that peptides in cosmetic creams marketed as anti-aging/anti-wrinkle may penetrate into the deep layers of the skin and potentially stimulate biological activity. Claims for one cosmetic peptide, acetyl hexapeptide-8 (Ac-EEMQRR-amide), suggest interference with neuromuscular signaling as its anti-wrinkle mechanism of action. Therefore, the skin penetration of commercially available Ac-EEMQRR-amide from a cosmetic formulation (oil-in-water (O/W) emulsion) was determined in hairless guinea pig (HGP) and human cadaver skin assembled into in vitro diffusion cells. An O/W emulsion containing 10% Ac-EEMQRR-amide was applied to skin at a dose of 2?mg/cm². After a 24-h exposure, the skin surface was washed to remove unabsorbed peptide. Skin disks were tape stripped to determine the amount of peptide in the stratum corneum. Removal of the stratum corneum layers was verified by confocal microscopy. The epidermis was heat separated from the dermis and each skin fraction was homogenized. Skin penetration of Ac-EEMQRR-amide was measured in skin layers by hydrophilic interaction liquid chromatography with tandem mass spectrometry using electrospray ionization (ESI) in the positive mode. Stable isotopically labeled hexapeptides were used as internal standards for the quantitation of native hexapeptides to correct for matrix effects associated with ESI. The results (percent of applied dose) showed that the majority of the Ac-EEMQRR-amide was washed from the surface of both HGP and human skin. Ac-EEMQRR-amide that penetrated skin remained mostly in the stratum corneum of HGP (0.54%) and human (0.22%) with the peptide levels decreasing as each layer was removed by tape stripping. Total Ac-EEMQRR-amide found in the epidermis of HGP and human skin was similar at 0.01%. No peptide was detected in the dermis or buffer collected underneath the skin for both human and HGP. There was no hexapeptide metabolite (H₂N-EEMQRR-amide) detected in any layers of HGP skin, human skin or buffer collected underneath the skin. This skin penetration data will be useful for evaluating the safety of cosmetic products containing small peptide cosmetic ingredients.     

In Vitro Percutaneous Absorption of Arildone,a Highly Lipophilic Drug,and the Apparent No-Effect of the Penetration Enhancer Azone in Excised Human Skin

Purpose. Arildone, a novel lipophilic antiviral drug when evaluated in Clinical Trials showed limited skin absorption and antiviral efficacy. These studies were conducted to explain the apparent poor absorption characteristics and attempt to promote skin absorption by using Azone, a penetration enhancer. Methods. Standard in vitro skin permeation methods using excised human skin were employed to characterise the absorption of Arildone. ¹⁴C-Arildone was used to estimate the distribution in skin layers by scintigraphic and autoradiographic procedures. Results. The aqueous solubility and distribution constant values for Arildone were 2 µg ml^–1 and 5 × 10⁵ (isopropyl myristate/water), respectively. Absorption through full thickness skin or stratum corneum-viable epidermal membranes (diffusional resistant dermis removed), from a propylene glycol vehicle, was slow and the addition of Azone had no effect on the permeation rate. Distribution studies showed accumulation of Arildone in the stratum corneum. The concentration of Arildone in the viable epidermis was estimated from sectioning the skin and was found to be in sufficient amounts (400 µg cm^–3) to have potential antiviral activity. Conclusions. The apparent accumulation of Arildone in the stratum corneum suggested that the hydrophilic skin region presented the main barrier to permeation. Azone which affected the permeability of the stratum corneum was therefore not effective at enhancing Arildone absorption. Vehicles which readily permeate and enhance the transfer of lipophilic drugs from the stratum corneum into the viable epidermis were recommended.     

Pimecrolimus: skin disposition after topical administration in minipigs in vivo and in human skin in vitro.

The dermal disposition of pimecrolimus, a non-steroid, anti-inflammatory calcineurin inhibitor used for the treatment of atopic dermatitis, was evaluated in minipigs in vivo and in human skin in vitro using tritium-radiolabeled compound, and in dermal toxicokinetic investigations in minipigs using unlabeled compound. Following topical application of pimecrolimus 1% market form (MF) cream to minipig skin, approximately 2% of the dose penetrated into the stratum corneum and part of it into deeper skin layers. The remainder of the dose was recovered non-absorbed on the skin surface. The total systemic absorption was or=94% of dose remained non-absorbed, 3.1% was found in the epidermis (including stratum corneum) and 2.9% in the dermis. There was no indication of metabolism of pimecrolimus in human skin in vitro or minipig skin in vivo. No drug accumulation was observed in minipig skin after up to 13 weeks of once daily topical application of 0.1% or 0.3% pimecrolimus cream.     

Percutaneous and Systemic Disposition of Hexamethylene Lauramide and Its Penetration Enhancement Effect on Hydrocortisone in a Rat Sandwich Skin-Flap Model

The percutaneous absorption and distribution profile of hexamethylene lauramide (hexahydro-1-lauroyl-lH-azepine) were examined using a rat skin-flap model. After a topical dose to the skin flap, the drug concentrations in the vasculature at the site of drug application and in the systemic blood were monitored simultaneously. Hexamethylene lauramide penetrated the skin and reached a steady state in stratum corneum, viable epidermis, dermis, and cutaneous blood in 3 hr. Its concentration in the skin was much higher than that in the blood. Its apparent concentration in the epidermis was 19 times that in the dermis and about 3000 times that in the cutaneous blood. The percutaneous absorption of ¹⁴C-hexamethylene lauramide resulted in ascending systemic blood concentrations throughout the experimental period, whereas the cutaneous blood levels remained steady. The topically absorbed hexamethylene lauramide was quantitatively recovered in urine (85%) and feces (13%). The half-lives of urinary and fecal excretion of ¹⁴C-hexamethylene lauramide were 17 and 30 hr, respectively. Hexamethylene lauramide, when topically coadministered in an experimental formulation, enhanced the skin penetration of hydrocortisone with increased drug contents in the stratum corneum (2-fold) and with increased hydrocortisone concentrations in the cutaneous blood (3.4-fold) and the systemic blood (3.5-fold). The results indicated that the high concentration and retention of hexamethylene lauramide in stratum corneum and viable epidermis may contribute to its penetration enhancement effect in the skin. A steady state in percutaneous tissues was observed before the drug reached distribution equilibrium systemically. The systemic blood concentration of a topically applied agent therefore may not reflect its percutaneous kinetic processes before a systemic distribution equilibrium is reached. Temporal profiles of a topical penetration enhancer in the skin and in the body are important information for the development of dematologic preparations for the treatment of skin disorders.     

Dermal Stability and In Vitro Skin Permeation of Collagen Pentapeptides (KTTKS and palmitoyl-KTTKS)

Collagen pentapeptide (Lys-Thr-Thr-Lys-Ser, KTTKS) and its palmitoylated derivative (pal-KTTKS) have received a great deal of attention as cosmeceutical ingredients for their anti-wrinkle effects. The objective of this study was to evaluate stability and permeability of KTTKS and pal-KTTKS in hairless mouse skin. In this study, a liquid chromatography-tandem mass spectrometric method was developed for the quantification of pal-KTTKS, and used for stability and permeability studies. Stability studies were performed using skin extracts and homogenates. Both KTTKS and pal-KTTKS were rapidly degraded, but pal-KTTKS was more stable than KTTKS. When protease inhibitors were added, the stability of both compounds (KTTKS and pal-KTTKS) improved significantly. In the skin permeation study, neither KTTKS nor pal-KTTKS was detected in the receptor solution, which indicates that neither compound could permeate through the full-thickness hairless mouse skin in the experimental conditions of this study. While KTTKS was not detected in any of the skin layers (the stratum corneum, epidermis, and dermis), pal-KTTKS was observed in all skin layers: 4.2 ± 0.7 μg/cm² in the stratum corneum, 2.8 ± 0.5 μg/cm² in the epidermis, and 0.3 ± 0.1 μg/cm² in the dermis. In conclusion, this study indicated that pal-KTTKS had greater stability and permeability than that of un-modified KTTKS, and may be a useful anti-wrinkle and anti-aging cosmeceutical agent.     

Prediction of the percutaneous penetration and metabolism of dodecyl decaethoxylate in rats using in vitro models

 Percutaneous absorption of a lipophilic surfactant, dodecyl decaethoxylate, can be predicted using in vitro models. In vivo, dermal penetration of dodecyl decaethoxylate was found to be 22.9% in 48 h. All of the absorbed dodecyl decaethoxylate in the rat was metabolised and excreted in expired air as carbon dioxide, or in the urine and faeces. Using rat skin mounted in the unoccluded flow-through diffusion cell with MEM as receptor fluid, in vivo absorption was predicted by the percentage of the applied dose recovered in the stratum corneum, epidermis, dermis and receptor fluid at 24 h (25%). Conversely, the penetration of dodecyl decaethoxylate was over-predicted in the unoccluded static diffusion cell using aqueous ethanol (50% v/v) as the receptor fluid where 49.4% recovered in the receptor fluid at 24 h. In vitro models may be used to predict percutaneous absorption and reduce animal use, provided a suitable receptor fluid is used in which the penetrant is soluble. Dermal metabolism of dodecyl decaethoxylate was low and not considered to influence dermal absorption. Received: 9 November 1994/Accepted: 7 February 1995     

Partition coefficient and diffusion coefficient determinations of 50 compounds in human intact skin,isolated skin layers and isolated stratum corneum lipids

A standard protocol was used to determine partition (K) and diffusion (D) coefficients in dermatomed human skin and isolated human skin layers for 50 compounds relevant to cosmetics ingredients. K values were measured in dermatomed skin, isolated dermis, whole epidermis, intact stratum corneum (SC), delipidized SC and SC lipids by direct measurements of the radioactivity in the tissue layers/lipid component vs. buffer samples. D determinations were made in dermatomed skin, isolated dermis, whole epidermis and intact SC using a non-linear regression of the cumulative receptor fluid content of radiolabeled compound, fit to the solution of Fick's 2nd Law. Correlation analysis was completed between K, D, and physicochemical properties. The amount of interindividual (donor) and intraindividual (replicate) variability in the K and D data was characterized for each skin layer and chemical. These data can be further used to help inform the factors that influence skin bioavailability and to help improve in silico models of dermal penetration.     

Safety assessment by multiphoton fluorescence/second harmonic generation/hyper-Rayleigh scattering tomography of ZnO nanoparticles used in cosmetic products

Zinc oxide nanoparticles (ZnO NPs) are commonly used as UV filters in commercial sunscreen products. Their penetration into the skin is intensively discussed in the literature. In the present in vivo study, penetration of ZnO NPs (30 nm in size) into human skin was investigated by multiphoton tomography. Based on the non-linear effects of a second harmonic generation and hyper-Rayleigh scattering, the distribution of ZnO NPs in the horny layers of the epidermis, as well as the furrows, wrinkles and orifice of the hair follicles was analyzed. This method permitted distinguishing between the particulate and dissolved forms of Zn. A detection limit of 0.08 fg/μm(3) was estimated. Taking advantage of this sensitivity, it was clearly shown that ZnO NPs penetrate only into the outermost layers of stratum corneum, furrows and into the orifices of the hair follicles and do not reach the viable epidermis.     

Permeation enhancer dodecyl 6-(dimethylamino)hexanoate increases transdermal and topical delivery of adefovir: Influence of pH, ion-pairing and skin species

Adefovir (9-(2-phosphonomethoxyethyl)adenine) is an acyclic nucleoside phosphonate currently used for the treatment of hepatitis B. The aim of this study was to evaluate the effect of permeation enhancer DDAK (6-dimethylaminohexanoic acid dodecyl ester) on the transdermal and topical delivery of adefovir. In porcine skin, DDAK enhanced adefovir flux 42 times with maximum at pH 5.8 suggesting ion pair formation. DDAK increased thermodynamic activity and stratum corneum/vehicle distribution coefficient of adefovir, as well as it directly decreased the skin barrier resistance. Maximal flux was observed already at 2% adefovir + 1% DDAK. The results were confirmed in freshly excised human skin where DDAK enhanced adefovir flux 179 times to 8.9 μg/cm²/h. This rate of percutaneous absorption would allow for reaching effective plasma concentrations. After the topical application, adefovir concentrated in the stratum corneum with low penetration into the deeper skin layers from either aqueous or isopropyl myristate vehicle without the enhancer. With 1% DDAK, adefovir concentrations in the viable epidermis and dermis were 33–61 times higher. These results offer an attractive alternative to established routes of administration of adefovir and other acyclic nucleoside phosphonates.     

Permeation enhancer dodecyl 6-(dimethylamino)hexanoate increases transdermal and topical delivery of adefovir: Influence of pH,ion-pairing and skin species

Adefovir (9-(2-phosphonomethoxyethyl)adenine) is an acyclic nucleoside phosphonate currently used for the treatment of hepatitis B. The aim of this study was to evaluate the effect of permeation enhancer DDAK (6-dimethylaminohexanoic acid dodecyl ester) on the transdermal and topical delivery of adefovir. In porcine skin, DDAK enhanced adefovir flux 42 times with maximum at pH 5.8 suggesting ion pair formation. DDAK increased thermodynamic activity and stratum corneum/vehicle distribution coefficient of adefovir, as well as it directly decreased the skin barrier resistance. Maximal flux was observed already at 2% adefovir + 1% DDAK. The results were confirmed in freshly excised human skin where DDAK enhanced adefovir flux 179 times to 8.9 μg/cm²/h. This rate of percutaneous absorption would allow for reaching effective plasma concentrations. After the topical application, adefovir concentrated in the stratum corneum with low penetration into the deeper skin layers from either aqueous or isopropyl myristate vehicle without the enhancer. With 1% DDAK, adefovir concentrations in the viable epidermis and dermis were 33–61 times higher. These results offer an attractive alternative to established routes of administration of adefovir and other acyclic nucleoside phosphonates.     

Autoradiographic study of the localization of 2,2', 4,4', 5,5'-hexachlorobiphenyl in liver and skin tissue after in vitro uptake

Cellular distribution of the lipophilic environmental pollutant, 2,2', 4,4', 5,5'-hexachlorobiphenyl (6-CB), was determined by autoradiography after in vitro uptake of the 14C-labeled compound into liver and skin tissue preparation. Light microscopic data of cryostat sections showed that 6-CB is homogeneously distributed in liver tissue. In skin the distribution pattern depended on the conditions of incubation. If skin slices were incubated with 6-CB prior to preparation of cryostat thin sections, the epidermis was practically free of 6-CB and the radioactivity was found mainly in the stratum reticulare of the dermis. If, however, cryostat sections of skin were directly incubated, 6-CB was more homogeneously distributed with an accumulation in the epidermis. Liver and skin sections delipidated with acetone or 95% ethanol prior to incubation with 6-CB took up very little of the compound. Delipidation of sections preincubated with 6-CB resulted in total extraction of 6-CB. The results suggest that human stratum corneum is a barrier to the penetration of 6-CB and that this compound is located in lipid structures of liver and skin.     

Fate of chemicals in skin after dermal application: does the in vitro skin reservoir affect the estimate of systemic absorption?

Recent international guidelines for the conduct of in vitro skin absorption studies put forward different approaches for addressing the status of chemicals remaining in the stratum corneum and epidermis/dermis at the end of a study. The present study investigated the fate of three chemicals [dihydroxyacetone (DHA), 7-(2H-naphtho[1,2-d]triazol-2-yl)-3-phenylcoumarin (7NTPC), and disperse blue 1 (DB1)] in an in vitro absorption study. In these studies, human and fuzzy rat skin penetration and absorption were determined over 24 or 72 h in flow-through diffusion cells. Skin penetration of these chemicals resulted in relatively low receptor fluid levels but high skin levels. For DHA, penetration studies found approximately 22% of the applied dose remaining in the skin (in both the stratum corneum and viable tissue) as a reservoir after 24 h. Little of the DHA that penetrates into skin is actually available to become systemically absorbed. 7NTPC remaining in the skin after 24 h was approximately 14.7% of the applied dose absorbed. Confocal laser cytometry studies with 7NTPC showed that it is present across skin in mainly the epidermis and dermis with intense fluorescence around hair. For DB1, penetration studies found approximately 10% (ethanol vehicle) and 3% (formulation vehicle) of the applied dose localized in mainly the stratum corneum after 24 h. An extended absorption study (72 h) revealed that little additional DB1 was absorbed into the receptor fluid. Skin levels should not be considered as absorbed material for DHA or DB1, while 7NTPC requires further investigation. These studies illustrate the importance of determining the fate of chemicals remaining in skin, which could significantly affect the estimates of systemically available material to be used in exposure estimates. We recommend that a more conclusive means to determine the fate of skin levels is to perform an extended study as conducted for DB1.     

Skin disposition of menthol after its application in the presence of drug substances

Many drug products that are applied onto the skin contain menthol. Menthol plays a dual role in the analgesic and anti-inflammatory drugs: it causes cooling and local anesthetic effects and, being a penetration enhancer, it increases the skin permeation of the drug substances. However, there are no data concerning the skin penetration of menthol after its application in the most commonly used vehicles and in the presence of drug substances. Therefore, this study evaluated the ex vivo skin disposition of menthol after application of the commercially available drug products containing aluminum acetotartrate, methyl salicylate, ibuprofen and naproxen, using full human-skin mounted in flow-through diffusion cells. After 15, 30 and 60 min of application, the skin was progressively tape-stripped into three fractions of stratum corneum and the remaining epidermis with dermis. The content of menthol in the skin layers was determined by GC method. Varying degrees of penetration of menthol into the skin layers was observed, depending on its amount in the vehicle and the presence of drug substance. In the presence of aluminum acetotartrate, the skin penetration of menthol was limited only to the outer fraction of the stratum corneum. In the case of drug products containing naproxen, the concentration of the drug substance significantly influenced the skin penetration of menthol.     

In Vitro Skin Absorption of the Topically Applied Metal Ion-Chelating Agent Diethylenetriaminepenta-acetic Acid

Diethylenetriaminepenta-acetic acid (DTPA) is a metal ion-chelating agent that has antimicrobial properties and potential therapeutic properties against metal-induced toxicities such as nickel allergy. In this study, the absorption properties of DTPA applied topically to rat skin are investigated in vitro, using a flow-through diffusion skin absorption model. [¹⁴C]DTPA was applied in solution in 60% ethanol (pH 6). Overall skin penetration into receptor fluid resulting from a topical dose of 0.13 mg/cm² DTPA for 24 h was low at 1.27%. The local tissue distribution of DTPA was investigated using microautoradiography, and effects on the tissue were assessed by histology. Diethylenetriaminepenta-acetic acid was primarily associated with the stratum corneum and upper layers of the skin; minimal levels were observed in the dermis.     

Glucose Partition Coefficient and Diffusivity in the Lower Skin Layers

Purpose This work aims to estimate the diffusivity and partitioning of glucose in the dermis and the viable epidermis of human skin. Methods The partition coefficient of glucose between phosphate-buffered saline and dermis, tape-stripped epidermis (TSE), stratum corneum (SC), and split-thickness skin, was measured in vitro using human cadaver skin. Glucose permeability across dermis and tape-stripped split-thickness skin (TSS) was measured using side-by-side diffusion cells. Glucose desorption from TSE and human epidermal membrane (HEM) was measured. All measurements were conducted at 32°C. Results The partition coefficient for glucose [mean ± SD (no. of samples)] was 0.65 ± 0.09 (n = 25) for dermis, 0.81 ± 0.06 (n = 10) for TSE, and 0.53 ± 0.12 (n = 9) for SC. Glucose diffusivity in dermis was calculated to be 2.64 ± 0.42 × 10⁻⁶ cm²/s (n = 14). Glucose diffusivities in the viable epidermis estimated from TSS permeation, TSE desorption, and HEM desorption were 0.075 ± 0.050 × 10⁻⁶ cm²/s (n = 5), 0.037 ± 0.018 × 10⁻⁶ cm²/s (n = 4), and 1.0 ± 0.6 × 10⁻⁶ cm²/s (n = 4), respectively. Conclusion The tissue/buffer partition coefficient of glucose in all skin layers was found to be less than unity, suggestive of excluded volumes in each layer. Glucose diffusivity in human dermis was found to be one third of its value in water, indicative of hindered diffusion related to the structural components of the tissue. A substantially lower value for glucose diffusivity in viable epidermis is suggested.     

Quantum dot penetration into viable human skin

Systematic studies probing the effects of nanoparticle surface modification and formulation pH are important in nanotoxicology and nanomedicine. In this study, we use laser-scanning fluorescence confocal microscopy to evaluate nanoparticle penetration in viable excised human skin that was intact or tape-stripped. Quantum dot (QD) fluorescent nanoparticles with three surface modifications: Polyethylene glycol (PEG), PEG-amine (PEG-NH?) and PEG-carboxyl (PEG-COOH) were evaluated for human skin penetration from aqueous solutions at pH 7.0 and at pHs of solutions provided by the QD manufacturer: 8.3 (PEG, PEG-NH?) and 9.0 (PEG-COOH). There was some penetration into intact viable epidermis of skin for the PEG-QD at pH 8.3, but not at pH 7.0 nor for any other QD at the pHs used. Upon tape stripping 30 strips of stratum corneum, all QDs penetrated through the viable epidermis and into the upper dermis within 24 h.     

Effect of barrier disruption by acetone treatment on the permeability of compounds with various lipophilicities: implications for the permeability of compromised skin.

The permeability of compromised skin barrier was investigated in vitro using acetone-disrupted hairless mouse skin as a model membrane. The effect of compound lipophilicity was studied using sucrose, caffeine, hydrocortisone, estradiol, and progesterone as model compounds. The results demonstrated that permeability barrier disruption by acetone treatment significantly enhanced the permeability of the skin to both hydrophilic and amphipathic compounds, including sucrose, caffeine and hydrocortisone. This effect was more prominent with caffeine and hydrocortisone at different transepidermal water loss (TEWL) levels. Acetone treatment, however, didn't appear to alter the percutaneous penetration of highly lipophilic compounds, such as estradiol and progesterone. The characteristics of skin permeability were described by parabolic relationships between log P(WS) (permeability coefficient of whole skin) and log K(O/W) (octanol/water partition coefficient) at different degrees of permeability barrier disruption. The optimal log K(O/W) of compounds for skin penetration appeared to decrease with an increase in TEWL levels. The maximal permeability achieved was similar through skin displaying different TEWL levels. In an attempt to explore the underlying mechanisms for the changes in skin permeability, the stratum corneum/normal saline partition coefficients of water, caffeine, and hydrocortisone either decreased or remained unaffected with an increase in TEWL. Electron microscopic examinations have revealed reductions in stratum corneum lipid content and alterations in intercellular membrane structures as a result of acetone treatment, whereas negligible changes in the number of horny layers were observed by safranin staining of the stratum corneum. We have concluded that the enhancement in skin permeability to both hydrophilic and amphipathic compounds by acetone treatment arose mainly because of the increase in stratum corneum diffusivity at higher TEWL levels. The results imply the possibility of using both TEWL and drug lipophilicity to predict alterations in skin permeability and hence the dose adjustment of topically applied medication for patients with impaired skin barrier function.     

Topical bioavailability of triamcinolone acetonide: effect of occlusion

BACKGROUND/AIMS: Occlusion by covering the skin with an impermeable wrap enhances skin hydration, affects drug absorption and can induce the formation of a drug reservoir within the stratum corneum. This is desired in local therapy with topical corticosteroids. The aim of the study was to investigate the effect of occlusion before (experiment 1) and after (experiment 2) application on the penetration of triamcinolone acetonide (TACA) into the stratum corneum. METHODS: The experiments were conducted on the forearms of 10 healthy volunteers. In experiment 1, 100 microg/cm(2) TACA in acetone were applied on 3 sites per arm, one arm having been pre-occluded for 16 h. In experiment 2, the same dose was applied on 2 sites per arm, and one arm was occluded after application until skin sampling. Stratum corneum samples were removed by tape stripping at 0.5, 4 and 24 h (experiment 1) and 4 and 24 h (experiment 2) after application. Corneocytes and TACA were quantified by ultraviolet-visible spectroscopy and HPLC, respectively. The total TACA amount penetrated into the stratum corneum was evaluated by multifactor ANOVA. RESULTS: TACA penetration into the stratum corneum with and without pre-occlusion (experiment 1) showed no significant difference and decreased with time. Occlusion after application (experiment 2) produced a marked TACA accumulation within the stratum corneum, which persisted for 24 h. CONCLUSION: Pre-occlusion showed no effect on the topical bioavailability of TACA in the stratum corneum. In contrast, post-occlusion enhanced the TACA penetration by a factor of 2, favouring the development of a drug reservoir.     

In Vivo and in Vitro Analysis of Skin Penetration Enhancement Based on a Two-Layer Diffusion Model with Polar and Nonpolar Routes in the Stratum Corneum

In vitro and in vivo skin penetration of three drugs with different lipophilicities and the enhancing effects of l-geranylazacycloheptan-2-one (GACH) were studied in rats. In vivo drug absorption profiles obtained by deconvolution of urinary excretion profiles were compared to the corresponding in vitro data obtained with a diffusion experiment. In vivo skin penetration of lipophilic butylparaben was considerably greater than that observed in vitro, while hydrophilic mannitol and acyclovir showed low penetration in both systems without GACH pretreatment. On the other hand, GACH enhanced mannitol and acyclovir penetration, especially in the in vivo system. Analysis of absorption profiles, using a two-layer skin model with polar and nonpolar routes in the stratum corneum, suggested that the diffusion length of a viable layer (viable epidermis and dermis) was shorter in vivo than in vitro and the effective area of the polar route in the stratum corneum was larger in vitro without GACH pretreatment. GACH increased the partitioning of acyclovir into the nonpolar route to the same extent in both systems. In addition, GACH increased the effective area of the polar route in vivo, probably because of enhanced water permeability; however, this effect was smaller in vitro since the stratum corneum was already hydrated even without GACH pretreatment.