Comparative Analysis of Percutaneous Absorption Enhancement by d-Limonene and Oleic Acid Based on a Skin Diffusion Model

Percutaneous absorption-enhancing effects of d-limonene and oleic acid were investigated using three model drugs with different lipophilicities in in vitro diffusion experiments with guinea pig skin. Pretreatment of the skin with d-limonene resulted in a large penetration enhancement for the lipophilic butylparaben (BP) and amphiphilic 6-mercaptopurine (6-MP) but had little effect on the hydrophilic mannitol (MT). Oleic acid caused a large effect only on 6-MP penetration. The penetration profiles were analyzed with a two-layer skin diffusion model consisting of stratum corneum with polar and nonpolar routes and viable epidermis plus dermis. Through curve-fitting, six parameters corresponding to drug diffusivity and partitioning in these three regions of the skin were obtained, and the mechanisms of enhancers were assessed in comparison with those of l-geranylazacycloheptan-2-one (GACH) reported previously. Increased penetration was caused mainly by modification of the barrier property of the nonpolar route in the stratum corneum in all cases. In the nonpolar route, d-limonene increased mainly drug diffusivity, while GACH enhanced predominately drug partitioning. On the other hand, oleic acid moderately increased both parameters.     

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.     

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.     

Skin penetration of nonsteroidal antiinflammatory drugs out of a lipophilic vehicle: influence of the viable epidermis

The skin penetration of 10 nonsteroidal antiinflammatory drugs (NSAIDs) was investigated after application in the lipophilic vehicle light mineral oil. The skin permeabilities and maximum fluxes, which were calculated from the concentration decreases of the applied solutions in the steady state phases, were correlated with physicochemical parameters, mainly the vehicle solubilities and the partition coefficients of the model drugs according to the Fickian diffusion laws. The objective of the study was to characterize the barrier function of the stratum corneum and the viable epidermis and to predict their influences on the skin permeabilities and the maximum fluxes of the NSAIDs by model equations. The permeability of the human skin for NSAIDs applied in a lipophilic vehicle is a function of their hydrophilicity, while the maximum flux is primarily dependent on their vehicle solubilities. The viable epidermis was found to represent the decisive resistance to the drug transport.     

Supersaturated solutions evaluated with an in vitro stratum corneum tape stripping technique

The flux of a compound across a membrane from any formulation, whether it contains penetration enhancers or not, is limited by its saturated solubility in the vehicle. Under such conditions the concentration of the permeant in the outer layers of the stratum corneum is also saturated. Consequently, when the permeation of a drug from a supersaturated solution leads to enhanced penetration, the concentration of the drug in the outer layers of the membrane is also supersaturated. Therefore, the stratum corneum may possess antinucleant properties which inhibit or retard the crystallisation process. In this study, the enhanced in vitro permeation of supersaturated solutions of piroxicam across human skin in diffusion cells was demonstrated. The amount of permeant in the stratum corneum was determined using a tape stripping technique. Supersaturated solutions up to four degrees of saturation were investigated which produced a linear relationship between the degree of saturation and the amount of piroxicam in the stratum corneum (R² = 0.970). Furthermore, the amount of piroxicam in the viable layers of the skin also increased with increasing degree of saturation. An analysis of the results suggested that enhanced penetration across human skin from supersaturated solutions of piroxicam may occur as a result of the antinucleating ability of the intercellular lipids of the stratum corneum.     

On-Line Visualization of Dye Diffusion in Fresh Unfixed Human Skin

PURPOSE: The purpose of the current study was to develop a new method to examine the diffusion in fresh unfixed human skin on-line. METHODS: Full thickness skin samples were cut perpendicular to the skin surface (cutting plane facing upwards) with a new cutting device forming part of the final diffusion cell. The donor solution contained 0.1 mg/ml Bodipy FL C5 (moderately lipophilic) dissolved in citric acid buffer, pH 5.0, and the acceptor phase consisted of phosphate-buffered saline, pH 7.4. Images were taken with confocal laser scanning microscopy (CLSM) every 10 min for 8 h. RESULTS: This new method enabled for the first time visualization of concentration profiles in different skin layers simultaneously as a function of time. For this model penetrant, Bodipy FL C5 showed that the lower stratum corneum layer constitutes the greatest barrier to diffusion. Furthermore, there is preferred partitioning of this probe in epidermis vs. either stratum corneum or dermis. CONCLUSIONS: The on-line diffusion cell in combination with CLSM is a promising tool to study diffusion processes of dyes in fresh unfixed skin on-line. The method has the potential to access deeper skin layers as well as to visualize diffusion processes in cells.     

Analysis of the Combined Effect of 1-Menthol and Ethanol as Skin Permeation Enhancers Based on a Two-Layer Skin Model

The combined effects of 1-menthol and ethanol as a skin permeation enhancer were evaluated with two equations describing the permeability coefficient through full-thickness skin (P _FT) and the full-thickness skin/vehicle concentration ratio (C _FT/C _V) of drugs as a function of their octanol/vehicle partition coefficient (K _OV). A two-layer model was applied for skin, which consists of a stratum corneum (SC) with lipid and porous pathways and a viable epidermis and dermis (ED). The two equations contain one variable (K _OV) and nine coefficients, six of which (three diffusion coefficients, the porosity of the SC, and two terms of the linear free energy relationship) were considered different, dependent on the drug vehicle. In vitro permeation of four drugs (morphine hydrochloride, atenolol, nifedipine, and vinpocetine) was determined using excised hairless rat skin and four aqueous vehicles (water, 5% 1-menthol, 40% ethanol, and 5% l-menthol–40% ethanol) to measure each P _FT. Drug concentrations in full-thickness skin were also measured to obtain C _FT/C _V. A nonlinear least-squares method was employed to determine six coefficients using the two equations and experimentally obtained P _FT and C _FT/C _V. The addition of 1-menthol to water and 40% ethanol increased the diffusion coefficient of drugs in lipid and pore pathways of SC, whereas the addition of ethanol to water and 5% 1-menthol increased the drug solubility in the vehicle, decreased the skin polarity, and increased the contribution of the pore pathway to whole-skin permeation.     

Theoretical analysis of the effect of cutaneous metabolism on skin permeation of parabens based on a two-layer skin diffusion/metabolism model

The effect of cutaneous metabolism on the skin penetration of drugs was analyzed based on a two-layer skin diffusion/metabolism model. In vitro permeation studies of propylparaben and butylparaben with or without an esterase inhibitor, diisopropyl fluorophosphate (DFP), were performed. Pretreatment of the skin with DFP prolonged the lag time for the penetration of intact parabens. Additionally, DFP significantly decreased the total flux of butylparaben, but not that of propylparaben. Model analysis of the penetration profiles revealed that DFP inhibits the cutaneous metabolism without affecting any other processes. To comprehensively understand the relationships among lipophilicity, metabolic rate, and skin permeation of drugs, simulation studies were performed with newly derived equations concerning the permeability coefficient and the lag time for the penetration of both intact and metabolite forms. The analysis revealed that the lag time for the penetration of both intact and metabolite forms becomes shorter with increasing metabolic rate. As the metabolic rate of the drug increases, skin penetration of the intact form decreases whereas that of the metabolite increases. The total flux of intact and metabolite forms increases with increasing metabolic rate, being more obvious for highly lipophilic drugs. This indicates that the permeation of lipophilic compounds such as butylparaben is more highly affected by cutaneous metabolism in the viable layer because these compounds easily penetrate the stratum corneum layer. Consequently, the balance between the permeability of drug across the stratum corneum and the dermis has been implicated to impose a significant influence on the percutaneous absorption of drugs subjected to cutaneous metabolism.     

Unexpected Clobetasol Propionate Profile in Human Stratum Corneum After Topical Application in Vitro

PURPOSE: The validity of using drug amount-depth profiles in stratum corneum to predict uptake of clobetasol propionate into stratum corneum and its transport into deeper skin layers was investigated. METHODS: In vitro diffusion experiments through human epidermis were carried out using Franz-type glass diffusion cells. A saturated solution of clobetasol propionate in 20% (V/V) aqueous propylene glycol was topically applied for 48 h. Steady state flux was calculated from the cumulative amount of drug permeated vs. time profile. Epidermal partitioning was conducted by applying a saturated drug solution to both sides of the epidermis and allowing time to equilibrate. The tape stripping technique was used to define drug concentration-depth profiles in stratum corneum for both the diffusion and equilibrium experiments. RESULTS: The concentration-depth profile of clobetasol propionate in stratum corneum for the diffusion experiment is biphasic. A logarithmic decline of the drug concentration over the first four to five tape strips flattens to a relatively constant low concentration level in deeper layers. The drug concentration-depth profile for the equilibrium studies displays a similar shape. CONCLUSIONS: The shape of the concentration-depth profile of clobetasol propionate is mainly because of the variable partitioning coefficient in different stratum corneum layers.     

Diffusion modeling of percutaneous absorption kinetics: 2. Finite vehicle volume and solvent deposited solids

The diffusion model for percutaneous absorption is developed for the specific case of delivery to the skin being limited by the application of a finite amount of solute. Two cases are considered; in the first, there is an application of a finite donor (vehicle) volume, and in the second, there are solvent-deposited solids and a thin vehicle with a high partition coefficient. In both cases, the potential effect of an interfacial resistance at the stratum corneum surface is also considered. As in the previous paper, which was concerned with the application of a constant donor concentration, clearance limitations due to the viable eqidermis, the in vitro sampling rate, or perfusion rate in vivo are included. Numerical inversion of the Laplace domain solutions was used for simulations of solute flux and cumulative amount absorbed and to model specific examples of percutaneous absorption of solvent-deposited solids. It was concluded that numerical inversions of the Laplace domain solutions for a diffusion model of the percutaneous absorption, using standard scientific software (such as SCIENTIST, MicroMath Scientific software) on modern personal computers, is a practical alternative to computation of infinite series solutions. Limits of the Laplace domain solutions were used to define the moments of the flux-time profiles for finite donor volumes and the slope of the terminal log flux-time profile. The mean transit time could be related to the diffusion time through stratum corneum, viable epidermal, and donor diffusion layer resistances and clearance from the receptor phase. Approximate expressions for the time to reach maximum flux (peak time) and maximum flux were also derived. The model was then validated using reported amount-time and flux-time profiles for finite doses applied to the skin. It was concluded that for very small donor phase volume or for very large stratum corneum-vehicle partitioning coefficients (e.g., for solvent deposited solids), the flux and amount of solute absorbed are affected by receptor conditions to a lesser extent than is obvious for a constant donor constant donor concentrations.     

Finite dose transport of drugs in liquid formulations through stratum corneum: analytical solution to a diffusion model

Finite dose penetration of acetaminophen, through snake stratum corneum has been studied using modified Franz cells. The drug was formulated in a volatile: non-volatile vehicle system. Non-steady-state transport of the drug into the receptor compartment is observed. A diffusion model for the above situation is proposed. An analytical solution for this model has been obtained. This solution is used to obtain computer-simulated transport profiles. The experimental data is then fitted with the simulated profile to obtain a value for the apparent diffusion coefficient of the drug. Using steady-state experiments, an independent value of the diffusion coefficient is also obtained. The two values of the diffusion coefficient differ by a factor of 4.17.     

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.     

氢醌经不同皮肤层的吸收差异

目的 :评价皮肤角质层和真皮层对药物经皮吸收的差异。方法 :选择氢醌 (HQ)为模型药物 ,采用Franz吸收池法 ,考察药物经完整皮肤和剥离角质层皮肤的体外透皮能力Kp ,并比较吸收促进剂肉豆蔻酸异丙酯(IPM)共存时的促透能力大小。结果 :HQ经剥离角质层皮肤的Kp 是经完整皮肤的3 29倍 ,加入IPM后HQ的Kp 分别提高到原来的4 95倍 (经完整皮肤 )和7 49倍 (经剥离角质层皮肤 )。结论 :本实验为皮肤病态条件 ,如皮肤受伤或溃疡等时的药物经皮吸收规律研究提供了一种新的方法。     

Correlation between stratum corneum/water-partition coefficient and amounts of flufenamic acid penetrated into the stratum corneum

The stratum corneum of various donors differs in particular in the composition of the lipoidal phase. Considering the drug amounts penetrating into the stratum corneum a simple methodology to correlate these differences in the stratum corneum composition with the drug amounts detectable within the stratum corneum is desirable. Penetration experiments investigating several incubation times were carried out with three different skin flaps using the Saarbruecken penetration model and the lipophilic model drug flufenamic acid. The drug amounts within the stratum corneum were obtained with the tape-stripping technique, while the drug amounts present in the deeper skin layers were achieved by cryosectioning. The stratum corneum/water-partition coefficient was determined with the same three skin flaps to characterize the lipoidal stratum corneum phase in general, and the differences were attributed to the different amounts of ceramides and sterols. In addition, for the lipophilic drug flufenamic acid, a direct linear correlation was found between the stratum corneum/water-partition coefficients and the drug amounts penetrated into the stratum corneum for all investigated time intervals (correlation coefficients of r(30 min) = 0.998, r(60 min) = 0.998 and r(180 min) = 0.987). In contrast to the stratum corneum/water-partition coefficients, the determination of a corresponding relationship for the stratum corneum and the deeper skin layers failed due to the reason that steady-state conditions could not be achieved for the deeper skin layers during the investigated time intervals. In summary, the stratum corneum/water-partition coefficients offer the possibility to predict drug amounts within the stratum corneum of different donor skin flaps without a time consuming determination of the lipid composition of the stratum corneum.     

盐酸尼卡地平贴剂的制备及体外透皮释药

目的：制备盐酸尼卡地平透皮贴剂,研究其释药影响因素。方法：利用Ｆｒａｎｚ扩散池,进行离体皮肤体外渗透实验,通过改变皮肤的状态,研究其体外透皮释药。结果：盐酸尼卡地平贴剂中药物的渗透可用Ｈｉｇｕｃｈｉ方程来表达,去除角质层皮肤的药物渗透远大于完整皮肤,而完整皮肤的贮库效应大于去角质层皮肤。结论：盐酸尼卡地平在经皮渗透过程中存在贮库效应,渗透的主要屏障为角质层。     

Analysis of Simultaneous Transport and Metabolism of Ethyl Nicotinate in Hairless Rat Skin

Purpose. Simultaneous skin transport and metabolism of ethyl nicotinate (EN), a model drug, were measured and theoretically analyzed. Methods. Several permeation studies of EN or its metabolite nicotinic acid (NA) were done on full-thickness skin or stripped skin with and without an esterase inhibitor. Permeation parameters such as partition coefficient of EN from the donor solution to the stratum corneum and diffusion coefficients of EN and NA in the stratum corneum and the viable epidermis and dermis were determined by these studies. Enzymatic parameters (Michaelis constant K _m and maximum metabolism rate V _max were obtained from the production rate of NA from different concentrations of EN in the skin homogenate. Obtained permeation data were then analyzed by numerical method based on differential equations showing Fick's second law of diffusion in the stratum corneum and the law with Michaelis-Menten metabolism in the viable epidermis and dermis. Results. Fairly good steady-state fluxes of EN and NA through the skin were obtained after a short lag time for all the concentrations of EN applied. These steady-state fluxes were not proportional to the initial donor concentration of EN: EN and NA curves were concave and convex, respectively, which suggests that metabolic saturation from EN to NA takes place in the viable skin at higher EN application. The steady-state fluxes of EN and NA calculated by the differential equations with resulting permeation and enzymatic parameters were very close to the obtained data. Conclusions. The present method is a useful tool to analyze simultaneous transport and metabolism of many drugs and prodrugs, especially those showing Michaelis-Menten type-metabolic saturation in skin.     

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.     

Overcoming the stratum corneum: the modulation of skin penetration. A review

It is preferred that topically administered drugs act either dermally or transdermally. For that reason they have to penetrate into the deeper skin layers or permeate the skin. The outermost layer of the human skin, the stratum corneum, is responsible for its barrier function. Most topically administered drugs do not have the ability to penetrate the stratum corneum. In these cases modulations of the skin penetration profiles of these drugs and skin barrier manipulations are necessary. A skin penetration enhancement can be achieved either chemically, physically or by use of appropriate formulations. Numerous chemical compounds have been evaluated for penetration-enhancing activity, and different modes of action have been identified for skin penetration enhancement. In addition to chemical methods, skin penetration of drugs can be improved by physical options such as iontophoresis and phonophoresis, as well as by combinations of both chemical and physical methods or by combinations of several physical methods. There are cases where skin penetration of the drug used in the formulation is not the aim of the topical administration. Penetration reducers can be used to prevent chemicals entering the systemic circulation. This article concentrates on the progress made mainly over the last decade by use of chemical penetration enhancers. The different action modes of these substances are explained, including the basic principles of the physical skin penetration enhancement techniques and examples for their application.     

Supersaturation: enhancement of skin penetration and permeation of a lipophilic drug

Purpose. To increase the dermal delivery of a lipophilic model compound (LAP), and to deduce the underlying mechanism of enhanced absorption. Methods. Penetration of LAP from mixtures of up to four degrees of saturation into the stratum corneum was evaluated using a tape-stripping method; epidermal permeation of the drug was measured in Franz diffusion cells. The relative diffusion and stratum corneum-vehicle partition coefficients of LAP were determined by fitting the results to the appropriate solutions to Fick's second law of diffusion. Results. Both the skin permeation rate and the amount of LAP in the stratum corneum increased linearly with increasing degree of saturation. The apparent diffusivity and its partition coefficient deduced from the penetration experiments were independent of the degree of saturation of the drug in the applied formulation, and consistent with corresponding parameters derived from the permeation experiments. Conclusions. Supersaturation can increase the skin penetration and permeation of lipophilic drugs. The diffusion and partition parameters deduced for LAP indicate that supersaturation acts exclusively via increased thermodynamic activity without apparent effect on the barrier function of the skin per se.     

蛇床子素凝胶药物经皮吸收的体外研究

目的体外测定含有渗透促进剂的蛇床子素凝胶经人体皮肤的吸收.方法以离体人皮肤为渗透模型,应用Franz扩散池进行实验.样品以高效液相法测定蛇床子素的含量.结果与对照组相比,渗透促进剂Azone、薄荷醇、土荆芥油可以使得蛇床子素的稳态流量分别提高3.12、2.00、1.25倍.结论三种渗透促进剂的作用机理为破坏了皮肤角质层的屏障作用,降低了药物的扩散阻力,因而提高了蛇床子素的扩散系数.