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.     

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.     

Penetration Profile of Taurine in the Human Skin and Its Distribution in Skin Layers

PURPOSE: To measure in vitro release of taurine from a semisolid standard formulation (amphiphilic cream, DAC) containing 1% taurine, a multi-layer membrane system was used. The content and distribution of taurine in different healthy skin layers (stratum corneum, epidermis and dermis) before (native taurine) and after application of the DAC cream were determined using capillary electrophoresis. METHODS: The release of taurine from the DAC cream was studied using a multilayer membrane system. Due to the high hydrophilic properties of taurine, the artificial model membranes consisted of collodion as matrix and glycerol as the acceptor phase. In order to determine whether taurine shows the potential for dermal penetration a Franz diffusion cell system was used. The distribution of taurine in the skin layers was determined before and after application of the DAC cream followed by the incubation in a Franz diffusion cell. The excised skin sample was cut in horizontal sections using a cryomicrotome. In order to detect taurine, fluorescamine was used as a derivatization agent. RESULTS: Experiments with a multilayer membrane system were performed to verify the release of taurine at different times (1, 2 and 5 h). Approximately 42.5% taurine was released from the semisolid standard formulation, accumulating in the first membrane (17.63%). The native taurine content was quantified in human isolated skin layer before and after the application of the semisolid standard formulation followed by incubation in a Franz-type diffusion cell for 1 and 5 h. No statistically significant difference (p < 0.05) of the taurine content in the skin layers existed between exposure times (1 and 5 h) studied. The highest taurine content was found in the epidermis both before (256.01 microg taurine/g skin layer) and after (555.5 microg taurine/g skin layer) the application of the DAC cream. CONCLUSIONS: The distribution profile of taurine in the skin layers was very similar for the times studied, which suggests that taurine is accumulated in specific cells of the skin. The study suggests that taurine is effectively released from the semisolid standard formulation and can be used for topical application in dermatopharmaceutics.     

Analysis of arterial–venous blood concentration difference of drugs based on recirculatory theory with fast inverse laplace transform (FILT)

An arterial and venous blood (or plasma) concentration difference of drugs across the lung of rats was evaluated based on the recirculatory concept. The recirculatory system is given by the combination of the transfer functions for the pulmonary and the systemic circulations and is described by a Laplace-transformed equation, i.e., an image equation. For the manipulation of the image equations, the fast inverse Laplace transform (FILT) was adopted and MULTI(FILT) was used for the simultaneous curve fitting to estimate the pharmacokinetic parameters in the recirculatory model. Metoprolol as a test drug and cephalexin as a control drag were infused, respectively into the femoral vein for 30 min, and arterial and venous blood samples were collected simultaneously through the cannula at the femoral artery and at right atrium during and after the infusion. Exponential functions were assumed for the weight functions through both the pulmonary and systemic circulations. Results of the curve fitting showed that the single-pass extraction ratio through the pulmonary circulation (E_p)of meloprolol was about 0.2, whereas that of cephalexin was negligible. The mean transit times through the pulmonary circulation (¯t_p of metoprolol and cephalexin were both about 0.5 min, which is small. The singlepass extraction ratios through the systemic circulation (E_s)of metoprolol and cephalexin were both about 0.1. and the mean transit times through the systemic circulation (¯t_s were 11.5 min and 8.2 min, respectively.     

Spectral Reflectance Measurement of Evaporating Chemical Films: Initial Results and Application to Skin Permeation

The present study has 2 aims. First, the method of spectral reflectance was used to measure evaporation rates of thin (～25-300 μm) films of neat liquid volatile organic chemicals exposed to a well-regulated wind speed u. Gas-phase evaporation mass transfer coefficient (k_evap) measurements of 10 chemicals, 9 of which were measured at similar u, are predicted (slope of log-log data = 1.01; intercept = 0.08; R² = 0.996) by a previously proposed mass transfer correlation. For one chemical, isoamyl alcohol, the dependence of k_evap on u^0.52 was measured, in support of the predicted exponent value of ½. Second, measured k_evap of nicotine was used as an input in analytical models based on diffusion theory to estimate the absorbed fraction (F_abs) of a small dose (5 μL/cm²) applied to human epidermis in vitro. The measured F_abs was 0.062 ± 0.023. Model-estimated values are 0.066 and 0.115. Spectral reflectance is a precise method of measuring k_evap of liquid chemicals, and the data are well described by a simple gas-phase mass transfer coefficient. For nicotine under the single exposure condition measured herein, F_abs is well-predicted from a theoretical model that requires knowledge of k_evap, maximal dermal flux, and membrane lag time.     

Kinetic Analysis of the Drug Permeation Process Across the Intestinal Epithelium

The rat intestinal lumen and the blood vessel were simultaneously perfused to study drug permeation across the intestinal epithelium. On the basis of drug disappearance from the intestinal lumen and its appearance into the vascular outflow, the mean time required for permeation across the intestinal membrane (MPT) and the permeation clearance (CLp) were calculated. MPT values of water, antipyrine, propranolol, imipramine and mannitol, varied from 0.45 min to 9.91 min depending on their physicochemical property. From both MPT and CLp, five drugs were classified as being (i) highly and rapidly absorbed (water, antipyrine), (ii) highly but slowly absorbed (propranolol, imipramine) and (iii) low and slowly absorbed (mannitol). Permeation profiles of these drugs were analyzed based on the diffusion model which defined the parameter for each permeation process, i.e. partitioning to and diffusion through the epithelium and clearance into the blood flow. Propranolol and imipramine partitioned into the membrane at a higher level than the other drugs. However, the clearance of both drugs from the epithelium was extremely slow, suggesting that this process is the rate-limiting step in their permeation. On the other hand, the rate-limiting step in the permeation of water and antipyrine was found to be the diffusion process in the epithelial layer.     

Diffusion and Metabolism of Prednisolone Farnesylate in Viable Skin of the Hairless Mouse

The diffusion and metabolism of prednisolone 21-farnesylate were investigated in viable skin of the hairless mouse in vitro. The pro-drug ester was extensively metabolized in viable skin, while it was stable in the donor and receptor solutions. The rate of appearance of the prodrug and its metabolite prednisolone was markedly influenced by the direction of the skin placed between the in vitro diffusion half-cells. The rate of bioconversion of the prodrug was determined as a function of the distance from the surface of the skin. The prodrug was increasingly metabolized with the distance from the surface of the skin, indicating that the responsible enzymes are enriched in the lower layers of the viable skin. A model with linearly increasing enzyme activity in the viable skin accounts for the in vitro profiles of the diffusion/metabolism of the prodrug in the viable skin of hairless mouse.     

Microscopic Models of Drug/Chemical Diffusion Through the Skin Barrier: Effects of Diffusional Anisotropy of the Intercellular Lipid

Owing to the systematic alignment and ordering of fatty acid and ceramide chains, lipid layers in biological membranes have strongly anisotropic diffusion properties. The diffusivity $D_{\parallel }^{\mathrm{lip}}$ for solute transport in the direction parallel to the lipid layer is typically 10³-10⁵ times the diffusivity $D_{\perp }^{\mathrm{lip}}$ for the perpendicular direction. This article explores the consequences of this strong degree of anisotropy on solute diffusion through the stratum corneum (barrier) layer of the skin based on a realistic representation of a unit cell of the microstructure. Complementary numerical methods (smoothed particle hydrodynamics, finite differences) are used to solve the steady-state unit-cell diffusion problem leading to the average (homogenized, coarse-grained) diffusion tensor characterizing the tissue as an effective continuum. A parametric study is presented characterizing solute concentration profiles in detail for testosterone- and caffeine-like permeants, and it is shown that the results cannot be mimicked by calculations based on an isotropic lipid-phase diffusivity. The ratio of lateral to transdermal effective diffusivities calculated by the present model is of the order of 40 and 300 for fully hydrated (in vitro) and partially hydrated (in vivo) states, respectively. These values compare favorably with the results of recent experiments.     

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.     

In Vivo Evaluation of Acyclovir Prodrug Penetration and Metabolism Through Rat Skin Using a Diffusion/Bioconversion Model

Purpose. In order to evaluate the in vivo penetration of prodrugs which undergo metabolism in skin, we analyzed thein vivo penetration profiles of acyclovir prodrugs based on a two-layer skin diffusion model in consideration of metabolic process. Methods. Acyclovir prodrugs (e.g., valerate, isovalerate and pivarate) were used as model prodrugs and the amounts excreted in urine were measured after percutaneous application. In vivo penetration profiles were then estimated by employing a deconvolution method and the penetration of acyclovir prodrugs was analyzed using a diffusion model. Subsequently, diffusion, partitioning and metabolic parameters were compared under in vitro and in vivo conditions. Results. Although total penetration amounts at the end of the experiment were similar for the three prodrugs, the ratio of intact prodrug to total penetration amount differed significantly. Moreover, the excretion and absorption profiles were also very different for each prodrug. Enzymatic hydrolysis rate constants calculated under in vivo conditions were considerably larger than those obtained in the skin homogenate and in vitro penetration experiments. Conclusions. The present skin diffusion/bioconversion model combined with computer analysis enables us to comprehensively account for diffusion, partitioning and metabolism during in vivo percutaneous absorption. Nevertheless, different enzymatic hydrolysis rate constants obtained under bothin vivo and in vitro conditions demonstrate the difficulty of obtaining accurate values for in vivo enzymatic activity from related in vitro experiments.     

The Effect of Ultrasound on the in Vitro Penetration of Ibuprofen Through Human Epidermis

The objective of this study was to develop an in vitro method to investigate the effect of ultrasound on the in vitro absorption of ibuprofen from a propylene glycol/water vehicle through human epidermis. A diffusion cell was modified so ultrasound could be applied to the vehicle and skin. Since ultrasound can increase the temperature underneath the area of application, control representing temperature effects ran concurrently to the ultrasound experiment. The results demonstrate that ultrasound can increase the penetration of ibuprofen through human skin. This increase in diffusion was greater than for controls where an equivalent increase in temperature was utilized. The results also indicate that evaporation of vehicle components may alter the skin/vehicle partition coefficient, decreasing the effects of ultrasound on the penetration of ibuprofen through the skin.     

Binding of Prednisolone and Its Ester Prodrugs in the Skin

Purpose. Skin binding of prednisolone and its esters was investigated in the hairless mouse skin in vitro. Methods. The distribution of the amount of drugs bound in the skin was determined by a skin slicing technique. The model drugs used were prednisolone (PN, M.W. 360) and its esters, senesyonate (PN-C₅, M.W. 442), geranate (PN-C₁₀, M.W. 510), farnesylate (PN-C₁₅, M.W. 578), and geranylgeranate (PN-C₂₀, M.W. 646). Results. The distribution of bound drug was nonhomogeneous in the skin; the concentration of PN-C₁₀ and PN-C₁₅ in the skin increased gradually with the distance from the skin surface. The parent drug, PN, however, was hardly bound in the viable skin. Conclusions. These findings suggest that the prodrugs of prednisolone may prolong the dermal retention of the parent drug and minimize to delivery into the systemic circulation of the prodrug and metabolite.     

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.     

A Composite Model for the Transport of Hydrophilic and Lipophilic Compounds Across the Skin: Steady-State Behavior

A porous pathway feature has been added to an existing skin diffusion model to extend the range of applicability to highly polar solutes that do not readily diffuse across the stratum corneum (SC) lipid/corneocyte matrix. The porous pathway consists of 2 components: Pathway A is appendageal and is implemented as an array of aqueous shunts (the macropores), which themselves have microporous walls with transient aqueous pores (the micropores). Two varieties of shunts are discussed, one representing a terminal hair follicle and the other representing an eccrine sweat duct; however, the focus here is on the hair follicle. Pathway B is transcellular, with lipid-phase transport accomplished through defects or breaks in the bilayer lipid structure. The composite model admits polar solutes into the skin in a size-selective manner with an effective micropore radius of 1.6 nm. Steady-state permeabilities, desorption rates from isolated SC, and SC/water partition coefficients of both polar and lipophilic solutes are effectively explained.     

A theoretical consideration of percutaneous drug absorption

The percutaneous drug absorption process and its clinical significance are not fully known. In this article we propose a theoretical method to obtain two parameters (k _d and k_c) of percutaneous drug absorption from in vivodata. These parameters are related to diffusion of a drug through the skin and removal process at the skin-capillary boundary, respectively, characterizing several pharmacokinetic aspects of the drug applied to the skin. Moreover, by employing these two kinetic constants, a simulation of percutaneous drug absorption can be theoretically generated. On the basis of our theoretical considerations on the percutaneous drug absorption process described herein, we conclude that the percutaneous drug absorption process is better understood by employing two kinetic constants in a mathematical model and that its clinical application would be highly possible.     

Pharmacokinetic Analysis of Drug Absorption from Muscle Based on a Physiological Diffusion Model: Effect of Molecular Size on Absorption

In a rabbit hind leg perfusion experiment, the absorption of radiolabeled water and carbohydrates of various molecular sizes from muscle was analyzed using a physiological diffusion model and, also, by statistical moment analysis. The model takes into account diffusion in the interstitial space, transcap-illary movement, and removal by the blood circulation and pharmacokinetic parameters representing these processes were computed by curve-fitting. The apparent diffusion coefficients of water and small sugars in the interstitial space (D _m) were proportional to their free diffusion coefficients in water (D _t), whereas the diffusion of ¹⁴C-inulin was hampered by interstitial structures. The first moments of each absorption process were also determined to assess the quantitative contribution of each absorption process to overall absorption. For carbohydrate molecules, residence time in the depot (t _d) accounted for most of the absorption time after injection, whereas for ³H-water, residence times in muscle (t_m) and in the depot (t_d) were similar.     

Absorption of Sunscreens and Other Compounds Through Human Skin in Vivo: Derivation of a Method to Predict Maximum Fluxes

Purpose. The goal of this study was to quantify the transdermally absorbed amounts of the sunscreens octyl dimethyl p-aminobenzoic acid, oxybenzone, 4-isopropyl-dibenzoylmethane, 3-(4-methylben-zylidene)-camphor, isoamyl-4-methoxycinnamate, the repellent and plasticizer dibutyl phthalate, the antioxidant 3.5-di-t-butyl-4-hydroxyanisol, and the antimicrobial compounds butyl-4-hydroxybenzoate, biphenyl-2-ol, and 2,4,4-tri-chlor-2-hydroxydiphenylether (tri-closane). Permeabilities P _B and maximum fluxes J _max should be correlated with relevant physicochemical properties. Methods. Saturated solutions of the above-mentioned compounds in a propylene glycol/water mixture were applied to the skin using glass chambers which were fixed to the upper arms of volunteers. Maximum fluxes were calculated from concentration decreases in the vehicle. Results. A linear relationship between the logarithms of permeabilities P _B of the penetrants (0.02–0.28 cm h^–l) and the corresponding octanol/vehicle partition coefficients PC _Oct/v (166–186,208) was found. Consequently, the influence of aqueous boundary layers could be neglected. However, the slope of the resulting straight line of 0.38 is considerably smaller than unity indicating that PC _Oct/v does not represent the lipophilicity of the stratum corneum adequately. Maximum fluxes range from 0.5 to 130 µg cm^–2 h^–1. A general equation for the calculation of J _max was derived based on experimental data taking into account the PC _Oct/v and the solubilities c _sV of the respective penetrants in the vehicle.     

A diffusion-diffusion model for percutaneous drug absorption

Several theories describing percutaneous drug absorption have been proposed, incorporating the mathematical solutions of differential equations describing percutaneous drug absorption processes where the vehicle and skin are regarded as simple diffusion membranes. By a solution derived from Laplace transforms, the mean residence time MRTand the variance of the residence time VRTin the vehicle are expressed as simple elementary functions of the following five pharmacokinetic parameters characterizing the percutaneous drug absorption: (1) k _d, which is defined as the normalized diffusion coefficient of the skin, (2) k_c, which is defined as the normalized skin-capillary boundary clearance, (3) the apparent length of diffusion of the skin l_d, (4) the effective length of the vehicle l_v, and (5) the diffusion coefficient of the vehicle D_v. All five parameters can be obtained by the methods proposed here. Results of numerical computation indicate that: (1) concentrationdistance curves in the vehicle and skin approximate two curves which are simply expressed using trigonometric functions when sufficient time elapses after an ointment application; (2) the most suitable condition for the assumption that the concentration of a drug in the uppermost epidermis can be considered unchanged is the case where the partition coefficient between vehicle and skin is small, and the constancy of drug concentration is even more valid when the effective length of the vehicle is large; and (3) the amount of a drug in the vehicle or skin and the flow rate of the drug from vehicle into skin or from skin into blood becomes linear on a semilogarithmic scale, and the slopes of those lines are small when D_v is small, when the partition coefficient between vehicle and skin is small, when l_v is large, or when k_c is small. A simple simulation method is also proposed using a biexponential for the concentration-time curve for the skin near the skin-capillary boundary, that is, the flow rate-time curve for drug passing from skin into blood.     

Drug Delivery to the Skin From Sub-micron Polymeric Particle Formulations: Influence of Particle Size and Polymer Hydrophobicity

Purpose  To investigate the influence of particle size and polymer properties on the topical delivery of a lipophilic “active” species (Nile Red (NR)) from sub-micron polymeric particles. Methods  Three poly-(ε-caprolactone) (CAPA) formulations were examined to assess the impact of particle size. Three other formulations, based on cellulose acetate butyrate (CAB), CAPA and polystyrene were studied to address the role of polymer hydrophobicity. In vitro skin permeation, and confocal microscopy and stratum corneum (SC) tape-stripping were used to evaluate the cutaneous disposition of NR. Results  NR delivery into the SC was greater from the larger particles, the overall smaller surface area of which enhanced the “leaving tendency” of the lipophilic “active”. Skin uptake of NR (measured as “%payload released”) from polystyrene, CAPA and CAB particles increased with decreasing polymer hydrophobicity (polystyrene > CAPA > CAB) as expected. Confocal microscopy revealed that NR released from the particles accumulated in, and penetrated via, lipid domains between the SC corneocytes. The particles showed affinity for hairs, and concentrated on the skin surface at the follicular openings. Conclusions  Delivery of a model drug to the skin from sub-micron polymeric particle formulations is sensitive to the particle size and the relative hydrophobicity of the carrier.     

Percutaneous Absorption and Elimination of the Penetration Enhancer Azone in Humans

The percutaneous absorption and elimination of Azone, a new penetration enhancer, were investigated in humans. The distribution and accumulation of Azone in the skin were studied by means of tape stripping. These studies reveal that pure Azone is poorly absorbed. Furthermore, what little Azone is absorbed appears to be rapidly cleared from the circulation by the kidneys. In order to explain the urinary excretion profile, the formation of at least one metabolite is suggested. No accumulation of Azone in the skin was observed.