A pharmacokinetic model for percutaneous absorption

Topical drug delivery: History, percutaneous absorption, and product development

Topical products, widely used to manage skin conditions, have evolved from simple potions to sophisticated delivery systems. Their development has been facilitated by advances in percutaneous absorption and product design based on an increasingly mechanistic understanding of drug-product-skin interactions, associated experiments, and a quality-by-design framework. Topical drug delivery involves drug transport from a product on the skin to a local target site and then clearance by diffusion, metabolism, and the dermal circulation to the rest of the body and deeper tissues. Insights have been provided by Quantitative Structure Permeability Relationships (QSPR), molecular dynamics simulations, and dermal Physiologically Based PharmacoKinetics (PBPK). Currently, generic product equivalents of reference-listed products dominate the topical delivery market. There is an increasing regulatory interest in understanding topical product delivery behavior under 'in use' conditions and predicting in vivo response for population variations in skin barrier function and response using in silico and in vitro findings.

Mathematical models for dermal drug absorption

INTRODUCTION

Mathematical models of dermal transport offer the advantages of being much faster and less expensive than in vitro or in vivo studies. The number of methods used to create such models has been increasing rapidly, probably due to the steady rise in computational power. Although each of the various approaches has its own virtues and limitations, it may be difficult to decide which approach is best suited to address a given problem.

AREAS COVERED

Here we outline the basic ideas, drawbacks and advantages of compartmental and quantitative structure-activity relationship models, as well as of analytical and numerical approaches for solving the diffusion equation. Examples of special applications of the different approaches are given.

EXPERT OPINION

Although some models are sophisticated and might be used in future to predict transport through damaged or diseased skin, the comparatively low availability of suitable and accurate experimental data limits extensive usage of these models and their predictive accuracy. Due to the lack of experimental data, the possibility of validating mathematical models is limited.

Finite dose skin mass balance including the lateral part: comparison between experiment, pharmacokinetic modeling and diffusion models

This work investigates in vitro finite dose skin absorption of the model compounds flufenamic acid and caffeine experimentally and mathematically. The mass balance in different skin compartments (donor, stratum corneum (SC), deeper skin layers (DSL), lateral skin parts and acceptor) is analyzed as a function of time. For both substances high amounts were found in the lateral skin compartment after 6h of incubation, which emphasizes not to elide these parts in the modeling. Here, three different mathematical models were investigated and tested with the experimental data: a pharmacokinetic model (PK), a detailed microscopic two-dimensional diffusion model (MICRO) and a macroscopic homogenized diffusion model (MACRO). While the PK model was fitted to the experimental data, the MICRO and the MACRO models employed input parameters derived from infinite dose studies to predict the underlying diffusion process. All models could satisfyingly predict or describe the experimental data. The PK model and MACRO model also feature the lateral parts.

In-silico model of skin penetration based on experimentally determined input parameters. Part II: Mathematical modelling of in-vitro diffusion experiments. Identification of critical input parameters

This work describes a framework for in-silico modelling of in-vitro diffusion experiments illustrated in an accompanying paper [S. Hansen, A. Henning, A. Naegel, M. Heisig, G. Wittum, D. Neumann, K.-H. Kostka, J. Zbytovska, C.M. Lehr, U.F. Schaefer, In-silico model of skin penetration based on experimentally determined input parameters. Part I: experimental determination of partition and diffusion coefficients, Eur. J. Pharm. Biopharm. 68 (2008) 352-367 [corrected] A mathematical model of drug permeation through stratum corneum (SC) and viable epidermis/dermis is presented. The underlying geometry for the SC is of brick-and-mortar character, meaning that the corneocytes are completely embedded in the lipid phase. The geometry is extended by an additional compartment for the deeper skin layers (DSL). All phases are modelled with homogeneous diffusivity. Lipid-donor and SC-DSL partition coefficients are determined experimentally, while corneocyte-lipid and DSL-lipid partition coefficients are derived consistently with the model. Together with experimentally determined apparent lipid- and DSL-diffusion coefficients, these data serve as direct input for computational modelling of drug transport through the skin. The apparent corneocyte diffusivity is estimated based on an approximation, which uses the apparent SC- and lipid-diffusion coefficients as well as corneocyte-lipid partition coefficients. The quality of the model is evaluated by a comparison of concentration-SC-depth-profiles of the experiment with those of the simulation. Good agreements are obtained, and by an analysis of the underlying model, critical parameters of the models can be identified more easily.

Deep Percutaneous Penetration into Muscles and Joints

The transdermal absorption of drugs and its subsequent deep tissue delivery is a complex process, with many factors influencing the penetration mechanisms. Nonsteroidal antiinflammatory drugs (NSAIDs) are widely used in the treatment of joint and muscle diseases. However, the dangers associated with oral medications highlight the need for alternative methods of targeting and retaining drugs; one such means is through topical delivery. The drug's lipophilicity, permeability, and fraction unbound found in the viable skin are some physiochemical factors influencing the delivery mechanism after transdermal absorption. These and other variables play a role in determining whether the drug reaches the deep tissues via direct penetration or from systemic redistribution. Pharmacokinetic models have been developed to help elucidate the penetration routes and efficacy for various drugs. While there are still uncertainties regarding the deep tissue penetration kinetics, improvements to current research methodologies may bring about a greater understanding of percutaneous absorption into the deep muscle and joints.

Kinetics of finite dose absorption through skin 2: Volatile compounds

A diffusion model to account for the disposition of an arbitrary dose of a (potentially) volatile compound applied to skin from a volatile vehicle is presented. In its most general form, the model allows for variable diffusivity of the permeant in the stratum corneum (SC) and must be solved numerically. However, for permeants having a constant diffusivity, absorption, and evaporation is characterized in terms of four dimensionless parameters-a reduced time tau, a fractional deposition depth in the SC f, a ratio of membrane capacity for the permeant to the applied dose beta, and a ratio of evaporative mass transfer coefficient to diffusive permeability chi. An important combination of these parameters arises as the reduced dose M(r) = (fbeta)(-1). Two cases are distinguished. In Case 1, corresponding to M(r) < or = 1, the dose is less than that required to saturate the upper layers of the SC, and the shape of the absorption and evaporation profiles is independent of the dose. Analytical solutions to Case 1 may be derived for arbitrary initial distributions of the permeant; the solution for a square wave is presented. In Case 2, corresponding to M(r) > 1, absorption and evaporation approach steady-state values as the dose is increased. Numerical evaluations of this behavior are shown. Limiting behavior for the case of a highly volatile solvent applied to skin is discussed. A companion paper discusses the application of the model to the absorption and evaporation of benzyl alcohol from human skin in vitro.

Absorption and evaporation of N,N-diethyl-m-toluamide from human skin in vitro

The penetration of DEET through split-thickness cadaver skin was measured in non-occluded Franz cells placed either in a fume hood or on a laboratory workbench. DEET, dissolved in a small volume of ethanol and spiked with (14)C radiolabel was applied to skin at doses from 0.02 to 11000 microg/cm(2). DEET penetration was greater for cells placed on the workbench, and the percentage of radioactivity penetrated after 72 h increased gradually with dose, for doses up to 680 microg/cm(2). At higher doses, it declined. Percent penetration ranged from 11.5 +/- 3.2% for a dose of 0.021 microg/cm(2) in the fume hood to 71.9 +/- 5.5% for a dose of 260 microg/cm(2) on the workbench. Results were interpreted in terms of a diffusion/evaporation model having three parameters-a solubility value for the chemical in the upper stratum corneum, M(sat); a mass transfer coefficient for evaporation, k(evap); and a characteristic time for diffusion, h(2)/D. The parameters obtained from fitting the model to the data (normalized to the fume hood environment) were M(sat) = 18 microg/cm(2) and k(evap) = 2.6 x 10(-5) cm/h. The value of h(2)/D decreased from 16 h at a DEET dose of 25 microg/cm(2) to 10 h at 1480 microg/cm(2), consistent with an increase in skin permeability of about 1.5-fold over this dose range. This effect was confirmed by means of an additional study in which skin samples pretreated with increasing amounts of unlabeled DEET were washed and redosed with (14)C-benzyl alcohol. A small (1.7-fold), but significant, increase in benzyl alcohol penetration with increasing amount of DEET was obtained. Thus, DEET enhanced its own skin permeation rate as well as that of another compound, but the effect was modest and not likely to be a major concern for compounds coadministered with DEET.

Distributed diffusion-clearance model for transient drug distribution within the skin

Quantitative predictions of molecular transport rates through the skin are key to the development of topically applied and transdermally delivered drugs, as well as risk assessment associated with dermal exposure. Most research to date has focused on correlations for the permeability of the stratum corneum, and transient diffusion models that oversimplify vascular clearance processes in terms of a perfect-removal boundary condition at an artificially introduced lower boundary. Considerations of the spatially distributed nature and action of blood vessels have usually been limited to the steady-state case. This article describes a more comprehensive transient model of percutaneous absorption formulated in terms of volumetric dispersion and clearance coefficients reflecting the spatial distribution of vascular processes. The model was implemented through an analysis of published experimental results on in vivo permeation of salicylic acid (SA) in de-epidermized rat skin. With regard to the characterization of SA in rat dermis ("de") in vivo, it was found that: (i) SA is likely to have a dermal effective partition coefficient (relative to pH 7.4 aqueous buffer "pH7.4") around unity (K(de/pH7.4) = 0.9 +/- 0.3); (ii) vascular processes seem not to increase drug dispersion significantly beyond molecular diffusion [D(de) approximately (D(de))(mol) = (8 +/- 3) . 10(-7) cm(2) s(-1)]; and (iii) vascular clearance is characterized by a rate coefficient k(de) = (7 +/- 2) . 10(-4) s(-1). Application of a whole-skin variant of the model (including the stratum corneum and viable epidermis) allowed realistic predictions to be made of transient subsurface concentration levels after application from a finite dose.

Cellular microtransport processes: intercellular, intracellular, and aggregate behavior

Ionic and molecular transfer among cells occurs by a variety of transport processes operative at different length scales. Cell membrane permeability and electrical conductance derive from channel proteins producing pores at the molecular (ultrastructural) scale. Intracellular mobility involves the dynamics of motion through the complex ultrastructure of the cytoplasm. These phenomena unite in the larger-scale (microscopic) process of gross intercellular transfer. When such movement occurs among sufficiently many cells, it in turn begins to reflect their average collective (macroscopic) behavior as bulk tissue. This article surveys selected aspects of intercellular and intracellular transport, with emphasis on detailed mechanistic theory, experimental probes of cellular permeability, and systematic transcendence from small to large length scales.

Pharmacokinetic models of dermal absorption

Many studies have used pharmacokinetic (compartment) models for skin to predict or analyze dermal absorption of chemicals. Comparing these models is difficult because the relationships between rate constants and the physicochemical parameters were not always defined clearly, simplifying assumptions built into models sometimes were not stated, and which skin layers were included often were not specified. In this paper we review and compare published one- and two-compartment models for which rate constants were expressed in terms of the physicochemical and physical properties of the skin (i.e., diffusion coefficients, partition coefficients and thickness). Nine one-compartment and two two-compartment models are presented with a consistent nomenclature and clearly defined assumptions. In addition, methods used for estimating the physicochemical parameters required by the various are summarized. These eleven compartment models are compared with calculations from a two-membrane skin model that corresponds better with skin function. Many of the compartment models do not predict key characteristics of the two-membrane skin model, especially the effect of blood flow on skin concentration and penetration rates, even when the same input parameters were used. The compartment models developed by Kubota and by McCarley are better predictors of the two-membrane model results, because these models were developed to match characteristics of the membrane model.

Evaluation of percutaneous absorption and skin irritation of ketoprofen through rat skin: in vitro and in vivo study

The influences of different mechanisms of penetration enhancers (such as menthol, azone, ethanol and nonivarnide) regarding the percutaneous absorption and skin irritation of ketoprofen formulations through rat skin were investigated by in vitro and in vivo study. The skin irritation degree at the end of the experiment (10 h) was deterinined by pathologic biopsy and colorimetry methods. In vitro, the menthol showed the most potent enhancing effect. Furthermore, the enhancement effect of a combination of menthol and nonivamide was higher than that of their individual use alone. In vivo the formulation containing 0.05% nonivantide, 5% menthol and 20% ethanol showed a higher penetration rate and an acceptable degree of skin irritation compared to a commercial product (Formax plus gel containing 3% ketoprofen), indicating that it could be used in the clinical situation.

Kinetics of finite dose absorption through skin 1. Vanillylnonanamide

Despite the considerable success in predicting the steady-state dermal absorption rates of chemical compounds from large reservoirs applied to skin, correspondingly little progress has been made in predicting the absorption rate and extent for small doses of topically applied compounds. In the latter case, steady-state absorption rates are generally not obtained, and rapid evaporation or penetration of the dose solvent makes application of permeability coefficient models problematic. This report presents a new analysis of the finite dose problem in terms of a diffusion model with three parameters-a characteristic time for diffusion, h2/D; a skin solubility factor, S(m)h; and a capacity factor for absorption of the dose during the dry down period, M*. These parameters can be related to the molecular weight and oil and water solubilities of the permeant in a manner similar to models describing steady-state absorption from saturated solutions. Some variation of the parameter values based on the chemical nature and volume of the dose solvent is anticipated. The applicability of the model is demonstrated by analyzing the in vitro absorption rates of varying doses of vanillylnonamide (VN, synthetic capsaicin) applied to excised human skin from propylene glycol. The analysis shows that a three-parameter model that assigns all of the resistance to transport to diffusion through the stratum corneum is able to explain most of the significant features of VN absorption through skin.

Topical oleo-hydrogel preparation of ketoprofen with enhanced skin permeability

In an attempt to improve the skin penetration of ketoprofen, various transdermal formulations were prepared, and their in vitro skin permeability and in vivo percutaneous absorption were evaluated. In vitro permeation studies were performed using a modified Franz cell diffusion system in which permeation parameters such as cumulative amount at 8 hr Q8hr, steady-state flux Jss, or lag time tL were determined. In the in vivo percutaneous absorption study using the hairless mouse, maximum concentration Cmax and area under the curve at 24 hr AUC24h were measured. The optimal transdermal formulation (oleo-hydrogel formulation) of ketoprofen showed a Q8hr value of 227.20 micrograms/cm2, a Jss value of 29.61 micrograms/cm2/hr, and a tL value of 0.46 hr. The Q8hr and Jss values were about 10-fold (p < .01) higher than those (Q8hr = 19.61 micrograms/cm2; Jss = 2.66 micrograms/cm2/hr) from the K-gel and about 3.5-fold (p < .01) than those (Q8hr = 60.00 micrograms/cm2; Jss = 7.99 micrograms/cm2/hr) of the K-plaster. In the in vivo percutaneous absorption, the Cmax (6.82 micrograms/ml) and AUC24h (55.74 micrograms.hr/ml) values of the optimal formulation were significantly (p < .01) higher than those of K-gel and K-plaster. The relative bioavailability of the oleo-hydrogel following transdermal administration in reference to oral administration was about 37%, and the Cmax value (4.73 micrograms/cm2) in the hypodermis following topical administration was much higher than those from the conventional products (Cmax of K-gel and K-plaster were 0.92 +/- 0.19 microgram/cm2 and 1.27 +/- 0.37 microgram/cm2, respectively). These data demonstrate that the oleo-hydrogel formulation of ketoprofen was more beneficial than conventional products (K-gel and K-plaster) in enhancing transdermal permeation and skin absorption of ketoprofen. Furthermore, there was a good correlation between in vitro permeation parameters and in vivo percutaneous absorption parameters.

Percutaneous absorption and disposition of [14C]chlordecone in young and adult female rats

The objective of this study was to evaluate the effect of age and dosage on percutaneous absorption and disposition of [14C]chlordecone (Kepone) and to describe results using a physiological based pharmacokinetic (PBPK) model. Female Fischer 344 rats 33 and 82 days old were used as the young and adult animal models, respectively, and were studied over a 10-fold dose range. [14C]Chlordecone (0.286 micromol/cm2) was applied to dorsal skin (2. 3% BSA) and radioactivity was quantified in selected tissues and excreta up to 120 h. Absorption and disposition were also determined at three dose levels up to 2.68 micromol/cm2; fraction absorbed decreased as dose increased. In vitro percutaneous absorption was measured by static and flow-through methods; these yielded similar penetration rates, which were lower than those obtained in vivo. In vivo percutaneous absorption over 120 h was 14.4+/-0.99 and 14.2+/-1. 5% dose in young and adults, respectively. Organ and tissue content increased over time (carcass>liver>kidney), indicating prolonged absorption. Statistical differences between young and old were found for liver, skin, and urine, but not for absorption. Excretion occurred primarily in feces, but also in urine. A biophysically based percutaneous model was fitted to both young and adult in vivo absorption data. This was embedded in a whole body PBPK model which, upon optimization with SAAM II, estimated apparent tissue partition coefficients, urinary and fecal excretion rates, and parameters characterizing hepatic nonlinear uptake of bound chlordecone. The model reasonably predicted tissue chlordecone content at higher doses, when decreased absorption was accounted for.

Physiologically relevant one-compartment pharmacokinetic models for skin. 1. Development of models

Many studies have used pharmacokinetic (compartment) models for skin to predict or analyze absorption of chemicals through skin. In these studies, several different definitions of the rate constants were used. The purpose of this study was to develop a general procedure for relating compartment model rate constants to dermal absorption parameters, such as permeability and partition coefficients, and to assess whether different definitions of the rate constants produce different results. Rate constant expressions were developed by requiring a one-compartment model to match a one-membrane model at specific conditions. Because a membrane model contains more information than a compartment model, a compartment model cannot match the membrane model in all respects. Consequently, many compartment models (i.e., different definitions of the rate constants) can be developed which match the membrane model for different conditions. Using this procedure, 11 different compartment models were developed and compared to the membrane model for four different dermal absorption scenarios. The compartment model that most closely matches the membrane model depends on the specific exposure scenario and what is to be predicted. One of the new compartment models agrees reasonably well with the membrane model, for the cases considered.

Blood flow measurements in skin and underlying tissues by microsphere method: application to dermal pharmacokinetics of polar nonelectrolytes

Blood flow rates to skin and underlying tissues in rats were determined by the radioactive microsphere technique. The dermal and local tissue clearance of two polar solutes, tritiated water and 14C-labeled sucrose, was assessed in anesthetized and sacrificed rats. The dermal clearance of tritiated water in the presence of viable dermal blood supply (6.17 +/- 1.41 mL/min/100 g) was similar in magnitude but more variable than the blood flow to skin estimated by the microsphere technique (5.18 +/- 0.25 mL/min/100 g). The dermal clearance of sucrose (1.66 +/- 0.32 mL/min/100 g) was approximately one quarter the dermal clearance for water, the difference corresponding to the free diffusion coefficients of the two solutes. The permeability coefficients of tritiated water in various tissues were estimated by both a pseudo-steady-state mass balance approach and numerical integration of differential equations (for dermis only) with a "series compartmental" model, from the tissue concentrations obtained in sacrificed rats. The estimates from numerical integration were comparable to those obtained by mass balance, confirming the general structure of the model. The observed tissue concentrations of tritiated water and sucrose in the anesthetized animal compared well with values predicted from the model.

A random walk method for percutaneous drug absorption pharmacokinetics: application to repeated administration of a therapeutic timolol patch

A random walk method for predicting percutaneous drug absorption pharmacokinetics was proposed. The profiles predicted by this method were compatible with those predicted by the analytical method. The random walk method is particularly useful for predicting complex processes such as repeated topical application of a drug. The amount of a drug released into skin from four therapeutic timolol patches was measured when the patches were serially applied for 2.5 h each on the same site of six healthy male volunteers. On the average, 33.2, 23.4, 15.1, and 16.5% of the applied dose was released into skin from the first, second, third, and fourth patches, respectively. This pattern was comparable with the predicted profiles (43.9, 30.2, 24.4, and 21.1%) of amounts of drug which were expected to be released from the first to fourth patches into skin, respectively. The estimation method for the normalized skin-capillary boundary clearance is also described and applied in examining the percutaneous absorption of timolol. The estimated value for this parameter was much greater than the diffusion parameter, indicating that the removal process of timolol by the local circulation is much faster than the diffusion process through skin.

Finite dose percutaneous drug absorption: a BASIC program for the solution of the diffusion equation

A BASIC program to obtain the solution for the diffusion equation describing the finite dose percutaneous absorption pharmacokinetics is shown. Drug amount in the solution matrix, Av, that in the skin which is regarded as a simple diffusion membrane. As, and the drug flux from the skin which is provided by the local circulation per unit area, J, and the blood drug concentration. C, expressed as functions of time are obtained using Fourier series. Usually, a small number (less than 10) of series is enough to obtain the accurate values for Av, As, J, and C. It is also shown that the solution of the diffusion equation reported previously for the specific case where both the diffusion coefficient of a drug through the vehicle and skin-capillary clearance are very large is expressed as a limit.

A novel differentiation method of vehicle models for topically applied drugs: application to a therapeutic timolol patch

A novel method to differentiate basic vehicle models for topically applied drugs is proposed. In this method, the rate of drug release as a function of time, obtained by using a flow-through cell, is plotted on both semilogarithmic and logarithmic scales. In the Solution Case, where all of the drug is dissolved in the vehicle, the profiles become linear on the semilogarithmic scale. However, in the Suspension Case, where the initial drug amount per vehicle volume is greater than the solubility of the drug and the vehicle contains finely dispersed drug, the profiles are linear on the logarithmic scale with a slope of -0.5. They abruptly depart from this pattern upon depletion of the suspended phase. The different attributes of the profiles for the drug release rate-time curves in these two cases can be visualized more clearly when vehicle thickness and drug concentration are varied. The theoretical principles are illustrated in profiles for the drug release-rate time plots of therapeutic patches containing the beta blocker timolol. This was formulated at different concentrations in an acryl copolymer with varied thickness. The release profiles were best fitted to the Solution Case treatment of the data.

A pharmacokinetic model for the percutaneous absorption of indomethacin and the prediction of drug disposition kinetics

A pharmacokinetic model with two parallel absorption processes from two fractions has been derived for the percutaneous absorption of indomethacin. The simulation includes five first-order rate constants assigned the following significance: (a) rapid absorption (ka1) and slow absorption (ka2) through the skin; and (b) distribution into the tissue and subsequent elimination from the body. The model, applied to data obtained after percutaneous dosing of ointments under occlusion, successfully described the plasma profile observed in the presence and absence of laurocapram (Azone) and sorbitan monooleate. The penetration characteristics through stripped skin were also well described by the model. It was clarified that indomethacin was absorbed via both pathways, with rapid and slow absorption rate constants at the initial time stage, and via the latter pathway at the later period. Also, laurocapram and sorbitan monooleate increased the fraction absorbed via the rapid absorption pathway by about three times, without a significant alteration of either absorption rate constant, while the enhancers gave no significant effect on either absorption pathway in the stripped skin. The results suggest that the model is simple yet adaptable enough to prove of general use for the interpretation of percutaneous absorption data.

The isolated perfused porcine skin flap. III. Percutaneous absorption pharmacokinetics of organophosphates, steroids, benzoic acid, and caffeine

The isolated perfused porcine skin flap (IPPSF) has been developed as an alternative in vitro tool for examining the pharmacokinetics and mechanisms of percutaneous absorption. In this study, dosing solutions of seven 14C-radiolabeled compounds representing three chemical classes--organic acid/base [benzoic acid (B), caffeine (C)], organophosphate (OP) pesticides (diisopropylfluorophosphidate, malathion, parathion), and steroid hormones (progesterone, testosterone)--were prepared in ethanol and applied topically at a surface concentration of 40 micrograms cm-2 to the IPPSF. A three-compartment pharmacokinetic model used to stimulate mass transfer from the surface (C1), diffusion through epidermis and dermis (C2), and transfer into the capillary perfusate (C3), was developed based on flux through the IPPSF from 0 to 8 hr. This basic model accurately stimulated measured IPPSF fluxes for five of seven compounds, including the OPs and steroids. The model was modified to simulate the shunting of drug to fast and slow release pathways, which occurred for B 3-4 hr postapplication, and to account for flow-dependent flux increases seen for C at 6 hr postapplication. The latter may be due to a direct pharmacologic effect, since C is a known vasodilator. Extrapolated (to 6 days) areas under the curve from the model simulations were compared with in vivo percutaneous absorption estimates, obtained from 6-day excretion studies in pigs. The in vivo-in vitro correlation, based on simple linear regression across compounds, was excellent (R2 = 0.88, R = 0.94, p less than 0.002). These results suggest that xenobiotic penetration in the 8-hr IPPSF experiments is highly predictive of in vivo absorption totals (6-day studies). In addition, since pig and human skin are similar physiologically and pharmacologically, the IPPSF may eventually have applications in formulating human dermal risk assessment models.

Percutaneous penetration of nicotinates: in vivo and in vitro measurements

The relationship between chemical structure and percutaneous absorption has been explored with nicotinic acid and its methyl, ethyl, hexyl, and benzyl esters. Skin penetration has been measured in vitro across hairless mouse skin and in vivo in humans. In vitro, methyl and ethyl nicotinates (when applied in acetone) were delivered into skin such that the stratum corneum barrier was effectively bypassed. The lipophilic esters, on the other hand, were not solubilized in this way and penetrated more slowly. Nicotinic acid penetrated poorly, yielding essentially zero-order skin transport kinetics. Tape-stripping experiments, in which penetration was monitored across skin with no stratum corneum, confirmed these observations. In vivo absorption of the esters was determined from the urinary excretion of total radioactivity following topical administration of 14C-labeled penetrant. Kinetic analysis of the data yielded rate constants, the ratio of which correlated acceptably with the penetrant octanol-water partition coefficient (K). The dependence of the rate constants on K was interpreted in terms of the relative affinity of the substrate for the stratum corneum compared with the viable tissue; the relationship agrees well with a previous evaluation involving structurally unrelated molecules.

Skin permeability of various non-steroidal anti-inflammatory drugs in man

The skin permeabilities of a series of eight salicylates and ten non-steroidal anti-inflammatory drugs were investigated in human subjects. The logarithms of % absorption through intact skin and of n-octanol/water partition coefficients (log P) of the test compounds were plotted against one another, and a parabolic relationship was obtained in both compound series.

Influence of skin source, penetration cell fluid, and partition coefficient on in vitro skin penetration

Using split-thickness pig skin mounted on in vitro skin penetration-evaporation cells, standard conditions were developed to preserve the viability of the skin as judged by its ability to successfully graft to nude mice. The effects of variations from these conditions on the disposition of radioactivity of radiolabeled compounds were determined. No differences in percutaneous penetration were found for N,N-diethyl-m-toluamide, parathion, and progesterone when Tyrode's solution was used in place of tissue culture media. The percutaneous penetration of benzo(a)pyrene on human and pig skin was unaffected by the presence of sodium azide in the tissue culture media; however, with mouse skin, penetration was lower when sodium azide was present. The disposition of radioactivity following topical application of five radiolabeled compounds was similar on fresh skin compared with skin that had been frozen and exposed to ethylene oxide, although variability of the values was greater with the treated skin. The percutaneous penetration of several compounds was determined on skin with and without the epidermis. The penetration of compounds with a lower log P (log octanol-water partition coefficient) increased to a greater extent (e.g., benzoic acid, log P = 2, sixfold increase) than compounds with a higher log P (e.g., DDT, log P = 5, twofold increase). To further validate the use of pig skin, the percutaneous penetration of 11 compounds on pig skin were correlated (r = 0.79) with the values obtained for human skin under standardized in vitro conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption of materials by or through the living skin

Synopsis The skin comes into contact with a large range of materials either deliberately or inadvertently. It should be possible to predict the exact transport rates of these materials through the skin as a function of the physicochemical properties of the different compounds. With this sort of knowledge it is possible to predict the exact disposition of compounds and use this in the formulation of new products both in the pharmaceutical and cosmetic field. The information will also be useful from the standpoint of skin toxicology and environmental health. In order to be able to predict this complex process it is necessary to split the overall transport into different component parts. This article will identify these components and provide illustrations. The major areas discussed will be barrier function of the skin, the release properties of different topical formulations and how these may be monitored. Novel means of enhancing the penetration of drugs will be discussed and how some additives that are incorporated into formulations will perhaps alter the barrier function of skin. A mathematical model describing skin penetration has been developed and its use in predicting blood levels will be described. This model has been tested both in animal experiments and in limited human studies and its relevance to these situations will be highlighted. Absorption cutanée et transcutanée in vivo.