Passive and iontophoretic transport through the skin polar pathway.

The purpose of the present article is to briefly recount the contributions of Prof. William I. Higuchi to the area of skin transport. These contributions include developing fundamental knowledge of the barrier properties of the stratum corneum, mechanisms of skin transport, concentration gradient across skin in topical drug applications that target the viable epidermal layer, and permeation enhancement by chemical and electrical means. The complex and changeable nature of the skin barrier makes it difficult to assess and characterize the critical parameters that influence skin permeation. The systematic and mechanistic approaches taken by Dr. Higuchi in studying these parameters provided fundamental knowledge in this area and had a measured and lasting influence upon this field of study. This article specifically reviews the validation and characterization of the polar permeation pathway, the mechanistic model of skin transport, the influence of the dermis on the target skin concentration concept, and iontophoretic transport across the polar pathway of skin including the effects of electroosmosis and electropermeabilization.

The effect of temperature upon the permeation of polar and ionic solutes through human epidermal membrane.

The temperature dependence of in vitro permeation through human epidermal membrane (HEM) was determined for urea, mannitol, tetraethylammonium ion (TEA), and corticosterone. The effect of temperature upon HEM electrical resistance was also measured. The majority of the experiments involved measuring the permeability coefficients of a specific permeant at 27 degrees C and 39 degrees C for a given HEM sample, the electrical resistance was also measured at each temperature. Similar experiments were also conducted with a model synthetic porous membrane. The effect of temperature was quantitated as the ratio of the permeability at 39 degrees C to the permeability at 27 degrees C for each permeant. These ratios observed for HEM with urea, mannitol, and TEA as the permeants were 1.66 +/- 0.05, 1.76 +/- 0.14, and 1.71 +/- 0.11, respectively. The change in temperature was shown to have a similar effect upon the electrical conductance of the HEM samples. The observed ratio for corticosterone permeation was 4.5 +/- 0.4. The experimental ratios observed for the three polar/ionic permeants were shown to approach those obtained from the model porous membrane and differed greatly from the ratio observed for the more lipophilic corticosterone, indicating differences in the effective transport mechanism/pathway for these classes of permeants. The permeability of urea was also observed to be inversely proportional to the electrical resistance of the HEM samples; this relationship was shown to be independent of temperature over the temperature range studied. The temperature dependence data and the observed relationship between urea permeability and electrical resistance strongly support the existence of a porous permeation pathway through the HEM as an operative diffusional route for polar-ionic permeants.

Quantification of pore induction in human epidermal membrane during iontophoresis: the importance of background electrolyte selection.

It has been shown that significant pore induction (electroporation) occurs in human epidermal membrane (HEM) during iontophoresis even at moderate applied voltages (1-10 V). Recent efforts in our laboratory have been aimed at quantifying HEM electroporation by examining the proportionality between flux enhancement due to electroporation and electrical conductance changes during iontophoresis. The specific purpose of the present study was to test the hypothesis that by matching the background electrolyte ion sizes with the permeant ion sizes, the flux enhancement due to electroporation can be quantified by the change in HEM electrical conductance. In this study, radiolabeled tetraethylammonium (TEA(+)), methylammonium (MA(+)), and mannitol were the permeants. Potassium chloride (KCl), tetraethylammonium bromide (TEAB), tetraethylammonium pivalate (TEAP), and sodium fluoride (NaF) were the background electrolytes. Iontophoresis experiments were carried out over an applied voltage range of 1 to 3 V. The experimental flux enhancement results were compared with the theoretical predictions from the Nernst-Planck model after corrections were made: (a) for HEM pore induction during iontophoresis based on electrical conductance changes and (b) for electroosmosis employing mannitol as the neutral probe permeant. In experiments where the ion sizes of the background electrolyte and permeant were closely matched (e.g., TEA(+) as the permeant and TEAP as the background electrolyte), there was excellent agreement between experimental results and theoretical predictions of the modified Nernst-Planck model, with only modest data scatter. When the electrolyte and permeant sizes were quite different (e.g., TEA(+)/KCl and MA(+)/TEAP), the experimental flux data were inconsistent with model predictions and there were large variations in the experimental results. The results of the present study illustrate that permeant flux enhancement can be predicted by the modified Nernst-Planck model even during moderate voltage iontophoresis when electroporation is operative.

Quantitative description of the effect of molecular size upon electroosmotic flux enhancement during iontophoresis for a synthetic membrane and human epidermal membrane.

This study focused upon quantitatively determining the influence of permeant molecular size upon flux enhancement which results from electroosmosis. The first phase of the study involved validation of a fundamental model describing the molecular size dependence of flux enhancement which results from convective solvent flow. This was accomplished using a model synthetic membrane (stack of 50 Nuclepore membranes) and four model permeants with a molecular weight range of 60-504 (urea, mannitol, sucrose, and raffinose). The steady-state flux of each permeant was determined under passive conditions and applied voltages of 125, 250, 500, and 1000 mV using side-by-side diffusion cells and a four-electrode potentiostat system. On the basis of the permeability enhancement for each permeant at each applied voltage (relative to the passive permeability) it was possible to calculate the effective solvent flow velocity from each permeant at each field strength. An important finding was that the flux enhancement due to electroosmosis was strongly molecular weight dependent (i.e., the flux enhancement ratio was around 4 times greater for raffinose than for urea, with mannitol and sucrose yielding intermediate values), while the calculated effective flow velocity at each voltage was independent of the molecular weight of the permeant. This coupled with a linear correlation between flow velocity and applied voltage served to establish the validity of the method and model. The second phase of the study was an extension of the model to human epidermal membrane (HEM). These experiments involved simultaneously measuring the fluxes of [14C]urea and [3H]sucrose across HEM samples under passive, 250 mV, and 500 mV conditions. Similar to the Nuclepore system, the observed flux enhancement ratios with HEM were approximately 3 times greater for sucrose than for urea. A detailed analysis of the HEM data showed semiquantitative agreement between predictions of the model and experimental results.

Iontophoretic transport across a synthetic membrane and human epidermal membrane: a study of the effects of permeant charge.

The effects of permeant charge (z) on iontophoretic-enhanced transport were investigated with synthetic Nucleopore membranes and with human epidermal membranes using a four-electrode potentiostat with side-by-side diffusion cells. The modified Nernst-Planck model (Nernst-Planck theory with an additional transport term to correct for the effect of the convective solvent flow due to electroosmosis) was first examined in a Nuclepore membrane system with model permeants calcein (z = -4), salicylate (z = -1), and a series of polystyrene sulfonates (from monomer to molecular weight of approximately 8000 with a z range of -1 to approximately -40). The flux enhancement (E) for each permeant was determined at 470 mV. Mannitol (a neutral molecule) was used as a probe to determine a correction for convective solvent flow under the same applied voltage conditions. Good agreement between the experimental results and the predictions from the modified Nernst-Planck model was found for calcein, salicylate, and polystyrene sulfonates up to molecular weight of approximately 1800 (z approximately -8). The flux enhancements for the higher molecular weight polystyrene sulfonates with greater z values were more than a factor of three lower than theoretical predictions; the electrophoretic effect and counterion binding to the permeants are proposed as possible explanations for these discrepancies between experiment and the modified Nernst-Planck theory. In the studies with human epidermal membranes, iontophoretic flux enhancements for calcein, salicylate, and taurocholate were determined at 250 and/or 470 mV. The flux enhancements were generally consistent with the results calculated from the modified Nernst-Planck model.

Characterization of the transport pathways induced during low to moderate voltage iontophoresis in human epidermal membrane.

This report describes the results of iontophoresis experiments involving the transport of polar nonelectrolytes across human epidermal membrane (HEM) at a moderate applied voltage of 2.0 V and where the data are interpreted via a convective transport model and hindered transport theory. A principal finding is that although HEM iontophoresis at 2.0 V resulted in a large increase in HEM porosity, the pore radii of the newly induced pores in HEM as calculated from the iontophoresis data using the hindered transport theory were found to be in the range of 6-12 A. This supports the view that electroporation at these modest applied voltages results in pores with sizes the same order of magnitude but somewhat smaller than those estimated for the preexisting pores in HEM prior to electroporation. This outcome is also important from a practical standpoint, as flux enhancement for large molecules (such as oligonucleotides and polypeptides) arising from electroporation under these conditions would be expected to be significantly less than if the resulting pore sizes were much greater. Providing a "prepulse" of 4.0, 8.0, and 15 V prior to the 2.0 V iontophoresis generally gave greater increases in HEM conductance (and, therefore, in porosity) but did not significantly change the deduced effective pore radii (around 5-9 A). The alteration during and the recovery of HEM after iontophoresis was also investigated. The recovery behavior was found to be dependent upon both the duration of the applied voltage and the magnitude of its effects: the recovery for a HEM sample that experienced a large increase in electrical conductance during iontophoresis was generally poorer than that for a sample that was more resistant to the electric field. Incomplete recovery was generally observed in experiments with long iontophoresis duration (50 min) and with the higher voltages (4.0, 8.0 V, and 15 V). In these cases, the barrier properties of HEM were more greatly altered as indicated by larger increases in the electrical conductance and passive permeability of HEM after iontophoresis.

Mechanistic studies of the 1-alkyl-2-pyrrolidones as skin permeation enhancers.

The influences of 1-ethyl-, 1-butyl-, 1-hexyl-, and 1-octyl-2-pyrrolidone in their saline solutions on the transport of beta-estradiol, corticosterone, and hydrocortisone across hairless mouse skin under in vitro conditions have been investigated by the physical model approach. The experimental data were interpreted with a physical model that treats the stratum corneum as a diffusional barrier with a lipoidal pathway and a pore pathway. Enhancement factors (E values) for the lipoidal pathway were calculated from the permeability coefficients and solubility data as a function of the 1-alkyl-2-pyrrolidone concentration for all three permeants. A pattern of increasing E values with increasing 1-alkyl-2-pyrrolidone chain length was found, and the results were essentially the same for all three steroidal permeants. A nearly semilogarithmic linear relationship was also obtained between the enhancement potency and the carbon number of the alkyl chain; there was about an approximately 3.5-fold increase in the enhancement potency per 1-alkyl-2-pyrrolidone methylene group. An important outcome of this research is that the enhancement potencies of the 1-alkyl-2-pyrrolidones were essentially the same as those for the previously studied n-alkanols when compared at the same carbon numbers of the alkyl groups. This result is somewhat surprising as it suggests that the enhancer action resides (in its entirety) in the alkyl group, and the nature of the polar head group may not be intrinsically important in transdermal enhancement of the lipoidal pathway within a class of permeation enhancers.

Pore induction in human epidermal membrane during low to moderate voltage iontophoresis: A study using AC iontophoresis.

The present study aimed to investigate new pore induction as a flux-enhancing mechanism in human epidermal membrane (HEM) with low to moderate voltage electric fields. The extent of pore induction and the effective pore sizes of these induced pores were to be assessed using a low frequency (12.5 Hz) low to moderate voltage (2. 0 to 4.0 V) square-wave alternating current (ac) "passive" permeation method (ac iontophoresis). This ac approach was to allow for inducing and sustaining a state of pore induction in HEM while permitting no significant transport enhancement via electroosmosis; thus, transport enhancement entirely due to new pore induction (enhanced passive permeation) was to be assessed without any contributions from electroosmosis. Good proportionality between the increase in HEM permeability and its electrical conductance was found with the "passive" transport data obtained during square-wave ac iontophoresis using urea as the model permeant. Typically, at 3.0 to 4.0 V, HEM conductance increases (and permeability increases) ranged from around 3- to 30-fold. These results appear to be the first direct evidence that new pore induction in HEM is a significant flux enhancing mechanism under moderate voltage conditions. The extents of pore induction in HEM under low frequency moderate voltage (2.0 to 3.0 V) ac, pulsed direct current (dc), and continuous dc were also compared. The extents of pore induction from square-wave ac and pulsed dc were generally of the same order of magnitude but somewhat less than that observed during continuous dc iontophoresis at the same applied voltage and duration, suggesting less extent of pore induction with reversing polarity or when a brief delay is provided between pulses to allow for membrane depolarization. The average effective pore sizes calculated for the induced pores from the experimental data with urea and mannitol as probe permeants and the hindered transport theory were 12 +/- 2 A, which are of the same order of magnitude as those of preexisting pores determined from conventional passive diffusion experiments.

Flux enhancement effects of ionic surfactants upon passive and electroosmotic transdermal transport.

This study focused upon the enhancement effects of ionic surfactants upon passive and electroosmotic transdermal flux. The first phase of the study involved validating theories relating surface properties of a membrane to electroosmotic solvent flow under appropriate experimental conditions using a synthetic model membrane (stack of 50 Nuclepore membranes). Numerical solutions to the Poisson-Boltzmann equation and the equations of fluid motion served as the theoretical basis for the experimental studies. Important outcomes of the model membrane studies were that electroosmotic solvent flow velocity was enhanced by the addition of an anionic surfactant, sodium dodecyl sulfate, and reversed by the addition of a cationic surfactant, dodecyltrimethylammonium bromide. The effective membrane pore wall surface charge densities were determined under a variety of experimental conditions. Adsorption of dodecyl sulfate to the pore wall increased the net negative charge on the pore wall. A reversal of the net pore wall surface charge density resulted from the adsorption of dodecyltrimethylammonium. The interrelationship between electroosmosis, surfactant adsorption, and ionic strength was also evaluated. The second phase of the study was an investigation of the effects of sodium dodecyl sulfate upon the transport of neutral polar permeants through human epidermal membrane (HEM). Fluxes of [14C]urea and [3H]sucrose were simultaneously measured across HEM samples under passive and 250 mV conditions; flux measurements were made before, during, and after HEM exposure to sodium dodecyl sulfate. A systematic analysis of the experimental data made it possible to elucidate the specific contributions of sodium dodecyl sulfate and the applied electric potential to the overall flux enhancement. Sodium dodecyl sulfate enhanced the intrinsic passive permeability of the HEM, and it also enhanced the contribution of electroosmosis to the flux during iontophoresis.

Hindered diffusion of polar molecules through and effective pore radii estimates of intact and ethanol treated human epidermal membrane.

The in vitro passive transport of urea, mannitol, sucrose and raffinose across intact and ethanol treated human epidermal membrane was investigated. The intent of this study was to characterize the barrier properties and permeation pathways of these membranes for polar permeants under passive conditions. Based upon the relative permeabilities of these four solutes and hindered diffusion theory, the experimental data was adequately modeled for both membrane systems according to permeation through a porous membrane. Effective pore radii estimates for intact human epidermal membrane fell between 15 A to 25 A while similar estimates fell compactly between 15 A to 20 A for ethanol treated human epidermal membrane. Similarities between the relative permeabilities of human epidermal membrane for the four permeants studied and the relative permeabilities of these same permeants through ethanol pretreated human epidermal membrane indicate that significant similarities exist between the permeation pathways for both membrane systems. The results of this study have important implications for transdermal drug delivery in general and more specifically for strategies of designing effective chemical permeation enhancement systems.