Ionic Permeants Research Articles

Characterization of cornified oral mucosa for iontophoretically enhanced delivery of chlorhexidine

Topical administration of chlorhexidine for periodontal disease can provide advantages over systemic delivery, but is limited by the permeability of the cornified oral mucosal tissue. In the present study, passive and iontophoretic transport of tetraethylammonium, salicylate, mannitol, dexamethasone, fluoride, and chlorhexidine across bovine palate was investigated to (a) determine the intrinsic barrier properties of bovine palate for its eventual use as a model of human cornified oral mucosa, (b) examine the feasibility of iontophoretically enhanced transport of chlorhexidine into and across bovine palate, and (c) identify the transport mechanisms involved in iontophoretic transport across the palate. The histology study suggests that bovine and human palates have similar cornified epithelium structures; bovine palate could be a model tissue of human hard palate for drug delivery studies. Transport study of tetraethylammonium, salicylate, and mannitol suggests that bovine palate was net negatively charged and the cornified epithelial layer was the rate-determining barrier. The direct-field effect (electrophoresis) was shown to be the dominant flux-enhancing mechanism in iontophoretic transport of ionic compounds. Electroosmosis also contributed to the iontophoretic transport of both neutral and ionic permeants. Anodal iontophoresis enhanced the delivery of chlorhexidine into and across the palate, reduced the transport lag time, and provided tissue concentration above the drug minimum inhibitory concentration, and therefore could be a promising method to assist in the treatment of periodontal disease.

A pH-Tunable Nanofluidic Diode with a Broad Range of Rectifying Properties

The use of fixed charge nanopores in practical applications requires tuning externally the electrostatic interaction between the charged groups and the ionic permeants in order to allow integrating a variety of functions on the same nanostructure. We design, produce, and characterize, theoretically and experimentally, a single-track amphoteric nanopore functionalized with lysine and histidine chains whose positive and negative charges are very sensitive to the external pH. This nanofluidic diode with amphoteric chains attached to the pore surface allows for a broad set of rectification properties supported by a single nanodevice. A definite plus is to functionalize these groups on a conical nanopore with well-defined, controlled structural asymmetry which gives virtually every rectification characteristic that may be required in practical applications. Nanometerscaled amphoteric pores are of general interest because of the potential applications in drug delivery systems, ion-exchange membranes for separation of biomacromolecules, antifouling materials with reduced molecular adsorption, and biochemical sensors.

Transungual Iontophoretic Transport of Polar Neutral and Positively Charged Model Permeants: Effects of Electrophoresis and Electroosmosis

Transungual iontophoretic transport of model neutral permeants mannitol (MA), urea (UR), and positively charged permeant tetraethylammonium ion (TEA) across fully hydrated human nail plates at pH 7.4 were investigated in vitro. Four protocols were involved in the transport experiments with each protocol divided into stages including passive and iontophoresis transport of 0.1 and 0.3 mA. Water and permeant uptake experiments of nail clippings were also conducted to characterize the hydration and binding effects of the permeants to the nails. Iontophoresis enhanced the transport of MA and UR from anode to cathode, but this effect (electroosmosis) was marginal. The transport of TEA was significantly enhanced by anodal iontophoresis and the experimental enhancement factors were consistent with the Nernst-Planck theory predictions. Hindered transport was also observed and believed to be critical in transungual delivery. The barrier of the nail plates was stable over the time course of the study, and no significant electric field-induced alteration of the barrier was observed. The present results with hydrated nail plates are consistent with electrophoresis-dominant (the direct field effect) transungual iontophoretic transport of small ionic permeants with small contribution from electroosmosis.

Alternating Current (AC) Iontophoretic Transport across Human Epidermal Membrane: Effects of AC Frequency and Amplitude

As a continuing effort to understand the mechanisms of alternating current (AC) transdermal iontophoresis and the iontophoretic transport pathways in the stratum corneum (SC), the objectives of the present study were to determine the interplay of AC frequency, AC voltage, and iontophoretic transport of ionic and neutral permeants across human epidermal membrane (HEM) and use AC as a means to characterize the transport pathways. Constant AC voltage iontophoresis experiments were conducted with HEM in 0.10 M tetraethyl ammonium pivalate (TEAP). AC frequencies ranging from 0.0001 to 25 Hz and AC applied voltages of 0.5 and 2.5 V were investigated. Tetraethyl ammonium (TEA) and arabinose (ARA) were the ionic and neutral model permeants, respectively. In data analysis, the logarithm of the permeability coefficients of HEM for the model permeants was plotted against the logarithm of the HEM electrical resistance for each AC condition. As expected, linear correlations between the logarithms of permeability coefficients and the logarithms of resistances of HEM were observed, and the permeability data were first normalized and then compared at the same HEM electrical resistance using these correlations. Transport enhancement of the ionic permeant was significantly larger than that of the neutral permeant during AC iontophoresis. The fluxes of the ionic permeant during AC iontophoresis of 2.5 V in the frequency range from 5 to 1,000 Hz were relatively constant and were approximately 4 times over those of passive transport. When the AC frequency decreased from 5 to 0.001 Hz at 2.5 V, flux enhancement increased to around 50 times over passive transport. While the AC frequency for achieving the full effect of iontophoretic enhancement at low AC frequency was lower than anticipated, the frequency for approaching passive diffusion transport at high frequency was higher than expected from the HEM morphology. These observations are consistent with a transport model of multiple barriers in series and the previous hypothesis that the iontophoresis pathways across HEM under AC behave like a series of reservoirs interconnected by short pore pathways.

Examination of penetration routes and distribution of ionic permeants during and after transscleral iontophoresis with magnetic resonance imaging

Previously, transscleral and transcorneal iontophoretic delivery was studied and compared to passive delivery and intravitreal injection using nuclear magnetic resonance imaging (MRI). The objective of the present study was to employ MRI to further investigate the factors affecting transscleral iontophoretic delivery. In the present study, anodal and cathodal constant current transscleral iontophoresis were conducted with excised sclera in side-by-side diffusion cells in vitro and with rabbits in vivo. The total current and duration of application were 2 and 4mA (current density 10 and 20mA/cm2) and 20–60min, respectively. The delivery and distribution of the model permeants manganese ion (Mn2+) and manganese ethylenediaminetetraacetic acid complex (MnEDTA2−) into the eye during iontophoresis were determined with MRI and compared with the results obtained in previous studies of subconjunctival injection and passive delivery. Both anodal and cathodal iontophoresis provided significant enhancement in ocular delivery compared to passive transport in the in vitro and in vivo experiments. Transscleral iontophoretic delivery was related to the position and duration of the iontophoresis application in vivo. Permeants were observed to be delivered primarily into the anterior segment of the eye when the pars plana was the application site. Extending the duration of iontophoresis at this site allowed the permeants to be delivered into the vitreous more deeply and to a greater extent than when the application site was at the back of the eye near the fornix. The present results show that electrode placement was an important factor in transscleral iontophoresis, and the ciliary body (pars plana) was determined to be the pathway of least resistance for iontophoretic transport. These new findings continue to support the utility of MRI as a noninvasive technique in ocular drug delivery research and testing.

Quantitative assessment of tissue retention, lipophilicity, ionic valence and convective transport of permeant as factors affecting iontophoretic enhancement

Iontophoresis of the dipeptide tyrosine-phenylalanine (TyrPhe), the protected amino acid tyrosine- β -naphthylamide (Tyr- β -NA) and the glucose derivative benzyl-2-acetamido-2-deoxy-α-D-glucopyranoside (BAd-α-Glc) used as model compounds was investigated in human epidermis in vitro at pH 3 and pH 4.5 under constant voltage application, the purpose being to delineate the contribution of tissue retention, lipophilicity, ionic valence and convective transport of these compounds to iontophoretic enhancement in a quantitative fashion and ultimately gain an improved mechanistic understanding of iontophoresis. Retention of BAd-α-Glc in epidermal tissue during permeation was considerable and was reduced upon iontophoresis in the presence of extraneous tyrosine (Tyr) and phenylalanine (Phe) added to the donor solution, this evoking an increase of the apparent iontophoretic permeation. This suggests firstly, the occurrence of an interaction between BAd-α-Glc, Tyr and Phe impacting iontophoretic enhancement and secondly, the potential of using amino acids as adjuvants to modulate iontophoresis of selected compounds. This influence of tissue retention on iontophoretic permeation in the presence of Tyr and Phe was not observed for TyrPhe or Tyr-β-NA or when BAd-α-Glc, TyrPhe and Tyr-β-NA were used concomitantly rather than individually in the absence of Tyr and Phe. The effect of lipophilicity, ionic valence in the aqueous permeation pathway and convective transport by electroosmosis of the three permeants on iontophoresis was assessed simultaneously for all permeants by analyzing the experimental data using a theoretically derived model for iontophoretic enhancement that encompassed these factors. This model was based on an extension of the modified Nernst-Planck equation, whereas the difference of ionic valence between the aqueous domain of tissue and the bulk solution was evaluated on the basis of a pH shift due to the electrical double layer at the lipid/aqueous interface in the epidermis using the Poisson-Boltzmann equation. Using deduced parameter values characterizing the involved factors, a very good agreement between model calculated and experimental enhancement data was obtained. At pH 4.5, a very weak convective transport due to electroosmosis from cathode to anode was evident, indicating an isoelectric point of the epidermis slightly above 4.5. At pH 3, electroosmotic transport was approximately 10-fold stronger than at pH 4.5 which reflected a high positive surface charge density of the epidermis at pH 3 and a low one at pH 4.5. The pH in the aqueous permeation pathway, estimated to differ from that of the bulk in accordance with these charge densities, produced an ionic valence in this pathway dependent on pK a that accounted consistently for the flux due to the direct effect of the electric field on the ionic permeants. The ratio of lipid to aqueous pathway passive permeability coefficient was

Influence of Asymmetric Donor–Receiver ion Concentration Upon Transscleral Iontophoretic Transport

Recent in vitro and in vivo studies have suggested transscleral iontophoresis as a means for non-invasive drug delivery to the eye. However, there remains a lack of information of the iontophoretic transport behavior of the sclera. The objective of the present study was to investigate the effects of permeant concentration upon transscleral iontophoretic transport. Constant current direct current (DC) iontophoresis was conducted with rabbit sclera in vitro at permeant concentration ranging from 0.015 to 1.0 M in the donor chamber without background electrolyte at 0.4-4 mA (current density: 2-20 mA/cm2). PBS (0.15 M) was the receiver solution. Salicylate (SA) and tetraethylammonium (TEA) were the model ionic permeants, and mannitol was the neutral probe permeant. Conductivity experiments of SA and TEA solutions were performed to determine the effects of ion concentration upon SA and TEA electromobilities. Model simulations were carried out and compared with the experimental data. It was found that the fluxes of the ionic permeants increased linearly with the electric current but were relatively independent of their donor concentrations. Electric field-induced convective solvent flow (electroosmosis) in the sclera was observed to be from the anode to cathode, suggesting that the sclera is net negatively charge at neutral pH. For the studied permeants, electrophoresis was the main transport enhancing mechanism with electroosmosis as a secondary effect. No significant interaction between the permeants and sclera was observed that significantly altered electroosmosis in the membrane. Under the asymmetric donor and receiver conditions, the transference of the permeants could not be predicted by the concentrations of the ions in the donor and receiver chambers with the assumption of constant electric field in the membrane. The membrane ion concentrations were different from those in the chambers due to the requirement of charge neutrality in the membrane.

Effects of electrophoresis and electroosmosis during alternating current iontophoresis across human epidermal membrane

Previous studies in our laboratory have demonstrated that skin electrical resistance can be controlled by an alternating current (AC) electric field. By maintaining constant skin resistance, AC iontophoresis has been shown to reduce the iontophoretic flux variability of neutral permeants. Recently, it was found that symmetric square‐wave AC could enhance iontophoretic transport of both neutral and ionic permeants by means of electrophoresis and/or electroosmosis in a synthetic membrane system, and a model was presented to describe the experimental results. The objective of the present study was to assess the effects of AC voltage and frequency and direct current (DC) offset on the flux of neutral and ionic model permeants with human epidermal membrane (HEM). Experiments were conducted under two different conditions: constant AC voltage iontophoresis and iontophoresis using constant HEM resistance with DC offset voltage. The following are the main findings in these experiments. In the constant AC voltage study, when the permeability data were compared at the same HEM electrical resistance, it was demonstrated that AC even at high frequency (≈1 kHz) could enhance the transport of the ionic permeant (tetraethylammonium ion) across HEM, but no enhancement was observed for the neutral permeant (arabinose). For the ionic permeant flux enhancement, the higher the applied AC voltage, the greater the flux enhancement. There was little or no AC frequency dependence of the flux enhancement in the frequency range of 50–1000 Hz. In the constant HEM resistance study of AC with DC offset, approximately linear relationships were observed between flux enhancement and the DC offset voltage for both the neutral and ionic permeants, and these results were found to be consistent with predictions of the modified Nernst‐Planck model for conventional constant voltage DC iontophoresis. When the DC offset voltage was increased, the AC component of the flux enhancement for the ionic permeant decreased, eventually appearing to contribute negligibly to the total flux enhancement at high DC offset voltages. © 2005 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:547–558, 2005

Assessment of Subconjunctival Delivery with Model Ionic Permeants and Magnetic Resonance Imaging

The objective was to assess the permeation and clearance of model ionic permeants after subconjunctival injection with nuclear magnetic resonance imaging (MRI). New Zealand white rabbit was the animal model and manganese ion (Mn2+) and manganese ethylenediaminetetraacetic acid complex (MnEDTA2-) were the model permeants. The current study was divided into three parts: in vitro, postmortem, and in vivo. Transscleral passive permeation experiments were conducted with excised sclera in side-by-side diffusion cells in vitro. Subconjunctival delivery experiments were conducted with rabbits postmortem and in vivo. The distribution and elimination of the probe permeants from the subconjunctival space after subconjunctival injections were determined by MRI. The data of excised sclera in vitro suggest large effective pore size for transscleral transport and negligible pore charge effects upon the permeation of the ionic permeants. The permeability coefficients of Mn2+ and MnEDTA2- across the sclera in vitro were 3.6 x 10(-5) cm/s and 2.4 x 10(-5) cm/s, respectively. Although relatively high sclera permeability was observed in vitro, subconjunctival injections in vivo did not provide significant penetration of Mn2+ and MnEDTA2- into the globe; permeant concentrations in the eye were below the detection limit, which corresponds to less than 0.05% of the concentration of the injection solution (e.g., less than 0.02 mM when 40 mM injection solution was used). The volume of the subconjunctival pocket and the concentration of the permeants in the pocket were observed to decrease with time after the injection, and this could contribute to the lower than expected subconjunctival absorption in vivo. Different from the results in vivo, experiments with rabbits postmortem show significant penetration of Mn2+ and MnEDTA2- into the globe with the permeants primarily delivered into the anterior segment of the eye. This difference suggests blood vasculature clearance as a main barrier for passive transscleral transport. The data also show that the pars plicata/pars plana is the least resistance pathway for passive transscleral drug delivery of the polar permeants, and there are indications of the presence of another barrier, possibly the retinal epithelium and/or Bruch's membrane, at the back of the eye. Subconjunctival delivery of the ionic permeants in vivo cannot be quantitatively predicted by the in vitro results. MRI is a noninvasive complementary technique to traditional pharmacokinetic methods. It can provide insights into ocular pharmacokinetics without permeant redistribution that can occur in surgical procedure postmortem in traditional pharmacokinetic studies when the blood vasculature barrier is absent.

Mechanistic studies of flux variability of neutral and ionic permeants during constant current dc iontophoresis with human epidermal membrane

Although constant current iontophoresis is supposed to provide constant transdermal transport, significant flux variability and/or time-dependent flux drifts are observed during iontophoresis with human skin in vitro and human studies in vivo. The objectives of the present study were to determine (a) the causes of flux variability in constant current dc transdermal iontophoresis and (b) the relationships of flux variabilities among permeants of different physicochemical properties. Changes in the human epidermal membrane (HEM) effective pore size and/or electroosmosis during constant current dc iontophoresis were examined. Tetraethylammonium ion (TEA), urea, and mannitol were the model permeants. For the neutral permeants, the results in the present study showed a significant increase of fluxes with time in a given experiment and large HEM sample-to-sample variability. Although both effective pore size and pore charge density variations contributed to the time-dependent flux drifts observed in electroosmotic transport, the significant flux drifts observed were found to be primarily a result of the time-dependent increase in effective pore charge density. For the ionic permeant, the observed flux variability was smaller than that of the neutral permeants and was believed to be primarily due to effective pore size alteration in HEM during iontophoresis as suggested in a previous study. The different extents of flux variability observed between neutral and ionic permeants are consistent with the different iontophoretically enhanced transport mechanisms for the neutral and ionic permeants (i.e. electroosmosis and electrophoresis, respectively). The results of the present study also demonstrate that flux variability of two neutral permeants are inter-related, so the flux of one neutral permeant can be predicted if the permeability coefficient of the other neutral permeant is known.

External Control of Ion Transport in Nanoporous Membranes with Surfaces Modified with Self-Assembled Monolayers

We investigated the effect of external pH, ionic strength, and applied voltage to control the transport of ions through porous PCTE/Au/acid thiol membranes with pore radii of 4.0−5.9 nm. Electroless gold was deposited on the inside surfaces of the nanopores of the membrane. The thiols 11-mercaptoundecanoic acid (HSC10H20COOH) and 3-mercapto-1-propanesulfonic acid (HSC3H6SO3H) were used to form self-assembled monolayers (SAMs) on the gold surfaces of the pores. The ionic permeant through the pores of the membranes was the organic ion benzene sulfonate (BS). The transport of benzene sulfonate in porous membranes modified with SAMs of the HSC10H20COOH weak acid thiol was highly affected by external variation of the pH; the BS flux could be changed by a factor of 100. However, transport of benzene sulfonate in membranes modified with SAMs of the HSC3H6SO3H strong acid thiol was influenced less by external pH than transport in membranes modified with SAMs of the HSC10H20COOH weak acid thiol. Increasing the ion...

The role of stratum corneum in electrically facilitated transdermal drug delivery. I. Influence of hydration, tape-stripping and delipidization on the DC electrical properties of skin

The direct-current electrical properties of excised, full-thickness hairless rat skin were investigated as a function of hydration, tape-stripping and delipidization in order to determine the impact of physical and chemical perturbation of the stratum corneum on the barrier function of mammalian skin. Skin samples subjected to hypotonic conditions were most conductive, yielding the smallest electrical potential difference across the tissue. Such samples were also characterized by highly symmetrical currentpotential ( j- V) profiles with respect to the sign and magnitude of the applied current strength. In comparison, samples subjected to isotonic or hypertonic conditions were markedly less conductive, and were characterized by highly asymmetric j- V profiles. Similar differences in electrical properties were observed in the case of tape-stripped samples and those selectively delipidized with either petroleum ether or a 2:1 blend of chloroform and methanol. Despite the ability of both solvents to extract equal quantities of epidermal lipids, skin exposed to petroleum ether behaved similarly to untreated (intact) skin. Such treatment groups were poorly conductive and exhibited both time-dependent and current-dependent electrical behavior, yielding strongly asymmetric j- V profiles. Alternatively, samples subjected to tape-stripping or immersion in chloroform/ methanol were highly conductive and displayed neither of the above characteristics. Collectively, such findings would indicate that the SC serves as the principal barrier to the transport of ionic permeants into and through the skin, and that specific lipid components likely regulate the integrity of the intercellular lipid domain under the influence of electric current.

Variation of human skin permeation in vitro: Ionic vs neutral compounds

The variation of skin permeation data has been investigated for ionic vs neutral permeants through human cadaver skin. In contrast to neutral ones, ionic permeants produced highly variable flux data with a positively skewed asymmetrical distribution. This permeant-dependent flux variation may suggest that different mechanisms are involved in the in vitro skin transport for ionic and neutral compounds.

Interaction of some solutes with certain cationic polymers and its effect on film permeability

AbstractPermeabilities of cast films of certain cationic acrylate methacrylate copolymers have been determined using a double (donor and acceptor) compartment cell. Permeation rates of urea (nonionic permeant) and sodium chloride (ionic permeant) were determined as functions of the polymer cation content. A decrease in film permeability accompanied the decrease in polymer cation content generally; the change was more marked with sodium chloride compared with urea. The films were also generally more permeable to urea; the difference increased at lower polymer cation content to about 400‐fold. While urea promoted film swelling, sodium chloride decreased it. Electron micrographs of the film section displayed compact skin zones which must be swelled to allow solute permeation. Asymmetry characterized the flux, the film lower surface (in contact with the mold during casting) being more permeable than its upper surface. This effect was more pronounced when sodium chloride was used as permeant.

Percutaneous absorption enhancement of an ionic molecule by ethanol-water systems in human skin.

Ethanol-water systems enhance permeation of ionic solutes through human stratum corneum. Optimum enhancement of salicylate ion permeation has been observed with ethanol volume fractions near 0.63. The mechanism of action of ethanol-water systems enhancing skin permeation was investigated by in vitro skin permeation studies combined with Fourier transform infrared spectroscopy experiments. The increased skin permeation of the ionic permeant by the ethanol-water systems may be associated with alterations involving the polar pathway. Polar pathway alterations may occur in either or both the lipid polar head and proteinaceous regions of the stratum corneum. Ion-pair formation may also contribute to increased permeation. However, the decreased permeation of salicylate ion observed at higher volume fractions of ethanol may be attributed to decreased uptake of permeant into the stratum corneum.

Swellability of cast films of an acrylate-methacrylate copolymer in aqueous solutions of certain permeants and concentration-dependent permeability

Abstract Swellability of cast films of an acrylate-methacrylate copolymer in aqueous solutions of certain permeants has been determined by fluid uptake experiments. Swellability increased linearly with urea concentrations but decreased with concentrations of ionic permeants, sodium chloride and sulfacetamide sodium. Film permeability coefficients for urea also increased linearly with its concentration whereas those for the ionic permeants increased with increase in their concentrations up to 5%, but decreased at concentrations 5% to 10% w/v. This finding can be associated with the swellability behavior of the films.

Transport properties of asymmetric polyamide membranes

Transport properties of asymmetric copolymide membranes made by interfacial polycondensation were studied. The directional flow preference observed in diffusion experiments with ionic permeants can be explained by the swelling behavior of the membrane. As a consequence of the weak acidic and weak basic endgroups, the ion exchange characteristics of the membrane are a function of the pH value in the ambient solution and can be changed from cation to anion exchange properties. Like a bipolar double membrane, it is composed of an cationite and an anionite region. The current–voltage characteristics were found to be asymmetric.

The interactions of ionic dye permeants with poly(vinyl alcohol) membranes

The well-known interaction between Congo Red and poly(vinyl alcohol), PVA, was studied by equilibrium dialysis. The diffusion of Congo Red into PVA membranes was much more rapid in 0.1N NaCl solution than in water. The dye appeared to be practically immobilized by the membranes in both solvents. A short survey of the dialytic behavior of various classes of ionic dyes through PVA membranes with water as solvent was undertaken. Anionic dyes permeated the membranes only very slowly, whereas cationic dyes permeated the membranes relatively rapidly and dyed them considerably. The existence of negative charges on the PVA membranes was demonstrated by performing dialysis experiments with the anionic dye Orange II and the cationic dye Acridine Orange in water and in excess electrolyte (1N NaCl).