Iontophoresis of poly-L-lysines: the role of molecular weight?

Modulation of Electroosmotic Flow through Skin: Effect of Poly(Amidoamine) Dendrimers

The objective of this work is to evaluate the effect of polyamidoamine (PAMAM) dendrimers on electroosmotic flow (EOF) through skin. The effect of size and concentration of dendrimer was studied, using generation 1, 4 and 7 dendrimer (G1, G4 and G7, respectively). As a marker molecule for the direction and magnitude of EOF, a neutral molecule, acetoaminophen (AAP) was used. The visualization of dendrimer permeation into the current conducting pore (CCP) of skin was made using G4-fluorescein isothiocyanate (FITC) conjugate and confocal microscopy. Without dendrimer, anodal flux of AAP was much higher than cathodal or passive flux. When G1 dendrimer was added, anodal flux decreased, presumably due to the decrease in EOF by the association of G1 dendrimer with net negative charge in CCP. As the generation increased, larger decrease in anodal flux was observed, and the direction of EOF was reversed. Small amount of methanol used for the preparation of dendrimer solution also contributed to the decrease in anodal flux of AAP. Cross-sectional view perpendicular to the skin surface by confocal laser scanning microscope (CLSM) study showed that G4 dendrimer-FITC conjugate (G4-FITC) can penetrate into the viable epidermis and dermis under anodal current. The permeation route seemed to be localized on hair follicle region. These results suggest that PAMAM dendrimers can permeate into CCP and change the magnitude and direction of EOF. Overall, we obtained a better understanding on the mechanistic insights into the electroosmosis phenomena and its role on flux during iontophoresis.

Dendritic polymers for dermal drug delivery

Skin-mediated therapeutic delivery is a potential alternative to traditional drug delivery approaches. However, dermal drug delivery is limited to the molecules with optimal physico-chemical properties. To overcome this barrier for delivering ‘nonideal’ drug molecules across the skin, different drug carriers and penetration enhancement methods have been investigated. Conventional chemical and physical approaches for dermal drug delivery are limited by their skin irritation potential, complexity of application and poor patient compliance. In recent years, dendritic polymers have shown potential in improving the dermal delivery of various molecules. With minimal skin irritation potential and high drug loading capacity, dendrimers offer multiple advantages for improving delivery of drugs across the skin. The current review aims to provide an overview of dendritic polymers for dermal (topical and transdermal) drug delivery.

[Formula: see text]

Transdermal iontophoresis patch with reverse electrodialysis

Reverse electrodialysis (RED) technology generates energy from the salinity gradient by contacting waters with different salinity. Herein, we develop the disposable skin patch using this eco-friendly energy. The current density, which can be controlled easily without special circuit, is enough to iontophoretic drug delivery. In vitro study, this iontophoretic system enhanced the transdermal delivery of peptide, which is difficult to penetrate the skin barrier by simple diffusion. We design the disposable iontophoretic skin patch using RED system and suggest this patch can be apply on new cosmetic patch or disposable drug patch.

Transdermal reverse iontophoresis: A novel technique for therapeutic drug monitoring

Application of transdermal reverse iontophoresis for diagnostic purpose is a relatively new concept but its short span of research is full of ups and downs. In early nineties, when the idea was floated, it received a dubious welcome by the scientific community. Yet to the disbelief of many, 2001 saw the launching of GlucoWatch® G2 Biographer, the first device that could measure the blood sugar level noninvasively. Unfortunately, the device failed to match the expectation and was withdrawn in 2007. However, the concept stayed on. Research on reverse iontophoresis has diversified in many fields. Numerous in vitro and in vivo experiments confirmed the prospect of reverse iontophoresis as a noninvasive tool in therapeutic drug monitoring and clinical chemistry. This review provides an overview about the recent developments in reverse iontophoresis in the field of therapeutic drug monitoring.

Use of electroporation and reverse iontophoresis for extraction of transdermal multibiomarkers

Background

Monitoring of biomarkers, like urea, prostate-specific antigen (PSA), and osteopontin, is very important because they are related to kidney disease, prostate cancer, and ovarian cancer, respectively. It is well known that reverse iontophoresis can enhance transdermal extraction of small molecules, and even large molecules if reverse iontophoresis is used together with electroporation. Electroporation is the use of a high-voltage electrical pulse to create nanochannels within the stratum corneum, temporarily and reversibly. Reverse iontophoresis is the use of a small current to facilitate both charged and uncharged molecule transportation across the skin. The objectives of this in vitro study were to determine whether PSA and osteopontin are extractable transdermally and noninvasively and whether urea, PSA, and osteopontin can be extracted simultaneously by electroporation and reverse iontophoresis.

Methods

All in vitro experiments were conducted using a diffusion cell assembled with the stratum corneum of porcine skin. Three different symmetrical biphasic direct currents (SBdc), five various electroporations, and a combination of the two techniques were applied to the diffusion cell via Ag/AgCl electrodes. The three different SBdc had the same current density of 0.3 mA/cm², but different phase durations of 0 (ie, no current, control group), 30, and 180 seconds. The five different electroporations had the same pulse width of 1 msec and number of pulses per second of 10, but different electric field strengths of 0 (ie, no voltage, control group), 74, 148, 296, and 592 V/cm. Before and after each extraction experiment, skin impedance was measured at 20 Hz.

Results

It was found that urea could be extracted transdermally using reverse iontophoresis alone, and further enhancement of extraction could be achieved by combined use of electroporation and reverse iontophoresis. Conversely, PSA and osteopontin were found to be extracted transdermally only by use of reverse iontophoresis and electroporation with a high electrical field strength (>296 V/cm). After application of reverse iontophoresis, electroporation, or a combination of the two techniques, a reduction in skin impedance was observed.

Conclusion

Simultaneous transdermal extraction of urea, PSA, and osteopontin is possible only for the condition of applying reverse iontophoresis in conjunction with high electroporation.

Combined strategies for enhancing the transdermal absorption of midazolam through human skin

Objectives

Midazolam administration by intravenous or intramuscular injection produces pain and stress. For this reason, alternative methods of administration have been proposed. The transdermal administration of midazolam could improve patient comfort, which is especially important for children in the pre-operative period. We aimed to assess the effect of iontophoresis and chemical percutaneous enhancers applied individually and together, to determine if a synergistic effect is achieved when both enhancement techniques are simultaneously employed.

Methods

This work reports the characterization of the passive diffusion of midazolam hydrochloride through human skin in vitro and evaluates the effect of iontophoresis application and chemical percutaneous enhancers on said diffusion when employed both individually and in combination.

Key findings

Percutaneous absorption assays demonstrated that the physical technique of iontophoresis, when applied alone, moderately increased midazolam hydrochloride permeation flux through human skin, producing a similar effect to that obtained with R-(+)-limonene chemical enhancer. Among the strategies assayed, it was observed that Azone produced the most pronounced enhancement effect when applied separately. The combination of pre-treatment with Azone and iontophoresis exhibited a higher capacity for enhancing the transdermal flux of midazolam through human skin than Azone alone.

Conclusions

In conclusion, when applied individually, Azone exhibited the greatest enhancement effect on the transdermal diffusion of midazolam of the various strategies assayed. The combination of Azone and iontophoresis produce the highest transdermal steady-state flux of midazolam but no synergic effect was achieved when the two enhancement strategies were applied in combination, showing that although selecting the best conditions for iontophoresis application, it is less effective for augmenting the transdermal delivery of midazolam than the chemical enhancer Azone.

Potential use of iontophoresis for transdermal delivery of NF-kappaB decoy oligonucleotides

Topical application of nuclear factor-kappaB (NF-kappaB) decoy appears to provide a novel therapeutic potency in the treatment of inflammation and atopic dermatitis. However, it is difficult to deliver NF-kappaB decoy oligonucleotides (ODN) into the skin by conventional methods based on passive diffusion because of its hydrophilicity and high molecular weight. In this study, we evaluated the in vitro transdermal delivery of fluorescein isothiocyanate (FITC)-NF-kappaB decoy ODN using a pulse depolarization (PDP) iontophoresis. In vitro iontophoretic experiments were performed on isolated C57BL/6 mice skin using a horizontal diffusion cell. The apparent flux values of FITC-NF-kappaB decoy ODN were enhanced with increasing the current density and NF-kappaB decoy ODN concentration by iontophoresis. Accumulation of FITC-NF-kappaB decoy ODN was observed at the epidermis and upper dermis by iontophoresis. In mouse model of skin inflammation, iontophoretic delivery of NF-kappaB decoy ODN significantly reduced the increase in ear thickness caused by phorbol ester as well as the protein and mRNA expression levels of tumor necrosis factor-alpha (TNF-alpha) in the mice ears. These results suggest that iontophoresis is a useful and promising enhancement technique for transdermal delivery of NF-kappaB decoy ODN.

Hydration effects on skin microstructure as probed by high-resolution cryo-scanning electron microscopy and mechanistic implications to enhanced transcutaneous delivery of biomacromolecules

Although hydration is long known to improve the permeability of skin, penetration of macromolecules such as proteins is limited and the understanding of enhanced transport is based on empirical observations. This study uses high-resolution cryo-scanning electron microscopy to visualize microstructural changes in the stratum corneum (SC) and enable a mechanistic interpretation of biomacromolecule penetration through highly hydrated porcine skin. Swollen corneocytes, separation of lipid bilayers in the SC intercellular space to form cisternae, and networks of spherical particulates are observed in porcine skin tissue hydrated for a period of 4-10 h. This is explained through compaction of skin lipids when hydrated, a reversal in the conformational transition from unilamellar liposomes in lamellar granules to lamellae between keratinocytes when the SC skin barrier is initially established. Confocal microscopy studies show distinct enhancement in penetration of fluorescein isothiocyanate-bovine serum albumin (FITC-BSA) through skin hydrated for 4-10 h, and limited penetration of FITC-BSA once skin is restored to its natively hydrated structure when exposed to the environment for 2-3 h. These results demonstrate the effectiveness of a 4-10 h hydration period to enhance transcutaneous penetration of large biomacromolecules without permanently damaging the skin.

Molecular charge mediated transport of a 13 kD protein across microporated skin

Transport of proteins across the skin is highly limited owing to their hydrophilic nature and large molecular size. This study was conducted to assess the skin transport abilities of a model protein across hairless rat skin during iontophoresis alone and in combination with microneedles as a function of molecular charge. The effect of microneedle pretreatment on electroosmotic flow was also investigated. Skin permeation experiments were carried out in vitro using daniplestim (DP) (MW, 12.76 kD; isoelectric point, 6.2) as a model protein molecule. The effect of molecular charge on protein transport was evaluated by performing studies in two different buffers--TRIS (pH 7.5) and acetate (pH 4.0). Iontophoretic transport mechanisms of DP varied with respect to molecular charge on the protein. The combination approach (iontophoresis and microneedles) gave much higher flux values compared to iontophoresis alone at both pH 4.0 and pH 7.5, however, the delivery in this case was also found to be charge dependent. The findings of this study indicate that electroosmosis persisted upon microporation, thus retaining skin's permselective properties. This enables us to explore the combination of microneedles and iontophoresis as a potential approach for delivery of proteins.

Investigation of drug delivery by iontophoresis in a surgical wound utilizing microdialysis

PURPOSE

This study investigated the penetration of lidocaine around and through a sutured incision following the application of iontophoretic and passive patches in the CD Hairless rat.

MATERIALS AND METHODS

Concentrations in localized areas (suture, dermis, subcutaneous, and vascular) were determined using microdialysis sampling followed by analysis using liquid chromatography with UV detection.

RESULTS

Iontophoresis significantly enhanced the dermal penetration of lidocaine. In an intact skin model, dermal concentrations were 40 times greater following iontophoretic delivery compared to passive delivery. In a sutured incision model, iontophoresis enhanced localized concentrations in the dermis, suture, and subcutaneous regions by 6-, 15-, and 20-fold, respectively. Iontophoretic delivery to a region containing a sutured incision was focused to the incision resulting in a greater increase in the suture concentration and in the subcutaneous region directly below the incision.

CONCLUSIONS

The four microdialysis probe design was successful in the determination of localized drug penetration in a sutured incision model. Iontophoresis enhanced skin penetration and allowed for site specific delivery when applied to a sutured incision.

Quantitative structure-permeation relationship for iontophoretic transport across the skin

The objective was to relate the efficiency of a charged drug to carry current across the skin during iontophoresis to its structural and/or physicochemical properties. The corollary was the establishment of a predictive relationship useful to predict the feasibility of iontophoretic drug delivery, and for the selection and optimization of drug candidates for this route of administration. A dataset of 16 cations, for which iontophoretic fluxes have been measured under identical conditions, with no competition from exogenous co-ions, was compiled. Maximum transport numbers correlated with ion mobilities and decreased with ionic size, the dependence indicating that the electromigration mechanism of iontophoresis would become negligible for drugs of hydrodynamic radius greater than about 8 A. Validation of the model was demonstrated by successfully predicting the transport numbers of three structurally distinct dipeptides, the iontophoretic data for which had been determined under distinctly different experimental conditions. Finally, for the "training" set of cations, a strong linear dependence between their transport numbers in skin and those in aqueous solution was demonstrated; the former were larger by approximately a factor of 1.4 consistent with skin's cation permselectivity. In conclusion, this research offers a practical contribution to the development of a predictive structure-transport model of iontophoresis.

Transdermal Delivery of Cytochrome C—A 12.4 kDa Protein—Across Intact Skin by Constant–Current Iontophoresis

PURPOSE

To demonstrate the transdermal iontophoretic delivery of a small (12.4 kDa) protein across intact skin.

MATERIALS AND METHODS

The iontophoretic transport of Cytochrome c (Cyt c) across porcine ear skin in vitro was investigated and quantified by HPLC. The effect of protein concentration (0.35 and 0.7 mM), current density (0.15, 0.3 or 0.5 mA.cm(-2) applied for 8 h) and competing ions was evaluated. Co-iontophoresis of acetaminophen was employed to quantify the respective contributions of electromigration (EM) and electroosmosis (EO).

RESULTS

The data confirmed the transdermal iontophoretic delivery of intact Cyt c. Electromigration was the principal transport mechanism, accounting for approximately 90% of delivery; correlation between EM flux and electrophoretic mobility was consistent with earlier results using small molecules. Modest EO inhibition was observed at 0.5 mA.cm(-2). Cumulative permeation at 0.3 and 0.5 mA.cm(-2) was significantly greater than that at 0.15 mA.cm(-2); fluxes using 0.35 and 0.7 mM Cyt c in the absence of competing ions (J ( tot ) = 182.8 +/- 56.8 and 265.2 +/- 149.1 microg.cm(-2).h(-1), respectively) were statistically equivalent. Formulation in PBS (pH 8.2) confirmed the impact of competing charge carriers; inclusion of approximately 170 mM Na(+) resulted in a 3.9-fold decrease in total flux.

CONCLUSIONS

Significant amounts ( approximately 0.9 mg.cm(-2) over 8 h) of Cyt c were delivered non-invasively across intact skin by transdermal electrotransport.

Transdermal delivery of interferon alpha-2B using microporation and iontophoresis in hairless rats

PURPOSE

To demonstrate transdermal delivery of interferon alpha-2b (IFNalpha2b) in hairless rats through aqueous microchannels (micropores) created in the skin and enhanced by iontophoresis.

MATERIALS AND METHODS

The Altea Therapeutics PassPort System was configured to form an array of micropores (2.0 cm(2); 72 micropores/cm(2)) on the rat abdomen. The transdermal patch (Iomed TransQ1-GS-hydrogel) was saturated with an IFNalpha2b solution (600 microg/ml) and applied for 4 h. Delivery was evaluated with and without cathodic iontophoresis (0.1 mA/cm(2)). Intravenous delivery (0.4 microg/100 g body weight) was performed to support pharmacokinetic calculations.

RESULTS

IFNalpha2b was not delivered through intact skin by itself (passive delivery) or during iontophoresis. However, passive delivery through micropores was achieved in vivo in rats. A dose of 397 +/- 67 ng was delivered over 6 h, with steady state serum concentrations reaching a plateau at 1 h post-patch application. These levels dropped rapidly after patch removal, and returned to baseline within 2 h of patch removal. Iontophoresis-enhanced delivery through micropores resulted in a two-fold increase in the dose delivered (722 +/- 169 ng) in the hairless rat.

CONCLUSIONS

In vivo delivery of IFNalpha2b was demonstrated through micropores created in the outer layer of the skin. Iontophoresis enhanced delivery through microporated skin in hairless rats.

Effect of electroporation and pH on the iontophoretic transdermal delivery of human insulin

The synergistic effect of electroporation (EP) and iontophoresis (IP) on the in vivo percutaneous absorption of human insulin was evaluated in rats. Passive diffusion and IP alone (0.4 mA/cm(2)) resulted in almost no skin permeation of insulin at pH 7, whereas EP treatment (150 or 300 V, 10 ms, and 10 pulses) resulted in a high plasma level of insulin and the combined use of EP and IP led to a further increase of the plasma level of insulin compared with that measured after EP alone. Interestingly, a much higher plasma level was observed when the pH of the insulin solution at 7 was increased to 10. One of the reasons was the different aggregation properties of insulin at pH 7 and pH 10. The nonassociation ratio of insulin was significantly higher at pH 10 than at pH 7. Insulin monomers and dimers were observed in addition to the normal form of insulin, hexamer, albeit in low percentages, at pH 10, whereas most of the insulin was in the hexamer form at pH 7. To confirm the influence of the aggregation properties of insulin, the commercially available human insulin analogue insulin lispro was then evaluated. Its skin permeation was found to be extremely high compared to that of conventional human insulin without increasing the solution pH. Marked decreases in blood glucose levels reflecting the increases in the plasma concentration of insulin were also observed after EP/IP treatment. The present study suggests that percutaneous absorption of insulin is synergistically enhanced by a combined use of EP and IP and that altering the aggregation properties of insulin is important to enhance the percutaneous absorption of insulin by IP and/or EP.

Comparative Study of the Skin Penetration of Protein Transduction Domains and a Conjugated Peptide

PURPOSE

We examined the ability of a protein transduction domain (PTD), YARA, to penetrate in the skin and carry a conjugated peptide, P20. The results with YARA were compared to those of a well-known PTD (TAT) and a control, nontransducing peptide (YKAc). The combined action of PTDs and lipid penetration enhancers was also tested.

METHODS

YARA, TAT, YKAc, P20, YARA-P20, and TAT-P20 were synthesized by Fmoc chemistry. Porcine ear skin mounted in a Franz diffusion cell was used to assess the topical and transdermal delivery of fluorescently tagged peptides in the presence or absence of lipid penetration enhancers (monoolein or oleic acid). The peptide concentrations in the skin (topical delivery) and receptor phase (transdermal delivery) were assessed by spectrofluorimetry. Fluorescence microscopy was used to visualize the peptides in different skin layers.

RESULTS

YARA and TAT, but not YKAc, penetrated abundantly in the skin and permeated modestly across this tissue. Monoolein and oleic acid did not enhance the topical and transdermal delivery of TAT or YARA but increased the topical delivery of YKAc. Importantly, YARA and TAT carried a conjugated peptide, P20, into the skin, but the transdermal delivery was very small. Fluorescence microscopy confirmed that free and conjugated PTDs reached viable layers of the skin.

CONCLUSIONS

YARA and TAT penetrate in the porcine ear skin in vitro and carry a conjugated model peptide, P20, with them. Thus, the use of PTDs can be a useful strategy to increase topical delivery of peptides for treatment of cutaneous diseases.

Visualization of skin penetration using confocal laser scanning microscopy

The use of skin as an alternative route for administering systemically active drugs has attracted considerable interest in recent years. However, the skin provides an excellent barrier, which limits the number of drug molecules suitable for transdermal delivery. Thus, in order to improve cutaneous delivery, it is necessary to adopt an enhancement method, either (i) passively using novel formulations, e.g. microemulsions, liposomes, and colloidal polymeric suspensions, or more conventional skin permeation enhancers, or (ii) with a physical approach, such as, iontophoresis, sonophoresis or electroporation. Although there has been much progress, the precise modes of action of the different techniques used are far from well-understood. The objective of this review, therefore, is to evaluate how confocal laser scanning microscopy may contribute to the determination of the mechanisms of diverse skin penetration enhancement strategies.

Transdermal reverse iontophoresis of valproate: a noninvasive method for therapeutic drug monitoring

PURPOSE

The objectives of this work were (a) to explore the potential of transdermal reverse iontophoresis for therapeutic drug monitoring and (b) to develop an "internal standard" calibration procedure so as to render the technique completely noninvasive.

METHODS

A series of in vitro iontophoresis experiments was performed in which the subdermal concentration of sodium valproate was varied from 21 microM to 1 mM. Glutamic acid was also introduced into the subdermal donor at a fixed concentration to act as an "internal standard" for the calibration method.

RESULTS

Both valproate and glutamate anions were recovered, as expected, at the anodal receptor chamber. The iontophoretic extraction flux of valproate was linearly correlated with the subdermal concentration. Glutamate flux was constant. It follows that the ratio of extracted fluxes (valproate/glutamate) was directly dependent upon (a) the subdermal valproate concentration and (b) the subdermal concentration ratio (valproate/glutamate), offering a means, thereby, to a completely noninvasive methodology.

CONCLUSIONS

This work demonstrates the potential of reverse iontophoresis for noninvasive therapeutic monitoring. The simultaneous quantification of the analyte of interest and of an "internal standard" renders the withdrawal of a blood sample unnecessary.

Skin permeability enhancement by low frequency sonophoresis: lipid extraction and transport pathways

The objective of this study was to shed light on the mechanism(s) by which low-frequency ultrasound (20 KHz) enhances the permeability of the skin. The physical effects on the barrier and the transport pathway, in particular, were examined. The amount of lipid removed from the intercellular domains of the stratum corneum following sonophoresis was determined by infrared spectroscopy. Transport of the fluorescent probes nile red and calcein, under the influence of ultrasound, was evaluated by laser-scanning confocal microscopy. The results were compared with the appropriate passive control data and with data obtained from experiments in which the skin was exposed simply to the thermal effects induced by ultrasound treatment. A significant fraction ( approximately 30%) of the intercellular lipids of the stratum corneum, which are principally responsible for skin barrier function, were removed during the application of low-frequency sonophoresis. Although the confocal images from the nile red experiments were not particularly informative, ultrasound clearly and significantly (again, relative to the corresponding controls) facilitated transport of the hydrophilic calcein via discrete permeabilized regions, whereas other areas of the barrier were apparently unaffected. Lipid removal from the stratum corneum is implicated as a factor contributing the observed permeation enhancement effects of low-frequency ultrasound. However, microscopic observations imply that sonophoresis induces localized (aqueous?) permeation pathways at discrete sites.

Iontophoresis-based transdermal delivery systems

Transdermal iontophoresis is the administration of ionic therapeutic agents through the skin by the application of a low-level electric current. This article presents an overview of transdermal iontophoretic delivery of drugs, including peptides and oligonucleotides. Recent advances in the area of iontophoretic delivery, including devices, hydrogel formulations, safety, clinical relevance and future prospects, are discussed. Electroporation, another method of electrically assisted drug delivery, is also briefly reviewed. Transdermal iontophoresis appears to be a promising technique for the delivery of a variety of compounds in a controlled and preprogrammed manner. Transdermal iontophoresis would be particularly useful in the delivery of hydrophilic drugs produced by biotechnology (peptides and oligonucleotides). However, because of the complex physicochemical properties of peptides, many factors must be carefully considered for the proper design of an iontophoretic drug delivery system for peptides. Iontophoresis has been successfully used in the delivery of small peptides, such as leuprolide and calcitonin analogues, in humans. However, it appears that transdermal iontophoresis may not be a suitable method for the systemic delivery of larger peptides (>7,000D). The combined use of iontophoresis and electroporation may be more effective in the delivery of peptides, proteins, genes and oligonucleotides. The long-term safety of iontophoresis, patient compliance with the technique and the commercial success of this technology are yet to be demonstrated. Iontophoretic delivery of drugs would be beneficial in the treatment of certain skin disorders such as skin cancer, psoriasis, dermatitis, venous ulcers, keloid and hypertrophic scars. Investigations on reverse iontophoresis may yield interesting results that would be useful in the noninvasive measurement of clinically important molecules in the body.

Electroosmotic pore transport in human skin

PURPOSE

To determine the pathways and origin of electroosmotic flow in human skin.

METHODS

Iontophoretic transport of acetaminophen in full thickness human cadaver skin was visualized and quantified by scanning electrochemical microscopy. Electroosmotic flow in the shunt pathways of full thickness skin was compared to flow in the pores of excised stratum corneum and a synthetic membrane pore. The penetration of rhodamine 6G into pore structures was investigated by laser scanning confocal microscopy.

RESULTS

Electroosmotic transport is observed in shunt pathways in full thickness human skin (e.g., hair follicles and sweat glands), but not in pore openings of freestanding stratum corneum. Absolute values of the diffusive and iontophoretic pore fluxes of acetaminophen in full thickness human skin are also reported. Rhodamine 6G is observed to penetrate to significant depths (approximately 200 microm) along pore pathways.

CONCLUSIONS

Iontophoresis in human cadaver skin induces localized electroosmotic flow along pore shunt paths. Electroosmotic forces arise from the passage of current through negatively charged mesoor nanoscale pores (e.g., gap functions) within cellular regions that define the pore structure beneath the stratum corneum.

Distribution and quantification of polyethylenimine oligodeoxynucleotide complexes in human skin after iontophoretic delivery using confocal scanning laser microscopy

Iontophoresis may be a potentially useful technique for the delivery of oligonucleotides into the skin. To enhance intracellular uptake during iontophoresis, we investigated the dermal delivery of oligodeoxynucleotides (ODN) as a polyelectrolyte complex with polyethylenimine (PEI). Perpendicular cross-sectioning was performed to visualize and quantify the penetration properties of double labeled PEI/ODN complexes across full thickness human skin. Due to the net positive charge of the complexes, anodal iontophoresis was expected to enhance skin delivery by electrorepulsion compared to passive diffusion. Confocal laser scanning microscopy demonstrated that non-complexed ODN could penetrate the skin after 1 h of cathodal iontophoresis but not by passive diffusion or anodal iontophoresis. However, extensive degradation occurred as documented by a dramatic decrease of fluorescence intensity within viable skin tissue after 10 h. Anodal iontophoresis of the complexes led to a deep penetration of both the TAMRA-labeled ODN and the Oregon Green-labeled PEI. A constant increase in fluorescence indicated a protective effect of the polymer against nuclease degradation. Co-localization of red and green fluorescence was noted within numerous nuclei of epidermal keratinocytes. In contrast, passive diffusion of the complexes did not lead to successful uptake into keratinocytes and was limited to the stratum corneum. Complexation of ODN by PEI, therefore, seems to be a promising method to enhance both the transport of charged complexes into the skin and to facilitate intracellular uptake, which may potentially be useful for the local treatment of skin diseases using ODN.

Transdermal delivery of poly-l-lysine by sonomacroporation

A feasibility study of using high-amplitude ultrasound (US) to deliver large molecules transdermally was undertaken. US (20 kHz) of intensity in the range between 2 to 50 W/cm(2) was used to increase the permeability of skin in vitro to large size molecules. For example, when 20-kHz, 5% duty cycle US at the spatial average and pulse-average intensity I(SAPA) = 19 W/cm(2) was applied for 10 min and the distance between the US source and the surface of a skin specimen was 2 mm, the skin permeability was calculated to be 0.5 +/- 0.2 cm/h and 8.5 +/- 4.2 cm/h, respectively, for poly l-lysine-fluorescein isothiocyanate (FITC) (51 kDa) and octa-1-lysine-FITC (2.5 kDa). Without application of US, the skin permeability of the above-mentioned molecules would be essentially zero. A transdermal flux enhancement occurring during the process reported here was much higher than that due to sonophoresis (I(SAPA) < 2 W/cm(2)) as reported in the literature. For comparison, for example, the skin permeability for delivering erythropoeitin (48 kDa) and insulin (6 kDa) reached 9.8 x 10(-6) and 3.3 x10(-3) cm/h, respectively, by using sonophoresis for 1 h US exposure. Experimental results from transdermal flux kinetics, and confocal microscopic cross-sectional and optical images, suggested that the formation of pores in the stratum corneum, whose size varies with skin samples, may be in the range of 1 to 100 microm. The confocal images also suggest the formation of microm-size pathways in epidermis during US exposure.

Transdermal drug delivery: overcoming the skin's barrier function

The skin represents an extraordinary evolutionary feat. Not only does it physically encapsulate the organism and provide a multifunctional interface between us and our surroundings, but it is perpetually engaged in the assembly of a highly efficient homeostatic barrier to the outward loss of water(1). In so doing, it furnishes a membrane that is equally adept at limiting molecular transport both from and into the body. Overcoming this barrier function then, for the purpose of transdermal drug delivery, has been a necessarily challenging task for the pharmaceutical scientist, and one that boasts significant progress.

Transdermal macromolecular delivery: real-time visualization of iontophoretic and chemically enhanced transport using two-photon excitation microscopy

PURPOSE

To investigate the transdermal delivery of a model macromolecule by passive and iontophoretic means following pretreatment with C12-penetration enhancers and to visualise transport across human stratum corneum (SC) in real time.

METHODS

Transport studies of dextran, labelled with fluorescent Cascade Blue (D-CB: M(R) = 3 kDa) across human stratum corneum, were conducted during passive and iontophoretic modes of delivery following pretreatment with either dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulphate (SDS) or Azone. Size-exclusion chromatography was used to assess maintenance of dextran structural integrity throughout experimental lifetime. Two-photon excitation microscopy was employed to visualise real-time dextran transport during current application.

RESULTS

The positively charged C12-enhancer DTAB elevated passive D-CB steady-state flux (J(ss)) and was the only enhancer to do so above control during iontophoresis. The negatively charged SDS had the least effect during both stages. On-line macromolecular transport was visualised, indicating both inter- and intra-cellular pathways across SC during current application. No transport was visible across untreated SC during passive transport.

CONCLUSIONS

Use of a positively charged enhancer may improve J(ss) of anionic macromolecular penetrants during passive and iontophoretic delivery. On-line visualisation of iontophoresis across SC was possible and can provide mechanistic insight into SC transport pathways.

Transdermal delivery of macromolecules using skin electroporation

PURPOSES

(1) To evaluate the feasibility of transdermal delivery of macromolecules by skin electroporation. (2) To assess the influence of the molecular weight of the permeant on transport and examine whether there exists a "cut-off" value of molecular weight. (3) To localize the transport pathways of the macromolecules in the skin.

METHODS

FITC-dextran (FD) of increasing molecular weight (4.4, 12 and 38 kDa) were used as model macromolecules to study the extent of transport across hairless rats skin in vitro and to localize their distribution in the skin by confocal scanning laser microscopy.

RESULTS

Electroporation enhanced the transport of the macromolecules as compared to passive diffusion. The transdermal delivery by skin electroporation of FITC and FD 4.4 was equivalent whereas transport of higher molecular weight FD was lower but significant. FITC and FD 38 were observed in the epidermis both around and in the keratinocytes.

CONCLUSIONS

Transdermal and topical delivery of macromolecules of at least 40 kDa can be achieved by skin electroporation.

Electrorepulsion versus electroosmosis: effect of pH on the iontophoretic flux of 5-fluorouracil

PURPOSE

To delineate the contributions of electrorepulsion and electroosmosis to the iontophoretic flux of 5-FU across porcine skin in vitro. Also, the isoelectric point (pI) of the skin model was determined.

METHODS

The electrotransport of 5-FU, anode-to-cathode ("anodal") and cathode-to-anode ("cathodal") was determined as a function of the pH of the electrolyte bathing the skin.

RESULTS

At pH 8.5, the drug (pKa approximately 8) is negatively charged and "cathodal", viz. electrorepulsive, transport is much greater than that in the opposite direction. At pH 7.4, where approximately 25% of 5-FU is charged, electrorepulsive and electroosmotic ("anodal") fluxes are balanced. Decreasing the pH to 6, and then 5, reduces the percentage of ionized 5-FU such that "anodal" electroosmosis dominates across the negatively-charged membrane. But, at pH 4, "anodal" and "cathodal" fluxes are again equal suggesting neutralization of the skin (i.e., pI approximately 4). This is confirmed at pH 3, where "cathodal" electroosmosis dominates across the now net-positively charged barrier.

CONCLUSIONS

Electrotransport is sensitive, mechanistically, to the properties of the permeant and of the skin; interactions of, for example, the drug or constituents of a formulation, that alter the barrier's net charge, can affect iontophoretic delivery. The pI of porcine ear skin is approximately 4.

Effects of iontophoresis and electroporation on the stratum corneum. Review of the biophysical studies

This review focuses on the effects induced by iontophoresis and electroporation on the stratum corneum of the skin. Hence, the aims were: (1) to contribute to the understanding of the mechanisms of drug transport by these methods; (2) to evaluate the safety issues associated with current application. Complementary biophysical methods were used to provide a complete picture of the stratum corneum. Even though the mechanism of drug transport is believed to be different, i.e., electrophoresis for iontophoresis and creation of new aqueous pathways for electroporation, the effects on the stratum corneum detected minutes after current application are very similar. For both methods, the major findings were: (1) a disorganisation of the lipid bilayers of the stratum corneum; (2) an increase in skin hydration; (3) a larger decrease in skin resistance induced by electroporation as compared to iontophoresis. These changes were partly reversible and depended on the amount of electrical charges transferred. The mechanisms of stratum corneum perturbations are discussed. These perturbations could explain partly the increase in drug transport. If iontophoresis is considered as a safe method of drug delivery, the data augurs for the safety of electroporation.