Transdermal therapy and diagnosis by iontophoresis

Noninvasive Glucose Sensing In Vivo

Blood glucose monitoring is an essential aspect of disease management for individuals with diabetes. Unfortunately, traditional methods require collecting a blood sample and thus are invasive and inconvenient. Recent developments in minimally invasive continuous glucose monitors have provided a more convenient alternative for people with diabetes to track their glucose levels 24/7. Despite this progress, many challenges remain to establish a noninvasive monitoring technique that works accurately and reliably in the wild. This review encompasses the current advancements in noninvasive glucose sensing technology in vivo, delves into the common challenges faced by these systems, and offers an insightful outlook on existing and future solutions.

Influencing factors and drug application of iontophoresis in transdermal drug delivery: an overview of recent progress

Transdermal drug delivery is limited by the stratum corneum of skin, which blocks most molecules, and thus, only few molecules with specific physicochemical properties (molecular weight < 500 Da, adequate lipophilicity, and low melting point) are able to penetrate the skin. Recently, various technologies have been developed to overcome the strong barrier properties of stratum corneum. Iontophoresis technology, which uses a small current to improve drug permeation through skin, is one of the effective ways to circumvent the stratum corneum. This approach not only provides a more efficient, noninvasive, and patient-friendly method of drug delivery but also widens the scope of drugs for transdermal delivery. In this review, the mechanisms underlying iontophoresis and affecting factors are outlined. The focus will be on the latest advancements in iontophoretic transdermal drug delivery and application of iontophoresis with other enhancing technologies. The challenges of this technology for drug administration have also been highlighted, and some iontophoretic systems approved for clinical use are described.

Transdermal iontophoretic drug delivery: advances and challenges

The stratum corneum continues to pose considerable impediment to transdermal drug delivery. One of the effective ways of circumventing this challenge is through the use of iontophoresis. Iontophoresis uses low-level current to drive charged compounds across the skin. This review discusses progress made in the field of iontophoretic transport of small and large molecules. The major obstacles are also touched upon and advances made in the last few decades described. A number of iontophoretic systems approved for clinical use by regulatory authorities is also discussed.

Influence of sonophoresis and chemical penetration enhancers on percutaneous transport of penbutolol sulfate

The effect of ultrasound and chemical penetration enhancers on transcutaneous flux of penbutolol sulfate across split-thickness porcine skin was investigated. Penbutolol sulfate is a potent, noncardioselective beta-blocker, which is used for the management of hypertension. The drug is one of the most lipid soluble of the β-adrenoceptor antagonists used clinically. It has an n-octanol/pH 7.4 buffer partition coefficient of 179 compared to a value of 22 for propranolol. The amount of penbutolol sulfate transported across the skin is low. In this project, we studied the effect of sonophoresis and chemical penetration enhancers on transdermal delivery of penbutolol sulfate. Low-frequency sonophoresis at a frequency of 20 kHz increased transcutaneous flux of penbutolol sulfate by 3.5-fold (27.37 ± μg cm^-2 h^-1) compared to passive delivery (7.82 ± 1.72 μg cm^-2 h^-1). We also investigated the effect of 50% ethanol, 1% limonene and 2% isopropyl myristate (IPM) on transcutaneous permeation of penbutolol sulfate. IPM, ethanol and limonene at the concentration of 1%, 50% and 2%, respectively, increased the steady-state flux values of penbutolol sulfate 2.2- (17.07 ± 3.24 μg cm^-2 h^-1), 2.6 - (19.40 ± 6.40 μg cm^-2 h^-1) and 3.4-times (26.38 ± 5.01 μg cm^-2 h^-1) compared to passive delivery (7.76 ± 2.9 μg cm^-2 h^-1). The results demonstrate that although there were slight increases in flux values, ultrasound, ethanol, limonene and IPM did not significantly enhance the transdermal delivery of penbutolol sulfate. Future studies will examine ways of optimizing sonophoretic and chemical enhancer parameters to achieve flux enhancement.

A new approach for noninvasive transdermal determination of blood uric acid levels

The aims of this study were to investigate the most effective combination of physical forces from laser, electroporation, and reverse iontophoresis for noninvasive transdermal extraction of uric acid, and to develop a highly sensitive uric acid biosensor (UAB) for quantifying the uric acid extracted. It is believed that the combination of these physical forces has additional benefits for extraction of molecules other than uric acid from human skin. A diffusion cell with porcine skin was used to investigate the most effective combination of these physical forces. UABs coated with ZnO₂ nanoparticles and constructed in an array configuration were developed in this study. The results showed that a combination of laser (0.7 W), electroporation (100 V/cm²), and reverse iontophoresis (0.5 mA/cm²) was the most effective and significantly enhanced transdermal extraction of uric acid. A custom-designed UAB coated with ZnO₂ nanoparticles and constructed in a 1×3 array configuration (UAB-1×3-ZnO₂) demonstrated enough sensitivity (9.4 μA/mM) for quantifying uric acid extracted by the combined physical forces of laser, electroporation, and RI. A good linear relationship (R²=0.894) was demonstrated to exist between the concentration of uric acid (0.2–0.8 mM) inside the diffusion cell and the current response of the UAB-1×3-ZnO₂. In conclusion, a new approach to noninvasive transdermal extraction and quantification of uric acid has been established.

Prospects of iontophoresis in cardiovascular drug delivery

Clinical benefits, industry interest, regulatory precedence, and strong market potential have made transdermal research the fastest growth area in drug delivery. As most drugs permeate poorly through skin, a major challenge is achieving the therapeutic level by enhancement of permeation rate. Iontophoresis, utilizing a minimal amount of current, is found to affect the skin permeation process drastically. Ideally suited for protein drugs, attempts have been made to utilize the technology for accelerating the low-molecular-weight drugs for chronic administration. However, because of the difficulty associated with the energy supply, commercialization was not feasible until recent times. Fortunately, the unprecedented growth of microelectronics has bridged this lacuna and brought the technology right into limelight. This article analyses the advantages of electrically assisted drug delivery in relation to passive permeation, with special reference to some cardiovascular drugs, for which there is already a demand in the market.

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.

Simultaneous, noninvasive, and transdermal extraction of urea and homocysteine by reverse iontophoresis

Background:

Cardiovascular and kidney diseases are a global public health problem and impose a huge economic burden on health care services. Homocysteine, an amino acid, is associated with coronary heart disease, while urea is a harmful metabolic substance which can be used to reflect kidney function. Monitoring of these two substances is therefore very important. This in vitro study aimed to determine whether homocysteine is extractable transdermally and noninvasively, and whether homocysteine and urea can be extracted simultaneously by reverse iontophoresis.

Methods:

A diffusion cell incorporated with porcine skin was used for all experiments with the application of a direct current (dc) and four different symmetrical biphasic direct currents (SBdc) for 12 minutes via Ag/AgCl electrodes. The dc and the SBdc had a current density of 0.3 mA/cm².

Results:

The SBdc has four different phase durations of 15 sec, 30 sec, 60 sec, and 180 sec. It was found that homocysteine can be transdermally extracted by reverse iontophoresis. Simultaneous extraction of homocysteine and urea by reverse iontophoresis is also possible.

Conclusion:

These results suggest that extraction of homocysteine and urea by SBdc are phase duration-dependent, and the optimum mode for simultaneous homocysteine and urea extraction is the SBdc with the phase duration of 60 sec.

Non-invasive and transdermal measurement of blood uric acid level in human by electroporation and reverse iontophoresis

The aim of this study was to find out the optimum combination of electroporation (EP) and reverse iontophoresis (RI) on noninvasive and transdermal determination of blood uric acid level in humans. EP is the use of high-voltage electric pulse to create nano-channels on the stratum corneum, temporarily and reversibly. RI is the use of small current to facilitate both charged and uncharged molecule transportation across the skin. It is believed that the combination of these two techniques has additional benefits on the molecules' extraction across the human skin. In vitro studies using porcine skin and diffusion cell have indicated that the optimum mode for transdermal uric acid extraction is the combination of RI with symmetrical biphasic direct current (current density = 0.3 mA/cm²; phase duration = 180 s) and EP with 10 pulses per second (voltage = 100 V/cm²; pulse width = 1 ms). This optimum mode was applied to six human subjects. Uric acid was successfully extracted through the subjects' skin into the collection solution. A good correlation (r² = 0.88) between the subject's blood uric acid level and uric acid concentrations in collection solutions was observed. The results suggest that it may be possible to noninvasively and transdermally determine blood uric acid levels.

The effects of pH and ionic strength on topical delivery of a negatively charged porphyrin (TPPS4)

Meso-tetra-[4-sulfonatophenyl]-porphyrin (TPPS(4)) is a charged porphyrin derivate used in photodynamic therapy (PDT) by parenteral administration. This study means to investigate potential enhancement for its topical delivery by determining the TPPS(4) dependence on the environmental characteristics and applying iontophoresis. In order to accomplish this task, cathodal and anodal iontophoresis as well as passive delivery of the drug were studied in vitro and in vivo in function of its concentration, pH and ionic strength. A reduction in drug concentration as well as the NaCl elimination from donor formulation at pH 2.0 increased TPPS(4) passive permeation through the skin in vitro. Iontophoresis improved TPPS(4) delivery across the skin when applied in solutions containing NaCl at pH 2.0, regardless electrode polarity. However, at pH 7.4, the amount of TPPS(4) permeated by iontophoresis was not different from that one permeated after passive experiments from a solution containing NaCl. Despite the fact that iontophoresis did not improve TPPS(4) transdermal delivery at this specific condition, in vivo experiments showed that 10 min of iontophoresis quickly and homogeneously delivered TPPS(4) to deeper skin layers when compared to passive administration, which is an important condition for topical treatment of skin tumors with PDT.

Simultaneous transdermal extraction of glucose and lactate from human subjects by reverse iontophoresis

This study investigated the possibility of simultaneously extracting glucose and lactate from human subjects, at the same skin location, using transdermal reverse iontophoresis. Transdermal monitoring using iontophoresis is made possible by the skin’s permeability to small molecules and the nanoporous and microporous nature of the structure of skin. The study was intended to provide information which could be used to develop a full, biosensor-based, monitoring system for multiple parameters from transdermal extraction. As a precursor to the human study, in vitro reverse iontophoresis experiments were performed in an artificial skin system to establish the optimum current waveforms to be applied during iontophoresis. In the human study, a bipolar DC current waveform (with reversal of the electrode current direction every 15 minutes) was applied to ten healthy volunteers via skin electrodes and utilized for simultaneous glucose and lactate transdermal extraction at an applied current density of 300 μA/cm². Glucose and lactate were successfully extracted through each subject’s skin into the conducting gel that formed part of each iontophoresis electrode. The results suggest that it will be possible to noninvasively and simultaneously monitor glucose and lactate levels in patients using this approach and this could have future applications in diagnostic monitoring for a variety of medical conditions.

Anti-hyperalgesic and anti-inflammatory effects of ketorolac tromethamine gel using pulsed ultrasound in inflamed rats

The aim of this study was to determine if a ketorolac tromethamine (KT) gel solution could be administered in vivo via phonophoretic transdermal delivery using pulsed ultrasound by examining its anti-hyperalgesic and anti-inflammatory effects in a rat carrageenan inflammation model. 1% carrageenan was injected into the plantar surface of the right hindpaw of a rat, and anti-hyperalgesic and anti-inflammatory effects of KT via phonophoretic transdermal delivery were examined. The changes in the mechanical and thermal hyperalgesia, nociceptive flexor reflex (NFR), as well as the swelling changes were determined. According to the anti-hyperagesia and anti-inflammation tests, which were used to determine the change in the pain threshold, NFR and swelling showed that the group given the phonophoretic transdermal delivery of KT exhibited significantly more noticeable anti-hyperalgesic and anti-inflammatory effects than those treated with the simple application of a KT gel. The transdermal application of KT gel using phonophoresis had significant anti-hyperalgesic and anti-inflammatory effects. These findings suggest that the transdermal administration of a KT gel using phonophoresis using pulsed ultrasound might be useful for treating acute inflammation and pain.

Electrically Assisted Ocular Gene Therapy

Electrotransfer and iontophoresis are being developed as innovative non-viral gene delivery systems for the treatment of eye diseases. These two techniques rely on the use of electric current to allow for higher transfection yield of various ocular cell types in vivo. Short pulses of relatively high-intensity electric fields are used for electrotransfer delivery, whereas the iontophoresis technique is based on the application of low voltage electric current. The basic principles of these techniques and their potential therapeutic application for diseases of the anterior and posterior segments of the eye are reviewed. Iontophoresis has been found most efficient for the delivery of small nucleic acid fragments such as antisense oligonucleotides, siRNA, or ribozymes. Electrotransfer, on the other hand, is being developed for the delivery of oligonucleotides or custom designed plasmids. The wide range of strategies already validated and the potential for targeting specific types of cells confirm the promising early observations made using electrotransfer and iontophoresis. These two nonviral delivery systems are safe and can be used efficiently for targeted gene delivery to ocular tissues in vivo. At the present, their application for the treatment of ocular human diseases is nearing its final stages of adaptation and practical implementation at the bedside.

Transdermal delivery of aminoglycosides: Amikacin transport and iontophoretic non-invasive monitoring

The aim of this paper was to try to single out new administration strategies for aminoglycoside antibiotics. The objectives of the work were to reduce the systemic absorption in the case of topical application and to achieve plasma levels within the therapeutic window in case of systemic administration. Amikacin (AK) was chosen as a model aminoglycoside, as it has a broad spectrum of activity against Gram-negative bacteria. Additionally, since the therapeutic use of aminoglycosides requires careful monitoring, the feasibility of noninvasive monitoring of AK by reverse iontophoresis was explored in preliminary experiments. Permeation experiments were performed in vitro using rabbit ear skin as barrier. From the results obtained, it can be concluded that topical delivery of amikacin is possible for the treatment of local diseases, both using a commercial gel formulation and an innovative transdermal film, the latter being able to reduce in a significant way the risks of systemic absorption. When anodal iontophoresis at pH 4.0 was applied, amikacin transport and, to a limited extent, accumulation were increased. Reverse iontophoresis gave promising results, since AK could be extracted across the skin at the cathode, and this can be taken as a reference point to develop and optimize the technique.

Release characteristics of anionic drug compounds from liquid crystalline gels

Liquid crystalline gels (LCG) offer the formulator dynamic and flexible vehicles, into which actives, enhancers and other adjuvants with a wide range of physicochemical properties can be incorporated. This is achievable because of the biphasic oil/water composition of the gel. In this paper, the suitability of an isotropic liquid crystalline gel is investigated for a range of anionic drug molecules, with particular emphasis on sodium diclofenac. Parameters, which have been investigated, include the mode of vehicle preparation, the effect of the concentration of the drug and how buffering the gel and/or the receptor medium affect the release profiles. Such profiles have been measured for the sodium salts of benzoate, salicylate and indomethacin. The passive release from the standard system was found to adhere to matrix-controlled diffusion. An increase in concentration leads to a non-linear increase in the cumulative release of sodium diclofenac from the gels. In direct contrast to the result reported for cationic salbutamol base, optimum release from the gel was achieved when neither the receptor medium nor the aqueous phase of the gel was buffered. The percentages released of the sodium salts of benzoate, salicylate and indomethacin, after 24 h, were determined to be 25, 26 and 19%, respectively, and these are significantly greater than the release of sodium diclofenac. This suggests that diclofenac undergoes ion-pairing or complexation within the gel, which inhibits its diffusion from the vehicle. Future papers will report on the incorporation of enhancers and the effects of iontophoresis on the release profiles of drugs from these gels, and ultimately on the transdermal transport of drugs from these vehicles across human and porcine skin.

Emerging technologies that overcome biological barriers for therapeutic protein delivery

In the past decade, genomic research and the nascent field of proteomics have exponentially increased the number of potential protein therapeutic molecules for treating medical needs that were previously unmet. To realise the full clinical potential of many of the novel protein drug entities arising from these intense research efforts, emerging protein delivery technologies may be required. Advanced delivery technologies may offer the ability to overcome biochemical and anatomical barriers to protein drug transport, without incurring adverse events, to deliver the agent(s) at a certain desired rate and duration, to protect therapeutic macromolecules from in situ or systemic degradation, as well as increase their therapeutic index by targeting the drug action to a specific site. This review will cover a myriad of novel and emerging technologies that are directed at bypassing biological barriers and that have shown promise in advancing the therapeutic potential of protein drugs.

Reverse iontophoresis for non-invasive transdermal monitoring

Iontophoresis is the application of a small electric current to enhance the transport of both charged and polar, neutral compounds across the skin. Manipulation of either the total charge delivered and/or certain electrode formulation parameters allows control of electromigration and electroosmosis, the two principal mechanisms of transdermal iontophoresis. While the approach has been mainly used for transdermal drug delivery, 'reverse iontophoresis', by which substances are extracted to the skin surface, has recently been the subject of considerable effort. Glucose monitoring has been extensively studied and other applications, including therapeutic drug monitoring, are contributing to the development of the technique. An internal standard calibration procedure may ultimately render this novel monitoring technique completely non-invasive.

Wavelet analysis of blood flow dynamics: effect on the individual oscillatory components of iontophoresis with pharmacologically neutral electrolytes

Iontophoresis currents are used in the transcutaneous delivery of vasoactive substances for noninvasive assessment of skin vascular properties. The blood flow rate can be recorded by laser Doppler flowmetry (LDF), its average value and the amplitudes of its oscillatory components being used to evaluate the effect of the drugs. Because non-drug-specific, current-induced, vasodilation could confound the interpretation of the response, we have investigated the effect of currents of both polarities on the spectral components of the LDF signal in the absence of vasoactive substances. It was recorded for healthy volunteers with both high conductance (5 mol/l NaCl) and low conductance (deionized water) electrolytes. The oscillatory components were analysed by wavelet transform within 0.0095-1.6 Hz, divided into five sub-intervals. Only cathodal iontophoresis with deionized water increased the oscillatory energy and amplitude. It did so at all frequencies, but none of the sub-intervals associated with vasodilation (0.0095-0.145 Hz) was selectively affected compared to the others.

Improvement of dermatitis by iontophoretically delivered antisense oligonucleotides for interleukin-10 in NC/Nga mice

IL-10 is overexpressed in skin lesions of atopic dermatitis (AD) patients and believed to be an important factor in the pathogenesis of the disease. Thus the regulation of IL-10 production is a potential solution for immunotherapeutic intervention in AD. We examined the topical delivery of an antisense oligonucleotide for mouse IL-10 (AS6) and the therapeutic effect on the skin lesions of NC/Nga mice, a human AD model. Using an iontophoresis system, about 30% of the applied dose of AS6 penetrated the skin and was distributed in the epidermis and upper dermis. Topically delivered AS6 decreased the levels of mRNA and protein of IL-10 in the lesions of NC/Nga mice, with no effect on IL-4 levels. The dorsal lesions of NC/Nga mice disappeared with repeated topical application of AS6. Topically delivered AS6 showed an inhibitory effect on the production of IL-10 in the skin lesions of NC/Nga mice and had a therapeutic effect on the established dermatitis.

Reverse iontophoretic monitoring in premature neonates: feasibility and potential

Premature neonates represent a fragile patient population, often subjected to intensive clinical care and multiple drug therapy, which must be monitored carefully and continuously. The difficult and painful nature of repetitive blood sampling, particularly in this population, has provided considerable impetus for the development of noninvasive methods for monitoring blood analytes. Reverse iontophoresis, a relatively new technology already used for the transdermal monitoring of blood glucose levels in adults, may be particularly well-suited to exploit the unique properties of preterm neonatal skin. The underdevelopment of the premature infant's epidermis, and more specifically the stratum corneum (SC), results in an increased permeability to molecular transport. In this study, we have investigated the feasibility of reverse iontophoretic monitoring of two model drugs, caffeine and theophylline, which are often administered to premature neonates. To this purpose, tape-stripped porcine skin in vitro, which has been previously demonstrated to be an excellent model for premature neonatal skin, was employed. Reverse iontophoresis across intact membranes enabled a quantifiable extraction of both drugs predominantly at the cathode compartment. The mechanism of extraction of these essentially neutral drugs (caffeine and theophylline being uncharged at pH 7.4) was electroosmosis. However, when the SC was removed by progressive tape-stripping, the amounts of drugs extracted by reverse iontophoresis were equivalent to those obtained by passive diffusion. In these circumstances, therefore, the benefit and usefulness of the applied electric field had been lost. In summary, the absence of an at least partially functional skin barrier obviates, in the case of neutral molecules, the control (and directional transport) offered by iontophoresis; in contrast, for ionized species, where the principal iontophoretic transport mechanism is electromigration, the approach should be valid.

Non-invasive assessment of the effects of iontophoresis on human skin in-vivo

The stratum corneum (SC), the outermost layer of the skin, presents a formidable barrier to transdermal drug delivery. As a result, different strategies have been developed to enhance drug transport into and through skin. Iontophoresis involves the application of a small electrical current which drives molecules across the skin and controls relatively well the rate of delivery. Although the technique has been widely investigated in-vitro, the evaluation of skin integrity in-vivo after iontophoresis is absolutely necessary for the future clinical application of this approach. This paper reviews the non-invasive biophysical techniques which have been used to assess the effects of current application on human skin in-vivo. Specifically, transepidermal water loss, infrared spectroscopy, impedance spectroscopy and skin blood flow measurements are discussed. After first presenting the basic principles of these methods, their application to the determination of SC barrier function and skin integrity is addressed, and the criteria for selecting the most appropriate approach are considered.

Peptides as receptor ligand drugs and their relationship to G-coupled signal transduction

Peptides act as effector agents that regulate and/or mediate physiological processes, serving as hormones, neurotransmitters and signal transducing factors. The low molecular weight peptides affect receptor-mediated events, which influence cardiovascular, gastrointestinal and neurocranial systems. While some peptides have been marketed as drugs, many have served as leads or templates for the development of non-peptide drugs that mimic peptide actions. This review presents the advantages and disadvantages of using peptides as drugs that bind as ligands to cell-surface receptors and considers their applications in such events. The value of both the peptides and their mimics is based on their participation in the biomodulation of physiological processes, which frequently employ scaffolding proteins acting in a cascading sequence of protein-to-protein interactions. The peptides bind to G-coupled surface receptors to initiate a signal that is transduced to the interior of the cell through multiple layers of phosphorylating enzymes and binding proteins. Peptides have been further employed to identify the molecular targets of signal transduction, the uncoupling of which might provide a means for various disease therapies. The exploitation of such peptide-mediated signal pathways, which are of primary importance to tumour cells, may provide an attractive strategy for anticancer therapy in the future.

Microfabrication of individual 200 microm diameter transdermal microconduits using high voltage pulsing in salicylic acid and benzoic acid

We describe an extension of semiconductor fabrication methods that creates individual approximately 200 microm diameter aqueous pathways through human stratum corneum at predetermined sites. Our hypothesis is that spatially localized electroporation of the multilamellar lipid bilayer membranes provides rapid delivery of salicylic acid to the keratin within corneocytes, leading to localized keratin disruption and then to a microconduit. A microconduit penetrating the isolated stratum corneum supports a volumetric flow of order 0.01 ml per s with a pressure difference of only 0.01 atm (about 10(2) Pa). This study provides a method for rapidly microengineering a pathway in the skin to interface future devices for transdermal drug delivery and sampling of biologically relevant fluids.

Methods for evaluation of peripheral neurovascular dysfunction

Measurement of skin blood flow is a sensitive marker of C-fiber neurovascular dysfunction. It precedes development of abnormalities in diabetes mellitus, correlates with in vivo indices of the metabolic syndrome, and may be a "benchmark" for future studies on agents to improve microvascular dysfunction in diabetes mellitus. Skin blood flow can be measured under basal and stimulated conditions. There are different methods of evaluation. Iontophoresis and microdialysis are novel methods of drug delivery and the latter may be used as a means of extracting analytes in the skin. Theses methods are not invasive (iontophoresis) or minimally invasive (microdialysis). They can be performed repeatedly and safely in most patients. The use of microdialysis may be limited by sampling only water-soluble molecules. An alternative to microdialysis is iontophoresis, which works better with polar molecules. A combination of microdialysis and iontophoresis techniques can be useful in assessment of the pharmacokinetics of polar and nonpolar agents and the physiology and pathophysiology of the skin neurovascular system.

Creation of transdermal pathways for macromolecule transport by skin electroporation and a low toxicity, pathway-enlarging molecule

A combined electrical (HV, "high voltage", pulsing) and chemical (topical sodium thiosulfate) intervention is hypothesized to create enlarged aqueous pathways that allow large quantities of macromolecules to be transported through human skin's stratum corneum (SC), the dominant barrier for transdermal drug delivery and biochemical analyte extraction. This expectation is based on the known structure and composition of the SC, and previous models and experiments for local transport regions (LTRs) due to transdermal HV pulsing. In vitro experiments demonstrated that transdermal macromolecule fluxes of 10(-9) to 10(-8) mol h(-1) cm(-2) (10 to 100 microg h(-1) cm(-2)) or greater are possible for lactalbumin and an antibody (IgG), which are potentially therapeutic values for peptides, proteins and nucleic acids. In the absence of sodium thiosulfate, only a small molecule (sulforhodamine) flux increased significantly, consistent with many previous studies. Significant macromolecule transdermal fluxes occurred only if a pathway enlarging molecule (sodium thiosulfate) was present. Our results also provide support for the mechanism hypothesis that HV pulses leading to transdermal voltages U(skin) > 50 V create straight-through aqueous pathways that penetrate multilamellar bilayer membranes, corneocyte envelopes and corneocyte interiors within the SC.

Spatially constrained skin electroporation with sodium thiosulfate and urea creates transdermal microconduits

Controlled transport of molecules through the skin's main barrier, the stratum corneum (SC), is a long standing goal of transdermal drug delivery. Traditional, needle-based injection provides delivery of almost any water soluble compound, by creating a single large aqueous pathway in the form of the hollow core of a needle, through which drug is delivered by pressure-driven flow. We extend previous work to show that SC-spanning microconduits (here with diameters of about 200 microm) can be created in vivo by skin electroporation and low-toxicity, keratolytic molecules (here sodium thiosulfate and urea). A single microconduit in isolated SC can support volumetric flow of the order of 0.01 ml s(-1) by a pressure difference of only 0.01 atm (about 10(2) Pa), demonstrating that the SC barrier has been essentially eliminated within this microscopic area.

Noninvasive sampling of phenylalanine by reverse iontophoresis

While iontophoresis is typically associated with drug delivery across the skin, the symmetry of the technique permits its application to the essentially noninvasive withdrawal of biologically important analytes from the subcutaneous space to the body's surface. The identification of other substances which can be monitored by this procedure, and to its optimization and development as a more general clinical chemistry tool, is a long-term objective. In this paper, we describe a preliminary in vitro investigation into the feasibility of extracting and analyzing the amino acid, phenylalanine, with the ultimate aim to develop a diagnostic test for phenylketonuria, a potentially fatal metabolic disease in infants. Over a subdermal concentration range of 1-10 mM phenylalanine, reverse iontophoretic extraction was rapid, easily detectable and highly linear. Manipulation of the electrolyte composition surrounding the cathode (i.e., the site of collection of the iontophoretically-extracted material) enabled phenylalanine to be electrotransported at a rate of approximately 6 nmol/cm(2)/h when present subdermally at 1 mM. The potential exists, therefore, to use this approach for the noninvasive detection of systemic amino acid levels in vivo. However, such a development will necessitate a suitable and convenient analytical approach (e.g., a biosensor), with a sensitivity about 10-times greater than that used in this work, which can be combined successfully with the reverse iontophoretic extraction technology.

Iontophoresis and electroporation: comparisons and contrasts

The techniques of iontophoresis and electroporation can be used to enhance topical and transdermal drug delivery. Iontophoresis applies a small low voltage (typically 10 V or less) continuous constant current (typically 0.5 mA/cm2 or less) to push a charged drug into skin or other tissue. In contrast, electroporation applies a high voltage (typically, ?100 V) pulse for a very short (micros-ms) duration to permeabilize the skin. This electric assistance of drug delivery across skin will expand the scope of transdermal delivery to hydrophilic macromolecules such as the drugs of biotechnology. These two techniques differ in several aspects such as the mode of application and pathways of transport but can be used together for effective drug delivery. Iontophoresis is already used clinically in physical therapy clinics and is close to commercialization for development of a systemic delivery patch with miniaturized circuits and similar in overall size to a passive patch. The use of electroporation for drug delivery is relatively new and is being actively researched.

Theory of electrical creation of aqueous pathways across skin transport barriers

Experimental studies have shown that application of electrical pulses to human skin that result in U(skin)>30 V for durations of about 1 ms or longer causes a large decrease in electrical resistance within microseconds, followed in seconds by an increase in molecular transport of water-soluble molecules. Local transport regions (LTRs), within which molecular transport is concentrated, mostly form away from the skin's appendages and rete pegs. Theoretical attempts to explain this behavior involve electrically created aqueous pathways ("pores"). For short (about 1 ms) "high voltage" (HV) pulses leading to about U(skin)>50 V, it was hypothesized that such pulses cause electroporation of the multilamellar lipid bilayer membranes of the skin's stratum corneum (SC). Much of the present experimental evidence supports the more specific hypothesis that such pulses create "straight through aqueous pathways", mostly within LTRs, that perforate the SC lipid bilayers and pass through the interiors of hydrated corneocytes. Theoretical estimates of the localized heating within LTRs predict relatively small temperature rises. The theory of LTR formation is incomplete, with both stochastic and deterministic models under consideration. Moderate voltage (MV) pulses leading to about 5<U(skin)<50 V, are consistent with appendageal activation and electroporation. The largest molecular fluxes occur for HV pulses, for which theory predicts large numbers of straight-through aqueous pathways. Both appendageal and stratum corneum electroporation are different from iontophoresis, which occurs at U(skin)<5 V.

Iontophoresis--recent developments

This paper addresses the development and optimization of the technology of iontophoresis as a non-invasive approach to transdermal diagnosis and therapy. The rationale for this work is that the skin offers a unique and easily accessible body surface through which drugs can be delivered and general clinical monitoring information can be extracted.