Assessment of Drug Delivery Kinetics to Epidermal Targets In Vivo.

It has proven challenging to quantify 'drug input' from a formulation to the viable skin because the epidermal and dermal targets of topically applied drugs are difficult, if not impossible, to access in vivo. Defining the drug input function to the viable skin with a straightforward and practical experimental approach would enable a key component of dermal pharmacokinetics to be characterised. It has been hypothesised that measuring drug uptake into and clearance from the stratum corneum (SC) by tape-stripping allows estimation of a topical drug's input function into the viable tissue. This study aimed to test this idea by determining the input of nicotine and lidocaine into the viable skin, following the application of commercialised transdermal patches to healthy human volunteers. The known input rates of these delivery systems were used to validate and assess the results from the tape-stripping protocol. The drug input rates from in vivo tape-stripping agreed well with the claimed delivery rates of the patches. The experimental approach was then used to determine the input of lidocaine from a marketed cream, a typical topical product for which the amount of drug absorbed has not been well-characterised. A significantly higher delivery of lidocaine from the cream than from the patch was found. The different input rates between drugs and formulations in vivo were confirmed qualitatively and quantitatively in vitro in conventional diffusion cells using dermatomed abdominal pig skin.

Stratum Corneum Sampling to Assess Bioequivalence between Topical Acyclovir Products.

PURPOSE: To examine the potential of stratum corneum (SC) sampling via tape-stripping in humans to assess bioequivalence of topical acyclovir drug products, and to explore the potential value of alternative metrics of local skin bioavailability calculable from SC sampling experiments. METHODS: Three acyclovir creams were considered in two separate studies in which drug amounts in the SC after uptake and clearance periods were measured and used to assess bioequivalence. In each study, a "reference" formulation (evaluated twice) was compared to the "test" in 10 subjects. Each application site was replicated to achieve greater statistical power with fewer volunteers. RESULTS: SC sampling revealed similarities and differences between products consistent with results from other surrogate bioequivalence measures, including dermal open-flow microperfusion experiments. Further analysis of the tape-stripping data permitted acyclovir flux into the viable skin to be deduced and drug concentration in that 'compartment' to be estimated. CONCLUSIONS: Acyclovir quantities determined in the SC, following a single-time point uptake and clearance protocol, can be judiciously used both to objectively compare product performance in vivo and to assess delivery of the active into skin tissue below the barrier, thereby permitting local concentrations at or near to the site of action to be determined.

Topical bioavailability of diclofenac from locally-acting, dermatological formulations.

Assessment of the bioavailability of topically applied drugs designed to act within or beneath the skin is a challenging objective. A number of different, but potentially complementary, techniques are under evaluation. The objective of this work was to evaluate in vitro skin penetration and stratum corneum tape-stripping in vivo as tools with which to measure topical diclofenac bioavailability from three approved and commercialized products (two gels and one solution). Drug uptake into, and its subsequent clearance from, the stratum corneum of human volunteers was used to estimate the input rate of diclofenac into the viable skin layers. This flux was compared to that measured across excised porcine skin in conventional diffusion cells. Both techniques clearly demonstrated (a) the superiority in terms of drug delivery from the solution, and (b) that the two gels performed similarly. There was qualitative and, importantly, quantitative agreement between the in vitro and in vivo measurements of drug flux into and beyond the viable skin. Evidence is therefore presented to support an in vivo - in vitro correlation between methods to assess topical drug bioavailability. The potential value of the stratum corneum tape-stripping technique to quantify drug delivery into (epi)dermal and subcutaneous tissue beneath the barrier is demonstrated.

A mechanistic approach to modelling the formation of a drug reservoir in the skin.

It has been shown that prolonged systemic presence of a drug can cause a build-up of that drug in the skin. This drug 'reservoir', if properly understood, could provide useful information about recent drug-taking history of the patient. We create a pair of coupled mathematical models which combine to explore the potential for a drug reservoir to establish based on the kinetic properties of the drug. The first compartmental model is used to characterise time-dependent drug concentrations in plasma and tissue following a customisable drug regimen. Outputs from this model provide boundary conditions for the second, spatio-temporal model of drug build-up in the skin. We focus on drugs that are highly bound as this will restrict their potential to move freely into the skin but which are lipophilic so that, in the unbound form, they would demonstrate an affinity to the outer layers of the skin. Buprenorphine, a drug used to treat opiate addiction, is one example of a drug satisfying these properties. In the discussion we highlight how our study might be used to inform future experimental design and data collection to provide relevant parameter estimates for reservoir formation and its potential to contribute to enhanced drug monitoring techniques.

Richard Guy and his collaborators: 'crackling' the skin code.

This paper aims to summarize the contributions of Richard Guy and collaborators to the skin field. Major contributions have been areas such as the modelling of skin absorption, use of spectroscopic techniques (Fourier transform infrared spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy and impedance spectroscopy) to characterize the skin barrier and the effects of enhancing techniques on the membrane, dermatopharmacokinetics and assessment of topical bio-availability, iontophoresis and reverse iontophoresis, and use of imaging techniques to elucidate pathways of penetration and the skin disposition of nanoparticles. The field of topical and transdermal drug delivery has benefitted incalculably from the extensive work of Guy and his group: their findings about mechanisms of drug delivery and permeation enhancement, and the development of methodologies which are now accepted as gold standards by skin scientists.

Hydration and N-acetyl-l-cysteine alter the microstructure of human nail and bovine hoof: implications for drug delivery.

This work aimed to (a) characterize the microstructure and porosity of human nail and bovine hoof by mercury intrusion porosimetry and SEM image analysis, (b) study the effects of hydration and of N-acetyl-l-cysteine treatment on the microstructure of both membranes, and (c) determine whether the microstructural modifications were associated with changes in drug penetration measured by standard diffusion studies. Bovine hoof surface is more porous than nail surface although there were no differences between the mean surface pore sizes. Hydration and N-acetyl-l-cysteine increased the roughness and apparent surface porosity, and the porosity determined by mercury intrusion porosimetry of both membranes. Pore-Cor™ was used to generate tridimensional structures having percolation characteristics comparable to nail and hooves. The modeled structures were horizontally banded having an inner less-porous area which disappeared upon treatment. Treatment increased the predicted permeability of the simulated structures. Triamcinolone permeation increased significantly for hooves treated N-acetyl-l-cysteine, i.e., the membranes for which microstructural and permeability changes were the largest. Thus, microstructural changes determined via mercury intrusion porosimetry and subsequently modeled by Pore-Cor™ were related to drug diffusion. Further refinement of the technique will allow fast screening of penetration enhancers to be used in ungual drug delivery.

Extraction and quantification of amino acids in human stratum corneum in vivo.

BACKGROUND: Amino acid (AA) levels in stratum corneum (SC) are potential biomarkers of skin health while their systemic levels may be used to diagnose inherited metabolic diseases. OBJECTIVES: To examine reverse iontophoresis, in human volunteers, as a minimally invasive tool to analyse AAs within the skin and subdermally. METHODS: In four volunteers, the amounts of iontophoretically extracted AAs were compared with those determined in the SC following repetitive tape stripping and with the plasma concentrations. Glucose levels, evaluated in the different compartments, were used as a control. RESULTS: SC concentrations of 13 essentially zwitterionic AAs were ∼100-fold higher than the respective plasma levels. Passive and reverse iontophoretic extraction for 4 h did not deplete the SC depot of AAs, a fact reinforced by postextraction tape stripping, which revealed that AAs remained in the SC at this time. In contrast, glucose was much less abundant in the SC and was fully and relatively quickly extracted by reverse iontophoresis. CONCLUSIONS: It follows that reverse iontophoresis is useful for quantifying AAs in the SC and these data are highly correlated with levels obtained by tape stripping. However, reverse iontophoresis is impractical for the routine monitoring of AA plasma concentrations (unlike the situation for glucose, the skin reservoir of which is much smaller).

Iontophoresis of dexamethasone phosphate: competition with chloride ions.

The objective was to study the competition of chloride released from a Ag/AgCl cathode on the iontophoretic delivery of dexamethasone phosphate (Dex-Phos). Iontophoresis of Dex-Phos was performed in side-by-side diffusion cells (0.78 cm(2)) using pig skin. A 0.3 mA constant current was applied via Ag/AgCl electrodes. The amounts of Dex-Phos and dexamethasone (Dex) were also quantified in the stratum corneum (SC), using tape-stripping, after passive and iontophoretic delivery. The profiles of Dex-Phos and Dex, as a function of position in the SC, were deduced. The iontophoretic delivery of Dex-Phos from pure water was unaffected by the accumulation of Cl- released by the donor cathode when the drug's concentration was 4.25 mM to 17 mM. At 0.85 mM, however, Cl- competition was significant and the drug flux was significantly reduced. Formulation of the drug in the presence of Cl- resulted in a non-linear dependence of flux on the molar fraction of the drug. Tape-stripping experiments confirmed the enhanced delivery of Dex-Phos by iontophoresis relative to passive diffusion, with Dex-Phos concentration greater inside the barrier post-iontophoresis than that in the donor. The latter observation could explain the robustness of Dex-Phos delivery to the presence of Cl- in the donor solution.

Reverse iontophoresis of L-lactate: in vitro and in vivo studies.

This work investigates the reverse iontophoretic extraction of lactate, a widely used marker of tissue distress in critically ill patients and of sports performance. In vitro experiments were performed to establish the relationship between subdermal lactate levels and lactate iontophoretic extraction fluxes. Subsequently, the iontophoretic extraction of lactate was performed in vivo in healthy volunteers. Lactate was quickly and easily extracted by iontophoresis both in vitro and in vivo. During a short initial phase, iontophoresis extracts the lactate present in the skin reservoir, providing information of relevance, perhaps, for dermatological and cosmetic applications. In a second step, lactate is extracted from the interstitial subdermal fluid allowing local lactate kinetics to be followed in a completely non-invasive way. The simultaneous in vivo extraction of chloride, and its possible role as an internal standard to calibrate lactate reverse iontophoretic fluxes, was also demonstrated. Despite these positive findings, however, considerably more research is necessary to eliminate potential artefacts and to facilitate interpretation of the data.

Iontophoretic transport across the skin.

There have been many attempts to define the key relationships between passive drug diffusion across the skin and the molecular and physicochemical properties of the permeant. At the present time, the importance of lipophilicity (or hydrogen bond donor and acceptor properties) and of molecular volume are well established, and useful predictive relationships for passive percutaneous permeability exist. With respect to iontophoresis, on the other hand, the situation is far less clear and the mechanisms involved have not been completely defined. The roles of electromigration and electroosmosis (current-induced convective solvent flow) are now beginning to be understood and experimentally separated. In turn, this allows the manner in which certain physicochemical parameters influence the efficiency of drug electrotransport to be deduced. An initial examination of a database drawn from the literature and from our own work (for which the experimental conditions employed were reasonably constant) suggests a rather sharp dependence of cationic drug delivery via electromigration upon molecular size. We suggest that the analysis reveals useful paths for further investigation.

Iontophoretic delivery of 5-aminolevulinic acid (ALA): effect of pH.

PURPOSE: To examine the iontophoretic delivery of ALA as a function of pH and to determine the principal mechanisms responsible for its electrotransport. METHODS: Anodal iontophoretic transport of ALA was measured as a function of its concentration and pH of the donor solution. Experiments were performed in vitro using skin excised from porcine ears as the membrane. To deduce mechanism, the concomitant transport of the electroosmotic marker, mannitol, was also assessed. RESULTS: ALA iontophoresis at pH 7.4 is a linear function of concentration over the range 1-100 mM. The mechanism was deduced to be electroosmosis. By reducing the pH from 7.4 to 4.0, the dominant mechanism of ALA transport was shifted from electroosmosis to electrorepulsion as the skin's net negative charge was progressively neutralized. However, the total delivery of the compound was not altered by lowering the pH suggesting that the increased electrorepulsive contribution was essentially balanced by the concomitantly reduced electroosmosis. CONCLUSIONS: Significant ALA delivery at pH 7.4 can be achieved by increasing the drug concentration in the anodal formulation to 100 mM. Lowering the pH does not result in increased ALA transport. Alternative strategies are therefore required to maximize and optimize ALA delivery by iontophoresis.

Passive and iontophoretic transdermal penetration of methotrexate.

The in vitro iontophoretic transdermal delivery of methotrexate (MTX) across pig skin was investigated. Cathodal iontophoresis considerably increased MTX skin permeation and accumulation as compared to the passive controls. The effect of NaCl and MTX concentrations in the vehicle were also studied. As expected, MTX iontophoretic transport decreased with NaCl content. On the other hand, MTX concentration did not modify its electrotransport in the range of concentrations considered (4.4-6.6 mM). The influence of the current density (0.25-0.5 mA/cm2) was also investigated. The iontophoretic transport of MTX tends to increase with current density although this effect was not always statistically significant. Finally, the possibility of using anodal iontophoresis from an acid (pH 4.0-5.0) donor solution to deliver MTX was explored. This was limited due to the low solubility of MTX in acid pH. On the whole, this work that iontophoresis may be used to improve the topical application of MTX for the treatment of psoriasis.

Characterization of the iontophoretic permselectivity properties of human and pig skin.

The objectives of this research were (a) to characterize the permselective properties of human and porcine skin and (b) to assess the validity of the latter as a model membrane in iontophoresis studies. The electroosmotic transport of [14C]mannitol was followed in vitro across human and porcine skin as a function of pH, in both "anode-to-cathode" and "cathode-to-anode" directions. At physiological pH, mannitol electrotransport dominated in the anode-to-cathode direction, clearly indicating the net negative charge and the corresponding cation-permselectivity of the skin. By lowering the pH to 3.5 the direction of electroosmosis progressively reverses, indicating that the skin is becoming net positively-charged, and thus anion-selective. The degree of permselectivity (DP) of the skin at each pH value was quantified by dividing mannitol electrotransport in the predominant direction (i.e. either anodal or cathodal) by that in the opposite sense. The net charge on the skin is zero when DP equals unity, corresponding to the isoelectric point (pI) of the membrane (approximately 4.4 for pig skin and approximately 4.8 for human skin). The consistent pIs and similar pH-dependent permselectivities observed for human and pig demonstrate that porcine skin is an appropriate model for iontophoresis studies. Finally, the characterization of the permselective properties of human skin is crucial to optimize the iontophoresis of large peptides and uncharged species, which are transported primarily by electroosmosis.

Optimizing iontophoretic drug delivery: identification and distribution of the charge-carrying species.

PURPOSE: To identify and quantify, in vitro and in vivo (in humans), the charge-carrying species during transdermal iontophoresis of lidocaine hydrochloride as a function of the concentration of drug relative to that of sodium chloride in the anodal solution. METHODS: In vitro experiments in standard diffusion cells quantified lidocaine delivery and the outward migration of chloride across the skin. Electrotransport of Na+ was inferred by difference, allowing transport numbers of the three main charge-carrying species to be deduced. In vivo, outward electrotransport of Cl- was measured and compared to the corresponding in vitro results. RESULTS: The transport number of lidocaine increased linearly with increasing mole fraction and reached 0.15-0.20 at X(L) = 1.0. In the absence of Na+, most of the charge was carried by Cl- (>80%) despite the skin retaining its net negative charge and cation permselectivity. In vivo data correlated very well with in vitro results. CONCLUSIONS: The mole faction of drug (relative to competing ions of like polarity) is the crucial determinant of the extent to which it can carry charge across the skin during iontophoresis. The outward electromigration of Cl-, in the sense opposite to drug delivery, may offer a useful means by which to optimize iontophoretic efficiency in the absence of competing cations in the anode formulation.

Contributions of electromigration and electroosmosis to iontophoretic drug delivery.

PURPOSE: To determine the electromigration and electroosmotic contributions to the iontophoretic delivery of lidocaine hydrochloride, in addition to the more-lipophilic quinine and propranolol hydrochlorides, in the presence and absence of background electrolyte. METHODS: In vitro experiments, using excised pig ear skin and both vertical and side-by-side diffusion cells, were performed as a function of drug concentration and with and without background electrolytes in the anodal formulation. Concomitantly, the contribution of electroosmosis in each experimental configuration was monitored by following the transport of the neutral, polar marker molecule, mannitol. RESULTS: Electromigration was the dominant mechanism of drug iontophoresis (typically representing approximately 90% of the total flux). In the presence of background electrolyte, lidocaine delivery increased linearly with concentration as it competed more and more effectively with Na+ to carry the charge across the skin. However, iontophoretic delivery of quinine and propranolol increased non-linearly with concentration. Without electrolytes, on the other hand, electrotransport of the three drugs was essentially independent of concentration over the range 1-100 mM. Transport efficiency of lidocaine was approximately 10%, whereas that of the more lipophilic compounds was significanly less, with the major charge carrier being Cl- moving from beneath the skin into the anodal chamber. Both quinine and propranolol induced a concentration-dependent attenuation of electroosmotic flow in the normal anode-to-cathode direction. CONCLUSION: Dissecting apart the mechanistic contributions to iontophoretic drug delivery is key to the optimization of the formulation, and to the efficient use of the drug substance.

Iontophoretic delivery of ropinirole hydrochloride: effect of current density and vehicle formulation.

PURPOSE: The objectives of this work were 1) to establish the feasibility of the transdermal iontophoretic delivery of ropinirole hydrochloride; 2) to investigate the possibility of delivering therapeutic doses of this drug; and 3) to determine the key factors that control ropinirole electrotransport. METHODS: A series of in vitro transdermal iontophoretic experiments were instituted to study the effects of drug concentration, co-ion concentration, intensity of current, and application time on ropinirole flux. The convective contribution to ropinirole electrotransport was evaluated by following the transport of the electroosmotic marker mannitol. RESULTS: Ropinirole flux decreased dramatically in the presence of competing ions. This effect was observed even when the molar fraction of the two competing cations was kept constant. Anodal flux of mannitol decreased with drug concentration, indicating a possible alteration of the skin permselectivity. In the absence of competing co-ions, ropinirole transport number reached a maximum value (8-13%). In these conditions, the main factor controlling drug delivery was the intensity of current applied. CONCLUSIONS: Transdermal iontophoresis allowed the delivery of therapeutic doses of ropinirole. The dose administered and the input rate were controlled by the judicious choice of the key delivery factors here described.

Microemulsions for topical delivery of 8-methoxsalen.

8-Methoxsalen (8-MOP) and related furocumarins have been extensively used for the treatment of hyperproliferative skin diseases in association with long-wavelength UVA light. In order to develop alternative formulations for the topical administration of 8-MOP, microemulsions were evaluated as delivery vehicles. Six microemulsion formulations were prepared using water, isopropyl myristate (IPM) and Tween((R)) 80: Span((R)) 80: 1,2-Octanediol (3:1:1.2 w/w). The microemulsions were characterized using conductimetric and dynamic light scattering analyses. The ability of the systems to deliver 8-MOP into and through the skin was evaluated in vitro using newborn pig-skin. The in vitro permeation data showed that the novel microemulsions increased the 8-MOP total penetration through the skin by order of 1.9-4.5, as compared with IPM. In general, the accumulation of 8-MOP into the skin was increased by a factor of 1.5-4.5 by the microemulsion systems with respect to their total amount of drug delivered across the skin. These results suggest that the studied microemulsion systems may be appropriate vehicles for the topical delivery of 8-MOP.

Iontophoresis: electrorepulsion and electroosmosis.

Over the last 10-15 years, the electrical enhancement of drug delivery across the skin has undergone intense investigation. During this period, considerable amounts of experimental data have been generated, and the successful enhancement of a diverse array of molecules has been achieved. Indeed, the commercial exploitation of the method can be envisaged within the next few years. Despite this progress, however, the mechanistic understanding of iontophoresis remains a challenging scientific question that is yet to be fully resolved. The routes of permeation under the influence of an applied electrical potential, and the molecular interactions of the transporting drug with these pathways, have resisted unequivocal and unambiguous identification. Equally, the relative contributions of electrorepulsion and electroosmosis to the total iontophoretic flux have proven difficult to quantify, due to the difficulty of designing appropriate experiments. The situation is further complicated by the fact that it has now been established that certain lipophilic cations, in particular, can associate strongly with the skin during their iontophoretic delivery, thereby altering the electrical properties of the membrane, and changing the mechanism of transport. In this short communication, the roles of electrorepulsion and electroosmosis have been reconsidered from a simple theoretical point of view, and experimental approaches by which their relative importance may be estimated have been proposed and subjected to initial evaluation.

Iontophoretic permselectivity of mammalian skin: characterization of hairless mouse and porcine membrane models.

PURPOSE: To evaluate the transport number of Na+, and the isoelectric point, of two skin membranes frequently used for iontophoretic in vitro research. METHODS: Na+ transport numbers were determined by the Hittorf method or by the measurement of membrane potential. The skin isoelectric point was deduced from the electroosmosis of mannitol (a polar non-electrolyte) as a function of pH. RESULTS: The Na+ transport number across porcine skin, like that for hairless mouse, indicated a modest cation permselectivity. Consistent with this observation, the isoelectric points of porcine and hairless mouse skin were determined to be in the ranges of 3.5-3.75 and 4.5-4.6, respectively. That is, at physiological pH, both of these model membranes supports a net negative charge. CONCLUSIONS: The permselective properties of porcine and hairless mouse skin are similar (but with the porcine membrane having apparently fewer basic or more weakly-acidic groups than that of the mouse) and consistent with the characteristics, which have been deduced elsewhere, of human skin.

Characterization of convective solvent flow during iontophoresis.

During iontophoresis under neutral pH conditions, there is a net convective flow of volume (electroosmosis) from anode to cathode leading to the enhanced transport of dissolved polar (but uncharged) solutes in the same direction. The objective of this study was to address the following unresolved questions with respect to electroosmotic transport: [1] Whether the efficiency of electroosmotic transport is solute size-dependent and, if so, how severe is this dependence? [2] Is electroosmosis linearly related to current density in the same way that the iontophoretic flux of charged species appears to be? [3] Are positively charged permeants able to influence their own electrotransport across the skin (by modifying the net charge on the membrane and altering, as a result, the permselectivity) and, if so, why and to what extent? Electroosmosis was assessed from the iontophoreically driven fluxes of mannitol, sucrose and lactose across hairless mouse skin in vitro. It was found that:- (a) The electroosmotic transport rate of mannitol is similar to that of the disaccharides, sucrose and lactose, when examined under identical conditions. The dependence of electroosmotic flux upon molecular size requires study of solutes having a wider range of MW than those considered here. (b) Electroosmotic flow from anode-to-cathode increases with applied current density; similarly, convective flow in the opposite direction diminishes with increasing current density. Apparently, there is correlation between the net movement of solvent and the total flux of ions across the skin. (c) The permselectivity of skin can be 'neutralized' by driving, iontophoretically, a cationic, lipophilic peptide (specifically the leutinizing hormone releasing hormone (LHRH) analog, Nafarelin) into the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)