An analysis of solute structure-human epidermal transport relationships in epidermal iontophoresis using the ionic mobility: pore model

Surging footprints of mathematical modeling for prediction of transdermal permeability

In vivo skin permeation studies are considered gold standard but are difficult to perform and evaluate due to ethical issues and complexity of process involved. In recent past, a useful tool has been developed by combining the computational modeling and experimental data for expounding biological complexity. Modeling of percutaneous permeation studies provides an ethical and viable alternative to laboratory experimentation. Scientists are exploring complex models in magnificent details with advancement in computational power and technology. Mathematical models of skin permeability are highly relevant with respect to transdermal drug delivery, assessment of dermal exposure to industrial and environmental hazards as well as in developing fundamental understanding of biotransport processes. Present review focuses on various mathematical models developed till now for the transdermal drug delivery along with their applications.

Iontophoretic skin permeation of peptides: an investigation into the influence of molecular properties, iontophoretic conditions and formulation parameters

The transdermal route offers advantages for delivery of peptides and proteins. However, these polar and large molecules do not permeate the skin barrier well. Various enhancement methods have been employed to address this problem. Iontophoresis is one of the methods that shows promise but its application to peptide delivery has yet to be fully explored. This study investigates the effects of different molecular properties and iontophoretic conditions on the skin permeation of peptides. In this study, the permeation of alanine-tryptophan dipeptide (MW 276 Da), alanine-alanine-proline-valine tetrapeptide (MW 355 Da), Argireline® (Acetyl hexapeptide-3, MW 889 Da) and Triptorelin acetate (decapeptide, MW 1311 Da) through excised human skin under passive or iontophoretic current of 0.4 mA was investigated. The effects of pH change (3.0-7.4, to provide different net negative, neutral, and positive charges) to the peptide, donor concentration (1-10 mg/ml), background electrolyte (34-137 mM NaCl and/or 5-20 mM HEPES) and current direction (anodal vs cathodal) were also studied. Peptides were analysed by high-performance liquid chromatography or liquid scintillation counting. Iontophoresis led up to a 30 times increase in peptide permeation relative to passive permeation for the peptides. Electroosmosis was an important determinant of the total flux for the high molecular weight charged peptides. Electrorepulsion was found to be considerable for low molecular weight charged moieties. Permeation was decreased at lower pH, possibly due to decreased electroosmosis. Results also showed that 10 times increase in donor peptide concentration increases permeation of peptides by about 2-4 times and decreases iontophoretic permeability coefficients by about 2.5-5 times. The addition of extra background electrolyte decreased the iontophoretic permeation coefficient of peptides by 2-60 times. This study shows that iontophoretic permeation of peptides is affected by a number of parameters that can be optimized for effective transdermal peptide delivery.

Mathematical models to describe iontophoretic transport in vitro and in vivo and the effect of current application on the skin barrier

The architecture and composition of the stratum corneum make it a particularly effective barrier against the topical and transdermal delivery of hydrophilic molecules and ions. As a result, different strategies have been explored in order to expand the range of therapeutic agents that can be administered by this route. Iontophoresis involves the application of a small electric potential to increase transport into and across the skin. Since current flow is preferentially via transport pathways with at least some aqueous character, it is ideal for hydrosoluble molecules containing ionisable groups. Hence, the physicochemical properties that limit partitioning and passive diffusion through the intercellular lipid matrix are beneficial for electrically-assisted delivery. The presence of fixed ionisable groups in the skin (pI 4-4.5) means that application of the electric field results in a convective solvent flow (i.e., electroosmosis) in the direction of ion motion so as to neutralise membrane charge. Hence, under physiological conditions, cation electrotransport is due to both electromigration and electroosmosis-their relative contribution depends on the formulation conditions and the physicochemical properties of the permeant. Different mathematical models have been developed to provide a theoretical framework in order to explain iontophoretic transport kinetics. They usually involve solutions of the Nernst-Planck equation - using either the constant field (Goldman) or electroneutrality (Nernst) approximations - with or without terms for the convective solvent flow component. Investigations have also attempted to elucidate the nature of ion transport pathways and to explain the effect of current application on the electrical properties of the skin-more specifically, the stratum corneum. These studies have led to the development of different equivalent circuit models. These range from simple parallel arrangements of a resistor and a capacitor to the inclusion of the more esoteric "constant phase element"; the latter provides a better mathematical description of the "non-ideal" behaviour of skin impedance. However, in addition to simply providing a "mathematical" fit of the observed data, it is essential to relate these circuit elements to biological structures present in the skin. More recently, attention has also turned to what happens when the permeant crosses the epidermis and reaches the systemic circulation and pharmacokinetic models have been proposed to interpret data from iontophoretic delivery studies in vivo. Here, we provide an overview of mathematical models that have been proposed to describe (i) the effect of current application on the skin and the implications for potential iontophoretic transport pathways, (ii) electrotransport kinetics and (iii) the fate of iontophoretically delivered drugs once they enter the systemic circulation.

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 iontophoresis

Iontophoresis is a technique used to enhance the transdermal delivery of compounds through the skin via the application of a small electric current. By the process of electromigration and electro-osmosis, iontophoresis increases the permeation of charged and neutral compounds, and offers the option for programmed drug delivery. Interest in this field of research has led to the successful delivery of both low (lidocaine) and high molecular drugs, such as peptides (e.g., luteinising hormone releasing hormone, nafarelin and insulin). Combinations of iontophoresis with chemical enhancers, electroporation and sonophoresis have been tested in order to further increase transdermal drug permeation and decrease possible side effects. In addition, rapid progress in the fields of microelectronics, nanotechnology and miniaturisation of devices is leading the way to more sophisticated iontophoretic devices, allowing improved designs with better control of drug delivery. Recent successful designing of the fentanyl E-TRANS iontophoretic system have provided encouraging results. This review will discuss basic concepts, principles and applications of this delivery technique.

Description of transdermal transport of hydrophilic solutes during low-frequency sonophoresis based on a modified porous pathway model

Application of low-frequency ultrasound has been shown to increase skin permeability, thereby facilitating delivery of macromolecules (low-frequency sonophoresis). In this study, we sought to determine a theoretical description of transdermal transport of hydrophilic permeants induced by low-frequency sonophoresis. Parameters such as pore size distribution, absolute porosity, and dependence of effective tortuosity on solute characteristics were investigated. Pig skin was exposed to low-frequency ultrasound at 58 kHz to achieve different skin resistivities. Transdermal delivery of four permeants [mannitol, luteinizing hormone releasing hormone (LHRH), inulin, dextran] in the presence and absence of ultrasound was measured. The porous pathway model was modified to incorporate the permeant characteristics into the model and to achieve a detailed understanding of the pathways responsible for hydrophilic permeant delivery. The slopes of the log kp(p) versus log R graphs for individual solutes changed with solute molecular area, suggesting that the permeability-resistivity correlation for each permeant is related to its size. The tortuosity that a permeant experiences within the skin also depends on its size, where larger molecules experience a less tortuous path. With the modified porous pathway model, the effective tortuosities and skin porosity were calculated independently. The results of this study show that low-frequency sonophoresis creates pathways for permeant delivery with a wide range of pore sizes. The optimum pore size utilized by solutes is related to their molecular radii.

Modeling skin permeability to hydrophilic and hydrophobic solutes based on four permeation pathways

Barrier properties of skin originate from low permeability of stratum corneum. The objective of this paper is to compile fundamentally-based analytical expressions that can be used to predict skin permeability to hydrophilic as well as hydrophobic solutes. Solute permeation through four possible routes in stratum corneum including free-volume diffusion through lipid bilayers, lateral diffusion along lipid bilayers, diffusion through pores, and diffusion through shunts was analyzed. Contribution of free-volume diffusion through lipid bilayers was determined using Scaled Particle Theory. This theory relates solute partition and diffusion coefficients to the work required to create cavities in a lipid bilayers to allow solute incorporation and motion. Contribution of lateral lipid diffusion was determined from the literature data. Contribution of pores was estimated using hindered transport theory. This theory assumes that hydrophilic solutes permeate across the skin through imperfections in the lipid bilayers modeled as pores. Finally, contribution of shunts was determined using a simple diffusion model. The model yielded a series of equations to predict skin permeability based on solute radius and octanol-water partition coefficient. Predictions of the model compare well with the experimental data.

Effect of lipophilicity on in vivo iontophoretic delivery. I. NSAIDs

The effect of drug lipophilicity on in vivo iontophoretic transdermal absorption was evaluated. Non-steroidal anti-inflammatory drugs (NSAIDs) were selected as model drugs with a wide range of lipophilicity: salicylic acid (SA), ketoprofen (KP), naproxen (NP) and indomethacin (IM). Cathodal iontophoresis of NSAIDs was conducted in rats (0.625 mA/cm2; 90 min), and drug concentrations in skin, cutaneous vein and systemic vein were determined. Skin concentrations of NSAID were higher in the case of lipophilic drugs (SA=KP=NP<IM), whereas cutaneous plasma concentrations decreased with an increase in lipophilicity (SA>KP=NP>IM). Additionally, the dependence of drug lipophilicity on systemic plasma concentration was similar to cutaneous plasma concentration. The transfer rate from skin to cutaneous vein (R(SC)) was calculated from the arterio-venous plasma concentration difference of drug in the skin. Normalized R(SC) by skin concentration (R(SC)/X(S)) yielded a negative correlation with the logarithm of n-octanol/buffer partition coefficient (Log P at pH 7.4), suggesting that transfer of NSAIDs from skin to cutaneous vein decreased with increasing lipophilicity (SA>KP=NP>IM). This correlation means that drug partitioning between stratum corneum and viable epidermis might be a dominant step.

Transdermal iontophoresis revisited

Iontophoresis evolved as a transdermal enhancement technique in the 20th century, primarily for the delivery of large and charged molecules. Significant achievements have been made in the understanding of underlying mechanisms of iontophoresis and these have contributed to the rational development of iontophoretic delivery systems. The major challenges in this area are the development of portable, cost effective devices and suitable semi-solid formulations that are compatible with the device and the skin. Some of the obstacles in transdermal iontophoresis can be overcome by combining iontophoresis with other physical and chemical enhancement techniques for the delivery of macromolecules. Iontophoresis also offers an avenue for extracting information from the body through the use of reverse iontophoresis, which has potential application in diagnosis and monitoring. The current research is focussed towards resolving the skin toxicity issues and other problems in order to make this technology a commercial reality.

Quantitative structure-property relationships in pharmaceutical research - Part 2

Part one of this two-part review described the advantages and limitations of quantitative structure-property relationships (QSPR), and offered an overview of the components involved in the development of correlations1. Part two provides a discussion of a few notable examples of relationships with organoleptic, physicochemical and pharmaceutical properties.