Recovery of human skin impedance in vivo after iontophoresis: effect of metal ions

Effect of formulation pH on transport of naltrexone species and pore closure in microneedle-enhanced transdermal drug delivery

Microneedle-enhanced transdermal drug delivery greatly improves the subset of pharmacologically active molecules that can be transported across the skin. Formulation pH plays an important role in all drug delivery systems; however, for transdermal delivery it becomes specifically significant since a wide range of pH values can be exploited for patch formulation as long as it does not lead to skin irritation or sensitization issues. Wound healing literature has shown significant pH effects on barrier recovery. Stability and solubility of the drug, and thus transport across skin, are all affected by formulation pH. The current study examined the role of ionization state of the drug naltrexone on transdermal flux and permeability across microneedle treated skin, as compared to intact skin. Impedance spectroscopy was done in pigs in vivo to assess the role of formulation pH on the rate of micropore closure under the influence of three different pH conditions. The data indicated that while there was significant advantage of using a lower pH formulation in terms of total transport across microneedle treated skin, the pH however did not have any significant effect on the rate of micropore closure beyond the first 24 h.

Iontophoretic drug delivery across the nail

INTRODUCTION

Topical drug delivery to treat nail diseases such as onychomycosis and psoriasis is receiving increasing attention. Topical nail delivery is challenged by the complicated structure of the nail and the low permeability of most drugs across the nail plate. Considerable effort has been directed at developing methods to promote drug permeation across the nail plate. Iontophoresis efficiently enhances molecular transport across the skin and the eye and is now being tested for its potential in ungual delivery.

AREAS COVERED

This review covers the basic mechanisms of transport (electro-osmosis and -migration) and their relative contribution to nail iontophoresis as well as the key factors governing nail permselectivity and ionic transport numbers. Methodological issues concerning research in this area are summarized. The data available in vivo on nail iontophoresis of terbinafine specifically are reviewed in separate sections.

EXPERT OPINION

Our understanding of nail iontophoresis has improved considerably since 2007; most decisively, the feasibility of nail iontophoresis in vivo has been clearly demonstrated. Future work is required to establish the adequate implementation of the technique so that its clinical efficacy to treat onychomycosis and nail psoriasis can be unequivocally determined.

Kinetics of Membrane Raft Formation: Fatty Acid Domains in Stratum Corneum Lipid Models

The major barrier to permeability in skin resides in the outermost layer of the epidermis, the stratum corneum (SC). The major SC lipid components are ceramides, free fatty acids, and cholesterol. Ternary mixtures containing these constituents are widely used for physicochemical characterization of the barrier. Prior X-ray diffraction and IR spectroscopy studies have revealed the existence of ordered lipid chains packed in orthorhombic subcells. To monitor the kinetics of formation of regions rich in fatty acids, the current study utilizes a modification of the method (J. Phys. Chem. 1992, 96, 10008) developed to monitor component demixing in n-alkane mixtures. The approach is based on changes in the scissoring or rocking mode contours in the IR spectra of (orthorhombically packed) ordered chains. In the current study, equimolar mixtures of ceramides (either non-hydroxy fatty acid sphingosine ceramide or alpha-hydroxy fatty acid sphingosine ceramide) with chain perdeuterated fatty acids (either palmitic or stearic acid) and cholesterol reveal a time evolution of the scissoring contour of the deuterated fatty acid chains following quenching from relatively high temperatures where random mixing occurs. Segregation of domains enriched in the fatty acid component is observed. The kinetics of segregation are sensitive to the quenching temperature and to the chemical composition of the mixture. The kinetic regimes are conveniently catalogued with a power law of the form P=Ktalpha where P is a (measured) property related to domain composition. The time scales for demixing in these experiments are similar to times observed in several studies that have tracked the restoration of the in vivo permeability barrier following nonthermal challenges to SC integrity. Further evidence for the physiological importance of the current measurements is the detection of these phases in native SC. The current work constitutes the first direct, structure-based determination of the kinetics of barrier formation in relevant skin lipid barrier models.

Post-iontophoresis recovery of human skin impedance in vivo

The objective of this study was to better understand the recovery of human skin impedance following iontophoresis in vivo. Volunteers were subjected to a 15-min period of iontophoresis in the presence of aqueous solutions of either NaCl, KCl, CaCl(2) or MgCl(2) at 133 mM. Subsequently, the low-frequency impedance (at 1 Hz) recovery was followed for a further 30 min. Assuming direct proportionality between the reciprocal impedance and the ion concentration in the membrane, the experimental data were fitted to the appropriate solutions of Fick's second law of diffusion to derive characteristic diffusion parameters (D/L(2)), apparent diffusivities (D), diffusion pathlengths (L) and mobilities, and ion concentrations in the skin immediately post-iontophoresis. Ion fluxes out of the membrane after termination of current flow were also deduced. In general, recovery was relatively independent of the background electrolyte as previously reported, and the data were consistent with ion transport in predominantly aqueous pathways. Compared to its mobility in aqueous solution, however, the apparent Cl- mobility in the skin was smaller, presumably due to the fact that, under normal physiological conditions, the human skin barrier supports a net negative charge. In parallel, the initial "release" of Na+ and K+ from the skin post-iontophoresis was faster than that of Ca(2+) and Mg(2+), the latter cations of higher charge density being able to associate more strongly, it seems, with the negatively-charged skin. The simple physicochemical analysis of the data presented serves to emphasize that a decrease in skin impedance is not a manifestation of damage to the barrier--rather, it is a natural response to the relevant electrical potential and ion concentration gradients involved.

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.