Ultrasound-mediated transdermal protein delivery.

Transdermal drug delivery offers a potential method of drug administration. However, its application has been limited to a few low molecular weight compounds because of the extremely low permeability of human skin. Low-frequency ultrasound was shown to increase the permeability of human skin to many drugs, including high molecular weight proteins, by several orders of magnitude, thus making transdermal administration of these molecules potentially feasible. It was possible to deliver and control therapeutic doses of proteins such as insulin, interferon gamma, and erythropoeitin across human skin. Low-frequency ultrasound is thus a potential noninvasive substitute for traditional methods of drug delivery, such as injections.

In vitro visualization and quantification of oleic acid induced changes in transdermal transport using two-photon fluorescence microscopy.

In a novel application of two-photon scanning fluorescence microscopy, three-dimensional spatial distributions of the hydrophilic and hydrophobic fluorescent probes, sulforhodamine B and rhodamine B hexyl ester, in excised full-thickness human cadaver skin were visualized and quantified. Both sulforhodamine B and rhodamine B hexyl ester were observed to lie primarily in the lipid multilamellae region surrounding the corneocytes within the stratum corneum. From the two-photon scanning fluorescence microscopy scans, the changes in the concentration gradient and the vehicle to skin partition coefficient of each probe induced by the oleic acid enhancer action were calculated relative to the control sample (not exposed to oleic acid), and subsequently applied to theoretically derived mathematical expressions of transdermal transport to quantitatively characterize the oleic acid-induced relative changes in the skin diffusion coefficient and the skin barrier diffusion length of the permeant. For the hydrophobic probe rhodamine B hexyl ester, the permeability enhancement was primarily driven by an increase in the vehicle to skin partition coefficient, leading to an increase in the steepness of the concentration gradient across the skin. The primary oleic acid-induced changes in the transport properties of the hydrophilic probe sulforhodamine B included increases in the vehicle to skin partition coefficient and the skin diffusion coefficient. These findings utilizing the two-photon scanning fluorescence microscopy methodology and data analysis described here demonstrate that, in addition to providing three-dimensional images that clearly delineate probe distributions in the direction of increasing skin depth, the subsequent quantification of these images provides additional important insight into the mechanistic changes in transdermal transport underlying the visualized changes in probe distributions across the skin.

Thermodynamic prediction of active ingredient loading in polymeric microparticles.

The growing use of microparticles as a controlled-delivery system for pharmaceutical and non-pharmaceutical active ingredients (AIs) has prompted a costly trial-and-error development of new and effective microparticle systems. In order to facilitate a more rational design and optimization of AI loadings in microparticles, we have developed a molecular-thermodynamic theory to predict the loading of liquid AIs in polymeric microparticles that are manufactured by a solvent evaporation process. This process involves the emulsification of a liquid polymer solution (consisting of polymer and AI dissolved in a volatile solvent) in an aqueous surfactant solution. The theory describes the equilibrium distribution of the AI between the aqueous phase and the dispersed polymeric droplets. The universal functional activity coefficient (UNIFAC) and UNIFAC-Free Volume (FV) group-contribution methods are utilized to model the nonidealities in the water and polymeric droplet phases, respectively. The inputs to the theory are: (i) the chemical structures, densities and total masses of the manufacturing ingredients, (ii) the manufacturing temperature and (iii) the glass transition temperature of the polymer. Since surfactant concentrations exceeding the critical micellar concentration (CMC) are often required in order to stabilize the dispersed polymeric droplets during the emulsion manufacturing process, the theory also accounts for AI solubilization in surfactant micelles present in the manufacturing solution. To test the AI loading predictions, we compare theoretical predictions of AI loadings in poly(lactic acid), poly(methyl methacrylate) and polystyrene microparticles to experimentally measured ones for five model AIs with varying degrees of hydrophobicity (benzyl alcohol, n-octanol, geraniol, farnesol and galaxolide). We also demonstrate how the developed theory can be utilized to screen polymers with respect to their abilities to load a given AI, as well as to provide guidelines for manufacturing microparticles having the desired AI loading.

An explanation for the variation of the sonophoretic transdermal transport enhancement from drug to drug.

Over the last few decades, application of therapeutic ultrasound (frequency between 1 and 3 MHz and intensity between 1 and 2 W/cm2) has been attempted to enhance transdermal transport of several drugs, a method referred to as sonophoresis. The sonophoretic enhancement of transdermal drug transport was found to vary significantly from drug to drug. In certain cases, ultrasound did not induce any enhancement of transdermal drug transport. This variation in the efficacy of sonophoresis has raised a controversy regarding its applicability as a transdermal delivery enhancer. The objective of this paper is to provide a summary of the literature data on sonophoresis and an explanation for the observed variation of the sonophoretic enhancement from drug to drug. This paper also presents an equation to qualitatively predict whether therapeutic ultrasound may enhance transdermal transport of a given drug based on knowledge of the drug passive skin permeability and octanol-water partition coefficient.

Evaluation of solute permeation through the stratum corneum: lateral bilayer diffusion as the primary transport mechanism.

Solute permeation across human stratum corneum (SC) was examined in terms of the fundamental bilayer transport properties. A mathematical model was developed to describe the macroscopic SC permeation via the interkeratinocyte lipid domain in terms of (i) the structure and dimensions of the SC, and (ii) the microscale lipid bilayer transport properties, which include the bilayer/water partition coefficient, the lateral diffusion coefficient, the interfacial transbilayer mass transfer coefficient, and the intramembrane transbilayer mass transfer coefficient. The relative importance of the diffusive resistances associated with the bilayer transport properties was evaluated with the model and experimental data. Lateral diffusion coefficients in SC lipid bilayers were calculated from 120 human skin permeability measurements, and compared with previously reported measurements made in SC-extracted lipids. Good qualitative and quantitative agreement was observed, indicating that, in the context of the model, the diffusive resistance associated with lateral diffusion is sufficient to explain the overall resistance of solute permeation through the SC. A similar analysis shows that the diffusive resistance associated with interfacial transbilayer transport is not capable of explaining the experimental permeation values, thus supporting this finding. The lateral diffusion analysis also revealed a bifunctional size dependence of transport within the SC, with a strong size dependence for small solutes (<300 Da) and a weak size dependence for larger solutes.

Theoretical description of transdermal transport of hydrophilic permeants: application to low-frequency sonophoresis.

Application of ultrasound enhances transdermal transport of drugs (sonophoresis). The enhancement may result from enhanced diffusion due to ultrasound-induced skin alteration and/or from forced convection. To understand the relative roles played by these two mechanisms in low-frequency sonophoresis (LFS, 20 kHz), a theory describing the transdermal transport of hydrophilic permeants in both the absence and the presence of ultrasound was developed using fundamental equations of membrane transport, hindered-transport theory, and electrochemistry principles. With mannitol as the model permeant, the role of convection in LFS was evaluated experimentally with two commonly used in vitro skin models- human cadaver heat-stripped skin (HSS) and pig full-thickness skin (FTS). Our results suggest that convection plays an important role during LFS of HSS, whereas its effect is negligible when FTS is utilized. The theory developed was utilized to characterize the transport pathways of hydrophilic permeants during both passive diffusion and LFS with mannitol and sucrose as two probe molecules. Our results show that the porous pathway theory can adequately describe the transdermal transport of hydrophilic permeants in both the presence and the absence of ultrasound. Ultrasound alters the skin porous pathways by two mechanisms: (1) enlarging the skin effective pore radii, or (2) creating more pores and/or making the pores less tortuous. During passive diffusion, both HSS and FTS exhibit the same skin effective pore radii (r = 28 +/- 13 A). In contrast, during LFS, r within HSS is greatly enlarged (r > 125 A), whereas r within FTS does not change significantly (23 +/- 10 A). The observed different roles of convection during LFS across HSS and FTS can be attributed to the different degrees of structural alteration that these two types of skin undergo during LFS.

Permeation of steroids through human skin.

The permeabilities of many steroids through human skin have been previously measured and reported in the literature. Analysis of these data reveals that significant discrepancies exist between the measurements of Scheuplein et al. and those of other groups. Six of the 14 steroids which were examined by Scheuplein et al., aldosterone, corticosterone, estradiol, hydrocortisone, progesterone, and testosterone, have also been examined by other groups. For each of these steroids, the permeability measurements of Scheuplein et al. are lower than those reported by other groups by factors of between 5.0 and 77. Eight independent measurements of the permeability of estradiol are in good agreement with one another, but are greater than the value reported by Scheuplein et al. by factors of between 11 and 20. Several possible sources of experimental error, including the variability of the skin samples, the differences in the experimental temperature, the establishment of steady-state conditions, the use of radiolabeled drugs, and the skin preparation technique, have been considered and do not appear to account for the magnitude of the observed discrepancies nor for the fact that the data of Scheuplein et al. are consistently lower than those reported by other groups.

A mechanistic study of ultrasonically-enhanced transdermal drug delivery.

Although ultrasound has been shown to enhance the transdermal transport of a variety of drugs, the mechanisms underlying this phenomenon are not clearly understood. In this paper, we evaluate the roles played by various ultrasound-related phenomena, including cavitation, thermal effects, generation of convective velocities, and mechanical effects, in the ultrasonic enhancement of transdermal drug delivery (sonophoresis). Our experimental findings suggest that among all the ultrasound-related phenomena evaluated, cavitation plays the dominant role in sonophoresis using therapeutic ultrasound (frequency range, 1-3 MHz; intensity range, 0-2 W/cm2). Furthermore, confocal microscopy results indicate that cavitation occurs in the keratinocytes of the stratum corneum upon ultrasound exposure. It is hypothesized that oscillations of the cavitation bubbles induce disorder in the stratum corneum lipid bilayers, thereby enhancing transdermal transport. Evidence supporting this hypothesis is presented using skin electrical resistance measurements. Finally, a theoretical model is developed to predict the effect of ultrasound on the transdermal transport of drugs. The model predicts that sonophoretic enhancement depends most directly on the passive permeant diffusion coefficient, rather than on the permeability coefficient through the skin. Specifically, permeants passively diffusing through the skin at a relatively slow rate are expected to be preferentially enhanced by ultrasound. The experimentally measured sonophoretic transdermal transport enhancement for seven permeants, including estradiol, testosterone, progesterone, corticosterone, benzene, butanol, and caffeine, agree quantitatively with the model predictions. These experimental and theoretical findings provide quantitative guidelines for estimating the efficacy of sonophoresis in enhancing transdermal drug delivery.

Synergistic effects of chemical enhancers and therapeutic ultrasound on transdermal drug delivery.

The effects of (i) a series of chemical enhancers and (ii) the combination of these enhancers and therapeutic ultrasound (1 MHz, 1.4 W/cm2, continuous) on transdermal drug transport are investigated. A series of chemical enhancer formulations, including (i) polyethylene glycol 200 dilaurate (PEG), (ii) isopropyl myristate (IM), (iii) glycerol trioleate (GT), (iv) ethanol/pH 7.4 phosphate buffered saline in a 1:1 ratio (50% EtOH), (v) 50% EtOH saturated with linoleic acid (LA/EtOH), and (vi) phosphate buffered saline (PBS), as a control, are evaluated using corticosterone as a model drug. LA/EtOH is the most effective of these enhancers, increasing the corticosterone flux by 900-fold compared to that from PBS. Therapeutic ultrasound (1 MHz, 1.4 W/cm2, continuous) increases the corticosterone permeability from all of the enhancers examined by up to 14-fold (LA/EtOH) and increases the corticosterone flux from the saturated solutions by up to 13,000-fold (LA/EtOH), relative to that from PBS. Similar enhancements are obtained with LA/EtOH with and without ultrasound for four other model drugs, dexamethasone, estradiol, lidocaine, and testosterone. The permeability enhancements for all of these drugs resulting from the addition of linoleic acid to 50% EtOH increase with increasing drug molecular weight. Likewise, the permeability enhancement attained by ultrasound and LA/EtOH relative to passive EtOH exhibits a similar size dependence. A mechanistic explanation of this size dependence is provided. It is suggested that bilayer disordering agents, such as linoleic acid and ultrasound, transform the SC lipid bilayers into a fluid lipid bilayer phase or create a separate bulk oil phase. The difference in diffusivity of a given solute in SC bilayers and in either fluid bilayers or bulk oil is larger for larger solutes, thereby producing greater enhancements for larger solutes.

Synergistic effect of low-frequency ultrasound and sodium lauryl sulfate on transdermal transport.

Application of low-frequency ultrasound has been shown to enhance transdermal transport of drugs (low-frequency sonophoresis). In this paper, we show that the efficacy of low-frequency ultrasound in enhancing transdermal transport can be further increased by its combination with sodium lauryl sulfate (SLS), a well-known surfactant. The dependence of the ultrasound-SLS-mediated transport on ultrasound parameters, including intensity, net exposure time, and duty cycle, is discussed. The transdermal transport enhancement is proportional to ultrasound intensity as well as to exposure time, and is independent of duty cycle as long as the net exposure time is the same. The synergistic effect of SLS and ultrasound on transdermal transport increases linearly with SLS concentration. The enhancement is also proportional to the ultrasound energy density beyond a threshold value, which suggests that a certain minimum amount of energy density is required before noticeable changes in skin permeability occur. A similar dependence of the transdermal transport enhancement on energy density is observed in the case of the enhancement induced by ultrasound alone. Although the threshold energy density value in the presence of SLS is about 10 times lower than that in the case of ultrasound alone, the relationship between enhancement and energy density in the presence and in the absence of SLS is otherwise similar. Possible mechanisms for the synergistic effect of ultrasound and SLS are also discussed.