The nature of ultrasound–SLS synergism during enhanced transdermal transport

Enhancement of the transdermal delivery of zidovudine by pretreating the skin with two physical enhancers: microneedles and sonophoresis

BACKGROUND

Zidovudine (AZT) has been the most widely used drug for antiretroviral therapy. In order to improve the therapy with this drug, different alternatives have been proposed, such as the transdermal administration. The use of permeation enhancers is necessary to favor the passage of this drug through the skin, due to its physicochemical properties and to the natural permeation barrier imposed by the skin.

OBJECTIVES

To evaluate the effect of two permeation enhancers, sonophoresis and microneedles, on the permeability of AZT through the skin.

METHODS

Permeation studies with an AZT solution were performed using pigskin clamped in Franz-type cells. Sonophoresis was applied under different conditions (i.e., amplitude, duty cycle and application time), selected according to an experimental design, where the response variables were the increase in temperature of the skin surface and the increase in transepidermal water loss. ATR-FTIR was also used to demonstrate the effect of enhancers on membrane components.

RESULTS

The permeability of AZT through intact skin was very poor, with a very long lag time. Pretreatment of the skin with sonophoresis increased AZT transport significantly, reducing the lag time. The maximum flux (27.52 µgcm^-2 h^-1) and the highest total amount permeated (about 624 µg/cm²) were obtained when applying sonophoresis in continuous mode, with an amplitude of 20%, and an application time of 2 min. Sonophoresis appears to have an impact on stratum corneum proteins. The use of microneedles further increased the flux (30.41 µgcm^-2 h^-1) and the total amount permeated (about 916 µg/cm²), relative to sonophoresis.

CONCLUSION

The results are encouraging in terms of promoting AZT transport through the skin using sonophoresis or microneedles as permeation enhancers.

Circulation Cooling in Continuous Skin Sonoporation at Constant Coupling Fluid Temperatures

Exposure of the skin to low-frequency ultrasound in the Franz diffusion cell has been found to increase the permeability of the skin to molecular transport. In many cases, significant heating of the coupling fluid requires the use of duty cycles that extend the total experimental time. This is a methodological study in which the coupling fluid is circulated between a modified Franz diffusion cell and a heat exchanger to allow for the continuous application of low-frequency ultrasound while the coupling fluid temperature is held constant. Dermatomed porcine skin was exposed to continuous ultrasound at 20 kHz for 10 min at an intensity of 55 W/cm² while the coupling fluid was maintained at one of three target temperatures (13°C, 33°C or 46°C). Foil pitting and passive cavitation detection revealed that inertial cavitation activity decreased with increasing coupling fluid target temperature. Transport measurements revealed an increase in mean donor calcein concentration with increasing coupling fluid temperature, though these were not statistically significant. Taken together these findings suggest that the weakened stratum corneum lipid structure at higher temperatures is more susceptible to the introduction of defects from the jetting of cavitation.

A Thermoelectric Device for Coupling Fluid Temperature Regulation During Continuous Skin Sonoporation or Sonophoresis

During skin sonoporation and sonophoresis, time-consuming duty cycles or fluid replacement is often required to mitigate coupling fluid temperature increases. This study demonstrates an alternative method for temperature regulation: a circulating, thermoelectric system. Porcine skin samples were sonoporated continuously for 10 min at one of three intensities (23.8, 34.2, 39.4 W/m²). A caffeine solution was then applied to the skin and left to diffuse for 20 h. During sonoporation, the system was able to maintain the temperature between 10 and 16°C regardless of the intensity. No increase in transdermal transport was achieved with an intensity of 23.8 W/m². Intensities of 34.2 and 39.4 W/m² resulted in 3.5-fold (p < 0.05) and 3.7-fold (p < 0.05) increases in mean transport, relative to a control case with no ultrasound. From these results, it is concluded that a significant transport increase can be achieved with a system that circulates and cools the coupling fluid during ultrasound application. Relative to the previous methods of temperature control (duty cycles and fluid replacement), use of this circulation system will lead to significant time savings in future experimental studies.

Synthesis, Comparison, and Optimization of a Humic Acid-Quat10 Polyelectrolyte Complex by Complexation-Precipitation versus Extrusion-Spheronization

A novel humic acid and polyquaternium-10 polyelectrolyte complex (PEC) was synthesized utilizing two methods and the solubility and permeability of efavirenz (EFV) were established. Complexation-precipitation and extrusion-spheronization were used to synthesize and compare the drug-loaded PECs. The chemical integrity, thermo-mechanical differences, and morphology between the drug-loaded PECs produced by the two methods were assessed by attenuated total reflectance-Fourier transform infrared, differential scanning calorimetry, and SEM. The extent of drug solubilization was determined using the saturation solubility test while the biocompatibility of both PECs was confirmed by cytotoxicity studies on human adenocarcinoma cells (caco2). Bio-relevant media was used for the solubility and permeability analysis of the optimized PEC formulations for accurate assessment of formulation performance. Ritonavir (RTV) was loaded into the optimized formulation to further corroborate the impact of the PEC on the solubility and permeability properties of a poorly soluble and poorly permeable drug. The optimized EFV-loaded PEC and the RTV-loaded PEC exhibited 14.16 ± 2.81% and 4.39 ± 0.57% increase in solubility, respectively. Both PECs were compared to currently marketed formulations. Intestinal permeation results revealed an enhancement of 61.24 ± 6.92% for EFV and 38.78 ± 0.50% for RTV. Although both fabrication methods produced PECs that enhanced the solubility and permeability of the model Biopharmaceutics Classification System Class II and IV drugs, extrusion-spheronization was selected as most optimal based on the higher solubility and permeability improvement and the impact on caco2 cell viability.

Ex vivo evaluation of a microneedle array device for transdermal application

A new approach of transdermal drug delivery is the use of microneedles. This promising technique offers the potential to be broadly used for drug administration as it enables the dramatic increase in permeation of medicaments across the stratum corneum. The potential of microneedles has evolved to spawn a plethora of potential transdermal applications. In order to advance the microneedle capabilities and possibly revolutionize advanced drug delivery, this study introduces a novel transdermal electro-modulated hydrogel-microneedle array (EMH-MNA) device composed of a nano-porous, embeddable ceramic microneedle array as well as an optimized EMH for the electro-responsive delivery of indomethacin through the skin. The ex vivo permeation as well as drug release experiments were performed on porcine skin tissue to ascertain the electro-responsive capabilities of the device. In addition, the microbial permeation ability of the microneedles across the viable epidermis in both microneedle-punctured skin as well as hypodermic needle-punctured skin was determined. Ex vivo evaluation of the EMH-MNA device across porcine skin demonstrated that without electro-stimulation, significantly less drug release was obtained (±0.4540mg) as compared to electro-stimulation (±2.93mg).

Treatment of chest wall tuberculosis with transdermal ultrasound-mediated drug delivery

Chest wall tuberculosis (TB) is an endemic disease with a large number of variants. The condition affects numerous parts of the body and can penetrate the skin to form chronic open ulcers. Current treatment methods include oral anti-TB drugs and surgery. However, conventional drug treatments are not effective due to the difficulty in achieving an effective local concentration, and certain patients are unable to tolerate surgery. The recurrence rate for chest wall TB is high following surgery, and may result in the prolonged healing of wounds in certain patients, as well as chronic sinusitis and fistula formation. To identify a safe, simple, less invasive and more clinically effective treatment method, the present study investigated transdermal ultrasound-mediated anti-TB drug delivery. A total of 186 patients were selected and randomly divided into transdermal ultrasound, surgery and oral anti-TB drug only groups. Rifampicin was the drug delivered by transdermal ultrasound. The cure and efficiency rates were shown to be 87.10 and 93.55%, respectively, in the ultrasound treatment group. No statistically significant difference was observed in the cure rates between the transdermal ultrasound and surgery groups; however, a statistically significant difference was identified in the cure rates between the transdermal ultrasound and oral anti-TB drug only groups. Therefore, transdermal ultrasound technology was shown to deliver anti-TB drugs quickly and directly, which resulted in a high local concentration of the drug, overcoming the problem of obtaining an effective local drug concentration. The observations demonstrated that transdermal ultrasound-mediated drug delivery is an effective method by which to control TB, particularly when compared with traditional oral anti-TB therapy and surgery.

Ultrasound mediated transdermal drug delivery

Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injections. However, the stratum corneum serves as a barrier that limits the penetration of substances to the skin. Application of ultrasound (US) irradiation to the skin increases its permeability (sonophoresis) and enables the delivery of various substances into and through the skin. This review presents the main findings in the field of sonophoresis in transdermal drug delivery as well as transdermal monitoring and the mathematical models associated with this field. Particular attention is paid to the proposed enhancement mechanisms and future trends in the fields of cutaneous vaccination and gene therapy.

Rapid skin permeabilization by the simultaneous application of dual-frequency, high-intensity ultrasound

Low-frequency ultrasound has been studied extensively due to its ability to enhance skin permeability. In spite of this effort, improvements in enhancing the efficacy of transdermal ultrasound treatments have been limited. Currently, when greater skin permeability is desired at a given frequency, one is limited to increasing the intensity or the duration of the ultrasound treatment, which carries the risk of thermal side effects. Therefore, the ability to increase skin permeability without increasing ultrasound intensity or treatment time would represent a significant and desirable outcome. Here, we hypothesize that the simultaneous application of two distinct ultrasound frequencies, in the range of 20 kHz to 3 MHz, can enhance the efficacy of ultrasound exposure. Aluminum foil pitting experiments showed a significant increase in cavitational activity when two frequencies were applied instead of just one low frequency. Additionally, in vitro tests with porcine skin indicated that the permeability and resulting formation of localized transport regions are greatly enhanced when two frequencies (low and high) are used simultaneously. These results were corroborated with glucose (180 Da) and inulin (5000 Da) transdermal flux experiments, which showed greater permeant delivery both into and through the dual-frequency pre-treated skin.

A physical mechanism to explain the delivery of chemical penetration enhancers into skin during transdermal sonophoresis - Insight into the observed synergism

The synergism between low-frequency sonophoresis (LFS) and chemical penetration enhancers (CPEs), especially surfactants, in transdermal enhancement has been investigated extensively since this phenomenon was first observed over a decade ago. In spite of the identifying that the origin of this synergism is the increased penetration and subsequent dispersion of CPEs in the skin in response to LFS treatment, to date, no mechanism has been directly proposed to explain how LFS induces the observed increased transport of CPEs. In this study, we propose a plausible physical mechanism by which the transport of all CPEs is expected to have significantly increased flux into the localized-transport regions (LTRs) of LFS-treated skin. Specifically, the collapse of acoustic cavitation microjets within LTRs induces a convective flux. In addition, because amphiphilic molecules preferentially adsorb onto the gas/water interface of cavitation bubbles, amphiphiles have an additional adsorptive flux. In this sense, the cavitation bubbles effectively act as carriers for amphiphilic molecules, delivering surfactants directly into the skin when they collapse at the skin surface as cavitation microjets. The flux equations derived for CPE delivery into the LTRs and non-LTRs during LFS treatment, compared to that for untreated skin, explain why the transport of all CPEs, and to an even greater extent amphiphilic CPEs, is increased during LFS treatment. The flux model is tested with a non-amphiphilic CPE (propylene glycol) and both nonionic and ionic amphiphilic CPEs (octyl glucoside and sodium lauryl sulfate, respectively), by measuring the flux of each CPE into untreated skin and the LTRs and non-LTRs of LFS-treated skin. The resulting data shows very good agreement with the proposed flux model.

Ultrasound-mediated transdermal drug delivery: mechanisms, scope, and emerging trends

The use of ultrasound for the delivery of drugs to, or through, the skin is commonly known as sonophoresis or phonophoresis. The use of therapeutic and high frequencies of ultrasound (≥0.7MHz) for sonophoresis (HFS) dates back to as early as the 1950s, while low-frequency sonophoresis (LFS, 20-100kHz) has only been investigated significantly during the past two decades. Although HFS and LFS are similar because they both utilize ultrasound to increase the skin penetration of permeants, the mechanisms associated with each physical enhancer are different. Specifically, the location of cavitation and the extent to which each process can increase skin permeability are quite dissimilar. Although the applications of both technologies are different, they each have strengths that could allow them to improve current methods of local, regional, and systemic drug delivery. In this review, we will discuss the mechanisms associated with both HFS and LFS, specifically concentrating on the key mechanistic differences between these two skin treatment methods. Background on the relevant physics associated with ultrasound transmitted through aqueous media will also be discussed, along with implications of these phenomena on sonophoresis. Finally, a thorough review of the literature is included, dating back to the first published reports of sonophoresis, including a discussion of emerging trends in the field.

Low-frequency sonophoresis: application to the transdermal delivery of macromolecules and hydrophilic drugs

IMPORTANCE OF THE FIELD

Transdermal delivery of macromolecules provides an attractive alternative route of drug administration when compared to oral delivery and hypodermic injection because of its ability to bypass the harsh gastrointestinal tract and deliver therapeutics non-invasively. However, the barrier properties of the skin only allow small, hydrophobic permeants to traverse the skin passively, greatly limiting the number of molecules that can be delivered via this route. The use of low-frequency ultrasound for the transdermal delivery of drugs, referred to as low-frequency sonophoresis (LFS), has been shown to increase skin permeability to a wide range of therapeutic compounds, including both hydrophilic molecules and macromolecules. Recent research has demonstrated the feasibility of delivering proteins, hormones, vaccines, liposomes and other nanoparticles through LFS-treated skin. In vivo studies have also established that LFS can act as a physical immunization adjuvant. LFS technology is already clinically available for use with topical anesthetics, with other technologies currently under investigation.

AREAS COVERED IN THIS REVIEW

This review provides an overview of mechanisms associated with LFS-mediated transdermal delivery, followed by an in-depth discussion of the current applications of LFS technology for the delivery of hydrophilic drugs and macromolecules, including its use in clinical applications.

WHAT THE READER WILL GAIN

The reader will gain an insight into the field of LFS-mediated transdermal drug delivery, including how the use of this technology can improve on more traditional drug delivery methods.

TAKE HOME MESSAGE

Ultrasound technology has the potential to impact many more transdermal delivery platforms in the future due to its unique ability to enhance skin permeability in a controlled manner.

Development of an optimised application protocol for sonophoretic transdermal delivery of a model hydrophilic drug

It has now been known for over a decade that low frequency ultrasound can be used to effectively enhance transdermal drug penetration - an approach termed sonophoresis. Mechanistically, acoustic cavitation results in the creation of defects in the stratum corneum that allow accelerated absorption of topically applied molecules. The aim of this study was to develop an optimised sonophoresis protocol for studying transdermal drug delivery in vitro. To this end, caffeine was selected as a model hydrophilic drug while porcine skin was used as a model barrier. Following acoustic validation, 20kHz ultrasound was applied for different durations (range: 5 s to 10 min) using three different modes (10%, 33% or 100% duty cycles) and two distinct sonication procedures (either before or concurrent with drug deposition). Each ultrasonic protocol was assessed in terms of its heating and caffeine flux-enhancing effects. It was found that the best regimen was a concurrent 5 min, pulsed (10% duty cycle) beam of SATA intensity 0.37 W/cm². A key insight was that in the case of pulsed beams of 10% duty cycle, sonication concurrent with drug deposition was superior to sonication prior to drug deposition and potential mechanisms for this are discussed.

Application of the aqueous porous pathway model to quantify the effect of sodium lauryl sulfate on ultrasound-induced skin structural perturbation

This study investigated the effect of sodium lauryl sulfate (SLS) on skin structural perturbation when utilized simultaneously with low-frequency sonophoresis (LFS). Pig full-thickness skin (FTS) and pig split-thickness skin (STS) treated with LFS/SLS and LFS were analyzed in the context of the aqueous porous pathway model to quantify skin perturbation through changes in skin pore radius and porosity-to-tortuosity ratio (ε/τ). In addition, skin treatment times required to attain specific levels of skin electrical resistivity were analyzed to draw conclusions about the effect of SLS on reproducibility and predictability of skin perturbation. We found that LFS/SLS-treated FTS, LFS/SLS-treated STS, and LFS-treated FTS exhibited similar skin perturbation. However, LFS-treated STS exhibited significantly higher skin perturbation, suggesting greater structural changes to the less robust STS induced by the purely physical enhancement mechanism of LFS. Evaluation of ε/τ values revealed that LFS/SLS-treated FTS and STS have similar transport pathways, whereas LFS-treated FTS and STS have lower ε/τ values. In addition, LFS/SLS treatment times were much shorter than LFS treatment times for both FTS and STS. Moreover, the simultaneous use of SLS and LFS not only results in synergistic enhancement, as reflected in the shorter skin treatment times, but also in more predictable and reproducible skin perturbation.

The importance of microjet vs shock wave formation in sonophoresis

Low-frequency ultrasound application has been shown to greatly enhance transdermal drug delivery. Skin exposed to ultrasound is affected in a heterogeneous manner, thus mass transport through the stratum corneum occurs mainly through highly permeable localized transport regions (LTRs). Shock waves and microjets generated during inertial cavitations are responsible for the transdermal permeability enhancement. In this study, we evaluated the effect of these two phenomena using direct and indirect methods, and demonstrated that the contribution of microjets to skin permeability enhancement is significantly higher than shock waves.

Effect of gel composition and phonophoresis on the transdermal delivery of ibuprofen: in vitro and in vivo evaluation

The objective of this study was to evaluate the effect of formulation composition and continuous ultrasound at 1.5 W/Cm² intensity on the permeation of ibuprofen across polymeric and biological barriers. In vitro studies were performed using cellulose membrane and rabbit skin, whereas in vivo studies were performed on rabbits. Overall, higher transport data were obtained with the cellulose membrane. Nonetheless, with both barriers, transport was higher with the gel formulation than oleaginous or emulsion based preparations, which was also dependent on the viscosity of the gel and the concentration of the formulation additives. Transport increased with the increase in alcohol concentration and decreased with the addition of propylene glycol. Ultrasound had a significant effect on ibuprofen transport. For example, an 11-fold increase in drug permeation across cellulose membrane was observed after the application of ultrasound. Similar effect was observed in animal studies. Ibuprofen plasma concentration increased with the concentration of the drug in the gel, which significantly increased after the application of ultrasound. It was concluded that aqueous gel formulations act as ideal coupling agents for topical application of drugs. Nonetheless, in addition to ultrasound, the composition of the gel is expected to have a significant effect on the transdermal delivery of drugs.

Feasibility of sonophoretic delivery for effective skin optical clearing

In an attempt to find a noninvasive way to improve the efficacy of skin optical clearing with topical application of optical clearing agents (OCA), we exploited the effect of ultrasound on optical clearing of intact skin in vitro Changes in optical transmittance and diffuse reflectance of the porcine skin under different treatment procedures were measured by a UV/Vis/NIR spectrophotometer. Clearing capability of glycerol or propylene glycol was much improved with the simultaneous application of ultrasound with a frequency of 1 MHz and a power of 0.75 W over a 3-cm probe. Light transmittance at 470 and 1276 nm increased by 112%-128% and 54%-65%, respectively. The most significant effect was seen where skin was treated with the combination of ultrasound and the chemical enhancer azone, resulting in a 2.3-fold increase of optical clearing at 1276 nm. We demonstrate the feasibility of using sonophoretic delivery methods to improve skin optical clearing with topical application of OCA.

Synergetic effect of ultrasound and sodium dodecyl sulphate in the formation of CdS nanostructures in aqueous solution

High aspect ratio (>1000) CdS nanostructures were prepared via ultrasound treatment of parent nanowires (NWs) dispersed in sodium dodecyl sulfate (SDS) aqueous solution. The CdS parent NWs were prepared using ordered mesoporous silica, SBA-15, as a template. The elongated nanostructures (ENS), namely, NWs, nanoribbons and nanotubes, form stable dispersions in aqueous solutions. Electron microscopy and X-ray diffraction techniques were used to characterize both the parent NWs and the ENS. While the structure of the parent NWs is crystalline cubic, the ENS are amorphous. We show that the amorphous ENS bud from the parent bundled NWs. Ultrasound power and duration, presence of commensurate surfactant and calcination temperature of the templating SBA-15 are critical parameters in the formation of ENS in aqueous solution.

Investigation into the potential of low-frequency ultrasound facilitated topical delivery of Cyclosporin A

The potential for low-frequency ultrasound facilitated topical transport of Cyclosporin A was investigated using rat skin. Studies of intensity and exposure time acting on the deposition of Cyclosporin A into deeper skin of in vitro sonophoresis were performed. Low-frequency ultrasound increased the amount of Cyclosporin A retained in the skin only seven times than the passive diffusion. Furthermore, we also tested the synergistic effect of ultrasound and other approaches such as chemical enhancers and electroporation on topical drug delivery of Cyclosporin A. We found that the efficacy of low-frequency ultrasound in enhancing topical delivery could be further increased by pretreatment of skin with chemical enhancers, such as laurocapram (Azone) and sodium lauryl sulfate (SLS). Meanwhile only a small amount was seen to across the full skin into the receiver compartment. Trimodality treatment comprising of pretreatment with Azone+ultrasound in combination followed by electroporation was not effective in enhancing the topical delivery of Cyclosporin A. However, this combination strategy increased the penetration of Cyclosporin A through rat skin by order of 15. The histopathological findings revealed that there was almost no change observed in the structure of skin after ultrasound or combination with ultrasound and enhancers as compared with the control group. In general, the enhanced skin accumulation of Cyclosporin A by the combination of low-frequency ultrasound and chemical enhancers could help significantly to optimize the targeting of the drug without of a concomitant increase of the systemic side effects.

Bubble growth within the skin by rectified diffusion might play a significant role in sonophoresis

Low frequency ultrasound has successfully been used for enhancing transdermal transport of a variety of different molecules. This phenomenon is referred to as sonophoresis. Several attempts have been made to investigate the enhancing mechanism in order to modulate the overall process. In this study we assess whether rectified diffusion is a process that occurs within the skin, which could eventually lead to channeling and thereby to transdermal sonophoresis. The model presented in this paper is based on the following postulate: gas bubbles are randomly distributed within the lipid bilayers of the stratum corneum (SC). As the skin is subjected to ultrasound, gas bubbles grow by rectified diffusion. During this period, bubbles may merge with the outer or inner boundaries of the SC, or merge with neighboring bubbles. Eventually, channels are created, allowing drugs to easily penetrate through the most significant barrier to transdermal delivery, the SC. As a result, transdermal transport rate is enhanced. In this work, a mathematical model has been formulated, in which permeability enhancement of the SC is linked to channels, possibly created by means of rectified diffusion. Sonophoresis may result from various mechanisms that act in synergy. The present model predicts that rectified diffusion might be one of the factors that lead to sonophoresis during ultrasound treatment.

Insights into synergistic interactions in binary mixtures of chemical permeation enhancers for transdermal drug delivery

Chemical permeation enhancers (CPEs) are known to increase skin permeability to therapeutic drugs. Single chemicals, however, offer limited enhancements of skin permeability. Mixtures of chemicals can overcome this limitation owing to their synergistic interactions. However, identification of potent mixtures of chemicals requires screening of a large number of formulations. Discovery of CPE mixtures can be significantly accelerated by identifying patterns that occur in the existing data on CPEs. In this study, we systematically mine through a huge database on skin permeabilizing effect of over 4000 binary formulations generated by high throughput screening and extract general principles that govern the effect of binary combinations of chemicals on skin's barrier properties. Potencies and synergies of these formulations are analyzed to identify the role played by the formulation composition and chemistry. The analysis reveals several intuitive but some largely non-intuitive trends. For example, formulations made from enhancer mixtures are most potent when participating moieties are present in nearly equal fractions. Methyl pyrrolidone, a small molecule, is particularly effective in forming potent and synergistic enhancer formulations, and zwitterionic surfactants are more likely to feature in potent enhancers. Simple but invaluable rules like these will provide guiding principles for designing libraries to further speed up the formulation discovery process.