Transdermal Drug Delivery Using Low-Frequency Sonophoresis

Purpose. Application of therapeutic ultrasound (frequency: 1–3 MHz and intensity: 0–2 W/cm²) enhances transdermal drug transport, although typically by a factor of less than 10. In this paper, we show that application of ultrasound at 20 KHz induces transdermal transport enhancements of up to 1000 times higher than those induced by therapeutic ultrasound. Methods. In vitro (human cadaver epidermis) as well as in vivo (hairless rat skin) permeation experiments were performed to assess the effect of low-frequency ultrasound on transdermal transport. Results. Application of low-frequency ultrasound (20 KHz, 125 mW/cm², 100 msec pulses applied every second) enhanced transdermal transport of several permeants, including estradiol, salicylic acid, corticosterone, sucrose, aldosterone, water, and butanol, across human cadaver skin by a factor in the range of 3 to 3000 and that of salicylic acid across hairless rat skin in vivo by a factor of up to 300. Low-frequency ultrasound did not induce a long-term loss of the barrier properties of the skin (in vitro) or damage to living skin of hairless rats. At a mechanistic level, it is hypothesized that application of low-frequency ultrasound enhances transdermal transport through aqueous channels in the SC generated by cavitation-induced bilayer disordering. Support for this hypothesis is provided using experimental and theoretical analyses of low-frequency sonophoresis. Conclusions. Low-frequency ultrasound enhances transdermal transport of drugs more effectively than that induced by therapeutic ultrasound.     

Enhancing the transdermal delivery of rigid nanoparticles using the simultaneous application of ultrasound and sodium lauryl sulfate

The potential of rigid nanoparticles to serve as transdermal drug carriers can be greatly enhanced by improving their skin penetration. Therefore, the simultaneous application of ultrasound and sodium lauryl sulfate (referred to as US/SLS) was evaluated as a skin pre-treatment method for enhancing the passive transdermal delivery of nanoparticles. We utilized inductively coupled plasma mass spectrometry and an improved application of confocal microscopy to compare the delivery of 10- and 20-nm cationic, neutral, and anionic quantum dots (QDs) into US/SLS-treated and untreated pig split-thickness skin. Our findings include: (a) ～0.01% of the QDs penetrate the dermis of untreated skin (which we quantify for the first time), (b) the QDs fully permeate US/SLS-treated skin, (c) the two cationic QDs studied exhibit different extents of skin penetration and dermal clearance, and (d) the QD skin penetration is heterogeneous. We discuss routes of nanoparticle skin penetration and the application of the methods described herein to address conflicting literature reports on nanoparticle skin penetration. We conclude that US/SLS treatment significantly enhances QD transdermal penetration by 500-1300%. Our findings suggest that an optimum surface charge exists for nanoparticle skin penetration, and motivate the application of nanoparticle carriers to US/SLS-treated skin for enhanced transdermal drug delivery.     

Dual-channel two-photon microscopy study of transdermal transport in skin treated with low-frequency ultrasound and a chemical enhancer

Visualization of transdermal permeant pathways is necessary to substantiate model-based conclusions drawn using permeability data. The aim of this investigation was to visualize the transdermal delivery of sulforhodamine B (SRB), a fluorescent hydrophilic permeant, and of rhodamine B hexyl ester (RBHE), a fluorescent hydrophobic permeant, using dual-channel two-photon microscopy (TPM) to better understand the transport pathways and the mechanisms of enhancement in skin treated with low-frequency ultrasound (US) and/or a chemical enhancer (sodium lauryl sulfate--SLS) relative to untreated skin (the control). The results demonstrate that (1) both SRB and RBHE penetrate beyond the stratum corneum and into the viable epidermis only in discrete regions (localized transport regions--LTRs) of US treated and of US/SLS-treated skin, (2) a chemical enhancer is required in the coupling medium during US treatment to obtain two significant levels of increased penetration of SRB and RBHE in US-treated skin relative to untreated skin, and (3) transcellular pathways are present in the LTRs of US treated and of US/SLS-treated skin for SRB and RBHE, and in SLS-treated skin for SRB. In summary, the skin is greatly perturbed in the LTRs of US treated and US/SLS-treated skin with chemical enhancers playing a significant role in US-mediated transdermal drug delivery.     

Evaluation of hydrophilic permeant transport parameters in the localized and non-localized transport regions of skin treated simultaneously with low-frequency ultrasound and sodium lauryl sulfate

The porosity (epsilon), the tortuosity (tau), and the hindrance factor (H) of the aqueous pore channels located in the localized transport regions (LTRs) and the non-LTRs formed in skin treated simultaneously with low-frequency ultrasound (US) and the surfactant sodium lauryl sulfate (SLS), were evaluated for the delivery of four hydrophilic permeants (urea, mannitol, raffinose, and inulin) by analyzing dual-radiolabeled diffusion masking experiments for three different idealized cases of the aqueous pore pathway hypothesis. When epsilon and tau were assumed to be independent of the permeant radius, H was found to be statistically larger in the LTRs than in the non-LTRs. When a distribution of pore radii was assumed to exist in the skin, no statistical differences in epsilon, tau, and H were observed due to the large variation in the pore radii distribution shape parameter (3 A to infinity). When infinitely large aqueous pores were assumed to exist in the skin, epsilon was found to be 3-8-fold greater in the LTRs than in the non-LTRs, while little difference was observed in the LTRs and in the non-LTRs for tau. This last result suggests that the efficacy of US/SLS treatment may be enhanced by increasing the porosity of the non-LTRs.     

Transport Pathways and Enhancement Mechanisms Within Localized and Non-Localized Transport Regions in Skin Treated with Low-Frequency Sonophoresis and Sodium Lauryl Sulfate

Recent advances in transdermal drug delivery utilizing low-frequency sonophoresis (LFS) and sodium lauryl sulfate (SLS) have revealed that skin permeability enhancement is not homogenous across the skin surface. Instead, highly perturbed skin regions, known as localized transport regions (LTRs), exist. Despite these findings, little research has been conducted to identify intrinsic properties and formation mechanisms of LTRs and the surrounding less-perturbed non-LTRs. By independently analyzing LTR, non-LTR, and total skin samples treated at multiple LFS frequencies, we found that the pore radii (r_pore) within non-LTRs are frequency-independent, ranging from 18.2 to 18.5 Å, but significantly larger than r_pore of native skin samples (13.6 Å). Conversely, r_pore within LTRs increase significantly with decreasing frequency from 161 to 276 Å and to ∞ (> 300 Å) for LFS/SLS-treated skin at 60, 40, and 20 kHz, respectively. Our findings suggest that different mechanisms contribute to skin permeability enhancement within each skin region. We propose that the enhancement mechanism within LTRs is the frequency-dependent process of cavitation-induced microjet collapse at the skin surface, whereas the increased r_pore values in non-LTRs are likely due to SLS perturbation, with enhanced penetration of SLS into the skin resulting from the frequency-independent process of microstreaming.     

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 (≥ 0.7 MHz) for sonophoresis (HFS) dates back to as early as the 1950s, while low-frequency sonophoresis (LFS, 20-100 kHz) 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.     

Fluorescent penetration enhancers for transdermal applications

Chemical penetration enhancers are often used to enhance transdermal drug delivery. However, the fundamental mechanisms that govern the interactions between penetration enhancers and skin are not fully understood. Therefore, the goal of this work was to identify naturally fluorescent penetration enhancers (FPEs) in order to utilize well-established fluorescence techniques to directly study the behavior of FPEs within skin. In this study, 12 fluorescent molecules with amphiphilic characteristics were evaluated as skin penetration enhancers. Eight of the molecules exhibited significant activity as skin penetration enhancers, determined using skin current enhancement ratios. In addition, to illustrate the novel, direct, and non-invasive visualization of the behavior of FPEs within skin, three case studies involving the use of two-photon fluorescence microscopy (TPM) are presented, including visualizing glycerol-mitigated and ultrasound-enhanced FPE skin penetration. Previous TPM studies have indirectly visualized the effect of penetration enhancers on the skin by using a fluorescent dye to probe the transdermal pathways of the enhancer. These effects can now be directly visualized and investigated using FPEs. Finally, future studies are proposed for generating FPE design principles. The combination of FPEs with fluorescence techniques represents a useful novel approach for obtaining physical insights on the behavior of penetration enhancers within the skin.     

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.