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

In Silico Prediction of Skin Permeability Using a Two-QSAR Approach

Topical and transdermal drug delivery is an effective, safe, and preferred route of drug administration. As such, skin permeability is one of the critical parameters that should be taken into consideration in the process of drug discovery and development. The ex vivo human skin model is considered as the best surrogate to evaluate in vivo skin permeability. This investigation adopted a novel two-QSAR scheme by collectively incorporating machine learning-based hierarchical support vector regression (HSVR) and classical partial least square (PLS) to predict the skin permeability coefficient and to uncover the intrinsic permeation mechanism, respectively, based on ex vivo excised human skin permeability data compiled from the literature. The derived HSVR model functioned better than PLS as represented by the predictive performance in the training set, test set, and outlier set in addition to various statistical estimations. HSVR also delivered consistent performance upon the application of a mock test, which purposely mimicked the real challenges. PLS, contrarily, uncovered the interpretable relevance between selected descriptors and skin permeability. Thus, the synergy between interpretable PLS and predictive HSVR models can be of great use for facilitating drug discovery and development by predicting skin permeability.

Synergistic Effect of Chemical Penetration Enhancers on Lidocaine Permeability Revealed by Coarse-Grained Molecular Dynamics Simulations

The search for new formulations for transdermal drug delivery (TDD) is an important field in medicine and cosmetology. Molecules with specific physicochemical properties which can increase the permeability of active ingredients across the stratum corneum (SC) are called chemical penetration enhancers (CPEs), and it was shown that some CPEs can act synergistically. In this study, we performed coarse-grained (CG) molecular dynamics (MD) simulations of the lidocaine delivery facilitated by two CPEs-linoleic acid (LA) and ethanol-through the SC model membrane containing cholesterol, N-Stearoylsphingosine (DCPE), and behenic acid. In our simulations, we probed the effects of individual CPEs as well as their combination on various properties of the SC membrane and the lidocaine penetration across it. We demonstrated that the addition of both CPEs decreases the membrane thickness and the order parameters of the DPCE hydrocarbon chains. Moreover, LA also enhances diffusion of the SC membrane components, especially cholesterol. The estimated potential of mean force (PMF) profiles for the lidocaine translocation across SC in the presence/absence of two individual CPEs and their combination demonstrated that while ethanol lowers the free energy barrier for lidocaine to enter SC, LA decreases the depth of the free energy minima for lidocaine inside SC. These two effects supposedly result in synergistic penetration enhancement of drugs. Altogether, the present simulations provide a detailed molecular picture of CPEs' action and their synergistic effect on the penetration of small molecular weight therapeutics that can be beneficial for the design of novel drug and cosmetics formulations.

Non-invasive drug delivery technology: development and current status of transdermal drug delivery devices, techniques and biomedical applications

Pay-load deliveries across the skin barrier to the systemic circulation have been one of the most challenging delivery options. Necessitated requirements of the skin and facilitated skin layer cross-over delivery attempts have resulted in development of different non-invasive, non-oral methods, devices and systems which have been standardized, concurrently used and are in continuous upgrade and improvements. Iontophoresis, electroporation, sonophoresis, magnetophoresis, dermal patches, nanocarriers, needled and needle-less shots, and injectors are among some of the methods of transdermal delivery. The current review covers the current state of the art, merits and shortcomings of the systems, devices and transdermal delivery patches, including drugs' and other payloads' passage facilitation techniques, permeation and absorption feasibility studies, as well as physicochemical properties affecting the delivery through different transdermal modes along with examples of drugs, vaccines, genes and other payloads.

Transdermal Delivery of the Free Base of 3-Fluoroamphetamine: In Vitro Skin Permeation and Irritation Potential

This work aimed to continue our effort in establishing the feasibility of 3-fluoroamphetamine (also known as PAL-353) to be a transdermal drug candidate by studying the delivery of the base form through the human cadaver skin in lieu of the previously investigated salt form, and for the first time using an EPIDERM™-reconstructed human epidermal model to predict the skin irritation potential of PAL-353, in support of development for a matrix-type transdermal delivery system. Passive and enhanced (with chemical permeation enhancers) transdermal delivery were investigated via in vitro permeation studies that were performed on Franz diffusion cells with dermatomed human cadaver skin. After 24 h, PAL-353 free base revealed high passive permeation of 417.49 ± 30.12, 1577.68 ± 165.41, and 4295.16 ± 264.36 μg/cm², with applied formulation concentrations of 5.5 (F1), 20 (F2), and 40 (F3) mg/mL, respectively. Oleyl alcohol produced an approximately threefold steady-state flux enhancement at 5% or 10% w/w but may not be needed as the free base alone provided therapeutically relevant permeation. Further, it was predicted that therapeutically relevant delivery would be unlikely to cause skin irritation using the EPIDERM™-reconstructed human epidermal model. In conclusion, the present study further supported the development of PAL-353 transdermal delivery systems.

Passive delivery of protein drugs through transdermal route

Skin is the largest external organ in the human body but its use for therapeutic purposes has been minimal. Stratum corneum residing on the uppermost layer of the skin provides a tough barrier to transport the drugs across the skin. Very small group of drugs sharing Lipinski properties, i.e. drugs having molecular weight not larger than 500 Da, having high lipophilicity and optimum polarity are fortunate enough to be used on skin therapeutics. But, at a time where modern therapeutics is slowly shifting from use of small molecular drugs towards the use of macromolecular therapeutic agents such as peptides, proteins and nucleotides in origin, skin therapeutics need to be evolved accordingly to cater the delivery of these agents. Physical technologies like iontophoresis, laser ablation, micro-needles and ultrasound, etc. have been introduced to enhance skin permeability. But their success is limited due to their complex working mechanisms and involvement of certain irreversible skin damage in some or other way. This review therefore explores the delivery strategies for transport of mainly peptide and protein drugs that do not involve any injuries (non-invasive) to the skin termed as passive delivery techniques. Chemical enhancers, nanocarriers, certain biological peptides and miscellaneous approaches like prodrugs are also thoroughly reviewed for their applications in protein delivery.

Transdermal Delivery of Ibuprofen Utilizing a Novel Solvent-Free Pressure-sensitive Adhesive (PSA): TEPI® Technology

PURPOSE

The main objective of this present study was the investigation of potential novel transdermal patch technology (TEPI®) delivering ibuprofen as the active pharmaceutical ingredient (API) using a novel poly(ether-urethane)-silicone crosslinked pressure-sensitive adhesive (PSA) as the drug reservoir in a solvent-free manufacturing process.

METHODS

The patch was synthesized utilizing the hot-melt crosslinking technique without the addition of solvents at 80 °C in 100% relative humidity. Dissolution and permeation studies performed utilizing diffusion cells and subsequently HPLC validated methods were employed to determine the API content in the acceptor solution. Accelerated stability studies were also performed at 40 °C and 70% relative humidity. The adhesive performance of the fabricated patch was evaluated utilizing loop tack adhesion tests.

RESULTS

In vitro permeation experiments across both Strat-M® and human skin demonstrated that ibuprofen can easily be released from the adhesive matrix and penetrate through the studied membrane. A comparison on the permeation rates of the API across the two membranes indicated that there is not a strong correlation between the obtained data. The presence of chemical enhancers facilitated an increased flux of the API higher than observed in the basic formulation. Initial stability studies of the optimized formulation showed no degradation with respect to the drug content. Adhesion studies were also performed indicating higher values when compared with commercially available products.

CONCLUSIONS

The present study demonstrated the fabrication of an ibuprofen patch utilizing a versatile, solvent-free drug delivery platform. Upon optimization of the final system, the resulting patch offers many advantages compared to commercially available formulations including high drug loading (up to 25 wt%), good adhesion, and painless removal leaving no residues on the skin. This PSA offers many advantages over existing adhesive technology. Graphical Abstractᅟ.

Influence of peptide dendrimers and sonophoresis on the transdermal delivery of ketoprofen

The aim of this study was to determine the individual and combined effects of peptide dendrimers and low frequency ultrasound on the transdermal permeation of ketoprofen. Arginine terminated peptide dendrimers of varying charges (4⁺, 8⁺ and 16⁺, named as A4. A8 and A16 respectively) were synthesized and characterized. Ketoprofen was subjected to passive, peptide dendrimer-assisted and sonophoretic permeation studies (with and without dendrimer application) across Swiss albino mouse skin, both in vitro and in vivo. The studies revealed that the synthesized peptide dendrimers considerably increased the transdermal permeation of ketoprofen and displayed enhancement ratios of up to 3.25 (with A16 dendrimer), compared to passive diffusion of drug alone in vitro. Moreover, the combination of peptide dendrimer treatment and ultrasound application worked in synergy and gave enhancement ratios of up to 1369.15 (with ketoprofen-A16 dendrimer complex). In vivo studies demonstrated that dendrimer and ultrasound-assisted permeation of drug achieved much higher plasma concentration of drug, compared to passive diffusion. Comparison of transdermal and oral absorption studies revealed that transdermal administration of ketoprofen with A8 dendrimer showed comparable absorption and plasma drug levels with oral route. The excised mouse skin after in vivo permeation study with dendrimers and ultrasound did not show major toxic reactions. This study demonstrates that arginine terminated peptide dendrimers combined with sonophoresis can effectively improve the transdermal permeation of ketoprofen.

Transdermal and Skin-Targeted Drug Delivery

Background:

The application of therapeutic agents to the skin addresses three general objectives: (a) the treatment of a variety of dermatologic diseases; (b) the “targeted” delivery of drugs to deeper subcutaneous tissues, with a concomitant reduction in systemic exposure; and (c) socalled transdermal administration to elicit a systemic pharmacologic effect.

Objective:

Recently, significant progress towards all three goals has been recorded and the level of research and development activity remains high. We aim to discuss these advances from mechanistic and clinical standpoints.

Results:

For the topical treatment of skin disease, novel vehicles (e.g., stabilized, supersaturated systems and liposomal formulations) have led to dramatic improvements in local drug bioavailability. Transdermal delivery of drugs for systemic effect, though limited in terms of the number of compounds, is perhaps the most commercially successful (in terms of the number of products) of the controlled release technologies. Considerable activity continues to enhance drug delivery (and hence to extend the range of drugs for which transdermal delivery can be used). Existing patches use formulations that contain solvents and adjuvants capable of reducing the barrier function of the skin. Much effort is directed at iontophoresis (electrically enhanced transport), particularly for small peptides that are difficult to administer by other routes. “Reverse iontophoresis” may allow the extraction of glucose (without skin puncture) so that continuous, noninvasive monitoring of blood sugar in diabetics approaches realization.

Conclusion:

In the not too distant future, the skin may also play a role not only in drug delivery, but also with respect to measurements in clinical chemistry.

Effect of low-intensity pulsed ultrasound on biocompatibility and cellular uptake of chitosan-tripolyphosphate nanoparticles

Using low molecular weight chitosan nanoparticles (CNPs) prepared by an ionic gelation method, the authors report the effect of low-intensity pulsed ultrasound (US) on cell viability and nanoparticle uptake in cultured murine preosteoblasts. Particle size and zeta potential are measured using dynamic light scattering, and cell viability is evaluated using the of [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt; MTS] assay. Results show that 30 min delivery of CNPs at 0.5 mg/mL is able to prevent loss of cell viability due to either serum starvation or subsequent exposure to US (1 W/cm(2) or 2 W/cm(2), up to 1 min). Additionally, flow cytometry data suggest that there is a close association between cellular membrane integrity and the presence of CNPs when US at 2 W/cm(2) is administered.

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.

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.

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.

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.

Synergistic effect of ultrasound and thiazone-PEG 400 on human skin optical clearing in vivo

In this paper, we propose a new physical method in combination with mixed solution of thiazone and polyethylene glycol 400 (thiazone PEG 400 solution) penetration into tissue to assess the skin optical clearing. Four treatments were performed: (1) control group (C); (2) polyethylene glycol 400 (PEG400); (3) 0.25% thiazone (0.25%T); (4) 0.25% thiazone and 5-min ultrasound (0.25%T/SP). The diffuse reflectance spectra and imaging depth of human skin in vivo at different times were measured by spectroscopy and optical coherence tomography (OCT). The optical clearing efficacy of skin was qualitatively and quantitatively analyzed. The results showed that the diffuse reflectance at 540 nm of samples at 10 min after being treated by 0.25%T/SP decreased by approximately 15.51%, whereas, 0.46%, 4.73% and 5.75% were received in C, PEG400 and 0.25%T, respectively. And at 60 min, the decrease in diffuse reflectance of samples in 0.25%T/SP is about 2.22-fold, 1.20-fold compared with that of the samples in PEG 400 and 0.25%T, at 540 nm, respectively. Simultaneously, 0.25%T/SP results in 41.33% increase in OCT 1/e light penetration depth after 60 min. There was a significant difference in the optical clearing effect on skin between ultrasound-mixed solution of thiazone in combination with PEG 400 and the mixed solution (P < 0.05).

Penetration pathways induced by low-frequency sonophoresis with physical and chemical enhancers: iron oxide nanoparticles versus lanthanum nitrates

Low-frequency sonophoresis (LFS) has been shown to disrupt the structure of stratum corneum (SC) lipid bilayers and enhance SC permeability. In this study, we examined the penetration pathway of lanthanum nitrate (LaNO(3)) tracer in viable epidermis after combined treatment of LFS and tape stripping (TS), as a physical enhancer, or oleic acid (OA) application, as a chemical enhancer, using transmission electron microscopy (TEM). As a positive control, we visualized the passive diffusion pathway of LaNO(3) and iron oxide (Fe(3)O(4)) nanoparticles after the incision of hairless mouse skin. Next, we applied LFS immediately after TS or OA application and visualized the penetration pathway of LaNO(3). Each treatment showed restricted penetration to the SC-stratum granulosum (SG) interface or upper SG layer. However, the additional application of LFS induced diffuse intracellular distribution of LaNO(3) throughout the viable epidermis. Quantitative analysis also revealed that combined treatment significantly increases LaNO(3) penetration into viable epidermis when compared with each treatment. Our ultrastructural findings show the synergistic effect of LFS and TS or OA application on transdermal drug delivery. We also found that this combined treatment enhances the penetration of LaNO(3) through the viable epidermis through an intracellular pathway.

Efficacy of a metered-dose 8% lidocaine pump spray for patients with post-herpetic neuralgia

OBJECTIVE

Topical lidocaine patch is effective in the treatment of post-herpetic neuralgia (PHN), but not suited for paroxysmal pain because of the long latency of analgesia. Here, we examined the efficacy of 8% lidocaine pump spray (Xylocaine pump spray, XPS) for PHN.

DESIGN

Twenty-four patients with PHN were recruited into a randomized, double-blind, placebo-controlled, crossover study (study 1), and 100 patients with PHN were recruited into an open-labeled study (study 2). In study 1, patients were randomized to receive either XPS or saline placebo pump spray (PPS) applied to the painful skin areas. Following a 7-day period, patients were crossed over to receive the alternative treatment. In study 2, XPS was prescribed for patients who were advised to use the spray anytime, with a 2-hour gap between applications, for 2 weeks. The pain was assessed with a visual analogue scale (VAS). Details of use were noted in the diary.

RESULTS

In study 1, greater decreases in VAS of persistent pain followed application of XPS (baseline: 6.1 +/- 1.7 cm, 15-minute post-spray: 2.3 +/- 2.5 cm, mean +/- SD) than with PPS (6.1 +/- 1.7 cm, 5.7 +/- 1.6 cm, [P < 0.01]). The effect persisted for a median of 4.5 hours (range, 2 to 24 hours) after application. In study 2, 13 of 100 patients discontinued the treatment because of mild local side effects or insufficient effect. In the remaining 87 patients, XPS maintained significant pain relief relative to baseline throughout the 2-week period. Satisfaction with the therapy was reported by 79% of patients.

CONCLUSIONS

In both studies, XPS provided a significant improvement in PHN due to its prompt analgesia, lack of systemic side effects, and convenience of use.

Sonophoresis: recent advancements and future trends

OBJECTIVES

Use of ultrasound in therapeutics and drug delivery has gained importance in recent years, evident by the increase in patents filed and new commercial devices launched. The present review discusses new advancements in sonophoretic drug delivery in the last two decades, and highlights important challenges still to be met to make this technology of more use in the alleviation of diseases.

KEY FINDINGS

Phonophoretic research often suffers from poor calibration in terms of the amount of ultrasound energy emitted, and therefore current research must focus on safety of exposure to ultrasound and miniaturization of devices in order to make this technology a commercial reality. More research is needed to identify the role of various parameters influencing sonophoresis so that the process can be optimized. Establishment of long-term safety issues, broadening the range of drugs that can be delivered through this system, and reduction in the cost of delivery are issues still to be addressed.

SUMMARY

Sonophoresis (phonophoresis) has been shown to increase skin permeability to various low and high molecular weight drugs, including insulin and heparin. However, its therapeutic value is still being evaluated. Some obstacles in transdermal sonophoresis can be overcome by combination with other physical and chemical enhancement techniques. This review describes recent advancements in equipment and devices for phonophoresis, new formulations tried in sonophoresis, synergistic effects with techniques such as chemical enhancers, iontophoresis and electroporation, as well as the growing use of ultrasound in areas such as cancer therapy, cardiovascular disorders, temporary modification of the blood-brain barrier for delivery of imaging and therapeutic agents, hormone replacement therapy, sports medicine, gene therapy and nanotechnology. This review also lists patents pertaining to the formulations and techniques used in sonophoretic drug delivery.

Ultrasonic-activated micellar drug delivery for cancer treatment

The use of nanoparticles and ultrasound in medicine continues to evolve. Great strides have been made in the areas of producing micelles, nanoemulsions, and solid nanoparticles that can be used in drug delivery. An effective nanocarrier allows for the delivery of a high concentration of potent medications to targeted tissue while minimizing the side effect of the agent to the rest of the body. Polymeric micelles have been shown to encapsulate therapeutic agents and maintain their structural integrity at lower concentrations. Ultrasound is currently being used in drug delivery as well as diagnostics, and has many advantages that elevate its importance in drug delivery. The technique is noninvasive, thus no surgery is needed; the ultrasonic waves can be easily controlled by advanced electronic technology so that they can be focused on the desired target volume. Additionally, the physics of ultrasound are widely used and well understood; thus ultrasonic application can be tailored towards a particular drug delivery system. In this article, we review the recent progress made in research that utilizes both polymeric micelles and ultrasonic power in drug delivery.

The analgesic effect of a metered-dose 8% lidocaine pump spray in posttraumatic peripheral neuropathy: a pilot study

BACKGROUND

A topical lidocaine patch is effective in the treatment of posttraumatic peripheral neuropathy (PTPN), but it is not suited for breakthrough pain because of difficulty with an additional application. Here, we examined the effect of 8% lidocaine pump spray (Xylocaine pump spray, XPS) on peripheral neuropathic pain caused by surgery or injury.

METHODS

Thirty-one patients with PTPN were randomized to receive either XPS or saline placebo pump spray applied to painful skin areas. The optimal dose of up to 30 sprays (0.1 mL/single spray, 30 times) was individually determined as the dose which completely covered the painful site. After a 7-day period, the patients were crossed over to receive the optimal dose of the alternative spray. Pain was assessed with a visual analog scale.

RESULTS

XPS, but not placebo pump spray, significantly decreased the visual analog scale for continuing pain and tactile allodynia. The effect persisted for a median of 5 h (range, 2-60 h) after application. Mild side effects were reported in three patients with XPS consisting of local irritation (n = 3) and local flare (n = 1). All adverse events disappeared without medication within a few hours.

CONCLUSIONS

The present study suggests that XPS provides a significant improvement in PTPN due to its prompt analgesia, lack of systemic side effects and convenience.

Applications of ultrasonic skin permeation in transdermal drug delivery

Transdermal ultrasound-mediated drug delivery has been studied as a method for needle-less, non-invasive drug administration. Potential obstacles include the stratum corneum, which is not sufficiently passively permeable to allow effective transfer of many medications into the bloodstream without active methods. A general review of the transdermal ultrasound drug delivery literature has shown that this technology offers promising potential for non-invasive drug administration. Included in this review are the reported acoustic parameters used for achieving delivery, along with the known intensities and exposure times. Ultrasound mechanisms are discussed as well as spatial field characteristics. Accurate and precise quantification of the acoustic field used in drug delivery experiments is essential to ensure safety versus efficacy and to avoid potentially harmful bioeffects.

Dose comparison of ultrasonic transdermal insulin delivery to subcutaneous insulin injection

Prior studies have demonstrated the effectiveness of noninvasive transdermal insulin delivery using a cymbal transducer array. In this study the physiologic response to ultrasound mediated transdermal insulin delivery is compared to that of subcutaneously administered insulin. Anesthetized rats (350–550 g) were divided into four groups of four animals; one group representing ultrasound mediated insulin delivery and three representing subcutaneously administered insulin (0.15, 0.20, and 0.25 U/kg). The cymbal array was operated for 60 minutes at 20 kHz with 100 mW/cm² spatial-peak temporal-peak intensity and a 20% duty cycle. The blood glucose level was determined at the beginning of the experiment and, following insulin administration, every 15 minutes for 90 minutes for both the ultrasound and injection groups. The change in blood glucose from baseline was compared between groups. When administered by subcutaneous injection at insulin doses of 0.15 and 0.20 U/kg, there was little change in the blood glucose levels over the 90 minute experiment. Following subcutaneous administration of insulin at a dose of 0.25 U/kg, blood glucose decreased by 190 ± 96 mg/dl (mean ± SD) at 90 minutes. The change in blood glucose following ultrasound mediated insulin delivery was −262 ± 40 mg/dl at 90 minutes. As expected, the magnitude of change in blood glucose between the three injection groups was dependant on the dose of insulin administered. The change in blood glucose in the ultrasound group was greater than that observed in the injection groups suggesting that a higher effective dose of insulin was delivered.

An evaluation of mathematical models for predicting skin permeability

A number of mathematical models have been proposed for predicting skin permeability, mostly empirical and very few are deterministic. Early empirical models use simple lipophilicity parameters. The recent trend is to use more complicated molecular structure descriptors. There has been much debate on which models best predict skin permeability. This article evaluates various mathematical models using a comprehensive experimental dataset of skin permeability for 124 chemical compounds compiled from various sources. Of the seven models compared, the deterministic model of Mitragotri gives the best prediction. The simple quantitative structure permeability relationships (QSPR) model of Potts and Guy gives the second best prediction. The two models have many features in common. Both assume the lipid matrix as the pathway of transdermal permeation. Both use octanol-water partition coefficient and molecular size. Even the mathematical formulae are similar. All other empirical QSPR models that use more complicated molecular structure descriptors fail to provide satisfactory prediction. The molecular structure descriptors in the more complicated QSPR models are empirically related to skin permeation. The mechanism on how these descriptors affect transdermal permeation is not clear. Mathematically it is an ill-defined approach to use many colinearly related parameters rather than fewer independent parameters in multi-linear regression.

Current challenges in non-invasive insulin delivery systems: a comparative review

The quest to eliminate the needle from insulin delivery and to replace it with non- or less-invasive alternative routes has driven rigorous pharmaceutical research to replace the injectable forms of insulin. Recently, various approaches have been studied involving many strategies using various technologies that have shown success in delivering insulin, which are designed to overcome the inherent barriers for insulin uptake across the gastrointestinal tract, mucosal membranes and skin. This review examines some of the many attempts made to develop alternative, more convenient routes for insulin delivery to avoid existing long-term dependence on multiple subcutaneous injections and to improve the pharmacodynamic properties of insulin. In addition, this article concentrates on the successes in this new millennium in developing potential non-invasive technologies and devices, and on major new milestones in modern insulin delivery for the effective treatment of diabetes.

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.

In silico and in vitro filters for the fast estimation of skin permeation and distribution of new chemical entities

The development of in silico and in vitro tools to estimate or predict the passive human skin permeation and distribution of new chemical entities, useful in dermal drug delivery, in absorption studies of toxic compounds, and in the cosmetics industry, is presented. In vitro permeation parameters were measured using the artificial membrane PAMPA-skin. The Volsurf approach was then applied to extract pertinent descriptors from molecular interaction fields characterizing the molecular structure of tested compounds. Two useful three-dimensional solvatochromic models able to predict PAMPA permeation parameters directly from the molecular structure were obtained using the partial least squares analysis. The models also provide valuable information to understand the link between physicochemical and structural properties of tested compounds and their interactions with the artificial membrane PAMPA-skin and can be useful to rapidly estimate their permeation through the human skin.

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.

Ultrasound-induced cavitation: applications in drug and gene delivery

Ultrasound, which has been conventionally used for diagnostics until recently, is now being extensively used for drug and gene delivery. This transformation has come about primarily due to ultrasound-mediated acoustic cavitation - particularly transient cavitation. Acoustic cavitation has been used to facilitate the delivery of small molecules, as well as macromolecules, including proteins and DNA. Controlled generation of cavitation has also been used for targeting drugs to diseased tissues, including skin, brain, eyes and endothelium. Ultrasound has also been employed for the treatment of several diseases, including thromboembolism, arteriosclerosis and cancer. This review provides a detailed account of mechanisms, current status and future prospects of ultrasonic cavitation in drug and gene delivery applications.

Parallel Artificial Membrane Permeability Assay: A New Membrane for the Fast Prediction of Passive Human Skin Permeability

This work was devoted to the search for new artificial membranes allowing a rapid evaluation of passive human skin permeation of compounds with a parallel artificial membrane permeability assay (PAMPA). Effective permeability coefficients (Pe) determined for a set of compounds using the PAMPA technique with isopropyl myristate (IPM) and silicone oil, alone or in mixture, were compared to the corresponding human skin permeability coefficient values (Kp). A good correlation between Pe and Kp was found for compounds tested through a membrane consisting of 70% silicone and 30% IPM. Moreover, positive correlation between the membrane retention of compounds and stratum corneum/water partition coefficients (PSC) was established. These results showed that this new artificial membrane, defined as PAMPA-skin, is able to mimic the main barrier properties of human stratum corneum and can be used for the fast prediction of passive human skin permeability coefficients.

Evaluation of chemical enhancers in the transdermal delivery of lidocaine

The effect of various classes of chemical enhancers was investigated for the transdermal delivery of the anesthetic lidocaine across pig and human skin in vitro. The lipid disrupting agents (LDA) oleic acid, oleyl alcohol, butenediol, and decanoic acid by themselves or in combination with isopropyl myristate (IPM) showed no significant flux enhancement. However, the binary system of IPM/n-methyl pyrrolidone (IPM/NMP) improved drug transport. At 2% lidocaine dose, this synergistic enhancement peaked at 25:75 (v/v) IPM:NMP with a steady state flux of 57.6 +/- 8.4 microg cm(-2) h(-1) through human skin. This observed flux corresponds to a four-fold enhancement over a 100% NMP solution and over 25-fold increase over 100% IPM at the same drug concentration (p < 0.001). NMP was also found to co-transport through human skin with lidocaine free base and improve enhancement due to LDA. These findings allow a more rational approach for designing oil-based formulations for the transdermal delivery of lidocaine free base and similar drugs.

Limonene GP1/PG organogel as a vehicle in transdermal delivery of haloperidol

Penetration enhancers are a classical means for improving transdermal drug delivery (TDD). Enhancers permeate into the skin and reversibly decrease the barrier resistance. Basically, our aim is to formulate a transdermal gel containing an appropriate enhancer for a controlled drug release. Terpenes, namely limonene, linalool and cineole, in propylene glycol (PG) were first investigated in vitro for their capacity to enhance the percutaneous release of an anti-psychotic drug, haloperidol (HP). Relative to oxygenated linalool and cineole, hydrocarbon limonene was more effective as a skin enhancer; it increased human skin permeability and decreased lag time. Limonene was thus incorporated in an organogel comprised of gelator GP1 and PG. This skin-friendly gel in a transdermal patch could act as a long-acting formulation that delivers HP at a sustained percutaneous rate. The microscopic framework of the organogel is a branched network of interlocking fibres. Varying the gelator content modulates the fibre density and gel stiffness, and presents different degrees of resistance to drug diffusion on the vehicle side. Rheological and permeation studies demonstrated that an increase in gelator concentration increased gel moduli and decreased drug flux simultaneously. The rheology of the gel matrix influenced drug release rate in a manner described by several experimentally-derived correlations.

Mechanical properties of human stratum corneum: effects of temperature, hydration, and chemical treatment

An in vitro mechanics approach to quantify the intercellular delamination energy and mechanical behavior of isolated human stratum corneum (SC) in a direction perpendicular to the skin surface is presented. The effects of temperature, hydration, and a chloroform-methanol treatment to remove intercellular lipids were explored. The delamination energy for debonding of cells within the SC layer was found to be sensitive to the moisture content of the tissue and to the test temperature. Delamination energies for untreated stratum corneum were measured in the range of 1-8J/m(2) depending on test temperature. Fully hydrated specimen energies decreased with increasing temperature, while room-humidity-hydrated specimens exhibited more constant values of 2-4J/m(2). Lipid-extracted specimens exhibited higher delamination energies of approximately 12J/m(2), with values decreasing to approximately 4J/m(2) with increasing test temperature. The peak separation stress decreased with increasing temperature and hydration, but lipid-extracted specimens exhibited higher peak stresses than untreated controls. The delaminated surfaces revealed an intercellular failure path with no evidence of tearing or fracture of cells. The highly anisotropic mechanical behavior of the SC is discussed in relation to the underlying SC structure.

A comparison between povidone-ethylcellulose and povidone-eudragit transdermal dexamethasone matrix patches based on in vitro skin permeation

The present study was designed to develop a suitable matrix type transdermal drug delivery system (TDDS) of dexamethasone using blends of two different polymeric combinations, povidone (PVP) and ethylcellulose (EC) and Eudragit with PVP. Physical studies including moisture content, moisture uptake, flatness to study the stability of the formulations and in vitro dissolution of the experimental formulations were performed to determine the amount of dexamethasone present in the patches were performed and scanning electron microscopy (SEM) photographs of the prepared TDDS were taken to see the drug distribution pattern. Drug-excipient interaction studies were carried out using Fourier transform infrared (FTIR) spectroscopic technique. In vitro skin permeation study was conducted in a modified Franz's diffusion cell. All the formulations were found to be suitable for formulating in terms of physicochemical characteristics and there was no significant interaction noticed between the drug and polymers used. In vitro dissolution studies showed that the drug distribution in the matrix was homogeneous and the SEM photographs further demonstrated this. The formulations of PVP:EC provided slower and more sustained release of drug than the PVP:Eudragit formulations during skin permeation studies and the formulation PVP:EC (1:5) was found to provide the slowest release of drug. Based on the above observations, it can be reasonably concluded that PVP-EC polymers are better suited than PVP-Eudragit polymers for the development of TDDS of dexamethasone.

Characterisation of the water–isopropyl myristate system

Partition coefficients for compounds (solutes) from water to isopropyl myristate, IPM, have been obtained from the literature, either as directly determined partition coefficients or from solubilities in water and in IPM. The general solvation equation of Abraham has been applied to 141 such partition coefficients, as logPipm, and it is shown that the main solute factors that influence partition are dipolarity/polarisability, hydrogen bond acidity and hydrogen bond basicity that reduce partition, and volume that increases partition. These factors are quantitatively very similar to those that influence partition in the water to olive oil system, and indicate that IPM has the expected behaviour of a long chain, hydrophobic ester. It is shown that the water to IPM system is a poor model for partition between water and human stratum corneum and for permeation from water through human skin.

Role of n-methyl Pyrrolidone in the Enhancement of Aqueous Phase Transdermal Transport

The role of n-methyl pyrrolidone (NMP) as an enhancer for permeants delivered from an aqueous phase was investigated in the transdermal delivery of the local anesthetics lidocaine free base, lidocaine-hydrochloride (HCl), and prilocaine-HCl. Lidocaine free-base flux increased from H2O/NMP binary systems containing over 50% (v/v) NMP with significant flux enhancement observed above 80% NMP. In this range, drug flux was found to correlate with NMP flux. The addition of oleic acid (1% w/v) further enhanced lidocaine flux sixfold, in these formulations. The H2O/NMP (50% v/v) system enhanced the transport of water-soluble hydrochloride salt derivatives of lidocaine and prilocaine by factors of 4.3 and 2.6, respectively, indicating that NMP was capable of enhancing hydrophilic and hydrophobic drugs from an aqueous phase. These findings were consistent with the model that NMP flux across the stratum corneum improves the transport of formulation solutes.

Drug delivery across the skin

Since the introduction of the first through the skin (TTS) therapeutic in 1980, a total of 34 TTS products have been marketed and numerous drugs have been tested by more than 50 commercial organisations for their suitability for TTS delivery. Most of the agents which have been tested have had low molecular weights, due to the impermeability of the skin barrier. This barrier resides in the outermost skin layer, the stratum corneum. It is mechanical, anatomical, as well as chemical in nature; laterally overlapping cell multi-layers are sealed by tightly packed, intercellular, lipid multi-lamellae. Chemical skin permeation enhancers increase the transport across the barrier by partly solubilising or extracting the skin lipids and by creating hydrophobic pores. This is often irritating and not always well-tolerated. The TTS approach allows drugs (< 400 kDa in size) to permeate through the resulting pores in the skin, with a short lag-time and subsequent steady-state period. Drug bioavailability for TTS delivery is typically below 50%, avoiding the first pass effect. Wider, hydrophilic channels can be generated by skin poration, with the aid of a small electrical current (> 0.4 mA/cm2) across the skin (iontophoresis) or therapeutic ultrasound (few W/cm2; sonoporation). High-voltage (> 150 V, electroporation) widens the pores even more and often irreversibly. These standard poration methods require experience and equipment and are therefore, not practical; at best, charged/small molecules (< or = 4000 kDa in size) can be delivered efficiently across the skin. In spite of the potential harm of gadget-driven skin poration, this method is used to deliver molecules which conventional TTS patches are unable to deliver, especially polypeptides. Lipid-based drug carriers (liposomes, niosomes, nanoparticle microemulsions, etc.) were proposed as alternative, low-risk delivery vehicles. Such suspensions provide an improved drug reservoir on the skin, but the aggregates remain confined to the surface. Conventional carrier suspensions increase skin hydration and/or behave as skin permeation enhancers. The recently developed carriers; Transferomes, comprise pharmaceutically-acceptable, established compounds and are thought to penetrate the skin barrier along the naturally occurring transcutaneous moisture gradient. Transfersomes are believed to penetrate the hydrophilic (virtual) channels in the skin and widen the former after non-occlusive administration. Both small and large hydrophobic and hydrophilic molecules are deliverable across the stratum after conjugation with Transfersomes. Drug distribution after transdermal delivery probably proceeds via the lymph. This results in quasi-zero order kinetics with significant systemic drug levels reached after a lag-time of up to a few hours. The relative efficiency of TTS drug delivery with Transfersomes is typically above 50 %; with the added possibility of regional drug targeting.

Noninvasive ultrasonic transdermal insulin delivery in rabbits using the light-weight cymbal array

OBJECTIVE

Recent studies have shown that ultrasound-mediated transdermal drug delivery offers a promising potential for noninvasive drug administration. The purpose of this study was to demonstrate ultrasonic transdermal delivery of insulin in vivo using rabbits with a novel, low-profile two-by-two ultrasound array based on the cymbal transducer. As a practical device, the cymbal array (f = 20 kHz) was 37 x 37 x 7 mm3 in size and weighed less than 22 g. Using the same array on hyperglycemic rats, our previous experiments demonstrated that blood glucose would decrease 233.3 +/- 22.2 mg/dL in 90 min from 5 min of pulsed ultrasound exposure. With a similar intensity (Isptp = 100 mW/cm2, 20% duty cycle), our goal was to determine if the same effect could be achieved with rabbits.

METHODS

Experiments were performed in 16 New Zealand White rabbits (weighing 2.7-3.4 kg) divided into three groups: two controls and one ultrasound with insulin exposure. The rabbits were first anesthetized, and their thigh area was shaved for the exposure area. While the animal was lying in the lateral recumbent position, a 1-mm-thick, water-tight standoff (reservoir) that held insulin or saline was fastened between the thigh and the ultrasound array. The first control group (control 1: insulin-no ultrasound) had insulin placed in the reservoir with no ultrasound exposure, while the second control group (control 2: saline-ultrasound) had saline in the reservoir with ultrasound operating at Isptp = 100 mW/cm2 for 60 min. The third rabbit group (ultrasound-insulin) was subjected to insulin with ultrasound exposure for 60 min (Isptp = 100 mW/cm2). At the beginning of the experiment and every 15 min for 90 min, 0.3 mL of blood was collected from the ear vein to determine the blood glucose level (in mg/dL) using a glucose monitoring system. For comparison between individual rabbits, the change in the blood glucose level was normalized to a baseline value. The insulin reservoir was removed with the array after the ultrasound was turned off at 60 min of exposure.

RESULTS

For both controls, insulin-no ultrasound and saline-ultrasound, the blood glucose level varied from the initial baseline by approximately +75 mg/dL. However, for the ultrasound-insulin group, the glucose level was found to decrease to -132.6 +/- 35.7 mg/dL from the initial baseline in 60 min. Even after the array and insulin reservoir were removed, the blood glucose level of ultrasound-insulin group continued to decrease to -208.1 +/- 29 mg/dL from the initial baseline.

CONCLUSIONS

These results indicate the feasibility of using a low-cost, lightweight cymbal array for enhanced transdermal insulin delivery using ultrasound.

Experimental demonstration of the existence of highly permeable localized transport regions in low‐frequency sonophoresis

Recent advances in low-frequency sonophoresis have focused on the existence of hypothesized localized transport regions (LTRs). However, there has been no actual experimental demonstration that the hypothesized LTRs are, in fact, localized regions of high permeability. Through a series of low-frequency sonophoresis experiments conducted with full-thickness pig skin, in the presence of the surfactant sodium lauryl sulfate (SLS), in which we have separately measured the transport of calcein through the LTRs, which have areas ranging from 10 to 40 mm(2), and the surrounding regions of the skin (the non-LTRs) by means of a novel masking technique, we demonstrate that the calcein permeability through the LTRs is approximately 80-fold higher than the calcein permeability through the non-LTRs, suggesting that the LTRs are structurally perturbed to a greater extent than the non-LTRs from the exposure to the ultrasound/SLS system. In addition, we propose basic models to predict the total skin transdermal permeability from the transdermal permeabilities of the LTRs and the non-LTRs, and then compare the predictions to the experimental data obtained from the masking experiments. We also demonstrate that both the LTRs and the non-LTRs exhibit significant decreases in skin electrical resistivity relative to untreated skin ( approximately 5000-fold and approximately 170-fold, respectively), suggesting the existence of two levels of significant skin structural perturbation due to ultrasound exposure in the presence of SLS. Finally, an analysis of the porosity/tortuosity ratio (epsilon/tau) values suggests that trans-cellular transdermal transport pathways are present within the highly permeable, and highly structurally perturbed, LTRs.

Ultrasonic drug delivery--a general review

Ultrasound has an ever-increasing role in the delivery of therapeutic agents, including genetic material, protein and chemotherapeutic agents. Cavitating gas bodies, such as microbubbles, are the mediators through which the energy of relatively non-interactive pressure waves is concentrated to produce forces that permeabilise cell membranes and disrupt the vesicles that carry drugs. Thus, the presence of microbubbles enormously enhances ultrasonic delivery of genetic material, proteins and smaller chemical agents. Numerous reports show that the most efficient delivery of genetic material occurs in the presence of cavitating microbubbles. Attaching the DNA directly to the microbubbles, or to gas-containing liposomes, enhances gene uptake even further. Ultrasonic-enhanced gene delivery has been studied in various tissues, including cardiac, vascular, skeletal muscle, tumour and even fetal tissue. Ultrasonic-assisted delivery of proteins has found most application in transdermal transport of insulin. Cavitation events reversibly disrupt the structure of the stratus corneum to allow transport of these large molecules. Other hormones and small proteins could also be delivered transdermally. Small chemotherapeutic molecules are delivered in research settings from micelles and liposomes exposed to ultrasound. Cavitation appears to play two roles: it disrupts the structure of the carrier vesicle and releases the drug; and makes cell membranes and capillaries more permeable to drugs. There remains a need to better understand the physics of cavitation of microbubbles and the impact that such cavitation has on cells and drug-carrying vesicles.

Ultrasound and transdermal drug delivery

Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injection. However, the stratum corneum acts as a barrier that limits the penetration of substances through the skin. Application of ultrasound 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, namely transdermal drug delivery and transdermal monitoring. Particular attention is paid to proposed enhancement mechanisms and future trends in the field of cutaneous vaccination and gene delivery.

Surfactant-enhanced transdermal delivery by electroporation

The objective of the experiment was to study the influence of sodium dodecyl sulfate (SDS) on transdermal transport of diffusants by electroporation. The resistance of porcine epidermis in contact with SDS solution (0.2% w/v) dropped by 40% within 24 h. SDS improved the efficiency of transdermal delivery of glucose, dextrans of molecular weight (MW) 4 kDa (FD4K) and 10 kDa (FD10K) by electroporation. However, the transport of dextran MW 35 kDa (FD35K) was not influenced significantly. Pretreatment of epidermis with SDS solution reduced its electroporation threshold from 80 to 60 V. It appears that presence of SDS during electroporation helps in achieving the desired transport with less electrical exposure dose. SDS enhanced the transdermal delivery of molecules by electroporation most likely by facilitating the barrier disruption during pulse application and also by prolonging the lifetime of electropores created by the pulse.

Human Skin Permeation and Partition: General Linear Free‐Energy Relationship Analyses

Literature values of the permeability coefficient for permeation of human skin from water have been adjusted for ionization in water and adjusted for temperature. The obtained values of log K(p) for 119 solutes at 37 degrees C have been correlated with Abraham descriptors to yield an equation with R(2) = 0.832 and SD = 0.46 log units. Three separate test sets of 60 compounds had log K(p) predicted with an SD of 0.48 log units. The main factors that influence log K(p) are solute hydrogen bond basicity that lowers the permeability coefficient and solute volume that increases the permeability coefficient. Human skin-water partition coefficients, as log K(sc), have been collected for 45 compounds and yield an equation with R(2) = 0.926 and SD = 0.22 log units. We have compared the log K(p) equation to equations for various other processes, but have found no process that appears to be similar to that for skin permeation. The nearest process to skin-water partition is the isobutanol-water partition system. An equation for lateral diffusion in the stratum corneum is shown to be reasonably close to various diffusion-related processes.