Self promotion of deep tissue penetration and distribution of methylsalicylate after topical application

Essential Oils as Multicomponent Mixtures and Their Potential for Human Health and Well-Being

Essential oils (EOs) and their individual volatile organic constituents have been an inherent part of our civilization for thousands of years. They are widely used as fragrances in perfumes and cosmetics and contribute to a healthy diet, but also act as active ingredients of pharmaceutical products. Their antibacterial, antiviral, and anti-inflammatory properties have qualified EOs early on for both, the causal and symptomatic therapy of a number of diseases, but also for prevention. Obtained from natural, mostly plant materials, EOs constitute a typical example of a multicomponent mixture (more than one constituent substances, MOCS) with up to several hundreds of individual compounds, which in a sophisticated composition make up the property of a particular complete EO. The integrative use of EOs as MOCS will play a major role in human and veterinary medicine now and in the future and is already widely used in some cases, e.g., in aromatherapy for the treatment of psychosomatic complaints, for inhalation in the treatment of respiratory diseases, or topically administered to manage adverse skin diseases. The diversity of molecules with different functionalities exhibits a broad range of multiple physical and chemical properties, which are the base of their multi-target activity as opposed to single isolated compounds. Whether and how such a broad-spectrum effect is reflected in natural mixtures and which kind of pharmacological potential they provide will be considered in the context of ONE Health in more detail in this review.

Capsaicin-based analgesic balm attenuates the skeletal muscle metaboreflex in healthy humans

The exercise pressor reflex (EPR) is comprised of group III and IV skeletal muscle afferents and is one of the principal mediators of the cardiovascular response to exercise. In animals, capsaicin-based analgesic balm (CAP) attenuates the pressor response to muscle contraction, indicating the transient receptor potential vanilloid 1 (TRPv1) receptor (localized on the group IV afferent neuron) as an important mediator of the EPR. However, whether these findings can be extrapolated to humans remains unknown. Here, we tested the hypothesis that CAP would attenuate blood pressure (BP) and muscle sympathetic nerve activity (MSNA) responses to isolated muscle metaboreflex activation in healthy men. MSNA (microneurography) and beat-to-beat heart hate (HR, by electrocardiography), and BP (finger photoplethysmography) were continuously measured in eight healthy males (23 ± 5 yr) at rest, during isometric handgrip exercise, and during postexercise ischemia (PEI). Trials were performed before and 30 and 60 min after the topical application of CAP (0.1%, CAPZASIN-HP) over the volar forearm of the subject's exercising arm. Isometric exercise evoked increases in mean BP (∆32 ± 4 mmHg) and MSNA (∆26 ± 5 bursts/min; ∆19 ± 5 bursts/100 heart beats). The increases in BP during handgrip were not affected by CAP, but the increase in MSNA was lower after 60 min of CAP application. During PEI, the increases in BP and MSNA were all significantly less than those before CAP (all P < 0.05). In conclusion, CAP attenuated BP and sympathetic responses evoked by PEI in humans. These data provide evidence that transient receptor potential vanilloid 1 receptors potentially contribute to the EPR in humans, via its metabolic component. NEW & NOTEWORTHY We found that topical application of capsaicin-based analgesic balm attenuates arterial blood pressure and muscle sympathetic nerve activity during isolated muscle metaboreflex activation following isometric handgrip exercise in healthy humans. These findings suggest that the transient receptor potential vanilloid 1 may contribute to the exercise pressor reflex in humans via its metabolic component.

Convective and diffusive effects on particle transport in asymmetric periodic capillaries

We present here results of a theoretical investigation of particle transport in longitudinally asymmetric but axially symmetric capillaries, allowing for the influence of both diffusion and convection. In this study we have focused attention primarily on characterizing the influence of tube geometry and applied hydraulic pressure on the magnitude, direction and rate of transport of particles in axi-symmetric, saw-tooth shaped tubes. Three initial value problems are considered. The first involves the evolution of a fixed number of particles initially confined to a central wave-section. The second involves the evolution of the same initial state but including an ongoing production of particles in the central wave-section. The third involves the evolution of particles a fully laden tube. Based on a physical model of convective-diffusive transport, assuming an underlying oscillatory fluid velocity field that is unaffected by the presence of the particles, we find that transport rates and even net transport directions depend critically on the design specifics, such as tube geometry, flow rate, initial particle configuration and whether or not particles are continuously introduced. The second transient scenario is qualitatively independent of the details of how particles are generated. In the third scenario there is no net transport. As the study is fundamental in nature, our findings could engender greater understanding of practical systems.

Mechanistic Evaluation of Hydration Effects on the Human Epidermal Permeation of Salicylate Esters

We sought to understand when and how hydration enhances the percutaneous absorption of salicylate esters. Human epidermal membrane fluxes and stratum corneum solubilities of neat and diluted solutions of three esters were determined under hydrated and dehydrated conditions. Hydration doubled the human epidermal flux seen for methyl and ethyl salicylate under dehydrated conditions and increased the flux of neat glycol salicylate 10-fold. Mechanistic analyses showed that this hydration-induced enhancement arises mainly from an increase in the stratum corneum diffusivity of the three esters. Further, we showed that unlike methyl and ethyl salicylate, glycol salicylate is hygroscopic and the ∼10-fold hydration-induced flux enhancement seen with neat glycol salicylate may be due to its ability to hydrate the stratum corneum to a greater extent. The hydration-induced enhancements in in vitro epidermal flux seen here for glycol and ethyl salicylate were similar to those reported for their percutaneous absorption rates in a comparable in vivo study, whilst somewhat higher enhancement was seen for methyl salicylate in vivo. This may be explained by a physiologically induced self enhancement of neat methyl salicylate absorption in vivo which is not applicable in vitro.

The application of local hypobaric pressure - A novel means to enhance macromolecule entry into the skin

The local application of controlled hypobaric stress represents a novel means to facilitate drug delivery into the skin. The aims of this work were to understand how hypobaric stress modified the properties of the skin and assess if this penetration enhancement strategy could improve the percutaneous penetration of a macromolecule. Measurements of skin thickness demonstrated that the topical application of hypobaric stress thinned the tissue (p<0.05), atomic force microscopy showed that it shrunk the corneocytes in the stratum corneum (p<0.001) and the imaging of the skin hair follicles using multiphoton microscopy showed that it opened the follicular infundibula (p<0.001). Together, these changes contributed to a 19.6-fold increase in in vitro percutaneous penetration of a 10,000 molecular weight dextran molecule, which was shown using fluorescence microscopy to be localized around the hair follicles, when applied to the skin using hypobaric stress. In vivo, in the rat, a local hemodynamic response (i.e. a significant increase in blood flow, p<0.001) was shown to contribute to the increase in follicular transport of the dextran to produce a systemic absorption of 7.2±2.81fg·mL(-1). When hypobaric stress was not applied to the rat there was no detectable absorption of dextran and this provided evidence that this novel penetration enhancement technique can improve the percutaneous penetration of macromolecules after topical application to the skin.

Solute-vehicle-skin interactions in percutaneous absorption: the principles and the people

An appreciation of solute-vehicle-skin interactions underpins our current understanding of the processes of percutaneous absorption as well as in the prediction of the extent of absorption. This understanding has been reached through principles developed and validated over the last century through the work of a number of authors, including Dale Wurster, Takeru Higuchi, Irvin Blank, Robert Scheuplein, Gordon Flynn, Boyd Poulsen and Tom Franz, as well as by many scientists from my and younger generations. Their work has led to an appreciation of the rate-limiting steps in percutaneous penetration, the role played by the physicochemical properties of the solute, vehicle and skin and the variability that may arise from using various experimental/mathematical/pharmacokinetic models to quantify absorption as well as enabling the prediction of local and systemic efficacy and toxicity. In addition, unexpected behaviour may result from non-ideality in solute-vehicle-skin effects, including dehydration, chemical enhancement, supersaturation, metabolism, sequestration and vascular effects, including those of nanosystems on the local vasculature. In general, in vitro skin penetration profiles are predictive of in vivo profiles but a number of exceptions also exist.

Modeling the human skin barrier--towards a better understanding of dermal absorption

Many drugs are presently delivered through the skin from products developed for topical and transdermal applications. Underpinning these technologies are the interactions between the drug, product and skin that define drug penetration, distribution, and elimination in and through the skin. Most work has been focused on modeling transport of drugs through the stratum corneum, the outermost skin layer widely recognized as presenting the rate-determining step for the penetration of most compounds. However, a growing body of literature is dedicated to considering the influence of the rest of the skin on drug penetration and distribution. In this article we review how our understanding of skin physiology and the experimentally observed mechanisms of transdermal drug transport inform the current models of drug penetration and distribution in the skin. Our focus is on models that have been developed to describe particular phenomena observed at particular sites of the skin, reflecting the most recent directions of investigation.

Convective transport of highly plasma protein bound drugs facilitates direct penetration into deep tissues after topical application

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Many products are applied to human skin for local effects in deeper tissues. Animal studies suggest that deep dermal and/or subcutaneous delivery may be facilitated by both dermal diffusion and transport via the cutaneous vasculature. However, the relationship between the extent and pathways of penetration, drug physicochemical properties and deeper tissue physiology is not well understood.

WHAT THIS STUDY ADDS

We have used a physiologically based pharmacokinetic model to analyze published human cutaneous microdialysis data, complemented by our own in vitro skin penetration studies. We found that convective blood, lymphatic and interstitial flow led to significant deep tissue concentrations for drugs that are highly plasma protein bound. In such cases, deeper tissue concentrations will occur earlier and may be several orders of magnitude greater than predicted by passive dermal diffusion alone.

AIMS

To relate the varying dermal, subcutaneous and muscle microdialysate concentrations found in man after topical application to the nature of the drug applied and to the underlying physiology.

METHODS

We developed a physiologically based pharmacokinetic model in which transport to deeper tissues was determined by tissue diffusion, blood, lymphatic and intersitial flow transport and drug properties. The model was applied to interpret published human microdialysis data, estimated in vitro dermal diffusion and protein binding affinity of drugs that have been previously applied topically in vivo and measured in deep cutaneous tissues over time.

RESULTS

Deeper tissue microdialysis concentrations for various drugs in vivo vary widely. Here, we show that carriage by the blood to the deeper tissues below topical application sites facilitates the transport of highly plasma protein bound drugs that penetrate the skin, leading to rapid and significant concentrations in those tissues. Hence, the fractional concentration for the highly plasma protein bound diclofenac in deeper tissues is 0.79 times that in a probe 4.5 mm below a superficial probe whereas the corresponding fractional concentration for the poorly protein bound nicotine is 0.02. Their corresponding estimated in vivo lag times for appearance of the drugs in the deeper probes were 1.1 min for diclofenac and 30 min for nicotine.

CONCLUSIONS

Poorly plasma protein bound drugs are mainly transported to deeper tissues after topical application by tissue diffusion whereas the transport of highly plasma protein bound drugs is additionally facilitated by convective blood, lymphatic and interstitial transport to deep tissues.

Percutaneous absorption of topically applied NSAIDS and other compounds: role of solute properties, skin physiology and delivery systems

Topical NSAIDS and related solutes are often applied to the skin to target tissues directly below the application site. We have used both biopsy and microdialysis techniques to show that most solutes penetrate below dermal capillaries into the subcutaneous and deeper tissues of both rats and human subjects. The selectivity of local penetration is time related, the concentrations in underlying tissues at longer times often being defined by recirculation from the systemic blood supply. Increased depths of penetration may be achieved by the use of vasoactive agents. Iontophoretic and other delivery systems appear to increase the efficiency of drug delivery through the stratum corneum and do not appear to greatly facilitate penetration into tissues below the dermis. Vehicle polarity and solute properties such as size can be used to advantage in delivering NSAIDs to deeper tissues.The pharmacokinetics of NSAIDs in the dermis and other tissues appears to be related to the absorption of solutes through the stratum corneum, binding of the NSAIDs to dermal and other tissues and clearance of NSAIDs from these tissues through either diffusion into deeper tissues or removal by the systemic blood supply. The latter is dependent on the blood flow to the tissues and protein binding of the NSAIDs in the blood. Absorption of NSAIDs and other solutes through the stratum corneum is defined by their inherent hydrogen bonding ability, lipophilicity and size as well as the interactions between the solute, vehicle and skin.The literature contains a number of examples of pharmacological efficacy after topical application which can now be better explained in terms of our recently gained understanding of the pharmacokinetics of NSAIDs after topical application. A complicating aspect in this interpretation is the variation in efficacy between the various models used to date.

Microdialysis sampling and the clinical determination of topical dermal bioequivalence

Our objective was to determine whether the degree of variability associated with dermal microdialysis allows its practical application to determinations of bioequivalence of topically applied agents with a reasonable number of subjects. A statistical review of literature data was conducted to estimate the variances associated with subject-to-subject variability and the probe-to-probe variability within the subjects. In order to successfully utilise dermal microdialysis to establish bioequivalence of topically applied agents, particular care must be applied to study design. Due to the inherent variability between subjects, to maintain subject numbers at reasonable levels, each subject should act as their own control, thus removing the element of subject-to-subject variability from calculations of sample sizes. It is also recommended that measurements are made in duplicate in each subject to reduce the element of variability further. It is then possible to demonstrate, within 80-125% confidence limits and a subject population of approximately 20, that two formulations are bioequivalent.

The Role of Menthol in Skin Penetration from Topical Formulations of Ibuprofen 5% in vivo

In vivo plasma profiles from formulations containing 5% ibuprofen were compared after a single topical application in a randomised, double-blind, cross-over trial. Ibuleve gel (Dermal Laboratories, UK) contained only ibuprofen whilst Deep Relief gel (Mentholatum, UK) also contained 3% menthol. In contrast to results obtained when these products were compared under in vitro conditions, there was no statistically significant difference in vivo between delivery of ibuprofen. Estimated relative bioavailability fraction (Deep Relief gel/Ibuleve gel) from log-transformed AUC((0-24h)) was 0.99 (95% CI: 0.94-1.04), estimated C(max )ratio was 0.96 (95% CI: 0.91-1.00) and estimated t(max) ratio was 1.01 (95% CI: 0.81-1.20). Menthol produces local vasodilation, which reduces skin barrier function, and these data demonstrate that it is inappropriate to extrapolate from in vitro data where formulation components produce biologically-mediated enhancement of permeation which cannot be modelled ex vivo. In clinical use, these products deliver comparable amounts of ibuprofen, but only Deep Reliefgel provides the secondary immediate benefit of the direct analgesic action of menthol.

New Racemosol Derivatives as Potent Cyclooxygenase (COX) Inhibitors

Racemosol (1) and 10-O-demethylracemosol (2), natural products from Bauhinia malabarica Roxb., exhibit potent in vitro anti-inflammatory activities against cyclooxygenase-1 and -2 (COX-1 and -2) enzymes. To investigate the structure-activity relationship (SAR) of these molecules, we prepared and fully characterized 17 derivatives by functionalizing one, two, or all three OH group(s) of 2 (Scheme). Both the size and polarity of the substituents as well as the substitution pattern in compounds 3a-q were found to be critical for anti-inflammatory activity. The orientation of the drugs and their mode of binding were studied by molecular docking based on the known 3D structure of the complex between COX-2 and the drug SC-558. Whereas the monoacetoxy derivative 3h exhibited an equally potent inhibitory activity towards both COX-1 and -2 (Table 1), its diacetoxy congener 3i was slightly more selective toward COX-2. In vivo anti-inflammatory tests showed that 3i and 2 are slightly more active than the reference compound phenylbutazone (Table 2).

Regional differences in the in vitro penetration of methylsalicylate through equine skin

Commercial formulations of non-steroidal anti-inflammatory drugs (NSAIDs) are developed for human use but the extent to which they will pass through equine skin is unknown. Skin was harvested from five Thoroughbred geldings from the thorax, groin and leg (dorsal metacarpal) regions and frozen (-20 degrees C) until required. Two grams of methylsalicylate (MeSa) gel was applied to defrosted full-thickness samples in diffusion cells and the penetration of MeSa and its active metabolite, salicylate (Sa), through skin samples were measured over 24 h. Significantly higher (P < or = 0.02) total salicylate (AUC; MeSa + Sa) penetrated through skin from the leg region (5491.3 h mg/L), compared to thorax (3710.7 h mg/L) and groin (3571.5 h mg/L). In addition, there was a significantly higher (P0.01) rate of penetration of total Sa through leg skin in the first 6h after application. It was concluded that the commercial formulation of MeSa would achieve therapeutic levels of total salicylate beneath sites of topical application, with a faster and more pronounced response through the leg region, compared to the upper body.

Penetration of a topically applied nonsteroidal anti-inflammatory drug into local tissues and synovial fluid of dogs

OBJECTIVE

To investigate penetration of a topically applied nonsteroidal anti-inflammatory drug (NSAID) into tissues and synovial fluid.

ANIMALS

5 Greyhounds.

PROCEDURE

Dogs were anesthetized and microdialysis probes placed in the dermis and gluteal muscle over each coxofemoral (hip) joint. Methylsalicylate (MeSA) was applied topically over the left hip joint. Dialysate and plasma (blood samples from the cephalic and femoral veins) were obtained during the subsequent 5 hours. Dogs were euthanatized, and tissue samples and synovial fluid were collected and analyzed for salicylic acid (SA) and MeSA by use of high-pressure liquid chromatography.

RESULTS

SA and MeSA concentrations increased rapidly (< 30 minutes after application) in dialysate obtained from treated dermis. Salicylic acid also appeared in plasma within 30 minutes and reached a plateau concentration after 2 hours, although combined drug concentrations (SA plus MeSA) in plasma obtained from femoral vein samples were twice those measured in plasma obtained from the cephalic vein (SA only). Treated muscle had a progressive decrease in NSAID concentration with increasing depth (SA and MeSA), but it was significantly higher than the concentration in untreated muscle. Substantial amounts of SA and MeSA were also measured in synovial fluid of treated joints.

CONCLUSIONS AND CLINICAL RELEVANCE

Topically applied NSAIDs can penetrate deeply into tissues and synovial fluid. Local concentrations higher than circulating systemic concentrations are suggestive that direct diffusion and local blood redistribution are contributing to this effect. Systemic blood concentrations may be inadequate to describe regional kinetics of topically applied drugs.

A rabbit ear flap perfusion experiment to evaluate the percutaneous absorption of drugs

A rabbit ear flap single-pass perfusion system was examined as an experimental method for studying the relationship between the physiological conditions of tissues and drug disposition after topical applications. Tyrode solutions containing bovine serum albumin (BSA) and sucrose or flurbiprofen (FP), used as a model drug, were perfused through the vessel in the ear flap to evaluate the physiological conditions prior to the application of FP to the skin surface. The extracellular volume and distribution properties of FP in the perfused ear were similar to those in an in vivo experimental system. In addition, the perfused ear flap exhibited a pharmacological response to bradykinin (BK). The amount of FP in the outflow Tyrode solution containing BSA after application to the skin surface of the perfused ear decreased with the addition of BK, while that in the tissues under the application site increased. FP binds to BSA, which leaked from the intravascular space, and could be retained in the tissues under the application site. The protein binding also affected the redistribution of FP to other tissues in the ear flap after application to the skin. The rabbit ear perfusion system is a useful method for studying the percutaneous absorption of drugs especially variations in the disposition of drugs in oedematous tissues.

Topical Analgesics and Blood Pressure during Static Contraction in Humans

PURPOSE

In decerebrate cats, topical application of analgesic balm (AB) can attenuate the pressor response to electrically induced static contraction. We examined the possibility that this phenomenon also occurs in humans and determined whether such effects were limited to a local action on the contracting muscle (e.g., attenuations of the action of groups III and IV muscle afferents that cause the exercise pressor reflex) or whether they also may have affected active muscle blood flow and/or central command.

METHODS

Blood pressure (mean arterial pressure (MAP)), heart rate (HR), brachial artery blood flow (BABF), and relative perceived exertion (RPE) were assessed at rest and during 90 s of static handgrip contraction before and 50 min after application of a commercially available AB (1% capsaicin, 12.5% methyl salicylate) to the skin of the forearm muscles.

RESULTS

AB attenuated the MAP response to contraction (19 +/- 3 vs 27 +/- 5 mm Hg) but had no effect on the HR response. Absolute BABF was enhanced at rest and during contraction, but absolute (118 +/- 47 vs 114 +/- 47 mL x min) and percent increases (83 +/- 31 vs 55 +/-19%) were not statistically different between conditions. AB appeared to slightly enhance RPE, but this was also the case in a control protocol where only the vehicle (petroleum jelly) was used and no change in the blood pressure response was seen.

CONCLUSIONS

AB attenuates contraction-induced increases in MAP that do not seem to be related to alterations in perfusion of active muscle or central command. Effects appear to be localized to the active skeletal muscle and likely involve reductions in sensitivity of groups III and IV muscle afferents to local chemical and/or mechanical stimulation.

The role of stratum corneum and dermal microvascular perfusion in penetration and tissue levels of water-soluble drugs investigated by microdialysis

BACKGROUND

Hydrophilic drugs are poorly absorbed when applied topically, due to low partitioning through the lipid matrix of the stratum corneum. Cutaneous blood flow rapidly clears the absorbed drug, which may result in low tissue levels. This is of importance for topically applied drugs whose site of action is within the epidermis or dermis. Dermal drug levels can be measured using cutaneous microdialysis, which is a means of continuously sampling substances from the dermal extracellular fluid.

OBJECTIVES

To measure the contribution of stratum corneum barrier and microvascular perfusion in determining dermal tissue levels of hydrophilic drugs (aciclovir and penciclovir) in vivo.

METHODS

Studies were performed using microdialysis of the volar surface of the forearm of healthy volunteers (n = 55) over a 5-h collection period. Stratum corneum was removed by tape stripping, and barrier disruption quantified by measurement of transepidermal water loss (TEWL); dermal microvascular perfusion was modulated by inclusion of noradrenaline in the microdialysis perfusate.

RESULTS

With intact skin and normal cutaneous blood flow the concentration of penciclovir recovered was below assay threshold (0.05 ng x mL(-1). With noradrenaline-induced local vasoconstriction, the area under the curve of drug absorbed through normal skin (+/- SEM) was 13.3 +/- 2.9 ng mL(-1) h(0-5) for penciclovir and 27.6 +/- 10.6 ng mL(-1) h(0-5) for aciclovir. Removal of the stratum corneum (to glistening) by tape stripping increased penciclovir absorption by 1300-fold and aciclovir absorption by 440-fold, confirming the stratum corneum as the major barrier to hydrophilic drug absorption. Sequential barrier disruption by tape stripping gave a close correlation between penciclovir concentration absorbed per hour and barrier disruption measured by TEWL (r2 = 0.9283). There was a 15.6-fold difference in the recovery of penciclovir through barrier-deficient skin with and without cutaneous blood flow. There was no relationship between fibre depth and amount of drug dialysed, which suggests free movement of antiviral drug on reaching the aqueous environment of the dermis.

CONCLUSIONS

This study defines for the first time the relationship between the degree of mechanical barrier impairment and drug absorption at the same anatomical site in humans, and the role of blood flow in drug clearance in vivo.

Sensitive and selective gas chromatographic methods for the quantitation of camphor, menthol and methyl salicylate from human plasma

Analytical methods using gas chromatography-flame ionization detection (GC-FID) for the quantitation of camphor and menthol and GC-MS for the quantitation of methyl salicylate have been developed for measurement of low concentrations from human plasma. Anethole serves as the internal standard for camphor and menthol and ethyl salicylate serves as the internal standard for methyl salicylate. Plasma samples undergo multiple, sequential extractions with hexane in order to provide optimal recovery. For menthol and camphor, the extracting solvent is reduced in volume and directly injected onto a capillary column (Simplicity-WAX). Extracted methyl salicylate is derivatized with BSTFA prior to injection onto a capillary column (Simplicity-5). Between-day variation (% RSD) at 5 ng/ml varies from 6.2% for methyl salicylate to 13.5% for camphor. The limit of detection for each analyte is 1 ng/ml and the limit of quantitation is 5 ng/ml. These analytical methods have been used in a clinical study to assess exposure from dermally applied patches containing the three compounds.