Topical penetration of commercial salicylate esters and salts using human isolated skin and clinical microdialysis studies

Human Skin Drug Metabolism: Relationships between Methyl Salicylate Metabolism and Esterase Activities in IVPT Skin Membranes

The presence of esterase enzymes in human skin and their role in drug metabolism has been reported, but their distribution in the various skin layers and the relative contributions of those layers to metabolism is poorly defined. To gain further insight into esterase distribution, we performed in vitro skin permeation of a commercial 28.3% methyl salicylate (MeSA) cream (Metsal™) in Franz diffusion cells, using a range of human skin membranes, all from the same donor. The membranes were viable epidermis separated by a dispase II enzymatic method, heat separated epidermis, dermatomed skin, and dermis separated by a dispase II enzymatic method. Methyl salicylate and its metabolite, salicylic acid (SA), were measured by high-performance liquid chromatography. Alpha naphthyl acetate and Hematoxylin and Eosin staining provided qualitative estimations of esterase distribution in these membranes. The permeation of methyl salicylate after 24 h was similar across all membranes. Salicylic acid formation and permeation were found to be similar in dermatomed skin and dermis, suggesting dermal esterase activity. These results were supported by the staining studies, which showed strong esterase activity in the dermal-epidermal junction region of the dermis. In contrast with high staining of esterase activity in the stratum corneum and viable epidermis, minimal stained and functional esterase activity was found in heat-separated and dispase II-prepared epidermal membranes. The results are consistent with dispase II digesting hemidesmosomes, penetrating the epidermis, and affecting epidermal esterases but not those in the dermis. Accordingly, whilst the resulting dispase II-generated dermal membranes may be used for in vitro permeation tests (IVPT) involving esterase-based metabolic studies, the dispase II-generated epidermal membranes are not suitable for this purpose.

Effects of storage conditions on permeability and electrical impedance properties of the skin barrier

The aim of this study was to investigate the effect of various skin preservation protocols on in vitro drug permeation, epidermal-dermal drug distribution, and electrical impedance properties of skin membranes. Acyclovir (AC) and methyl salicylate (MS) were selected as model drugs due to their different physicochemical properties and skin metabolic profiles. In particular, AC is relatively hydrophilic (logP -1.8) and not expected to be affected by skin metabolism, while MS is relatively lipophilic (logP 2.5) and susceptible to metabolism, being a substrate for esterase residing in skin. Skin from pig ears was used and freshly excised into split-thickness membranes, which were divided and immediately stored at five different storage conditions: a) 4 °C overnight (fresh control), b) 4 °C for 4 days, c and d) -20 °C for 6 weeks and one year, respectively, and e) -80 °C for 6 weeks. Based on the combined results, general trends are observed showing that fresh skin is associated with lower permeation of both model drugs and higher skin membrane electrical resistance, as compared to the other storage conditions. Interestingly, in the case of fresh skin, significantly lower amounts of MS are detected in the epidermis and dermis compartments, implying higher levels of ester hydrolysis of MS (i.e., higher esterase activity). In line with this, the concentration of salicylic acid (SA) extracted from the dermis is significantly higher for fresh skin, as compared to the other storage conditions. Nevertheless, for all storage conditions, substantial amounts of SA are detected in the receptor medium, as well as in the epidermis and dermis, implying that esterase activity is maintained to some extent in all cases. For AC, which is not expected to be affected by skin metabolism, freeze storage (protocols c-e) is observed to result in higher accumulation of AC in the epidermis, as compared to the case of fresh skin, while the AC concentration in dermis is unaffected. These observations can be rationalized primarily by the observed lower permeability of fresh skin towards this hydrophilic substance. Finally, a strong correlation between AC permeation and electrical skin resistance is shown for individual skin membranes irrespective of storage condition, while the corresponding correlation for MS is inferior. On the other hand, a strong correlation is shown for individual membranes between MS permeation and electrical skin capacitance, while a similar correlation for AC is lower. The observed correlations between drug permeability and electrical impedance open up for standardizing in vitro data for improved analysis and comparisons between permeability results obtained with skin stored at different conditions.

Needleless administration of advanced therapies into the skin via the appendages using a hypobaric patch

Needleless delivery into the skin would overcome a major barrier to efficient clinical utilization of advanced therapies such as nanomaterials and macromolecules. This study demonstrates that controlled skin stretching (in porcine, rat, and mouse models) using a patch comprising a hypobaric chamber, to open the skin appendages, can increase the permeability of the tissue and provide a means to enable direct delivery of advanced therapies directly into the skin without the use of a needle or injection system. This technology can facilitate the self-administration of therapeutics including vaccines, RNA, and antigens, thus improving the translation of these products into effective clinical use.

Advanced therapies are commonly administered via injection even when they act within the skin tissue, and this increases the chances of off-target effects. Here we report the use of a skin patch containing a hypobaric chamber that induces skin dome formation to enable needleless delivery of advanced therapies directly into porcine, rat, and mouse skin. Finite element method modeling showed that the hypobaric chamber in the patch opened the skin appendages by 32%, thinned the skin, and compressed the appendage wall epithelia. These changes allowed direct delivery of an H1N1 vaccine antigen and a diclofenac nanotherapeutic into the skin. Fluorescence imaging and infrared mapping of the skin showed needleless delivery via the appendages. The in vivo utility of the patch was demonstrated by a superior immunoglobulin G response to the vaccine antigen in mice compared to intramuscular injection and a 70% reduction in rat paw swelling in vivo over 5 h with diclofenac without skin histology changes.

Topical drug delivery: History, percutaneous absorption, and product development

Topical products, widely used to manage skin conditions, have evolved from simple potions to sophisticated delivery systems. Their development has been facilitated by advances in percutaneous absorption and product design based on an increasingly mechanistic understanding of drug-product-skin interactions, associated experiments, and a quality-by-design framework. Topical drug delivery involves drug transport from a product on the skin to a local target site and then clearance by diffusion, metabolism, and the dermal circulation to the rest of the body and deeper tissues. Insights have been provided by Quantitative Structure Permeability Relationships (QSPR), molecular dynamics simulations, and dermal Physiologically Based PharmacoKinetics (PBPK). Currently, generic product equivalents of reference-listed products dominate the topical delivery market. There is an increasing regulatory interest in understanding topical product delivery behavior under 'in use' conditions and predicting in vivo response for population variations in skin barrier function and response using in silico and in vitro findings.

Microdialysis on Ex Vivo Porcine Ear Skin Can Validly Study Dermal Penetration including the Fraction of Transfollicular Penetration-Demonstrated on Caffeine Nanocrystals

Common ex vivo methods for penetration investigations often fail to monitor transfollicular penetration appropriately. In the present investigation, the validity of dermal microdialysis on the ex vivo porcine ear skin to investigate penetration kinetics, including transfollicular penetration, was studied. In setup A, a caffeine nanocrystal formulation was compared to a non-particular caffeine gel formulation. In setup B, two caffeine nanocrystal formulations of different sizes (200 nm, 700 nm) were compared to each other. Microdialysis samples were collected for 46 h. After sampling, the skin layers were separated, homogenized, and caffeine was quantified in all samples. In setup A the area under the curve (AUC) after crystal gel formulation application was 12 times higher than after non-particular formulation application. Setup B showed an increased AUC of 42% in the microdialysis data when the 700 nm caffeine crystals were applied compared to the 200 nm crystals. The microdialysis data was supported by the separation, homogenization and extraction data. Microdialysis performed on ex vivo porcine ear skin is a novel experimental setup. It is of high interest for further investigations since it is able to also capture the impact of follicular and transfollicular penetration kinetics as no other ex vivo setup can.

Relating transdermal delivery plasma pharmacokinetics with in vitro permeation test (IVPT) findings using diffusion and compartment-in-series models

Increasing emphasis is being placed on using in vitro permeation test (IVPT) results for topical products as a surrogate for their in vivo behaviour. This study sought to relate in vivo plasma concentration - time pharmacokinetic (PK) profiles after topical application of drug products to IVPT findings with mechanistic diffusion and compartment models that are now widely used to describe permeation of solutes across the main skin transport barrier, the stratum corneum. Novel in vivo forms of the diffusion and compartment-in-series models were developed by combining their IVPT model forms with appropriate in vivo disposition functions. Available in vivo and IVPT data were then used with the models in data analyses, including the estimation of prediction intervals for in vivo plasma concentrations derived from IVPT data. The resulting predicted in vivo plasma concentration - time profiles for the full models corresponded closely with the observed results for both nitroglycerin and rivastigmine at all times. In contrast, reduced forms of these in vivo models led to discrepancies between model predictions and observed results at early times. A two-stage deconvolution procedure was also used to estimate the in vivo cumulative amount absorbed and shown to be linearly related to that from IVPT, with an acceptable prediction error. External predictability was also shown using a separate set of in vitro and in vivo data for different nitroglycerin patches. This work suggests that mechanistic and physiologically based pharmacokinetic models can be used to predict in vivo behaviour from IVPT data for topical products.

Investigation of transfollicular caffeine penetration using microdialysis on ex vivo porcine ear skin

The aim of this study was to develop an ex vivo method that allows to quantify the transfollicular penetration of topically applied substances by combining microdialysis and selective follicular closure with varnish. An experimental setup with three skin areas on ex vivo intact porcine ear skin was designed (varnish on hair follicle, varnish next to hair follicle, no varnish). On each area, 10 µl/cm² caffeine-hydroxyethyl-cellulose-gel was applied. Samples were collected for 22 h by microdialysis. After sampling, the skin layers were separated, homogenized and caffeine was quantified by high pressure liquid chromatography (HPLC) in all samples. Potential impact of the varnish placed next to the follicle by tension on the follicle during the drying process was monitored by a microscopic setup and could be excluded. The microdialysis and homogenization study showed a significantly reduced penetration of caffeine when the hair follicles were closed. In areas with open hair follicles caffeine was detected already in the first ten minutes after application. The reported novel combination of two methods is suitable to investigate ex vivo transfollicular penetration. Possible impact of the closure material in the control area can be ruled out by adjusting the design of the control area in future studies.

Design and Evaluation of a Poly(Lactide-co-Glycolide)-Based In Situ Film-Forming System for Topical Delivery of Trolamine Salicylate

Trolamine salicylate (TS) is a topical anti-inflammatory analgesic used to treat small joint pain. The topical route is preferred over the oral one owing to gastrointestinal side effects. In this study, a poly(lactide-co-glycolide) (PLGA)-based in situ bio-adhesive film-forming system for the transdermal delivery of TS was designed and evaluated. Therefore, varying amounts (0%, 5%, 10%, 20%, and 25% (w/w)) of PLGA (EXPANSORB® DLG 50-2A, 50-5A, 50-8A, and 75-5A), ethyl 2-cyanoacrylate, poly (ethylene glycol) 400, and 1% of TS were dissolved together in acetone to form the bio-adhesive polymeric solution. In vitro drug permeation studies were performed on a vertical Franz diffusion cell and dermatomed porcine ear skin to evaluate the distinct formulations. The bio-adhesive polymeric solutions were prepared successfully and formed a thin film upon application in situ. A significantly higher amount of TS was delivered from a formulation containing 20% PLGA (45 ± 4 µg/cm2) and compared to PLGA-free counterpart (0.6 ± 0.2 µg/cm2). Furthermore, the addition of PLGA to the polymer film facilitated an early onset of TS delivery across dermatomed porcine skin. The optimized formulation also enhanced the delivery of TS into and across the skin.

Skin microdialysis: methods, applications and future opportunities-an EAACI position paper

Skin microdialysis (SMD) is a versatile sampling technique that can be used to recover soluble endogenous and exogenous molecules from the extracellular compartment of human skin. Due to its minimally invasive character, SMD can be applied in both clinical and preclinical settings. Despite being available since the 1990s, the technique has still not reached its full potential use as a tool to explore pathophysiological mechanisms of allergic and inflammatory reactions in the skin. Therefore, an EAACI Task Force on SMD was formed to disseminate knowledge about the technique and its many applications. This position paper from the task force provides an overview of the current use of SMD in the investigation of the pathogenesis of chronic inflammatory skin diseases, such as atopic dermatitis, chronic urticaria, psoriasis, and in studies of cutaneous events during type 1 hypersensitivity reactions. Furthermore, this paper covers drug hypersensitivity, UVB-induced- and neurogenic inflammation, and drug penetration investigated by SMD. The aim of this paper is to encourage the use of SMD and to make the technique easily accessible by providing an overview of methodology and applications, supported by standardized operating procedures for SMD in vivo and ex vivo.

The safety evaluation of food flavouring substances: the role of metabolic studies

The safety assessment of a flavour substance examines several factors, including metabolic and physiological disposition data. The present article provides an overview of the metabolism and disposition of flavour substances by identifying general applicable principles of metabolism to illustrate how information on metabolic fate is taken into account in their safety evaluation. The metabolism of the majority of flavour substances involves a series both of enzymatic and non-enzymatic biotransformation that often results in products that are more hydrophilic and more readily excretable than their precursors. Flavours can undergo metabolic reactions, such as oxidation, reduction, or hydrolysis that alter a functional group relative to the parent compound. The altered functional group may serve as a reaction site for a subsequent metabolic transformation. Metabolic intermediates undergo conjugation with an endogenous agent such as glucuronic acid, sulphate, glutathione, amino acids, or acetate. Such conjugates are typically readily excreted through the kidneys and liver. This paper summarizes the types of metabolic reactions that have been documented for flavour substances that are added to the human food chain, the methodologies available for metabolic studies, and the factors that affect the metabolic fate of a flavour substance.

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.

RIFM fragrance ingredient safety assessment, isoamyl salicylate, CAS registry number 87-20-7

The use of this material under current use conditions is supported by the existing information. This material was evaluated for genotoxicity, repeated dose toxicity, developmental toxicity, reproductive toxicity, local respiratory toxicity, phototoxicity, skin sensitization potential, as well as, environmental safety. Repeated dose toxicity was determined using to have the most conservative systemic exposure derived NOAEL of 47 mg/kg/day. A dietary 13-week subchronic toxicity study conducted in rats on a suitable read across analog resulted in a MOE of 2350 while considering 10.3% absorption from skin contact and 100% from inhalation. A MOE of >100 is deemed acceptable.

Transdermal patches: history, development and pharmacology

Transdermal patches are now widely used as cosmetic, topical and transdermal delivery systems. These patches represent a key outcome from the growth in skin science, technology and expertise developed through trial and error, clinical observation and evidence-based studies that date back to the first existing human records. This review begins with the earliest topical therapies and traces topical delivery to the present-day transdermal patches, describing along the way the initial trials, devices and drug delivery systems that underpin current transdermal patches and their actives. This is followed by consideration of the evolution in the various patch designs and their limitations as well as requirements for actives to be used for transdermal delivery. The properties of and issues associated with the use of currently marketed products, such as variability, safety and regulatory aspects, are then described. The review concludes by examining future prospects for transdermal patches and drug delivery systems, such as the combination of active delivery systems with patches, minimally invasive microneedle patches and cutaneous solutions, including metered-dose systems.

Analytical quality-by-design approach for sample treatment of BSA-containing solutions

The sample preparation of samples containing bovine serum albumin (BSA), e.g., as used in transdermal Franz diffusion cell (FDC) solutions, was evaluated using an analytical quality-by-design (QbD) approach. Traditional precipitation of BSA by adding an equal volume of organic solvent, often successfully used with conventional HPLC-PDA, was found insufficiently robust when novel fused-core HPLC and/or UPLC-MS methods were used. In this study, three factors (acetonitrile (%), formic acid (%) and boiling time (min)) were included in the experimental design to determine an optimal and more suitable sample treatment of BSA-containing FDC solutions. Using a QbD and Derringer desirability (D) approach, combining BSA loss, dilution factor and variability, we constructed an optimal working space with the edge of failure defined as D<0.9. The design space is modelled and is confirmed to have an ACN range of 83±3% and FA content of 1±0.25%.

Percutaneous absorption of salicylic Acid after administration of trolamine salicylate cream in rats with transcutol(®) and eucalyptus oil pre-treated skin

PURPOSE

This study was conducted to assess the effect of skin pre-treatment with Transcutol(®) and eucalyptus oil on systemic absorption of topical trolamine salicylate in rat.

METHODS

Pharmacokinetic parameters of salicylic acid following administration of trolamine salicylate on rat skin pre-treated with either Transcutol(®) or eucalyptus oil were determined using both non-compartmental and non-linear mixed effect modeling approaches and compared with those of control group.

RESULTS

Median (% of interquartile range/median) of salicylic acid AUC0-8hr (ng/mL/hr) values in Transcutol(®) or eucalyptus oil treated rats were 2522(139%) and 58976(141%), respectively as compared to the 3023(327%) of the control group. Skin pre-treatment with eucalyptus oil could significantly decrease extravascular volume of distribution (V/F) and elimination rate constant (k) of salicylic acid.

CONCLUSION

Unlike Transcutol(®), eucalyptus oil lead to enhanced transdermal absorption of trolamine salicylate through rat 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.

Dermal uptake of 18 dilute aqueous chemicals: in vivo disappearance-method measures greatly exceed in vitro-based predictions

Average rates of total dermal uptake (Kup ) from short-term (e.g., bathing) contact with dilute aqueous organic chemicals (DAOCs) are typically estimated from steady-state in vitro diffusion-cell measures of chemical permeability (Kp ) through skin into receptor solution. Widely used ("PCR-vitro") methods estimate Kup by applying diffusion theory to increase Kp predictions made by a physico-chemical regression (PCR) model that was fit to a large set of Kp measures. Here, Kup predictions for 18 DAOCs made by three PCR-vitro models (EPA, NIOSH, and MH) were compared to previous in vivo measures obtained by methods unlikely to underestimate Kup . A new PCR model fit to all 18 measures is accurate to within approximately threefold (r = 0.91, p < 10(-5) ), but the PCR-vitro predictions (r > 0.63) all tend to underestimate the Kup measures by mean factors (UF, and p value for testing UF = 1) of 10 (EPA, p < 10(-6) ), 11 (NIOSH, p < 10(-8) ), and 6.2 (MH, p = 0.018). For all three PCR-vitro models, log(UF) correlates negatively with molecular weight (r(2) = 0.31 to 0.84, p = 0.017 to < 10(-6) ) but not with log(vapor pressure) as an additional predictor (p > 0.05), so vapor pressure appears not to explain the significant in vivo/PCR-vitro discrepancy. Until this discrepancy is explained, careful in vivo measures of Kup should be obtained for more chemicals, the expanded in vivo database should be compared to in vitro-based predictions, and in vivo data should be considered in assessing aqueous dermal exposure and its uncertainty.

Synovial distribution of "systemically" administered acetylsalicylic acid in the isolated perfused equine distal limb

Background

This study investigated synovial concentrations of acetylsalicylic acid (ASA) and its metabolite salicylic acid (SA) in the equine fetlock joint following systemic administration of ASA. Salicylates were chosen because SA is the only nonsteroidal anti-inflammatory drug for which threshold levels exist for plasma and urine in equine sports. To avoid animal experiments, the study was conducted using an ex vivo model of the isolated perfused equine distal limb in combination with plasma concentrations obtained from literature.

Salicylate concentrations in the joint were determined using microdialysis and high performance liquid chromatography (HPLC). Any anti-inflammatory effect of synovial ASA concentrations was assessed using an ASA EC₅₀ (half maximal effective concentration) determined in equine whole blood.

Results

The ASA concentration in the synovial fluid (n = 6) reached a maximum of 4 μg/mL, the mean concentration over the entire perfusion period was 2 μg/mL. Maximum SA concentration was 17 μg/mL, the average was 14 μg/mL. ASA and SA concentration in the synovial fluid exceeded systemic concentrations 2 h and 3.5 h after “systemic” administration, respectively.

Conclusions

ASA and SA accumulated in the in the synovial fluid of the ex vivo model despite decreasing systemic concentrations. This suggests a prolonged anti-inflammatory effect within the joint that remains to be further elucidated.

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.

Modelling dermal drug distribution after topical application in human

PURPOSE

To model and interpret drug distribution in the dermis and underlying tissues after topical application which is relevant to the treatment of local conditions.

METHODS

We created a new physiological pharmacokinetic model to describe the effect of blood flow, blood protein binding and dermal binding on the rate and depth of penetration of topical drugs into the underlying skin. We used this model to interpret literature in vivo human biopsy data on dermal drug concentration at various depths in the dermis after topical application of six substances. This interpretation was facilitated by our in vitro human dermal penetration studies in which dermal diffusion coefficient and binding were estimated.

RESULTS

The model shows that dermal diffusion alone cannot explain the in vivo data, and blood and/or lymphatic transport to deep tissues must be present for almost all of the drugs tested.

CONCLUSION

Topical drug delivery systems for deeper tissue delivery should recognise that blood/lymphatic transport may dominate over dermal diffusion for certain compounds.

Assessment of topical bioequivalence using dermal microdialysis and tape stripping methods

PURPOSE

To assess the bioequivalence of two commercial topical formulations of oxytetracycline HCl by tape stripping and microdialysis in healthy volunteers.

METHODS

Tape stripping study was conducted on 12 healthy volunteers. After a 30-minute application of the formulations, adhesive tapes were used to sample stratum corneum at 0.25, 0.5, 1, 1.5, 2, 3, 4 hr. Ten of these volunteers were included in the microdialysis study with a period of 4 weeks between the experiments. Microdialysis probes were inserted into the dermis of the forearm. Following the application of the test and reference simultaneously, dialysates were collected in 30-minute sampling intervals up to 4 hr.

RESULTS

Pharmacokinetic evaluation by microdialysis yielded that the test could not be said to be bioequivalent to the reference at 90% CI. The intersubject variability of oxytetracycline content in stratum corneum was moderate when it was compared to the dermal levels. The test was found to be bioequivalent to reference according to the dermatopharmacokinetic evaluation by tape stripping.

CONCLUSIONS

No significant correlations were found between microdialysis and tape stripping methods as regarding the topical bioequivalence of oxytetracycline HCl formulations.

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.

In vivo methods for the assessment of topical drug bioavailability

This paper reviews some current methods for the in vivo assessment of local cutaneous bioavailability in humans after topical drug application. After an introduction discussing the importance of local drug bioavailability assessment and the limitations of model-based predictions, the focus turns to the relevance of experimental studies. The available techniques are then reviewed in detail, with particular emphasis on the tape stripping and microdialysis methodologies. Other less developed techniques, including the skin biopsy, suction blister, follicle removal and confocal Raman spectroscopy techniques are also described.

Fragrance material review on methyl salicylate

A toxicologic and dermatologic review of methyl salicylate when used as a fragrance ingredient is presented.

Emergency department management of the salicylate-poisoned patient

Salicylate toxicity continues to be encountered commonly in emergency medicine. This article portrays the signs and symptoms of salicylate toxicity, reviews the erratic absorption and elimination kinetics, describes the devastating physiologic effects of overdose, and illustrates the potentially subtle manifestations of chronic aspirin toxicity.

Evaluation of skin penetration of topically applied drugs in humans by cutaneous microdialysis: acyclovir vs. salicylic acid

BACKGROUND AND OBJECTIVE

Cutaneous drug application is used for both local drug therapy and systemic treatment. For both types of treatment, the drug concentration profile in, and transport across, the skin is important. To evaluate skin penetration of topically-applied drugs we recently used cutaneous microdialysis. The aim of this study was the use of this method for studying acyclovir and salicylic acid.

METHOD

Five per cent acyclovir cream was applied on intact and tape-stripped skin of healthy volunteers and 5% salicylic acid ointment-onto intact skin of other volunteers. Microdialysis probes with 2 kDa molecular weight cut-off were inserted intradermally and were perfused with Ringer solution. Drug concentrations were measured by high-performance liquid chromatography.

RESULTS

Following topical application of 5% acyclovir cream onto intact skin of eight healthy volunteers, no drug was determinable in the skin (cutaneous microdialysate) in any of the subjects studied. After partial removal of the stratum corneum the penetration of this drug into skin increased markedly. The mean maximum skin concentration was about 2 x 5 micromol/L after 2 x 4 +/- 0 x 7 h. Topically applied salicylic acid penetrated intact skin with a maximum concentration in the cutaneous microdialysate of 7 x 57 +/- 3 x 90 micromol/L after 5 x 3 +/- 0 x 4 h.

CONCLUSION

Cutaneous microdialysis is a valuable method for estimating skin concentration of topically-applied drug. It allows evaluation after application to a small skin area, of about 2 cm(2), thereby reducing the risk of systemic toxicity. The method may be helpful for evaluating the influence of skin condition on the transport process.

The effects of equine skin preparation on transdermal drug penetration in vitro

An increasing number of formulations are applied to equine skin, yet variable penetration can affect efficacy, or the incidence of adverse effects, or both. To investigate the effects of common methods of skin preparation on transdermal drug penetration in vitro, we clipped, harvested, and froze skin samples from 5 Thoroughbred geldings. Thawed samples were prepared as follows: control (no preparation); cleaned with aqueous chlorhexidine (Aq-C, 0.1% w/v); cleaned with alcoholic chlorhexidine (Al-C, 0.5% w/v); shaved (Sh); or tape-stripped (Ta) with the use of adhesive tape. The samples were then placed in diffusion cells, and 2 g of methylsalicylate (MeSa) gel (Dencorub) was applied to the stratum corneum side. The penetration of MeSa and its analyte, salicylate (Sa), through the skin samples was measured over 10 h. Compared with control skin, significantly more MeSa penetrated through skin prepared with Al-C or Sh (P < 0.01) or with Aq-C or Ta (P < 0.05), and significantly more Sa was recovered in the receptor phase from skin prepared with Aq-C, Al-C, or Sh (P < 0.05) or with Ta (P < 0.01). A significantly higher rate of penetration and shorter lag time were also noted for MeSa with all the prepared skin samples, compared with the control samples. The results show that clinical techniques routinely used to clean or prepare skin can significantly affect the rate and extent of penetration of a topically applied drug. This may result in greater systemic availability of active drug, which could lead to enhanced efficacy and, possibly, a higher incidence of adverse effects.

Kinetics of blood flow after topical application of benzyl nicotinate on different anatomic sites

Cutaneous characteristics, e.g., thickness of the SC and density of follicles, affect the penetration of topically applied substances. In the present study, the penetration of benzyl nicotinate, causing a vasodilation, was studied on three anatomic sites (forearm, forehead and calf) differing in their skin characteristics. Therefore, the blood flow of the superficial dermal plexus and that of the larger capillaries in the deeper skin layers was simultaneously measured using a laser Doppler flowmeter. In addition, the cutaneous temperature and redness were determined as a function of time. These four biological reactions were measured in turn on a skin area treated with a gel containing benzyl nicotinate and on an untreated control area. The highest basal levels were observed on the forehead. Topical application of benzyl nicotinate resulted in an increase in each biological response. Compared to the other sites, the maximal values were reached earliest on the forehead, which also showed the fastest decrease. No significant differences were obtained comparing the kinetic data of the calf with that of the forearm. The results indicate an additional contribution by the numerous vellus hair follicles of the forehead to the penetration and exposure of the drug.

Transdermal drug delivery: Basic principles for the veterinarian

The use of topical pharmaceutical formulations is increasingly popular in veterinary medicine. A potential concern is that not all formulations are registered for the intended species, yet current knowledge strongly suggests that simple extrapolation of transdermal drug pharmacokinetics and pharmacodynamics between species, including humans, cannot be done. In this review, an overview is provided of the underlying basic principles determining the movement of topically applied molecules into and through the skin. Various factors that may affect transdermal drug penetration between species, between individuals of a particular species and regional differences in an individual are also discussed. A good understanding of the basic principles of transdermal drug delivery is critical to avoid adverse effects or lack of efficacy when applying topical formulations in veterinary medicine.

Bioequivalence of topical formulations in humans: evaluation by dermal microdialysis sampling and the dermatopharmacokinetic method

The aim of this study was to evaluate the relationship between dermal microdialysis (DMD) sampling and the dermatopharmacokinetic method when employed simultaneously for bioequivalence (BE) investigations of topical formulations. Topical lidocaine cream and ointment (both 5%) was investigated in eight healthy human volunteers (four male, four female). On one forearm, four microdialysis probes in two penetration areas sampled for 5 hours, and on the other arm, tape stripping was performed 30 and 120 minutes after product application. Lidocaine content in samples was analyzed by HPLC-mass spectrometry. The two methods were in agreement showing 3- to 5-fold higher lidocaine penetration from cream formulation than from ointment. A rank-order correlation between the two methods was demonstrated for lidocaine contents in microdialysates versus tape strip at 120 minutes, significant for the ointment formulation and for both formulations analyzed together. Analysis of variance demonstrated reproducible lidocaine concentrations in microdialysates with an intrasubject variability of 19% between probes and 20% between the two penetration areas. Thus, intersubject variability accounted for 61% of the variance. DMD sampling proved effective and variability analyses demonstrated the feasibility of BE studies in as little as 18 subjects.

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.

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.

Differentiated in vivo skin penetration of salicylic compounds in hairless rats measured by cutaneous microdialysis

The purpose was to investigate the in vivo skin penetration of four 14C-salicylic compounds using microdialysis and to relate dermal concentrations to structural features. Furthermore, to compare two in vivo retrodialysis recovery methods for estimation of true unbound extracellular concentrations. Microdialysis probes were inserted in the dermis of hairless rats. Equimolal 14C-salicylic formulations were applied topically and dialysate sampled consecutively for 4h. True extracellular concentrations were estimated by retrodialysis by drug method (the 14C-salicylic compounds themselves) and by retrodialysis by calibrator method (3H-salicylic acid as internal standard). Probe depth was measured by ultrasound scanning. High dermal concentrations were found after application of 14C-salicylamide (low protein-binding) and the lipophilic ester 14C-butyl salicylate, which was completely hydrolysed to 14C-salicylic acid during skin diffusion. Protein binding and dissociation may explain the lower dermal concentrations of 14C-salicylic acid and 14C-diethylamine salicylate, respectively. Probe depth did not significantly influence dialysate concentrations. The two in vivo recovery correction methods did not reduce the variation in concentration-time curves. In conclusion, differentiated penetration kinetics was found ranking: 14C-salicylamide >/= 14C-butyl salicylate > 14C-salicylic acid > 14C-diethylamine salicylate. Dermal concentrations were related to structural features of the model compounds. The two correction methods performed alike; however, the calibrator method has the advantage of serving as a quality control during experiments.

The effect of topically applied salicylic compounds on serotonin-induced scratching behaviour in hairless rats

There is a strong need for antipruritic substances for treating itch in clinical dermatology. In one recent human study, topically applied acetylsalicylic acid has been described to rapidly decrease histamine-induced itch. We have established a model for periferally elicited pruritus by injecting serotonin into the rostral back area (neck) in rats. Using this model, we aimed to investigate the antipruritic potential of four different salicylic compounds, which all possess different skin penetration characteristics. Eighteen rats were studied for 6 weeks. Prior to serotonin injections (2 mg/ml, 50 micro l), 10 micro l of test substances was applied to a circular area 18 mm in diameter. The four substances were salicylic acid, butyl salicylate, diethylamine salicylate and salicylamide, all solubilized in dimethyl isosorbide to a concentration of 5% w/w. Diethylamine salicylate and salicylamide were previously shown to be slowly absorbed through rat skin in contrast to salicylic acid and butyl salicylate. After serotonin injections, scratching was monitored by video recording for 1.5 h. Compared with the vehicle, a lower number of scratch sequences were seen when diethylamine salicylate (P < 0.001) and salicylamide (P = 0.005) had been applied. The numbers of scratch sequences were lower with diethylamine salicylate and salicylamide than with the vehicle throughout the 1.5-h study period. We conclude that topical application of diethylamine salicylate and salicylamide could suppress serotonin-induced scratching in rats. The antipruritic effect seems to be related to the slow drug release of the two substances. The results may be clinically relevant as serotonin induces itch in humans.

Site-specific percutaneous absorption of methyl salicylate and VX in domestic swine

The site specificity of the percutaneous absorption of methyl salicylate (MeS) and the organophosphate nerve agent VX (O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothioate) was examined in anaesthetized domestic swine that were fully instrumented for physiological endpoints. Four different anatomical sites (ear, perineum, inguinal crease and epigastrium) were exposed to the MeS and the serum levels were measured over a 6-h time period. The dose absorbed at the ear region was 11 microg cm(-2) with an initial flux of 0.063 microg cm(-2)min(-1), whereas at the epigastrium region the dose absorbed was 3 microg cm(-2) with an initial flux of 0.025 microg cm(-2)min(-1). For this reason further studies were carried out with VX on the ear and the epigastrium only. In animals treated with agent on the epigastrium, blood cholinesterase (ChE) activity began to drop 90 min after application and continued to decline at a constant rate for the remainder of the experiment to ca. 25% of awake control activity. At this time there were negligible signs of poisoning and the medical prognosis was judged to be good. In contrast, the ChE activity in animals receiving VX on the ear decreased to 25% of awake control values within 45 min and levelled out at 5-6% by 120 min. Clinical signs of VX poisoning paralleled the ChE inhibition, progressing in severity over the duration of the exposure. It was judged that these animals would not survive. The dramatic site dependence of agent absorption leading to vastly different toxicological endpoints demonstrated in this model system has important ramifications for chemical protective suit development, threat assessment, medical countermeasures and contamination control protocols.