Percutaneous absorption

Topical drug delivery strategies for enhancing drug effectiveness by skin barriers, drug delivery systems and individualized dosing

Topical drug delivery is widely used in various diseases because of the advantages of not passing through the gastrointestinal tract, avoiding gastrointestinal irritation and hepatic first-pass effect, and reaching the lesion directly to reduce unnecessary adverse reactions. The skin helps the organism to defend itself against a huge majority of external aggressions and is one of the most important lines of defense of the body. However, the skin's strong barrier ability is also a huge obstacle to the effectiveness of topical medications. Allowing the bioactive, composition in a drug to pass through the stratum corneum barrier as needed to reach the target site is the most essential need for the bioactive, composition to exert its therapeutic effect. The state of the skin barrier, the choice of delivery system for the bioactive, composition, and individualized disease detection and dosing planning influence the effectiveness of topical medications. Nowadays, enhancing transdermal absorption of topically applied drugs is the hottest research area. However, enhancing transdermal absorption of drugs is not the first choice to improve the effectiveness of all drugs. Excessive transdermal absorption enhances topical drug accumulation at non-target sites and the occurrence of adverse reactions. This paper introduces topical drug delivery strategies to improve drug effectiveness from three perspectives: skin barrier, drug delivery system and individualized drug delivery, describes the current status and shortcomings of topical drug research, and provides new directions and ideas for topical drug research.

Influence of Dose, Particle Size and Concentration on Dermal Penetration Efficacy of Curcumin

The influence of size, particle concentration and applied dose (finite vs. infinite dose) on the dermal penetration efficacy of curcumin was investigated in this study. For this, curcumin suspensions with different particle sizes (approx. 20 µm and approx. 250 nm) were produced in different concentrations (0.625-5% (w/w)). The dermal penetration efficacy was determined semi-quantitatively on the ex vivo porcine ear model. The results demonstrated that the presence of particles increases the dermal penetration efficacy of the active compounds being dissolved in the water phase of the formulation. The reason for this is the formation of an aqueous meniscus that develops between particles and skin due to the partial evaporation of water from the vehicle after topical application. The aqueous meniscus contains dissolved active ingredients, and therefore creates a small local spot with a locally high concentration gradient that leads to improved dermal penetration. The increase in penetration efficacy depends on the number of particles in the vehicle, i.e., higher numbers of particles and longer contact times lead to higher penetration efficacy. Therefore, nanocrystals with a high particle concentration were found to be the most suitable formulation principle for efficient and deep dermal penetration of poorly water-soluble active ingredients.

Regional variation in percutaneous absorption in in vitro human models: a systematic review

Percutaneous absorption is of importance given its role in topical medicaments, transdermal drug systems, and dermatotoxicology. Many factors influence percutaneous penetration, including anatomical region, although little is currently known regarding this parameter. Hence, the aim of this study was to summarize existing data on regional variation in percutaneous penetration in in vitro human models. PubMed, Embase, Web of Science, and US patent literature were explored, and relevant data collected. Eight eligible articles were identified, which together, explored 15 anatomical locations. Four investigations compared percutaneous penetration between scalp and abdominal skin, and all concluded that the former was more permeable. Within those four studies, 10 penetrants of varying physical/chemical properties were tested indicating that in those particular study conditions, anatomical location exerted a greater effect on percutaneous absorption than the physicochemical properties of the penetrants. In addition, torso area was less absorptive than scrotum in both studies in which these sites were compared. In conclusion, the scrotum and scalp appear to be highly susceptible to percutaneous absorption compared to other locations such as the abdomen. This is postulated to be largely due to the high density of hair follicles in these areas, enabling greater penetration via the appendageal pathway. However, there is a paucity of conclusive data regarding the penetrability of other anatomical locations. Investigations testing and ranking the susceptibility of different anatomical regions is of vital importance given the importance of (1) transdermal drug delivery and decontamination protocols and (2) understanding the underlying mechanisms and degree of these variances might aid our pharmacologic/toxicologic judgments.

Nanotechnology-based Drug Delivery Systems as Potential for Skin Application: A Review

Administration of substances through the skin represents a promising alternative, in relation to other drug administration routes, due to its large body surface area, in order to offer ideal and multiple sites for drug administration. In addition, the administration of drugs through the skin avoids the first-pass metabolism, allowing an increase in the bioavailability of drugs, as well as reducing their side effects. However, the stratum corneum (SC) comprises the main barrier of protection against external agents, mainly due to its structure, composition and physicochemical properties, becoming the main limitation for the administration of substances through the skin. In view of the above, pharmaceutical technology has allowed the development of multiple drug delivery systems (DDS), which include liquid crystals (LC), cubosomes, liposomes, polymeric nanoparticles (PNP), nanoemulsions (NE), as well as cyclodextrins (CD) and dendrimers (DND). It appears that the DDS circumvents the problems of drug absorption through the SC layer of the skin, ensuring the release of the drug, as well as optimizing the therapeutic effect locally. This review aims to highlight the DDS that include LC, cubosomes, lipid systems, PNP, as well as CD and DND, to optimize topical skin therapies.

Enhancing Permeation of Drug Molecules Across the Skin via Delivery in Nanocarriers: Novel Strategies for Effective Transdermal Applications

The transdermal route of administration provides numerous advantages over conventional routes i.e., oral or injectable for the treatment of different diseases and cosmetics applications. The skin also works as a reservoir, thus deliver the penetrated drug for more extended periods in a sustained manner. It reduces toxicity and local irritation due to multiple sites for absorption and owes the option of avoiding systemic side effects. However, the transdermal route of delivery for many drugs is limited since very few drugs can be delivered at a viable rate using this route. The stratum corneum of skin works as an effective barrier, limiting most drugs' penetration posing difficulty to cross through the skin. Fortunately, some non-invasive methods can significantly enhance the penetration of drugs through this barrier. The use of nanocarriers for increasing the range of available drugs for the transdermal delivery has emerged as a valuable and exciting alternative. Both the lipophilic and hydrophilic drugs can be delivered via a range of nanocarriers through the stratum corneum with the possibility of having local or systemic effects to treat various diseases. In this review, the skin structure and major obstacle for transdermal drug delivery, different nanocarriers used for transdermal delivery, i.e., nanoparticles, ethosomes, dendrimers, liposomes, etc., have been discussed. Some recent examples of the combination of nanocarrier and physical methods, including iontophoresis, ultrasound, laser, and microneedles, have also been discussed for improving the therapeutic efficacy of transdermal drugs. Limitations and future perspectives of nanocarriers for transdermal drug delivery have been summarized at the end of this manuscript.

Unplanned absorption of sunscreen ingredients: Impact of formulation and evaluation methods

Permeation of sunscreens agents reduces its effectiveness and safety, leading to systemic circulation and causing unknown adverse effects. In order to maintain the sunscreen efficacy and safety, the filters must stay on the skin surface, with minimum penetration through dermis. Even facing the possibility of filters permeation, the use of sunscreen is important to avoid skin damage as erythema, free-radicals formation, skin ageing and skin cancer, caused by ultraviolet radiation. Aiming potential side effects caused by topical absorption of sunscreens, studies are carried to improve formulation characteristics and stability, reduce skin permeation and evaluate sun protections factor (SPF). Current assays to detect the permeation of sunscreens involve in vivo or in vitro studies, to simulate physiological conditions of use. The aim of this review is to revisit sunscreen skin permeation data over the last decade and the factors that can enhance skin permeation or improve the sunscreen efficacy.

Can cyanotoxins penetrate human skin during water recreation to cause negative health effects?

Cyanobacteria blooms and associated cyanotoxins pose significant public health risks during water recreation. Oral ingestion is the only recognized route of toxin exposure in water recreation guidelines. This review examines human skin as a barrier for the prevention of cyanotoxin absorption and investigates the likelihood of negative health effects through dermal exposure. Epidemiological studies of health effects from recreational exposure to algal blooms and toxins are summarized to highlight the importance of better understanding the toxicological effect of dermal exposure. The ability of a specific cyanotoxin to penetrate human skin is inferred by its physiochemical properties according to transdermal drug studies. The review identifies a disparity between the human health effects described in algal bloom exposure case studies and the toxicological skin exposure data. Skin penetration by algal toxins is likely and deserves further investigation.

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.

A Novel Method for Studying the Pharmacokinetics of [(14) C]Umeclidinium After Application to the Axilla or Palm of Healthy Male Subjects

Umeclidinium (UMEC), a long-acting muscarinic antagonist approved for chronic obstructive pulmonary disease (COPD), was investigated for primary hyperhidrosis as topical therapy. This study evaluated the pharmacokinetics, safety, and tolerability of a single dose of [(14) C]UMEC applied to either unoccluded axilla (UA), occluded axilla (OA), or occluded palm (OP) of healthy males. After 8 h the formulation was removed. [(14) C]UMEC plasma concentrations (Cp) were quantified by accelerator mass spectrometry. Occlusion increased systemic exposure by 3.8-fold. Due to UMEC absorption-limited pharmacokinetics, Cp data from the OA were combined with intravenous data from a phase I study. The data were described by a two-compartment population model with sequential zero and first-order absorption and linear elimination. Simulated systemic exposure following q.d. doses to axilla was similar to the exposure from the inhaled therapy, suggesting that systemic safety following dermal administration can be bridged to the inhaled program, and offering the potential for a reduced number of studies and/or subjects.

Evaluation of leishmanicidal activity and cytotoxicity of Ricinus communis and Azadirachta indica extracts from western Kenya: in vitro and in vivo assays

BACKGROUND

Despite advances to targeted leishmanicidal chemotherapy, defies around severe toxicity, recent emergence of resistant variants and absence of rational vaccine still persist. This necessitates search and/or progressive validation of accessible medicinal remedies including plant based. The study examined both in vivo and in vitro response of L. major infection to combined therapy of Ricinus communis and Azadirachta indica extracts in BALB/c mice as the mouse model. A comparative study design was applied.

RESULTS

BALB/c mice, treated with combination therapy resulted in significantly (p < 0.05) larger reduction of lesion than those treated with monotherapies. The spleno-somatic index was found to be significantly low with combination therapy than monotherapies. Antiparasitic effect of A. indica and R. communis on amastigote with a 50 % inhibitory concentration (IC50) was of 11.5 and 16.5 µg mL(-1) respectively while combination therapy gave 9.0 µg ml(-1) compared to the standard drugs, Pentostam and amphotericin B which had an IC50 of 6.5 and 4.5 µg ml(-1) respectively. Optimal efficacy of A. indica and R. communis was 72 and 59.5 % respectively, combination therapy gave 88 %, while Pentostam and amphotericin B had 98 and 92 % respectively against amastigotes. Against promastigotes A. indica and R. Communis gave an IC50 of 10.1, 25.5 µg mL(-1) respectively, while combination, 12.2 µg mL(-1) against 4.1 and 5.0 µg ml(-1) for Pentostam and amphotericin B respectively. The optimal efficacy of the compounds against promastigotes was 78.0, 61.5 and 91.2 % (A. indica, R. communis and A. indica + R. communis respectively) against 96.5 and 98 % for Pentostam and amphotericin B respectively. The concentrations at optimal efficacy were significantly different (p < 0.05) among the test compounds. An evaluation of the IC50 values of the combination therapies clearly reveals synergistic effects.

CONCLUSION

Combination therapy of A. indica and R. communis had best antileishmanial activity than the monotherapies. The active ingredients of both R. communis and A. indica need to be fractionated, and studied further for activity against Leishmania parasites.

Prodrug strategies for enhancing the percutaneous absorption of drugs

The transdermal application of drugs has attracted increasing interest over the last decade or so, due to the advantages it offers, compared to other delivery methods. The development of an efficient means of transdermal delivery can increase drug concentrations, while reducing their systemic distribution, thereby avoiding certain limitations of oral administration. The efficient barrier function of the skin, however, limits the use of most drugs as transdermal agents. This limitation has led to the development of various strategies to enhance drug-skin permeation, including the use of penetration enhancers. This method unfortunately has certain proven disadvantages, such as the increased absorption of unwanted components, besides the drug, which may induce skin damage and irritancy. The prodrug approach to increase the skin’s permeability to drugs represents a very promising alternative to penetration enhancers. The concept involves the chemical modification of a drug into a bioreversible entity that changes both its pharmaceutical and pharmacokinetic characteristics to enhance its delivery through the skin. In this review; we report on the in vitro attempts and successes over the last decade by using the prodrug strategy for the percutaneous delivery of pharmacological molecules.

An in vitro model for identifying skin-corrosive chemicals. I. Initial validation

An in vitro epidermal slice technique has been developed for identifying chemicals with the potential to cause a corrosive lesion in animal skin in vivo. Skin-corrosive potential has been correlated with the ability to reduce the skin's penetration barrier by lysis of the stratum corneum. This effect was measured as a lowering of the electrical resistance of an epidermal slice following chemical contact in vitro. An initial validation with 68 chemicals showed the technique to have a high sensitivity for corrosive chemicals. The model has potential as a pre-screen for conventional animal tests and, in contrast to in vivo screening methods, has the advantage of providing quantitative and objective data.

Iontophoretic permeation of lisinopril at different current densities and drug concentrations

PURPOSE

The purpose of the present work was to assess iontophoretic permeation of Lisinopril at different current densities and concentrations for development of patient-controlled active transdermal system.

METHODS

In vitro iontophoretic transdermal delivery of Lisinopril across the pigskin was investigated at three different drug concentrations and three different current densities (0.25- 0.75 mA/cm2) in the donor cell of the diffusion apparatus, using cathodal iontophoresis along with the passive controls.

RESULTS

For passive permeation, the steady state flux significantly increased with the increasing of donor drug concentration. At all concentration levels, iontophoresis considerably increased the permeation rate compared to passive controls. Iontophoretic transport of Lisinopril was to be found increase with current densities. Flux enhancement was highest at the lowest drug load and lowest at the highest drug load.

CONCLUSION

The obtained results indicate that permeation rate of Lisinopril across the pigskin can be considerably enhanced, controlled or optimized by the use of Iontophoresis technique.

Transdermal delivery enhancement of haloperidol from gel formulations by 1,8-cineole

The feasibility of using 10% 1,8-cineole as an enhancer for transdermal delivery of haloperidol has been examined. In-vitro transdermal delivery across full-thickness human, rabbit and hairless mouse skins was measured from three polymer gel systems, hypromellose (hydroxypropylmethylcellulose), Carbomer (Carbopol) 940 and macrogol (polyethylene glycol) using Franz cells. Values for the permeability coefficient kp, calculated as the product (Kh)x(D/h2) where these two factors were obtained from curve fitting of the non-steady-state equation over 24 h, were similar from the three formulations. The value of kp from hypromellose was significantly enhanced by cineole by factors of 6.2 (4.6-8.1), 5.6 (5.0-6.2) and 3.0 (2.6-3.4) for human, rabbit and mouse, respectively (mean and 95% confidence intervals). Enhancement ratios for K: 13.3 (8.3-20), 3.1 (2.5-3.9) and 2.0 (1.5-2.6), were higher than those for D: 0.47 (0.41-0.55), 1.8 (1.6-2.1) and 1.5 (1.3-1.8). This suggested that the barrier function of the skin lipids was marginally affected and the main effect was to increase the thermodynamic activity of the drug in the barrier. The enhancement achieved in human skin suggested that delivery could be safely enhanced by terpenoids.

Fatty acid interdigitation in stratum corneum model membranes: a neutron diffraction study

The influence of the chain length of the free fatty acid (FFA) in a stratum corneum (SC) lipid model membrane composed of N-(alpha-hydroxyoctadecanoyl)-phytosphingosine (CER [AP]), cholesterol (Ch), FFA and cholesterol sulphate (ChS) was investigated by neutron diffraction. The internal nanostructure of the SC lipid membrane in addition to the water distribution function was determined via calculation of the neutron scattering length density profile (Fourier profile). The Fourier profiles of the studied SC model membranes revealed that such membranes have a repeat distance approximately equal to the membrane thickness. Increasing the chain length of the FFA in the CER[AP] based model membrane did not cause an alteration of the internal nanostructure but led to a decrease in the membrane repeat distance from 45.6 angstroms (palmitic acid, C16:0) to 43.7 angstroms (cerotic acid, C26:0) due to a partial interdigitation of the FFA chains. Ceramide [AP] forces the long chain fatty acids to incorporate into the unchanged spacing of the bilayer, thereby obligating the FFA protrude partly through opposing leaflet. Furthermore, the longer chained free fatty acids tend to form a new separate so-called "fatty acid rich phase". Therefore, the elongation of the chain length of the FFA decreases the solubility of the FFA in the SC model membrane based on CER[AP].

Functional map and age-related differences in the human face: nonimmunologic contact urticaria induced by hexyl nicotinate

Variation in human skin reactivity to various irritants in association with age and body region has been reported. Hexyl nicotinate (HN), a lipophilic nicotinate ester, was used to induce nonimmunologic contact urticaria in human volunteers of 2 age groups: 10 young subjects [24-34 years, mean +/- standard deviation (SD) 29.8 +/- 3.9 years] and 10 older volunteers (66-83 years, mean +/- SD 73.6 +/- 17.4 years); and to define skin function and potential age-related differences in various facial areas. About 5 mM of HN in ethanol was applied to 8 locations on the face, neck, and volar forearm. A laser Doppler flowmeter was used to determine baseline blood flow and to monitor the skin blood flow changes after HN application. In the contralateral areas, stratum corneum turnover was determined using 5% dansyl chloride in petrolatum. In the young group, the perioral area exhibited the strongest reaction to HN. In the older group, the chin was the most sensitive site. In both the groups, the forearm was the least responsive. The older group demonstrated a stronger reaction than the younger group in 3 sites (forehead, cheek, and nasolabial area). Stratum corneum turnover was slower in the nasolabial area and in the forearm in both age groups, whereas the fastest was in the perioral area and the chin in the younger group and in the chin and the forehead in the older group. Compared to the older group, the younger group showed a slower stratum corneum turnover in the nose and the neck. This study demonstrates the regional and the age-related variability of the stratum corneum turnover and the skin reactions to HN. These observations may help explain some aspects of the cutaneous intolerance in skin care of the face.

In vitro release studies of flurbiprofen from different topical formulations

The release profiles of flurbiprofen (F) from different gel and ointment formulations were studied in order to evaluate factors governing the release process. Carbopol 934P (CAB), poloxamer 407 (POL), and eudragit S100 (EUD) gel bases were used, while emulsion (EML) and polyethylene glycol (PEG) ointments were employed. The release studies were conducted using membraneless diffusion cells and lipophilic receptor medium, isopropyl myristate (IPM). The effects of gelling agent concentrations and the initial drug load on drug release were determined. Hydrogels were observed to give higher amounts of drug release than hydrophobic EUD gel and ointments, despite the lower bulk viscosity of these bases. Flurbiprofen release from CAB gels was 3.06-1.56-fold higher than from other formulations. Over a 4-hr period, the amount of F released was 492.8 and 316.0 micrograms/cm2 from 2% CAB and 25% POL gels, while it was 213.05, 168.61, and 160.9 micrograms/cm2 from EML, 40% EUD, and PEG bases, respectively. The diffusivity of F in the gel bases was an inverse function of the polymer concentrations over the range of 1-3% CAB, 20-30% POL, and 35-45% EUD gels. Drug release was increased from the bases as the initial F concentration increased over the range 0.25-1.0%, while the diffusion coefficient observed an inverse relationship. The CAB and POL gels could be the vehicles of choice for the rapid release and onset of F after topical application.

Prediction of steady-state skin permeabilities of polar and nonpolar permeants across excised pig skin based on measurements of transient diffusion: characterization of hydration effects on the skin porous pathway

The applicability of a two-parameter Fickian diffusion model for predicting the skin steady-state permeability based on measurements of the transient transport of permeants across the skin was tested. Using five model permeants possessing different physicochemical properties and pig skin as the model membrane, the skin permeabilities predicted by the two-parameter Fickian diffusion model were compared with the measured skin permeabilities. Results show that the transient skin permeation profiles of the hydrophobic permeants, estradiol, testosterone, and dolichol, across split-thickness pig skin can be modeled adequately by the two-parameter Fickian diffusion model (with constant parameter values), and therefore, that this model can be utilized to shorten the experimental time required to determine the skin permeabilities of these compounds. However, the skin permeabilities of the highly hydrophilic permeants, mannitol and sucrose, predicted by the two-parameter Fickian diffusion model (with constant parameter values) were significantly lower than the experimentally determined values, indicating that the dominant skin pathway of polar permeants within the excised pig skin undergoes significant structural changes during the in vitro diffusion cell studies. Although the skin permeability values determined experimentally using the traditional steady-state method normally correspond to a highly hydrated skin sample, the two-parameter Fickian diffusion model enables an estimation of the skin permeability of the skin membrane at its less-hydrated state (a condition more representative of in vivo and clinical situations). Using the two-parameter Fickian diffusion model and a recently developed skin porous-pathway theory, the effects of skin hydration on the skin porous pathway within the excised pig skin were characterized. Specifically, we found that hydration leads to induction of new pores/reduction of the tortuosity of existing pores within the excised pig skin during the 48 h diffusion cell studies conducted, while the skin average pore radii remain relatively constant (approximately 26 A) for up to 48 h.

Quantitative structure-permeability relationships (QSPRs) for percutaneous absorption

Quantitative structure-permeability relationships (QSPRs) have been derived by many researchers to model the passive, diffusion-controlled, percutaneous penetration of exogenous chemicals. Most of these relationships are based on experimental data from the published literature. They indicate that molecular size (as molecular weight) and hydrophobicity (as the logarithm of the octanol-water partition coefficient; log k(ow)) are the main determinants of transdermal penetration. This article reviews the current state of the art in QSPRs for absorption of chemicals through the skin, and where this technology can be exploited in future research. The main shortfalls in QSPR models result from inconsistency and error of the experimental values used to derive them. This is probably caused by the manner in which they employ data from a variety of sources and, in some cases, slightly different experimental protocols. Further, most current models are based on data generated from either aqueous or ethanolic solution, where each penetrant is present at its saturated solubility or a fraction of its saturated solubility. No models currently account for the influences of formulation upon percutaneous penetration. Current QSPR models provide a significant tool for assessing the percutaneous penetration of chemicals. They may be important in determining the bioavailability of a range of topically applied exogenous chemicals, and in issues of dermal toxicology and risk assessment. However, their current use may be limited by their lack of applicability across different formulation types. As a consequence, their true value may be to make predictions within specific formulation types, as opposed to a general model based on a range of formulation types. In addition, the endpoint of models may be inappropriate for specific applications other than the systemic delivery of topically applied chemicals.

Chemomorphic analysis of malathion in skin layers of the rat: implications for the use of dermatopharmacokinetic tape stripping in exposure assessment to pesticides

The dermatopharmacokinetic (DPK) method of dermal tape stripping may prove to be a valuable addition to risk assessment protocols for toxic substances as it has been for the assessment of bioequivalence and bioavailability of topical dermatologic drugs. The measurement of drug penetration into stratum corneum (SC) with respect to time is thought to be comparable with drug distribution in underlying tissues. To examine this possibility, the dermal penetration and absorption characteristics of [(14)C]malathion in the Sprague-Dawley rat was examined by three analytical techniques. [(14)C]Malathion was applied in different vehicles for 30-min and 1-h periods of exposure. Penetration into the SC was assessed by tape stripping followed by instant electronic autoradiography (IEA). Also, the (14)C activity retained in three successive 16 microm sections of the skin application site was determined by IEA and malathion was identified by Fourier transform infrared microscopy (FTIR microscopy). Absorbed [(14)C]malathion was measured in selected tissues, organs, and the residual carcass by liquid scintillation counting (LSC). Penetration into the SC followed a linear trend. The capacity of the SC reservoir for malathion amounted to approximately 1% of the dermal dose, while approximately 6% of the dose was absorbed. Results from this study support the view that LSC remains the method of choice to efficiently and reliably quantify absorption of a radiolabeled test substance. IEA offers the ability of the user to visualize the extent and profile of dermal absorption. When IEA is combined with FTIR microscopy, an effectual tool for studying the penetration of chemicals into layers of the skin emerges. The combined use of the three analytical techniques can be used to test the validity of the DPK method in hazard evaluation and exposure assessment of the organophosphorus insecticides.

Lipid extraction and transport of hydrophilic solutes through porcine epidermis

The purpose of this study was to investigate the effect of delipidization of the stratum corneum (SC) on the in vitro percutaneous absorption of hydrophilic solutes (i.e. water, urea, and inulin). Fourier transform infrared (FT-IR) spectroscopy was employed to study the extent of delipidization of porcine SC due to chloroform:methanol (2:1) (C:M (2:1)) treatments for various time periods. In vitro percutaneous absorption of [3H] water, [14C] urea, and [3H] inulin were studied through C:M (2:1) treated epidermis in Franz diffusion cells. There was a greater decrease in peak areas of the asymmetric and symmetric C-H stretching absorbances (i.e. increase in lipid extraction) with increasing exposure times of the SC with C:M (2:1). After 40-min treatment, asymmetric and symmetric C-H stretching peak area showed a decrease of 75.9 and 89.9%, respectively. The permeability coefficient of water, urea, and inulin increased with increasing lipid extraction. Enhancement in the permeability coefficient, through 40 min C:M (2:1) treated epidermis in comparison to the control, for water, urea, and inulin was 48.72, 215.65, and 3.90, respectively. Log (permeability coefficient) and log (mol. wt.) for test solutes and leuprolide acetate were found to be inversely related (R(2)=0.9974). In conclusion, this study implies that penetration enhancers that are safe and extract the SC lipids can be selected in order to enhance the percutaneous absorption of polar solutes through the skin.

Interrelation of permeation and penetration parameters obtained from in vitro experiments with human skin and skin equivalents

In a comparative study, two different in vitro cutaneous test systems were examined: (1) The Franz diffusion cell (FD-C), a test system to study drug permeation through the skin and to obtain data like steady state flux and lag time as well as permeability and diffusion coefficients. (2) The Saarbruecken penetration model (SB-M), a test system to investigate drug penetration into different skin layers and after varying incubation times to acquire values about the quasi steady state drug amounts in the stratum corneum (SC). Three drug concentrations (0.9, 0.45 and 0.225%) of a lipophilic model drug preparation, flufenamic acid in wool alcohols ointment, were applied on the skin's surface using 'infinite dose' conditions. Trypsin-isolated SC, heat-separated epidermis, full-thickness skin and reconstructed human skin (RHS) served as skin membranes in the FD-C, while the SB-M experiments were only carried out using full-thickness skin. Increasing steady state flux data and m(ss) values (steady state drug amount in the SC) were detectable after the application of rising drug amounts. Concerning the permeability of the used skin membranes in establishing barrier properties, the following rank order was observed: RHS>SC> or =epidermis>full skin. The flux data of the FD-C experiments for isolated SC, separated epidermis and RHS were linearly related with the m(ss) values of the SB-M investigations, allowing a direct comparison of permeation with penetration parameters. Concerning the drug amount in the SC, previous investigations succeeded in the establishment of an in vivo/in vitro correlation. Based on the results presented here, the prediction of drug amounts present in the SC after different incubation times in vivo is now possible after penetration as well as permeation experiments using the lipophilic model drug preparation, flufenamic acid in wool alcohols ointment.

Histopathologic effects of cutaneous tape stripping in pigs

OBJECTIVE

The stratum corneum (SC) is the major barrier to topical absorption of medications. Skin tape stripping (TS) removes the stratum corneum, allowing more rapid absorption of drugs such as local anesthetics. Prior to evaluating TS in human clinical trials, this study was performed to evaluate its immediate and delayed histopathologic effects in swine.

METHODS

This was a prospective, interventional, longitudinal, experimental animal study using two isoflurane-anesthetized young swine. Cellophane tape was applied to the skin of clipped swine flanks and peeled away 10, 15, 20, 25, or 30 times. Each level of tape stripping was performed three times in each pig. Full-thickness biopsies were taken at 30 minutes and two weeks later for blinded histopathologic evaluation by a dermatopathologist using randomly ordered hematoxylin-and-eosin-stained tissue sections and conventional light microscopy. The absolute thickness of the cornified layer was measured and compared with normal (unstripped) cornified layer.

RESULTS

Tape stripping up to 30 times produces thinning of the SC without detectable changes in the underlying epidermis and dermis at 30 minutes and 14 days post-stripping. The degree of thinning was proportional to the number of tape applications. Complete recovery of the cornified layer was noted at two weeks. There were no adverse effects, such as infection, scarring, or inflammatory cell infiltrates.

CONCLUSIONS

Under the conditions studied, TS not only appears safe, but appears to produce no long-term sequelae.

Water disrupts stratum corneum lipid lamellae: damage is similar to surfactants

Using electron microscopy, we investigated the effect of (i) a dilute surfactant and of water alone on the ultrastructure of stratum corneum lipids in pig skin exposed in vitro at 46 degrees C, and (ii) of water alone on human skin exposed in vivo at ambient temperature. For pig skin, the surfactant sodium dodecyl sulfate disrupts stratum corneum intercellular lamellar bilayers, leading to bilayer delamination and "roll-up" in a water milieu after 1 h, extensive bilayer disruption after 6 h, and nearly complete dissociation of corneocytes after 24 h. Corneodesmosomes show progressive degradation with exposure time. Water alone also disrupts the stratum corneum, but with a slower onset. Alterations in intercellular lamellar bilayers, but not intercellular lamellar bilayer roll-up, are detected after 2 h. Intercellular lamellar bilayer roll-up occurs after 6 h. Extensive dissociation of corneocytes occurs after 24 h of water exposure. Unlike sodium dodecyl sulfate, water exposure results in the formation of amorphous intercellular lipid. Corneodesmosome degradation parallels intercellular lamellar bilayer disruption; calcium appears to offer some protection. Similar disruption of intercellular lamellar bilayers occurs in human skin in vivo at ambient temperature. Our studies show that water can directly disrupt the barrier lipids and are consistent with surfactant-induced intercellular lamellar bilayer disruption being due at least in part to the deleterious action of water. Intercellular lamellar bilayer disruption by water would be expected to enhance permeability and susceptibility to irritants; accordingly, increased attention should be given to the potential dangers of prolonged water contact. For common in vitro procedures, such as skin permeation studies or isolation of stratum corneum sheets, exposure to water should also be minimized.

Wet decontamination-induced stratum corneum hydration--effects on the skin barrier function to diethylmalonate

Decontamination of chemical agents from the skin uses both dry and wet decontamination processes. Recent studies have shown that wet decontamination frequently results in stratum corneum hydration. To evaluate the hydration effect of wet decontamination on the skin barrier function and hence on the decontamination efficiency, a series of comparative studies were carried out on human skin contaminated with the nerve agent simulant diethylmalonate, using decontamination media having different salinity and surfactants. The results showed that, compared to non-decontaminated skin, remnant diethylmalonate on decontaminated skin penetrated at an accelerated rate in the immediate 2 h following decontamination. This transient enhancement effect, ranging from 20 to 98%, was depended on the nature of the decontamination media used and was more obvious in skin samples that were decontaminated 1 h postexposure. All decontamination media exhibited this effect, with the greatest enhancement observed in the following order: anionic surfactant > cationic surfactant > non-ionic surfactant > deionized water > 0.9% saline > 9% saline.

Influences of bathing and hot weather on the pharmacokinetics of a new transdermal clonidine, M-5041T

The influences of bathing and hot weather on plasma concentrations of clonidine were examined during application of a new transdermal clonidine system, M-5041T, in eight healthy volunteers. An M-5041T patch containing 6 mg of clonidine was applied on the right chest for 96 hours during winter with or without bathing (40 degrees C for 5 minutes) and also during summer without bathing. Plasma concentrations and urinary excretion of clonidine were determined for a 160-hour period after application. Plasma concentrations of clonidine in the winter trials did not differ significantly with or without bathing. Plasma drug concentration as a whole was significantly higher in the summer trial than in the winter trial, however. The maximum plasma concentration (Cmax), area under the plasma concentration-time curve (AUC), and urinary excretion (Ae) of clonidine also tended to be higher in the summer trial. These results suggest that plasma levels of clonidine are higher after application of M-5041T during hot weather. Although the period of bathing used in this study was short, the influence of bathing seems to be negligible.

In vitro evaluation of a series of N-dodecanoyl-L-amino acid methyl esters as dermal penetration enhancers

A series of N-dodecanoyl-L-amino acid methyl esters (1-10) and n-pentyl N-acetylprolinate (11) were evaluated for dermal enhancement properties using an in vitro diffusion cell technique. Methods of synthesis of these compounds were described. Enhancers were applied 1 h prior to drug treatment. Hydrocortisone was used as the model drug and was applied to excised hairless mouse skin as a saturated suspension in propylene glycol. Enhancement ratios (ER) were determined for permeability coefficient, 24 h diffusion cell receptor concentration (Q24), and 24 h full-thickness skin steroid content. Controls received no enhancer pretreatment of the skin. N-Dodecanoyl-L-proline (10) showed the highest Q24 value for total steroid (ER 13.7) while N-dodecanoyl-L-phenylalanine (5) showed the highest total steroid skin retention (ER 16.5).

Serum concentrations of salicylic acid following topically applied salicylate derivatives

OBJECTIVE

To compare the rate and extent of systemic salicylate absorption following single and multiple applications of two topically applied analgesics, one containing methyl salicylate and the other containing trolamine salicylate.

DESIGN

Two-period, two-treatment, randomized, crossover, multiple-dose study in healthy men and women volunteers.

PARTICIPANTS

Six men and six women volunteers, 21-44 years of age.

INTERVENTIONS

Subjects applied 5 g of an ointment containing 12.5% methyl salicylate twice daily for 4 days (8 doses) or a cream containing trolamine 10% twice daily for two doses, to a 10-cm2 area on the thigh. Treatment order and leg (right or left) were assigned randomly. Subjects were crossed over to the alternate treatment on the other leg after a minimum washout period of 7 days.

MAIN OUTCOME MEASURES

The total amount of salicylate recovered in the urine during two dosing intervals (24 hours) on each study day, relative to the applied dose, was used to calculate the bioavailability of each product. Mean standard pharmacokinetic parameters including area under the curve, maximum concentration (Cmax), time to maximum concentration, and minimum concentrations at steady-state were determined from serum concentrations. Serum concentrations were fit to three pharmacokinetic models and the suitability of each model was evaluated. Estimates of absorption rate constant, clearance, volume, and fraction absorbed on day 1 were estimated by using the best-fitting model.

RESULTS

Salicylic acid could not be detected in serum after trolamine application. However, concentrations between 0.31 and 0.91 mg/L were detected within 1 hour of the first application of methyl salicylate and Cmax between 2 and 6 mg/L were observed following the seventh application on day 4. Both the extent and rate of absorption changed after the first 24 hours. The absorption rate constant increased significantly from the first to the seventh dose (first dose absorption rate constant: 0.16 h-1, seventh dose: 0.28 h-1; p < 0.035). Urinary recovery of total salicylate (salicylic acid and principal metabolites of salicylic acid) during the first 24 hours of the methyl salicylate phase averaged 175.2 mg, exceeding the 6.9 mg (p < 0.05) recovered during the trolamine phase. The recovery of salicylate in the urine in the first 24 hours after application of methyl salicylate was significantly greater than the 1.4% recovered after application of trolamine (p < 0.05). Furthermore, the fraction of methyl salicylate recovered in the urine increased significantly from 15.5% on day 1 to approximately 22% on the second, third, and fourth days.

CONCLUSIONS

A considerable amount of salicylic acid may be absorbed through the skin after topical application of methyl salicylate products and this may increase with multiple applications. Caution is warranted in patients for whom systemic salicylate may be hazardous or problematic.

Percutaneous absorption of ketoprofen. I. In vitro release and percutaneous absorption of ketoprofen from different ointment bases

Ketoprofen (KP) is a potent non-steroidal anti-inflammatory drug which is used for the treatment of rheumatoid arthritis. The oral administration of KP can cause gastric irritation and renal adverse effects. Topical application of the drug can bypass gastrointestinal disturbances and provide relatively consistent drug levels at the site of action. Since the efficacy of an ointment depends on the type of ointment base and the concentration of the drug, four different bases (white petrolatum, cold cream, hydrophilic ointment and Carbopol 940 gel) were used at 1, 3, 5, 7 and 10% concentrations of KP to evaluate the effect of ointment base and concentration. The general rank order of the drug release was found to be: Carbopol gel > hydrophilic ointment > cold cream > white petrolatum. There was a positive correlation between the concentration of KP and release rate for all bases except Carbopol gel. The in vivo percutaneous absorption of KP from different ointment bases at 3% concentration was studied by carrageenan-induced paw edema in mice. The rank order of the percent edema inhibition was as follows: Carbopol gel > or = hydrophilic ointment > cold cream > white petrolatum. There was a good correlation between the in vitro and in vivo results.

In vitro percutaneous absorption of piroxicam through synthetic membranes and abdominal rat skin

The in vitro release of piroxicam from carbomer gels and its penetration through isopropyl myristate impregnated membranes and abdominal rat skin were investigated. Attempts were made to relate the differences in the release rate with physicochemical properties of the drug and the vehicle. The results showed that piroxicam is released from the topical gel formulations and diffuses through skin. It is suggested that although piroxicam flux across abdominal rat skin was lower than through isopropyl myristate membranes, this kind of membranes can be used in preliminary screening among the different piroxicam formulations.

Transdermal delivery of drugs with differing lipophilicities using azone analogs as dermal penetration enhancers

Six model drugs were selected for this study based on their degree of lipophilicity as represented by their log P values (range = -0.95 to 3.51). They included 2,4-dihydroxy-5-fluoropyrimidine (5-fluorouracil); 1,3,7-trimethylxanthine (caffeine); [(2-hydroxybenzoyl)amino]-acetic acid (salicyluric acid); 2-hydroxybenzoic acid (salicylic acid); 9 alpha-fluoro-16 alpha-hydroxyprednisolone 16 alpha, 17 alpha-acetonide (triamcinolone acetonide); and alpha-methyl-4-[2-methylpropyl]benzeneacetic acid (ibuprofen). Six dermal penetration enhancers [Azone or 1-dodecylhexahydro-2H-azepin-2-one (1), N-dodecyl-2-pyrrolidinone (2), N-dodecyl-2-piperidinone (3), N-dodecyl-N-(2-methoxyethyl)acetamide (4), N-(2,2-dihydroxyethyl)dodecylamine (5), and 2-(1-nonyl)-1,3-dioxolane (6)] were tested in vitro across full-thickness hairless mouse skin with each of the drugs. The relationship between lipophilicity (log P) and efficacy (represented by the enhancement ratio of flux) of the drugs when coadministered with the enhancers was examined using linear regression. The three cyclic enhancers (1-3) exhibited linear relationships, indicating that they were more effective at enhancing the penetration of hydrophilic drugs R2 = 0.8997 for 1, 0.8801 for 2, and 0.804 for 3) when evaluating all the model drugs except triamcinolone acetonide (TA). The two acyclic enhancers (4 and 5) showed a similar relationship, but their correlation coefficients were lower at 0.6463 for 4 and 0.6213 for 5. Studies with the dioxolane (6) yielded no relationship between the lipophilicity of the drug and the efficacy of the enhancer, with an R2 of 0.002. Overall, 6 was the least effective enhancer studied. The steroid TA was not included in the linear regression analysis. Of the six model drugs studied, TA exhibited the largest increase in transdermal delivery when enhancers 1-6 were used.