Quantitative evaluation of ethanol effects on diffusion and metabolism of beta-estradiol in hairless mouse skin

Occupational exposure assessment with solid substances: choosing a vehicle for in vitro percutaneous absorption experiments

Percutaneous occupational exposure to industrial toxicants can be assessed in vitro on excised human or animal skins. Numerous factors can significantly influence skin permeation of chemicals and the flux determination. Among them, the vehicle used to solubilize the solid substances is a tricky key step. A "realistic surrogate" that closely matches the exposure scenario is recommended in first intention. When direct transposition of occupational exposure conditions to in vitro experiments is impossible, it is recommended that the vehicle used does not affect the skin barrier (in particular in terms of structural integrity, composition, or enzymatic activity). Indeed, any such effect could alter the percutaneous absorption of substances in a number of ways, as we will see. Potential effects are described for five monophasic vehicles, including the three most frequently used: water, ethanol, acetone; and two that are more rarely used, but are realistic: artificial sebum and artificial sweat. Finally, we discuss a number of criteria to be verified and the associated tests that should be performed when choosing the most appropriate vehicle, keeping in mind that, in the context of occupational exposure, the scientific quality of the percutaneous absorption data provided, and how they are interpreted, may have long-range consequences. From the narrative review presented, we also identify and discuss important factors to consider in future updates of the OECD guidelines for in vitro skin absorption experiments.

Time controlled pulsatile transdermal delivery of nicotine: A phase I feasibility trial in male smokers

Nicotine substitution is a mainstay component in smoking cessation schemes. Current products including patches are poorly effective mainly because they do not give smokers the same pharmacokinetic profile of nicotine as cigarette consumption. This work evaluates a new computer operated delivery system for time controlled pulsatile transdermal administration of nicotine in a phase I clinical trial with twelve heavy smoking male volunteers. The device was affixed to the ventral side of the leading lower arm of the subjects and was programmed to deliver two pulses of drug within 16h with three delivery rates in a consecutive dose escalation study. Tolerability of the three increasing doses of nicotine was established. Plasma concentration of nicotine exhibited two peaks and one trough and reached therapeutically effective levels that behaved linearly with the drug load concentration of the device. In vivo input rate, delivered amount and elimination kinetics were deduced by pharmacokinetic modeling to analyze device performance. Timing, dose and duration of delivery were controlled by system operation parameters. Hence, feasibility of controlled pulsatile delivery of nicotine at predetermined intervals was demonstrated. After additional optimization, preprogrammed or on demand administration to meet individualized and circadian replacement needs should improve smoking cessation efficacy.

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.

Design and performance of a spreadsheet-based model for estimating bioavailability of chemicals from dermal exposure

A comprehensive transient model of chemical penetration through the stratum corneum, viable epidermis and dermis formulated in terms of an Excel™ spreadsheet and associated add-in is presented. The model is a one-dimensional homogenization of underlying microscopic transport models for stratum corneum and dermis; viable epidermis is treated as unperfused dermis. The model's salient features are a detailed structural description of the skin layers, a combination of first-principles based transport equations and empirical partition and diffusion coefficients, and the capability of simulating a variety of exposure scenarios. Model predictions are compared with representative in vitro skin permeation data obtained from the literature using as summary parameters total absorption (Q(abs)), maximum flux (J(max)) and skin permeability coefficient (k(p)). The results of this evaluation demonstrate the current state-of-the-art in prediction of transient skin absorption and highlight areas in which further elaborations are needed to obtain satisfactory predictions.

Structure Enhancement Relationship of Chemical Penetration Enhancers in Drug Transport across the Stratum Corneum

The stratum corneum is a major barrier of drug penetration across the skin in transdermal delivery. For effective transdermal drug delivery, skin penetration enhancers are used to overcome this barrier. In the past decades, a number of research studies were conducted to understand the mechanisms of skin penetration enhancers and to develop a structure enhancement relationship. Such understanding allows effective prediction of the effects of skin penetration enhancers, assists topical and transdermal formulation development, and avoids extensive enhancer screening in the transdermal delivery industry. In the past two decades, several hypotheses on chemical enhancer-induced penetration enhancement for transport across the skin lipoidal pathway have been examined based on a systematic approach. Particularly, a hypothesis that skin penetration enhancement is directly related to the concentration of the enhancers in the stratum corneum lipid domain was examined. A direct relationship between skin penetration enhancer potency (based on enhancer aqueous concentration in the diffusion cell chamber) and enhancer n-octanol-water partition coefficient was also established. The nature of the microenvironment of the enhancer site of action in the stratum corneum lipid domain was found to be mimicked by n-octanol. The present paper reviews the work related to these hypotheses and the relationships between skin penetration enhancement and enhancer concentration in the drug delivery media and stratum corneum lipids.

Comparison of the steady-state pharmacokinetics, metabolism, and variability of a transdermal testosterone patch versus a transdermal testosterone gel in hypogonadal men

AIM

To compare the pharmacokinetics (PK), metabolism, intra- and inter-subject variability of a permeation-enhanced testosterone patch versus a topical testosterone gel.

METHODS

28 hypogonadal men were treated with a testosterone patch (5 mg/day applied at 2200 h) and a 1% testosterone gel (5 g/day applied at 0800 h; nominal delivery 5 mg/day), each for 14 days, in an open-label crossover design. PK profiles of total testosterone (TT) and calculated free testosterone (cFT) were measured on day 7 and day 14 of each treatment, with patches or gel applied to the abdomen; dihydrotestosterone (DHT) and estradiol (E2) profiles were measured on day 14. The time-average (Cavg), maximum (Cmax), time of maximum (Tmax) and minimum concentrations (Cmin) were derived from each profile. The intra- and inter-subject coefficients of variation (CVintra and CVinter) of the TT and cFT parameters were computed by ANOVA.

RESULTS

Nightly applications of the patch produced a mean TT profile that mimicked the circadian pattern of healthy men. Morning applications of the gel produced a flatter mean profile; though individual subjects exhibited significant peaks at variable times. For TT, the mean and 90% confidence intervals of the patch/gel ratio of Cavg (1.030; 0.936-1.133; P > 0.05) and Cmax (1.086; 0.974-1.211; P > 0.05) met the criteria for bioequivalence. Cmin was lower for the patch. DHT levels and DHT/T ratios were 2 to 3-fold higher for the gel (P < 0.0001). E2 levels and E2/T ratios were comparable. CVintra and CVinter for Tmax approached 100% for the gel and were 23% and 42%, respectively, for the patch (P < 0.0001). Other variability parameters were generally comparable. Both products were well tolerated, and the patches adhered well.

CONCLUSIONS

These findings reflect the different mechanisms of transdermal absorption from the patch and gel and provide new considerations for selecting testosterone replacement therapies in hypogonadal men.

Skin Metabolism in Transdermal Therapeutic Systems

Skin has at least two barriers with protective functions: the stratum corneum physical barrier and a biochemical barrier in the epidermis and dermis. Numerous chemical and physical enhancers exist for transdermal therapeutic systems; some cause irritation, and possibly influence enzyme deactivation. Knowledge of enzymatic skin reactions is important for developing safe and efficacious transdermal systems for treatment not only of skin diseases but also for systemic application. This paper overviews the effects of (a) chemical enhancers and additives, (b) drug structure, and (c) physical enhancement on skin metabolism.

Vitamin D3?Based Conjugates for Topical Treatment of Psoriasis: Synthesis, Antiproliferative Activity, and Cutaneous Penetration Studies

PURPOSE

The goals of the experiments reported in this paper were to explore skin bioavailability and cell growth inhibitory activity of new vitamin D3-based conjugates studied as a potential drug complex for psoriasis.

METHODS

Conjugation was made between polyunsaturated fatty acids (PUFAs), such as linolenic acid or gamma-linolenic acid, and calcipotriol--a vitamin D3 analogue clinically used for topical treatment of psoriasis. These complexes were prepared by coupling the corresponding fatty acid with calcipotriol in the presence of dicyclohexyl-carbodiimide (DCC) and 4-(dimethylamino)-pyridine (DMAP) to obtain an ester bond.

RESULTS

The conjugates were capable of enhancing the penetration of the vitamin into the skin as well as inhibiting proliferation of keratinocytes in cultures. The antiproliferative activity even increased after simulating the full hydrolysis of the conjugates. In vitro skin penetration studies revealed that the conjugates penetrated into the skin at higher levels relative to calcipotriol alone. It was also demonstrated that the conjugate containing n-3 fatty acid penetrated into the skin at higher levels as compared to the conjugate containing n-6 PUFA. High-performance liquid chromatography analysis has shown that after penetration, a major portion of calcipotriol-PUFA conjugate was first converted mainly into another isomer form, presumably by transesterification, and only then it was hydrolyzed to form apparently high local concentrations of both calcipotriol and PUFA.

CONCLUSIONS

The unique biotransformation that occurred after penetration into the skin indicates that these conjugates are mutual prodrugs that are able to be bioprocessed in the skin and fully converted to the parent therapeutic agents.

Transdermal delivery of paracetamol for paediatric use: effects of vehicle formulations on the percutaneous penetration

Paracetamol is a safe and effective analgesic and antipyretic agent, and is one of the most widely used medications for infants and children. The formulations currently available have been designed for oral and rectal administration. However, they are not practical in young patients with vomiting and diarrhoea, or in those who refuse to take the full dose. An alternative route of administration would be a significant contribution to the paediatric pharmacopoeia. The aim of this study was to develop a new transdermal system for optional therapeutic administration of paracetamol in infants and children. In-vivo studies were carried out in animals using a transdermal system of high-loaded, soluble paracetamol in a hydrogel patch, which was also tested in-vitro for 8 h. Although the beneficial contribution of glyceryl oleate to the transdermal penetration of paracetamol seemed to be significant in-vitro, it was shown to be insufficient in-vivo. To improve the penetration of the drug, 4% PEG-40 stearate and 10% ethanol were incorporated as absorption enhancers into the dermal patches. A few hours after application of the improved patches to rats, plasma drug concentrations were elevated to levels comparable with those obtained after oral and subcutaneous administration of a high dose of paracetamol. Since plasma drug concentrations did not reach a constant steady state (as a peak or plateau) during the short-term animal experiments, longer pharmacokinetic studies in conscious animals are necessary.

The effect of ethanol on the simultaneous transport and metabolism of methyl p-hydroxybenzoate in excised skin of Yucatan micropig

The effects of ethanol on the simultaneous transport and metabolism of methyl p-hydroxybenzoate (HBM) were investigated in the skin of Yucatan micropig in vitro. It was found that transesterification occurred in the permeation studies involving ethanol. This was confirmed by monitoring the flux of ethyl p-hydroxybenzoate (HBE) into the receptor phase, as well as by monitoring the fluxes of HBM and p-hydroxybenzoic acid (HBA). The apparent flux of total HBM was decreased. The solubility of HBM increased with ethanol concentration, thus, the activity of HBM in ethanol solution became low because we used 10 mM HBM solution for permeation studies. The enhancement factor (E) was calculated to correct the activity. E increased with increasing the flux of ethanol, thus, ethanol may function as an enhancer of HBM transport. The hydrolysis of HBM to HBA was inhibited, whereas transesterification of HBM to HBE was induced at all concentrations of ethanol used (10-40%). The formation of HBE occurred much more readily than that of HBA at all concentrations of ethanol used.

Influences of alkyl group chain length and polar head group on chemical skin permeation enhancement

Previous investigations in our laboratory on the influence of the n-alkanols and the 1-alkyl-2-pyrrolidones as skin permeation enhancers for steroid molecules as permeants demonstrated that the enhancer potencies (based on aqueous concentration values) of these two homologous series were the same when compared at the same alkyl chain length; that is, the contribution of the hydroxyl group and that of the pyrrolidone group to enhancer potency were the same. The purpose of the present study was to further investigate what was believed to be a somewhat surprising finding, and two additional homologous series, the 1,2-alkanediols and N,N-dimethylalkanamides, were selected for study as enhancers. Corticosterone (CS) flux enhancement along the lipoidal pathway of hairless mouse skin stratum corneum was determined with 1,2-hexane-, 1,2-octane-, and 1,2-decanediol and with N,N-dimethylhexanamide, N,N- dimethylheptanamide, N,N-dimethyloctanamide, and N,N-dimethylnonanamide as enhancers. The enhancement factor (E) for the lipoidal pathway was calculated from the CS permeability coefficient and the CS solubility data over a 4 to 100 range of E values. Comparisons of the enhancer potencies of all four homologous series revealed that the enhancer potencies of all were very nearly the same when compared at equal alkyl group chain length. Moreover, the contribution of each of the polar head groups toward the enhancer potency was essentially constant, independent of the alkyl group chain length. It was reasoned that this outcome was either the result of the random selection of four polar head groups making the same contribution to enhancer potency or the result of these particular polar head groups not contributing to enhancer potency. To test the hypothesis that the former was more likely than the latter and that a suitable semipolar organic phase may mimic the microenvironment of the polar head group at the site of enhancer action, n-octanol-phosphate buffered saline (PBS) and n-hexane-PBS partition coefficients were determined for all the enhancers. The n-octanol-PBS partition coefficients for the enhancers, but not the n-hexane-PBS partition coefficients, were very nearly the same when compared at equal alkyl group chain lengths; this result supports the hypothesis that each of the four polar head groups likely contributes the same toward the enhancer potency and locates in the semipolar region of the hairless mouse skin stratum corneum lipid bilayers, which is well-approximated by water-saturated n-octanol.

Biotransformation of estradiol in the human keratinocyte cell line HaCaT: metabolism kinetics and the inhibitory effect of ethanol

PURPOSE

The aim of our study was to investigate the kinetics of beta-estradiol (E2) metabolism in the human keratinocyte cell line HaCaT and to estimate the effect of the potential inhibitor ethanol on the biotransformation reaction.

METHODS

The formation rates of estrone (E1) in dependence on substrate concentrations were determined in HaCaT cells using tritium labelled E2. Experiments were conducted with and without addition of dehydroepiandrosterone (DHEA) and ethanol. Possible toxic effects on the cells due to ethanol were investigated by cytotoxicity tests.

RESULTS

The metabolism of E2 in HaCaT cells exhibited Michaelis-Menten kinetics with Km and Vmax values of 3.5 microM and 216 pmol x mg(-1) protein x h(-1), respectively. The reaction was inhibited by DHEA and ethanol. The alcohol showed a reversible competitive inhibition mechanism for concentrations of 4 to 8% (v/v). Lower ethanol concentrations had no effect, whereas levels > or =10% significantly decreased cell viability leading to a different inhibition mechanism.

CONCLUSIONS

The HaCaT cell line seems to be a suitable model for studying enzyme kinetics equivalent to the human skin. The concentration dependent inhibitory effect of ethanol observed in this cell line may be relevant for the transdermal E2 application in patients.

Percutaneous permeation of N,N-diethyl-m-toluamide (DEET) from commercial mosquito repellents and the effect of solvent

N,N-Diethyl-m-toluamide (DEET), the active ingredient in many commercial mosquito repellents, is thought to be responsible for a wide range of local and systemic adverse reactions following its use. Many investigators have studied the dermal absorption of pure DEET; however, there is only one report in the literature on the absorption of DEET from commercial mosquito repellents and the effect of concentration of DEET on its absorption through skin. The first objective of the present study was to evaluate the permeation characteristics of DEET from four commercial products, Everglades (95%), Repel Deerhunters (52.25%), Off Skintastic (6.65%), and Skedaddle (6.2%), as compared to pure DEET (approximately 100%). The second objective was to study the effects of ethanol, the solvent for DEET, on the permeation of DEET and investigate its potential for enhancing the dermal absorption of DEET. Permeation studies of DEET from commercial mosquito repellents and from solutions containing various percentages of ethanol were conducted across human skin using an infinite dose technique with a Franz diffusion cell. Permeation parameters such as steady-state flux (Jss), lag time (tL), diffusion coefficient (D), permeability (P), and skin/ vehicle partition coefficient (K) were obtained from the permeation profiles in each case. The cumulative amount of DEET permeated can be ranked according to the following order: neat DEET (100%) = Everglades (95%) > Repel (52.25%) > Skedaddle (6.2%) = Off Skintastic (6.65%). Pure DEET exhibited the highest flux value of 63.20 +/- 24.52 micrograms/cm2-h, while Off Skintastic had the lowest value of 21.12 +/- 14.75 micrograms/cm2-h. The tL and D values for each of the products were similar to that of pure DEET. The total amount of DEET permeated from 30-45% ethanolic solutions at the end of 36 h was significantly higher than that from pure DEET and from the 60-90% ethanolic solutions. The Jss, P, and K values of DEET from the 30-45% ethanolic solutions were significantly higher than those from the 75-90% ethanolic solutions, while the tL and D values were similar for each solution. Therefore, there is potential for significant absorption of DEET after the dermal application of commercial mosquito repellents, and ethanol, used as a solvent, may enhance the permeation of DEET.

Analysis of simultaneous transport and metabolism of ethyl nicotinate in hairless rat skin

PURPOSE

Simultaneous skin transport and metabolism of ethyl nicotinate (EN), a model drug, were measured and theoretically analyzed.

METHODS

Several permeation studies of EN or its metabolite nicotinic acid (NA) were done on full-thickness skin or stripped skin with and without an esterase inhibitor. Permeation parameters such as partition coefficient of EN from the donor solution to the stratum corneum and diffusion coefficients of EN and NA in the stratum corneum and the viable epidermis and dermis were determined by these studies. Enzymatic parameters (Michaelis constant Km and maximum metabolism rate Vmax) were obtained from the production rate of NA from different concentrations of EN in the skin homogenate. Obtained permeation data were then analyzed by numerical method based on differential equations showing Fick's second law of diffusion in the stratum corneum and the law with Michaelis-Menten metabolism in the viable epidermis and dermis.

RESULTS

Fairly good steady-state fluxes of EN and NA through the skin were obtained after a short lag time for all the concentrations of EN applied. These steady-state fluxes were not proportional to the initial donor concentration of EN: EN and NA curves were concave and convex, respectively, which suggests that metabolic saturation from EN to NA takes place in the viable skin at higher EN application. The steady-state fluxes of EN and NA calculated by the differential equations with resulting permeation and enzymatic parameters were very close to the obtained data.

CONCLUSIONS

The present method is a useful tool to analyze simultaneous transport and metabolism of many drugs and prodrugs, especially those showing Michaelis-Menten type-metabolic saturation in skin.

Quantitative evaluation of aqueous isopropyl alcohol enhancement on skin flux of terbutaline (sulfate). 2. Permeability contributions of equilibrated drug species across human skin in vitro

This paper demonstrates the usefulness of an equilibria-cotransport model for understanding the isopropyl alcohol-enhanced transport of an ionizable model compound, terbutaline in its sulfate salt form, through human skin in vitro. With the same isopropyl alcohol concentrations (0 - 80% v/v) present at both sides of skin, the permeation experiments were conducted using split-thickness skin and dermis membranes. The equilibria-cotransport model was consistent with total terbutaline flux and a terbutaline-to-sulfate flux ratio, both increased with increasing isopropyl alcohol and/or terbutaline sulfate concentrations. From the saturated drug solutions, aqueous isopropyl alcohol enhanced terbutaline skin flux about 10 - 100-fold with the maximum at 60 - 80% isopropyl alcohol. This overall flux enhancement was qualitatively separated into the contributions of isopropyl alcohol effects on both equilibrated donor concentrations and skin permeabilities of protonated terbutaline, terbutaline-sulfate ion pair anion, and neutral terbutaline-sulfate (2:1) ion triplet. In addition to altering the species equilibria, isopropyl alcohol was found to enhance the transport of both neutral and ionic species of terbutaline sulfate across stratum corneum.

Diffusion and metabolism of prednisolone farnesylate in viable skin of the hairless mouse

The diffusion and metabolism of prednisolone 21-farnesylate were investigated in viable skin of the hairless mouse in vitro. The prodrug ester was extensively metabolized in viable skin, while it was stable in the donor and receptor solutions. The rate of appearance of the prodrug and its metabolite prednisolone was markedly influenced by the direction of the skin placed between the in vitro diffusion half-cells. The rate of bioconversion of the prodrug was determined as a function of the distance from the surface of the skin. The prodrug was increasingly metabolized with the distance from the surface of the skin, indicating that the responsible enzymes are enriched in the lower layers of the viable skin. A model with linearly increasing enzyme activity in the viable skin accounts for the in vitro profiles of the diffusion/metabolism of the prodrug in the viable skin of hairless mouse.

Pharmacokinetic characterisation of transdermal delivery systems

The key aspects of the pharmacokinetics of transdermal delivery systems including time lag, steady-state plasma levels and decline phase are illustrated in this review. The 7 currently marketed transdermal systems [nitroglycerin (glyceryl trinitrate), estradiol, clonidine, fentanyl, nicotine, scopolamine (hyoscine) and estradiol/norethisterone acetate] are discussed, as are systems in development. Single-dose absolute bioavailability studies characterise the period of onset, the steady-state plateau and the declining phase, and typify transdermal delivery. More complex temporal profiles result from interactions with enhancers or removal of the system before steady-state conditions are achieved. Clinically these systems are used to achieve multiple peak serum estradiol concentrations after application of transdermal estradiol, and an initial peak systemic concentration of testosterone after application of transdermal testosterone. Multiple-dose, dose proportionality and skin site bioequivalence studies are needed for the full pharmacokinetic characterisation of a transdermal delivery system. The relationship of system design to variability is discussed. Although the data are limited, population factors, cutaneous metabolism and tolerance all appear to influence the disposition of drugs administered transdermally. For example, the route of delivery influences which nitroglycerin metabolite predominates. Furthermore, as a result of tolerance to nitrates, a transdermal delivery system must be removed for 8 to 12 hours for optimal effect. Therefore, transdermal delivery systems, designed on the basis of pharmacokinetic principles and concentration-effect relationships, have the potential to provide optimal therapy for the treatment of some conditions.

Cotransport of estradiol and ethanol through human skin in vitro: understanding the permeant/enhancer flux relationship

The thermodynamic and kinetic limits of ethanol-enhanced estradiol skin transport have been investigated by studying the relationship between estradiol and ethanol steady-state flux in the cotransport of permeant and enhancer in situations in which there exists an enhancer solvent gradient across the skin ("asymmetric" configuration). For aqueous ethanol solution saturated with estradiol, the flux of estradiol across the human epidermal membrane is empirically observed to be linear with the ethanol flux. A physical model approach has been used to determine the basis of this empirical linearity and to predict permeant/enhancer transport across the skin for the asymmetric configuration. Enhancement factors, determined with a balanced ethanol concentration across the skin ("symmetric" configurations), are used to predict fluxes in the asymmetric configurations. The model demonstrates that ethanol enhances the stratum corneum transport of estradiol and of itself by increasing the respective diffusion coefficients at lower concentrations (less than 50%) and by both increasing the diffusion coefficients and decreasing the membrane activity coefficients at moderate concentrations (50 to 75%). The model also demonstrates that the permeant flux, in general, is not linear with the cotransported enhancer flux.