Simple alkyl esters as skin permeation enhancers

Solvent-Containing Closure Material Can Be Used to Prevent Follicular Penetration of Caffeine and Fluorescein Sodium Salt on Porcine Ear Skin

AIM

The skin represents a drug delivery portal. The establishment of a skin model capable of distinguishing between the follicular and intercellular penetration pathways remains a challenge. The study described herein was aimed at showing the influence of two nail varnishes as closure material and four application techniques to spread the active pharmaceutical ingredient (API) on a successful follicular closure without inducing penetration-enhancing effects.

MATERIALS AND METHODS

For all experiments, ex vivo porcine ear skin was used. In study design A, a standard and a solvent-free nail varnish were compared. It was tested whether the different application techniques (spreading with pipette, careful finger massage, 5-Hz finger massage, 5-Hz automatic massage) potentially destroy an intact follicular closure. Laser scanning microscopy imaging was used to measure if the model drug (fluorescein sodium salt) penetrated into the hair follicles. Study design B investigated how the penetration is affected when applying standard nail varnish containing solvents to skin. It was tested if the varnish blocks the API (caffeine) on completely covered areas and if adjacent areas show increased penetration. Furthermore, lateral diffusion of the API was investigated. After 20 h, the skin layers were separated by tape stripping and heat separation. The tissue samples were homogenized. Caffeine was quantified by chromatography.

RESULTS

In study design A, the standard nail varnish showed a secure follicular closure, while the solvent-free nail varnish was not able to prevent follicular penetration. Moreover, rapid application techniques were found to destroy an intact follicular closure. Only the two most gentle application techniques kept the follicular closing intact. In study design B, no caffeine was detected in both skin areas that were completely covered. Since no significant difference in caffeine penetration between the two uncovered groups was found, any influence of the applied closure material on adjacent areas was excluded.

CONCLUSION

This study clearly demonstrates that a standard nail varnish in combination with a gentle application technique of the API provides a secure follicular closure. The presented study only investigated the closure for the substances caffeine and fluorescein sodium salt. The results might not be transferable to all kinds of APIs.

Bilayer structure and lipid dynamics in a model stratum corneum with oleic acid

The stratum corneum is the uppermost layer of the skin and acts as a barrier to keep out contaminants and retain moisture. Understanding the molecular structure and behavior of this layer will provide guidance for optimizing its biological function. In this study we use a model mixture comprised of equimolar portions of ceramide NS (24:0), lignoceric acid, and cholesterol to model the effect of the addition of small amounts of oleic acid to the bilayer at 300 and 340 K. Five systems at each temperature have been simulated with concentrations between 0 and 0.1 mol % oleic acid. Our major finding is that subdiffusive behavior over the 200 ns time scale is evident in systems at 340 K, with cholesterol diffusion being enhanced with increased oleic acid. Importantly, cholesterol and other species diffuse faster when radial densities indicate nearest neighbors include more cholesterol. We also find that, with the addition of oleic acid, the bilayer midplane and interfacial densities are reduced and there is a 3% decrease in total thickness occurring mostly near the hydrophilic interface at 300 K with reduced overall density at 340 K. Increased interdigitation occurs independent of oleic acid with a temperature increase. Slight ordering of the long non-hydroxy fatty acid of the ceramide occurs near the hydrophilic interface as a function of the oleic acid concentration, but no significant impact on hydrogen bonding is seen in the chosen oleic acid concentrations.

Efficacy of clobetasol spray: factors beyond patient compliance

Clobetasol 0.05% spray, a topical clobetasol propionate, is a non-greasy formulation that has shown increased clinical efficacy in a head-to-head comparison with foam formulation. Moreover, available data from randomized, controlled, double-blind trials suggests that clobetasol spray is, in fact, slightly more effective than most, if not all, other preparations of clobetasol. The fact that clobetasol spray is exceptionally easy to comply with may have played a major role in this outcome; however, other factors must be considered. These include vehicle metamorphosis post-application as well as vehicle and excipient effects on stratum corneum permeability. Basic concepts in topical drug delivery and how they apply to this spray vehicle may further explain the greater efficacy of clobetasol spray.

In vitro enhancement of lactate esters on the percutaneous penetration of drugs with different lipophilicity

Lactate esters are widely used as food additives, perfume materials, medicine additives, and personal care products. The objective of this work was to investigate the effect of a series of lactate esters as penetration enhancers on the in vitro skin permeation of four drugs with different physicochemical properties, including ibuprofen, salicylic acid, dexamethasone and 5-fluorouracil. The saturated donor solutions of the evaluated drugs in propylene glycol were used in order to keep a constant driving force with maximum thermodynamic activity. The permeability coefficient (K(p)), skin concentration of drugs (SC), and lag time (T), as well as the enhancement ratios for K(p) and SC were recorded. All results indicated that lactate esters can exert a significant influence on the transdermal delivery of the model drugs and there is a structure-activity relationship between the tested lactate esters and their enhancement effects. The results also suggested that the lactate esters with the chain length of fatty alcohol moieties of 10-12 are more effective enhancers. Furthermore, the enhancement effect of lactate esters increases with a decrease of the drug lipophilicity, which suggests that they may be more efficient at enhancing the penetration of hydrophilic drugs than lipophilic drugs. The influence of the concentration of lactate esters was evaluated and the optimal concentration is in the range of 5-10 wt.%. In sum, lactate esters as a penetration enhancer for some drugs are of interest for transdermal administration when the safety of penetration enhancers is a prime consideration.

Chemical penetration enhancers: a patent review

BACKGROUND

Ever since transdermal drug delivery came into existence, it has offered great promises, although most of them are yet to be fulfilled owing to some intrinsic restrictions of the transdermal route. On the positive side, transdermal drug delivery systems present advantages including non-invasiveness, prolonged therapeutic effect, reduced side effects, improved bioavailability, better patient compliance and easy termination of drug therapy. The greatest hindrance in the percutaneous delivery is the obstruction property of the stratum corneum, the outermost layer of the skin, in addition to usual problems such as skin binding, skin metabolism, cutaneous toxicity and prolonged lag times.

OBJECTIVE

This paper reviews investigations on the feasibility and application of penetration enhancers as described in recent patents, which help in the selection of a suitable sorption promoter(s) for enhanced delivery of medicaments through the skin.

METHOD

The patents granted under various categories of penetration enhancers have been discussed including fatty acids, terpenes, fatty alcohol, pyrrolidone, sulfoxides, laurocapram, surface active agents, amides, amines, lecithin, polyols, quaternary ammonium compounds, silicones, alkanoates and so on.

CONCLUSION

Scores of promising chemicals have been harnessed for their skin permeation promoting capacity as mentioned earlier. In future, many more chemicals and putative enhancers are likely be documented and patented.

Sorbitan Monopalmitate-Based Proniosomes for Transdermal Delivery of Chlorpheniramine Maleate

A proniosomal gel for transdermal drug delivery of chlorpheniramine maleate (CPM) was developed based on Span 40 and extensively characterized in vitro. The system was evaluated for the effect of composition of formulation, type of surfactants and alcohols on the drug loading, rate of hydration, vesicle size, polydispersity, entrapment efficiency, and drug release across cellulose nitrate dialysis membrane. The stability studies were performed at 4 degrees C and at room temperature. The results showed that lecithin produced more stable and larger vesicles with higher loading efficiency but lower dissolution efficiency than cholesterol (chol) and dicethyl phosphate (DCP). The type of alcohol had no significant effect on the stability of vesicles, but ethanol produced larger vesicles (approximately equal to 44 microm) and entrapped a greater amount of drug. Drug release from vesicles of lecithin followed a first-order kinetics whereas those with DCP or without lecithin fit better with a Higuchi model. The proniosomes that contained Span 40/lecithin/chol prepared by ethanol showed optimum stability, loading efficiency, and particle size and release kinetic suitable for transdermal delivery of CPM.

Percutaneous Penetration Enhancers: An Overview

Transdermal drug delivery is the controlled release of drugs through the skin to obtain therapeutic levels systematically. Several technological advances have been made in the recent decades to enhance percutaneous drug penetration. This overview focuses on the physical, biochemical, and chemical means of penetration enhancement, as well as the classification and mechanisms of chemical penetration enhancers, their application in transdermal drug delivery, and trends and development in penetration enhancement.

Novel transdermal drug penetration enhancer: synthesis and enhancing effect of alkyldisiloxane compounds containing glucopyranosyl group

The syntheses of alkyldisiloxanes containing sugar moiety with various alkyl chain length were investigated, in order to develop a silicone-based transdermal penetration enhancer which was expected to show a low irritation to the skin. 1-Alkyl-3-beta-D-glucopyranosyl-1,1,3,3-tetramethyldisiloxanes (Glc-SiCs) were prepared by two-step hydrosilylations of 1-alkene and 1-allyl-beta-D-glucose tetraacetate with 1,1,3,3-tetramethyldisiloxane in the presence of bis(benzonitrile)platinum dichloride as the catalyst, followed by hydrolysis of the acetyl groups with sodium methoxide. The enhancing effect of Glc-SiCs on the percutaneous drug penetration was evaluated by in vitro experiments using a two-chamber diffusion cell. Antipyrine (ANP) and indomethacin (IND) were used as hydrophilic and hydrophobic model drugs, respectively, and the amount of drug permeating through the rat abdominal skin with or without Glc-SiCs was estimated by HPLC. As a result, Glc-SiCs exhibited a enhancing effect on the permeation of both drugs through the skin, which was influenced by the alkyl chain length of Glc-SiCs. In addition, it was suggested that a suitable balance of polarity would be necessary to appear the high enhancing effect, where Glc-SiCs with octyl and decyl groups exhibited the highest enhancing effect. From the determination of kinetic parameters in the drug permeation, it was also found that this enhancing effect was due to the increase of both partition and diffusion coefficients of drug permeation through the skin. By experiments to determine the amount of cholesterol extracted from the skin, the defatting effect would be one of the functions of Glc-SiCs which resulted in the high enhancing activity. Furthermore, according to the Draize test, it was confirmed that Glc-SiCs showed a low irritation to the skin.

Veterinary drug delivery: potential for skin penetration enhancement

A range of topical products are used in veterinary medicine. The efficacy of many of these products has been enhanced by the addition of penetration enhancers. Evolution has led to not only a highly specialized skin in animals and humans, but also one whose anatomical structure and skin permeability differ between the various species. The skin provides an excellent barrier against the ingress of environmental contaminants, toxins, and microorganisms while performing a homeostatic role to permit terrestrial life. Over the past few years, major advances have been made in the field of transdermal drug delivery. An increasing number of drugs are being added to the list of therapeutic agents that can be delivered via the skin to the systemic circulation where clinically effective concentrations are reached. The therapeutic benefits of topically applied veterinary products is achieved in spite of the inherent protective functions of the stratum corneum (SC), one of which is to exclude foreign substances from entering the body. Much of the recent success in this field is attributable to the rapidly expanding knowledge of the SC barrier structure and function. The bilayer domains of the intercellular lipid matrices within the SC form an excellent penetration barrier, which must be breached if poorly penetrating drugs are to be administered at an appropriate rate. One generalized approach to overcoming the barrier properties of the skin for drugs and biomolecules is the incorporation of suitable vehicles or other chemical compounds into a transdermal delivery system. Indeed, the incorporation of such compounds has become more prevalent and is a growing trend in transdermal drug delivery. Substances that help promote drug diffusion through the SC and epidermis are referred to as penetration enhancers, accelerants, adjuvants, or sorption promoters. It is interesting to note that many pour-on and spot-on formulations used in veterinary medicine contain inert ingredients (e.g., alcohols, amides, ethers, glycols, and hydrocarbon oils) that will act as penetration enhancers. These substances have the potential to reduce the capacity for drug binding and interact with some components of the skin, thereby improving drug transport. However, their inclusion in veterinary products with a high-absorbed dose may result in adverse dermatological reactions (e.g., toxicological irritations) and concerns about tissue residues. These are important considerations when formulating a veterinary transdermal product when such compounds are added, either intentionally or otherwise, for their penetration enhancement ability.

Lipid extraction and iontophoretic transport of leuprolide acetate through porcine epidermis

The purpose of this study was to explore the effect of lipid extraction by the simple alkyl acetates of increasing carbon chain lengths (e.g. methyl, ethyl, propyl, butyl, pentyl, hexyl, and octyl acetates) and iontophoresis on the in-vitro transport of leuprolide acetate through porcine epidermis. The extent of lipid extraction from the stratum corneum (SC) by alkyl acetates was studied by Fourier transform infrared (FT-IR) spectroscopy. Ethyl, propyl, pentyl, hexyl, and octyl acetates significantly increased (P < 0.05) the permeability of leuprolide acetate through the epidermis in comparison to the control (epidermis without alkyl acetate treatment). Iontophoresis further increased (P < 0.05) the permeability of leuprolide acetate for all the alkyl acetates studied, when compared to their corresponding passive permeability. Ethyl acetate produced the maximum passive (13.47 microg/cm(2)/h) and iontophoretic (89.79 microg/cm(2)/h) flux among all the alkyl acetates studied. The SC treated with alkyl acetates showed a decrease in peak heights and areas of asymmetric and symmetric C--H stretching absorbances in comparison to untreated SC. A greater percentage decrease in peak heights and areas was obtained by ethyl acetate. Chloroform:methanol(2:1) [C:M(2:1)] was used as a positive control for lipid extraction. Our findings provide evidence that alkyl acetates cause lipid extraction, which leads to an enhancement in the passive and iontophoretic permeability of leuprolide acetate.

Pharmacokinetic profile of a new matrix-type transdermal delivery system: diclofenac diethyl ammonium patch

A transdermal delivery system containing the anti-inflammatory analgesic diclofenac diethyl ammonium in an ethyl hexyl acrylate and vinyl acetate pressure-sensitive adhesive system was developed for percutaneous absorption. These patches were subjected to in vitro permeation and permeation enhancement studies through rat skin using a specially designed diffusion cell. Further, the work deals with percutaneous absorption studies carried out on both animals and human volunteers. The pharmacokinetic parameters calculated from the blood levels of the drug reveal a profile typical of a sustained-release formulation, with the ability to maintain adequate plasma levels for 24 hr (i.e., up to the next application). (Area under the curve [AUC]: 4.356 +/- 1.3 mcg/ml.hr in animals and 0.442 +/- 0.053 mcg/ml.hr in humans; Tmax was 8 hr in both the cases, whereas Cmax was 0.288 +/- 0.088 mcg/ml in animals and 0.034 +/- .008 mcg/ml in human volunteers.) The amount of the drug bioavailable for targeting the sites of action is lower than via the oral route, but the absorbed dose appears to be adequate for therapeutic use, particularly because of the absence of side effects.

Proniosome based transdermal delivery of levonorgestrel for effective contraception

A proniosome based transdermal drug delivery system of levonorgestrel (LN) was developed and extensively characterized both in vitro and in vivo. The proniosomal structure was liquid crystalline-compact niosomes hybrid which could be converted into niosomes upon hydration. The system was evaluated in vitro for drug loading, rate of hydration (spontaneity), vesicle size, polydispersity, entrapment efficiency and drug diffusion across rat skin. The effect of composition of formulation, amount of drug, type of Spans, alcohols and sonication time on transdermal permeation profile was observed. The stability studies were performed at 4 degrees C and at room temperature. The biological assay for progestational activity included endometrial assay and inhibition with the formation of corpora lutea. The study demonstrated the utility of proniosomal transdermal patch bearing levonorgestrel for effective contraception.

Proliposome-based transdermal delivery of levonorgestrel

Mesophasic proliposomal system for levonorgestrel was developed and evaluated both in vitro and in vivo. The vesicles were mostly unilamellar, however, few vesicles were multilamellar which budded off spontaneously upon hydration. The release of drug from this system adhered to zero order kinetics. The effect of alcohols and volatile oils on transdermal flux was investigated. The flux was found to be the highest for alcohol, and followed by that for lemon oil. The in vivo studies indicate the requirement for a loading dose, since, a significant lag phase was observed before the therapeutic levels were reached. This system was, however, superior to the PEG-based ointment system which was employed as the control formulation. The results demonstrate the potential of proliposomal system for efficacious transdermal delivery of hydrophobic drugs.

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.

Facilitated transport of two model steroids by esters and amides of clofibric acid

A series of novel dermal penetration enhancers, esters and amides of clofibric acid, was synthesized. The permeation parameters and skin retention of two steroids (hydrocortisone-21-acetate and betamethasone-17-valerate) in propylene glycol were studied with athymic nude mouse skin by in vitro diffusion cell techniques in the presence of the novel enhancer compounds. Isopropyl myristate, dimethyl lauramide, and 1-dodecylazacycloheptan-2-one (laurocapram, Azone) were used as control enhancers. The most satisfactory enhancement of both the ester and amide series was observed with clofibric acid octyl amide; coadministration increased skin retention of hydrocortisone acetate after 24 h by 3.5-fold and that of betamethasone valerate by 2.9-fold. Diffusion cell receptor concentrations increased 51.6- and 10.3-fold, respectively, during the same time period. However, the enhancer compound in this case was applied to the skin 1 h prior to each of the steroids. The amide analogues were more effective than the equivalent ester compounds of the same carbon chain length. The best enhancer compounds (2c, 3d, 3e, and 3f) were nonirritating to athymic mouse skin in vivo.

Cutaneous effects of transdermal levonorgestrel

The irritation of transdermal devices delivering levonorgestrel and the permeation enhancer ethyl acetate with or without ethanol was evaluated in rabbits. Erythema and oedema were assessed 24, 48 and 72 hr and 7 days after application of the 24-hr delivery system. The devices were found to be mild to moderately irritating, with erythema the primary manifestation. No differences were observed between devices using pure ethyl acetate or ethyl acetate-ethanol (7:3, v/v) as enhancers. Devices using pure ethanol as an enhancer gave levels of irritation similar to those using ethyl acetate-ethanol (7:3) or pure ethyl acetate. Control devices containing only water (no drug) were also found to be mildly irritating to rabbits following a 24-hr exposure period. A histological evaluation of the application sites of two of the formulations confirmed the visual observations of mild subacute irritation. The changes produced by transdermal levonorgestrel were reversible. The problems of skin irritation of transdermal devices is discussed with particular reference to the use of ethyl acetate and ethanol as skin penetration enhancers.

Transdermal delivery of levonorgestrel. VII. In vivo studies

This paper describes the results of transdermal experiments performed in vivo using devices designed for delivery of levonorgestrel. To help evaluate the performance of the transdermal devices on rabbits, a constant infusion experiment was performed using osmotic pumps. Reservoir-type transdermal devices were prepared and tested on rabbits for their ability to deliver 30 to 35 micrograms/d of levonorgestrel. Ethyl acetate and ethanol (0.7:0.3) or neat ethyl acetate were used as permeation enhancers. The results indicated that for rabbits, a rate-limiting membrane was required to control the delivery of enhancer(s) to the skin so that constant plasma levels could be maintained. The devices induced mild erythema and very mild edema over the 24-hour exposure period. The potential use of such a transdermal delivery system in humans is discussed.