Synthesis and evaluation of N-acetylprolinate esters - novel skin penetration enhancers

Proline, hydroxyproline, and pyrrolidone carboxylic acid derivatives as highly efficient but reversible transdermal permeation enhancers

Overcoming the skin barrier properties efficiently, temporarily, and safely for successful transdermal drug delivery remains a challenge. We synthesized three series of potential skin permeation enhancers derived from natural amino acid derivatives proline, 4-hydroxyproline, and pyrrolidone carboxylic acid, which is a component of natural moisturizing factor. Permeation studies using in vitro human skin identified dodecyl prolinates with N-acetyl, propionyl, and butyryl chains (Pro2, Pro3, and Pro4, respectively) as potent enhancers for model drugs theophylline and diclofenac. The proline derivatives were generally more active than 4-hydroxyprolines and pyrrolidone carboxylic acid derivatives. Pro2-4 had acceptable in vitro toxicities on 3T3 fibroblast and HaCaT cell lines with IC₅₀ values in tens of µM. Infrared spectroscopy using the human stratum corneum revealed that these enhancers preferentially interacted with the skin barrier lipids and decreased the overall chain order without causing lipid extraction, while their effects on the stratum corneum protein structures were negligible. The impacts of Pro3 and Pro4 on an in vitro transepidermal water loss and skin electrical impedance were fully reversible. Thus, proline derivatives Pro3 and Pro4 have an advantageous combination of high enhancing potency, low cellular toxicity, and reversible action, which is important for their potential in vivo use as the skin barrier would quickly recover after the drug/enhancer administration is terminated.

Synthesis of a series of validoxylamine A esters and their biological activities

BACKGROUND

The worldwide reduction in food production due to pests and diseases is still an important challenge facing today. Validoxylamine A (VAA) is a natural polyhydroxyl compound derived from validamycin, acting as an efficient trehalase inhibitor with insecticidal and antifungal activities. To extend the application and discover green pesticide, a series of ester derivatives were prepared based on VAA as a lead compound. Their biological activities were investigated against three typically agricultural disease, Rhizoctonia solani, Sclerotinia sclerotiorum and Aphis craccivora.

RESULTS

This study involved 30 novel validoxylamine A fatty acid esters (VAFAEs) synthesized by Novozym 435 and they were characterized with high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) and proton nuclear magnetic resonance (¹ H-NMR). Of these 30 derivatives, most compounds showed improved antifungal activity, and 12 novel compounds showed improved insecticidal activity. When reacted with pentadecanoic acid, compound 14 showed the highest inhibitory activity against R. solani [median effective concentration (EC₅₀ ) 0.01 μmol L^-1 ], while the EC₅₀ value of VAA was 34.99 μmol L^-1 . Furthermore, 21 novel VAFAEs showed higher inhibitory activity against S. sclerotiorum. Validoxylamine A oleic acid ester, compound 21, exhibited the highest insecticidal activity against A. craccivora [median lethal concentration (LC₅₀ ) 39.63 μmol L^-1 ], while the LC₅₀ value of Pymetrozine was 50.45 μmol L^-1 , a commercialized pesticide against A. craccivora.

CONCLUSION

Combining our results, esterification of VAA by introducing different acyl donors was beneficial for the development of new eco-friendly drugs in the field of pesticides.

Current Status of Amino Acid-Based Permeation Enhancers in Transdermal Drug Delivery

Transdermal drug delivery (TDD) presents many advantages compared to other conventional routes of drug administration, yet its full potential has not been achieved. The administration of drugs through the skin is hampered by the natural barrier properties of the skin, which results in poor permeation of most drugs. Several methods have been developed to overcome this limitation. One of the approaches to increase drug permeation and thus to enable TDD for a wider range of drugs consists in the use of chemical permeation enhancers (CPEs), compounds that interact with skin to ultimately increase drug flux. Amino acid derivatives show great potential as permeation enhancers, as they exhibit high biodegradability and low toxicity. Here we present an overview of amino acid derivatives investigated so far as CPEs for the delivery of hydrophilic and lipophilic drugs across the skin, focusing on the structural features which promote their enhancement capacity.

Recent advances in follicular drug delivery of nanoparticles

Introduction: The improvement of percutaneous absorption represents a clear dermatopharmaceutical aim. Recently, the hair follicle was recognized to be an important penetration pathway. Especially nanoparticles show an enhanced intrafollicular penetration and can be utilized to target specific cell populations within the hair follicle.Areas covered: The present review briefly summarizes the recent advances in follicular drug delivery of nanoparticles. Moreover, the particularities of the hair follicle as a penetration pathway are summarized which include its structure and specific barrier properties. Recently, the mechanism of the follicular penetration process has been clarified.In the meantime, different strategies have been developed to successfully improve follicular drug delivery of nanoparticles. One approach is to equip the nanocarriers with a triggered release system enabling them to release their drug load at the right time and place.Expert opinion: Follicular drug delivery with smart nanocarrier-based drug delivery systems represents a promising approach to increase the percutaneous absorption of topically applied substances. Although technical achievements and efficacy proofs concerning an increased penetration of substances are already available, the practical implementation into clinical application still represents an additional challenge and should be in the focus of interest in future research.

Skin Delivery of EGCG and Silibinin: Potential of Peptide Dendrimers for Enhanced Skin Permeation and Deposition

The aim of the present study was to evaluate the ability of the peptide dendrimers to facilitate transdermal delivery of antioxidants, silibinin, and epigallocatechin-3-gallate (EGCG). Drug-peptide dendrimer complexes were prepared and evaluated for their ability to permeate across the skin. The data revealed the ready formation of complexes between drug and peptide dendrimer in a molar ratio of 1:1. In vitro permeation studies using excised rat skin and drug-peptide dendrimer complexes showed highest values for cumulative drug permeation at the end of 12 h (Q₁₂), with corresponding permeability coefficient (Kp) and enhancement ratio values also determined at this time point. With silibinin, 3.96-, 1.81-, and 1.06-fold increase in skin permeation was observed from silibinin-peptide dendrimer complex, simultaneous application of silibinin + peptide dendrimer, and pretreatment of skin with peptide dendrimer, respectively, in comparison with passive diffusion. With EGCG, 9.82-, 2.04-, and 1.72-fold increase in skin permeation was observed from EGCG-peptide dendrimer complex, simultaneous application of EGCG + peptide dendrimer, and pretreatment of skin with peptide dendrimer, respectively, in comparison with passive diffusion. The present study demonstrates the application of peptide dendrimers in effectively delivering antioxidants such as EGCG and silibinin into the skin, thus offering the potential to provide antioxidant effects when delivered via appropriately formulated topical preparations.

Efficacy of Combined Ultrasound-and-Microbubbles-Mediated Diclofenac Gel Delivery to Enhance Transdermal Permeation in Adjuvant-Induced Rheumatoid Arthritis in the Rat

A previous study that investigated the effect of ultrasound (US) on the transdermal permeation of the non-steroidal anti-inflammatory drug diclofenac found that therapeutic US can increase circulation in an inflamed joint and decrease arthritic pain. Transdermal drug delivery has recently been demonstrated by US combined with microbubbles (MB) contrast agent (henceforth referred to as "US-MB"). The present study evaluated the efficacy of US-MB-mediated diclofenac delivery for treating adjuvant-induced rheumatoid arthritis (RA) in rats. RA was induced by injecting 100 μL of complete Freund's adjuvant into the ankle joint of male Sprague-Dawley rats (250-300 g) that were randomly divided into five treatment groups: (i) carbopol gel alone (the control [group C]), (ii) diclofenac-carbopol gel (group D), (iii) US plus carbopol gel (group U), (iv) US plus diclofenac-carbopol gel (group DU) and (v) US-MB plus diclofenac-carbopol gel (group DUB). The ankle width was measured over 10 d using high-frequency (40-MHz) US B-mode and color Doppler-mode imaging, covering the period before and after treatment. Longitudinal US images of the induced RA showed synovitis and neovascularity. Only a small amount of neovascularity was observed after treatment. The recovery rate on day 10 was significantly higher in group DUB (97.7% ± 2.7%, mean ± standard deviation [SD]) than in groups C (1.0% ± 2.7%), D (37.5% ± 4.6%), U (75.5% ± 4.2%) and DU (87.3% ± 5.2%) (p < 0.05). The results obtained indicate that combining US and MB can increase the skin permeability and thereby enhance the delivery of diclofenac sodium gel and thereby inhibit inflammation of the tissues surrounding the arthritic ankle. Color Doppler-mode imaging revealed that US-MB treatment induced a rapid reduction in synovial neoangiogenesis in the arthritic area.

The application of anethole, menthone, and eugenol in transdermal penetration of valsartan: Enhancement and mechanistic investigation

CONTEXT

The main barrier for transdermal delivery is the obstacle property of the stratum corneum. Many types of chemical penetration enhancers have been used to breach the skin barrier; among the penetration enhancers, terpenes are found as the most highly advanced, safe, and proven category.

OBJECTIVE

In the present investigation, the terpenes anethole, menthone, and eugenol were used to enhance the permeation of valsartan through rat skin in vitro and their enhancement mechanism was investigated.

MATERIALS AND METHODS

Skin permeation studies of valsartan across rat skin in the absence and the presence of terpenes at 1% w/v, 3% w/v, and 5% w/v in vehicle were carried out using the transdermal diffusion cell sampling system across rat skin and samples were withdrawn from the receptor compartment at 1, 2, 3, 4, 6, 8, 10, 12, and 24 h and analysed for drug content by the HPLC method. The mechanism of skin permeation enhancement of valsartan by terpenes treatment was evaluated by Fourier transform infrared spectroscopy (FTIR) analysis and differential scanning calorimetry (DSC).

RESULTS

All the investigated terpenes provided a significant (p < 0.01) enhancement in the valsartan flux at a concentration of 1%, and less so at 3% and 5%. The effectiveness of terpenes at 1% concentration was in the following order: anethole > menthone > eugenol with 4.4-, 4.0-, and 3.0-fold enhancement ratio over control, respectively. DSC study showed that the treatment of stratum corneum with anethole shifted endotherm down to lower melting point while FTIR studies revealed that anethole produced maximum decrease in peak height and area than other two terpenes.

CONCLUSION

The investigated terpenes can be successfully used as potential enhancers for the enhancement of skin permeation of lipophilic drug.

Active compounds release from semisolid dosage forms

The aim of this paper is to review all the aspects of the in vitro release testing (IVRT) from semisolid dosage forms. Although none of the official dissolution methods has been specified for use with semisolid dosage forms, their utility for assessing release rates of drugs from semisolid dosage forms has become a topic of considerable interest. One can expect to overcome such complexity in the future, when the official "Topical and Transdermal Drug Products-Product Performance Tests" will be published in an issue of the Pharmacopeial Forum. Many factors such as type of the dissolution medium, membrane, temperature, and speed have an influence on the mechanism and kinetics of the release testing from gels, creams, and ointments; therefore, those parameters have been widely discussed.

Thermoanalytical methods applied to medicine

Thermoanalytical methods have found increasing application in medicine due to the improved sensitivity and usability of the available instrumentation. Studies have identified important findings applied to medicine, including information on the thermal properties of the skin and the effect of insertion into body cavities of implants and prosthetics. These studies have explored the thermal stability of various materials to provide insight into drug penetration in order to design drug delivery systems, which are not only safe but capable of delivering improved and predictable therapeutic outcomes for patients. Differential scanning calorimetry (DSC) has also been applied to the study of disease states such as diabetes, where changes in the collagen structure of the skin which may lead to long-term complications in these patients, can be detected. Although these results may at this stage not have significant clinical implications, they do provide medical researchers with a starting point for future investigations. The application of these techniques has been further extended to examinations of body systems and other disease states. The key for the future will be the ability of these techniques not only to provide information on alterations in these biological systems, but also to determine whether these alterations are clinically relevant.

Chemical Enhancers for the Absorption of Substances Through the Skin: Laurocapram and Its Derivatives

Absorption enhancers are substances used for temporarily increasing a membrane's permeability (e.g., the skin and mucosa), either by interacting with its components (lipids or proteins) or by increasing the membrane/vehicle partition coefficient. This article presents the results of biophysical and permeability studies performed with Laurocapram and its analogues. As shown, Laurocapram and its analogues present different enhancing efficacies, for most of both hydrophilic and lipophilic substances. The enhancing effect of Laurocapram (Azone) is attributed to different mechanisms, such as insertion of its dodecyl group into the intercellular lipidic bilayer, increase of the motion of the alkylic chains of lipids, and fluidization of the hydrophobic regions of the lamellate structure. Toxicological studies reveal a low toxicity for Laurocapram, and for some derivatives, a relationship exists between toxicity and the number of carbons in the alkylic chain. Very important, when applied to human skin, Laurocapram shows a minimal absorption, being quickly eliminated from circulation. However, although Laurocapram and its derivatives have been shown to provide enhancement, they have not been widely accepted because of their suspected pharmacological activity or questions about their safety.

Follicular Penetration: Development of a Method to Block the Follicles Selectively against the Penetration of Topically Applied Substances

Investigations into the penetration pathways of topically applied substances through the skin play an important role in dermatological science. Recently, the hair follicles have increasingly been recognized as an important pathway for percutaneous penetration, although the role of the follicles has still not been clarified in detail. Therefore, the aim of the present study was to develop an in vivo method for the analysis of the follicular rate of penetration processes. In order to reach this goal, the follicles were excluded from the penetration process of topically applied substances. Different study designs were evaluated, and we were able to show that the application of nail varnish to each follicular orifice represents an effective method of blocking the follicles selectively against the penetration of topically applied substances. In dermatological science, there is new information on the physiology of skin penetration processes, thus allowing examination of the follicular rate of penetration and improvement of the pharmacokinetics of topically applied substances.

Semiquantitative Determination of the Penetration of a Fluorescent Hydrogel Formulation into the Hair Follicle with and without Follicular Closure by Microparticles by Means of Differential Stripping

Recently, hair follicles were shown to be of great importance concerning the penetration and reservoir behavior of topically applied substances. Especially microparticles are known to be efficient drug carriers into the hair follicles. It has been shown in various investigations that, depending on their size, microparticles can be utilized to enhance the penetration of substances into the hair follicles. In the present study, it was investigated whether they could also be used to block the hair follicles to stop penetration. For this purpose, the follicular penetration of a sodium fluorescein-containing hydrogel and follicular reservoir depletion were investigated by means of differential stripping in vivo. In a second study design, the skin areas were pretreated with microparticles 5 microm in diameter. Subsequently, the follicular penetration of sodium fluorescein-containing hydrogel was likewise investigated by differential stripping. The results showed that the hair follicle infundibula contained a significant fluorescent signal when sodium fluorescein in hydrogel was applied. In contrast, in the case of pretreatment with microparticles, only a very low fluorescence signal was measurable in the hair follicle infundibula. The microparticles clearly blocked the follicular orifices so that a penetration of sodium fluorescein was almost completely inhibited. This was confirmed by histological investigations. Thus, depending on their size, microparticles act as efficient drug carriers or can be utilized as follicle blockers to stop the penetration of topically applied substances. This could be very useful in skin protection to prevent the storage of harmful topically applied substances in the hair follicles, known to represent long-term reservoirs.

Overcoming the Stratum Corneum: The Modulation of Skin Penetration

It is preferred that topically administered drugs act either dermally or transdermally. For that reason they have to penetrate into the deeper skin layers or permeate the skin. The outermost layer of the human skin, the stratum corneum, is responsible for its barrier function. Most topically administered drugs do not have the ability to penetrate the stratum corneum. In these cases modulations of the skin penetration profiles of these drugs and skin barrier manipulations are necessary. A skin penetration enhancement can be achieved either chemically, physically or by use of appropriate formulations. Numerous chemical compounds have been evaluated for penetration-enhancing activity, and different modes of action have been identified for skin penetration enhancement. In addition to chemical methods, skin penetration of drugs can be improved by physical options such as iontophoresis and phonophoresis, as well as by combinations of both chemical and physical methods or by combinations of several physical methods. There are cases where skin penetration of the drug used in the formulation is not the aim of the topical administration. Penetration reducers can be used to prevent chemicals entering the systemic circulation. This article concentrates on the progress made mainly over the last decade by use of chemical penetration enhancers. The different action modes of these substances are explained, including the basic principles of the physical skin penetration enhancement techniques and examples for their application.

Ethnic variation in vellus hair follicle size and distribution

It is a given fact that hair follicles play an important role in the penetration process of topically applied drugs and cosmetics. Since exact knowledge of the potential follicular reservoir is essential for understanding and calculating the penetration process, this knowledge may indeed help to optimize the development of topically applied drugs and cosmetic products. We know that variability in skin properties exists among the human population, but it is the deciphering and quantification of these differences that may help to explain racial disparities in dermatological disorders and provide a basic approach for treatment and prevention. Data pertaining to structural and functional differences within ethnic skin types (reviewed in this paper) support the assumption that hair follicles in the various skin types are not equal. Thus, the need to investigate follicular morphology in non-Caucasian skin types becomes more than apparent. The aim of the present study was to determine the potential reservoir of vellus hair follicles in non-Caucasian skin types in order to estimate their contribution to the penetration of topically applied substances. Cyanoacrylate skin surface biopsies were taken from seven body sites of Asians and African-Americans. To determine the follicular reservoir, the samples were evaluated under the light microscope. Follicular parameters were measured with the help of a digital imaging software program. In the evaluation and comparison of the results with those of Caucasians, significant differences were detected. Compared with Whites, follicular density on the forehead is significantly lower in Asians and African-Americans. Smaller values were detected for volume, surface, follicular orifice and hair shaft diameter on the thigh and in calf regions in Asians and African-Americans. Furthermore, the follicular reservoir, characterized by follicular volume, is generally higher in Caucasians. In comparison to African-Americans, larger follicular volumes on the forehead, back and on the upper arm were determined in Asians. The fairly similar density in these sites revealed that Asians have a higher follicular reservoir in these sites. Due to significant ethnic differences in the follicular reservoir, particularly on the calf and forehead, prospective skin absorption experiments need to be performed on different skin types in order to be representative. Likewise, the development of topically applied drugs should take place under adequate consideration of ethnic differences in skin physiology.

Follicular targeting--a promising tool in selective dermatotherapy

The penetration of topically applied compounds varies considerably in the different regions of the human body. The presence of hair follicles significantly contributes to this effect by an increase in surface area and a disruption of the epidermal barrier towards the lower parts of the hair follicle. The human hair follicle, hereby, serves not only as a reservoir, but also as a major entry point for topically applied compounds. Topical delivery of active compounds to specific targets within the skin may help reduce side-effects caused by unspecific reactions, and may help develop new strategies in the prevention and treatment of skin diseases. Various drug carrier and drug delivery systems are currently being investigated. The aim of these investigational efforts is to direct topically applied compounds to the different types of hair follicles and, ideally, to specific compartments and cell populations within the hair follicles. Follicular targeting offers opportunities for new developments, not only in hair therapy and in the treatment of hair follicle associated diseases but also in gene therapy and immunotherapy.

Differential Stripping: Determination of the Amount of Topically Applied Substances Penetrated into the Hair Follicles

The determination of penetration pathways of topically applied substances into the skin is the subject of several investigations. Recently, follicular penetration has become a major focus of interest. To date, a direct, non-invasive quantification of the amount of topically applied substance penetrated into the follicles had not been possible. The development of such a method was the aim of this study. Therefore, the advantages of both stripping techniques, tape stripping and cyanoacrylate skin surface biopsy, were combined and evaluated. Tape stripping was used to remove the part of the stratum corneum that contained the topically applied dye. Subsequently, the follicular contents were ripped off by cyanoacrylate skin surface biopsy. The combined method termed "differential stripping" was evaluated in vitro and in vivo, and the amount of topically applied fluorescent dye penetrated into the hair follicles was quantified after different penetration times. After 30 min, 5% of the recovered concentration of sodium fluorescein was found in the follicular infundibula, where it was still detectable after 48 h. Altogether, the results of this investigation revealed that differential stripping is a new method that can be used to study the penetration of topically applied substances into the follicular infundibula non-invasively and selectively.

Comparison of four different in vitro systems to study the reservoir capacity of the stratum corneum

Four in vitro test systems were used to study the reservoir capacity of porcine stratum corneum (SC) for flufenamic acid and its drainage via penetration into the deeper skin layers: Franz diffusion cell using full thickness skin and split skin of 300 mum; Saarbruecken penetration model (SB) and intact porcine tissue (IP). Each skin sample was segmented 1, 4 and 21 h after application of an 'infinite dose' of flufenamic acid. The lipophilic drug was extracted from the SC and the deeper skin layers (viable epidermis and dermis) and determined using high-performance liquid chromatography (HPLC). For each test system, an increase in the drug amount in the deeper skin layers and the acceptor fluid, respectively, was observed in combination with a decreased amount in the SC with increasing time after application. The drainage of the SC reservoir was only reflected by a linear correlation of the drug amount in the SC with the amount in the deeper skin layers in the case of IP. The absolute drug concentrations previously detected in human skin in vivo and in vitro were compared with the present data, affording the best accordance in the case of IP.

Biodegradable derivatives of tranexamic acid as transdermal permeation enhancers

The purpose of this work was to develop a novel approach to transdermal permeation enhancer design, based on utilizing some favorable properties of their metabolites. As an example of this concept, a series of carbamic acid salts of tranexamic acid (TXA) esters was synthesized, because TXA was previously shown to improve skin barrier homeostasis. Enhancement activities of 1% TXA derivatives dispersed in both hydrophilic and lipophilic vehicles were evaluated in vitro using human skin and theophylline as a model drug. Dispersed in an aqueous donor vehicle, the dodecyl ester showed the enhancement ratio (ER) of 4.3+/-0.9, which is almost 2 times higher than that of 1-dodecylazepan-2-one (Azone; 2.2+/-0.7). From an isopropyl-myristate suspension, the decyl ester was the most effective enhancer (4.9+/-1.4), while Azone was inactive. Decomposition of the carbamate in a slightly acidic environment was shown by FTIR; hydrolysis of the pertinent ester by porcine esterase was monitored by TLC and HPLC. Biodegradable enhancers of this type could mediate easier and faster recovery of the skin barrier after transdermal delivery through the action of the released TXA.

Mechanistic study of chemical skin permeation enhancers with different polar and lipophilic functional groups

In a previous study, the enhancement effects on the transport of a steroidal permeant along the hairless mouse skin (HMS) stratum corneum (SC) lipoidal pathway were investigated for two homologous series of chemical enhancers: the 1-alkyl-2-pyrrolidones and the 1-alkyl-2-azacycloheptanones. The objective of the present study was to extend this investigation to a broader range of enhancers in order that generalizations with regard to the mechanistic aspects of enhancer function might be established. Specific questions to be addressed included: (a) what is the nature of the microenvironment of the enhancer site of action? (b) what is the extent of the equilibrium uptake of the enhancer from its E = 10 aqueous enhancer solution (the aqueous concentration for which the enhancer induces a tenfold transport enhancement) into the HMS SC intercellular lipid "phase"? and (c) are the microenvironment of the enhancer site of action and that for the equilibrium enhancer uptake at E = 10 relatively independent of the molecular characteristics of the enhancers (as suggested by the earlier study)? Enhancers selected for this study included: a wide range of polar head group size and polarity; n-alkyl group chain lengths from C(4) to C(12); and enhancers in which a double bond is substituted for a single bond in the hydrocarbon chain (3-alkenols) from C(5) to C(9). In addition to the main study, an ancillary set of experiments were to be conducted on the partitioning of a surrogate permeant (estradiol) into the intercellular lipid "phase" under E = 10 isoenhancement conditions to assess the extent to which the permeant partition coefficient may contribute to the permeation enhancement. The following were the principal findings of this research. First, there was very good correlation between the E = 10 isoenhancement aqueous enhancer concentrations and K(octanol/water) for all the studied enhancers. Second, the partitioning of the enhancer from the E = 10 aqueous enhancer solution into the HMS SC intercellular lipid "phase" was found to be relatively independent of the molecular characteristics for all studied enhancers, and the partition coefficients also correlated well with K(octanol/water). These results may have the following meanings: both the microenvironment of the enhancer site of action and the SC intercellular lipid "phase" involved in the enhancer partitioning experiments are well mimicked by liquid n-octanol, and the "intrinsic" potencies (as assessed by the equilibrium enhancer concentration in the microenvironment at the site of action) of the enhancers are relatively independent of the molecular characteristics of the studied enhancers. Finally, the estradiol partitioning experiments suggest the permeant partitioning into the HMS SC intercellular lipid "phase" is enhanced around five- to seven-fold when permeation is enhanced ten-fold for most of the studied enhancers; therefore, the enhancement of the permeant partition coefficient rather than the permeant diffusion coefficient seems to be more important in permeation enhancement of the SC barrier lipoidal pathway.

The effect of penetration enhancers on drug delivery through skin: a QSAR study

Skin penetration enhancers are used to allow formulation of transdermal delivery systems for drugs that are otherwise insufficiently skin-permeable. A full understanding of the mode of action could be beneficial for the design of potent enhancers and for the choice of the enhancer to be used in the topical formulation of a special drug. In this study, the structural requirements of penetration enhancers have been investigated using the Quantitative Structure-Activity Relationship (QSAR) technique. Activities of naturally occurring terpenes, pyrrolidinone and N-acetylprolinate derivatives on the skin penetration of 5-fluorouracil, diclofenac sodium (DFS), hydrocortisone (HC), estradiol and benazepril have been considered. The resulting QSARs indicated that for 5-fluorouracil and diclofenac sodium, less hydrophobic enhancers were the most active. More precisely, molecular descriptors in the corresponding QSARs indicated the possible involvement of intermolecular electron donor-acceptor interactions. This was in contrast to the skin permeation promotion of hydrocortisone, estradiol and benazepril by enhancers, where a linear relationship between enhancement activity and n-octanol/water partition coefficients of enhancers was evident. The possible mechanisms of penetration enhancement as suggested by the QSARs will be discussed.

Synergistic penetration enhancement effect of ethanol and phospholipids on the topical delivery of cyclosporin A

In the present study, ethanol was used with a commercially available lipid mixture, NAT 8539, to improve the topical delivery of cyclosporin A (CyA). The vesicles formed from this solution ranged from 56.6 to 100.6 nm in diameter, depending on the amount of ethanol added in the formulation. In-vitro skin penetration studies were carried out with Franz diffusion cell using human abdominal skin. There was a decrease in average size of vesicles, as the amount of ethanol in formulation increased from 0% to 3.3% and a further addition of ethanol resulted in an increase in average diameter of vesicles. CyA vesicles containing 10% and 20% ethanol showed statistically enhanced deposition of CyA into the stratum corneum (SC), as compared to vesicles prepared without ethanol. CyA vesicles prepared with NAT 8539/ethanol (10/3.3) showed a 2.1-fold, CyA vesicles with NAT 8539/ethanol (10/10) showed a 4.4-fold, and CyA vesicles with NAT 8539/ethanol (10/20) showed a 2.2-fold higher deposition of CyA into SC, as compared to vesicles made of NAT 8539 without ethanol [NAT 8539/ethanol (10/0)]. The efficiency of the formulations was sequenced in the order of: NAT 8539/ethanol (10/10)>NAT 8539/ethanol (10/20)>NAT 8539/ethanol (10/3.3)>ethanol>NAT 8539/ethanol (10/0). These results can be considered a step forward for the topical delivery of problematic molecules like CyA using liposomes as a tool for the treatment of inflammatory skin diseases like psoriasis, atopic dermatitis, and diseases of the hair follicle like alopecia areata, etc.

Variations of hair follicle size and distribution in different body sites

For the evaluation and quantification of follicular penetration processes, the knowledge of variations of hair follicle parameters in different body sites is basic. Characteristics of follicle sizes and potential follicular reservoir were determined in cyanoacrylate skin surface biopsies, taken from seven different skin areas (lateral forehead, back, thorax, upper arm, forearm, thigh, and calf region). The highest hair follicle density and percentage of follicular orifices on the skin surface and infundibular surface were found on the forehead, whereas the highest average size of the follicular orifices was measured in the calf region. The highest infundibular volume and therefore a potential follicular reservoir was calculated for the forehead and for the calf region, although the calf region showed the lowest hair follicle density. The calculated follicular volume of these two skin areas was as high as the estimated reservoir of the stratum corneum. The lowest values for every other parameter were found on the forearm. The present investigation clearly contradicts former hypothesis that the amount of appendages of the total skin surface represents not more than 0.1%. Every body region disposes its own hair follicle characteristics, which, in the future, should lead us to a differential evaluation of skin penetration processes and a completely different understanding of penetration of topically applied drugs and cosmetics.

Synthetic ceramide analogues as skin permeation enhancers: structure–Activity relationships

The study presents new information about the structure-activity relationships of the skin permeation enhancers. A series of ceramide analogues including eight different polar head groups and six different chain lengths was synthesised. The compounds were evaluated as permeation enhancers in vitro using porcine skin. The physico-chemical parameters of the tested compounds obtained by computer modelling were used to evaluate, by multiple linear regression, the enhancement ratios (ERs) of the compounds. The regression analysis suggests that the hydrogen bonding ability of the compounds is inversely related to the ER values and that the molecular size and lipophilicity must be well balanced. In the studied enhancers having the same chain length, the enhancement activity is dependent only on their permeability coefficients. This finding confirms the Warner's hypothesis that the polar head of an enhancer is responsible for the permeation and anchoring of the molecule into the stratum corneum lipids and that it does not influence the mechanism of action. For the specific action of enhancers, that is disordering of the intercellular lipid packing, the length of the hydrophobic chain(s) and not the lipophilicity is important. Furthermore, the examination of the FTIR spectra indicated that the most active substances possess the most ordered chains. The described relationships could bring more rational approaches in designing new potent enhancers for transdermal formulations.

[Follicular penetration. An important pathway for topically applied substances]

The knowledge of penetration pathways in and through the skin is a prerequisite for the development and optimization of topically applied drugs and cosmetics. Skin penetration has been assumed to occur via diffusion though the lipid domains of the stratum corneum. Recent studies show that the skin appendages, especially the hair follicles, play an important role in skin penetration processes. Topically applied substances cannot enter all follicles. We discuss the reasons for the phenomenon of open and closed hair follicles.

T cell repertoire in mice with alopecia areata

It has become clear that skin infiltrating autoreactive CD4+ T helper cells play a crucial role in the initiation of alopecia areata. However, the natures of the pathogenic T cell clones as well as of the skin antigen they recognize remain obscure. Here, we analyzed the T cell receptor repertoire expressed in the spleen of diseased mice. We consistently observed the dominant expansion of a limited set of T cell clones expressing Vbeta8.2/Jbeta2.5 T cell receptor rearrangement. We conclude that T cell response in mice alopecia areata is markedly oligoclonal; a feature that may permit the design of selective immunotherapy.