Percutaneous penetration enhancement and its quantification

Topical delivery of Cyclosporine A into the skin using SPACE-peptide

Cyclosporine A (CsA) is used for the treatment of psoriasis; however systemic administration of CsA is potentially life threatening and there are long-term side effects. Topical application of CsA has the potential to overcome this hurdle; however, its use is limited by poor water solubility and low permeability. Here, we report the use of a physical mixture of SPACE-peptide and CsA in an aqueous ethanol solution to enhance the dermal absorption of the drug. The aqueous ethanol solution (hydroethanolic solution) containing 5mg/mL CsA and 50mg/mL of free SPACE-peptide (SP50) delivered about 30% of topically applied CsA into the porcine skin in vitro and led to an approximately 9-fold (p<0.01) increase in accumulation in viable epidermis compared to the hydroethanolic solution without SPACE-peptide (control group). In vivo biodistribution and pharmacokinetic studies performed using SKH1 hairless mice also confirmed the efficacy of SP50 in dermal delivery of CsA and also demonstrated its advantages over other routes in terms of minimizing its systemic absorption. Topical application of SP50 significantly increased the localization of CsA in the target skin (113.1±13.6(μg/g)/mg) compared to all other groups (p<0.01). In addition, SP50 led to significantly higher skin/blood ratio (443.4±181.5) and skin/liver ratio (1059.5±110.8) of CsA compared to all other groups (p<0.01). The SP50 formulation reported here offers a promising approach for the dermal delivery of CsA.

Confocal Raman microscopic investigation of the effectiveness of penetration enhancers for procaine delivery to the skin

A methodology that employs confocal Raman microscopy (CRM) on ex vivo skin samples is proposed for the investigation of drug content and distribution in the skin. To this end, the influence of the penetration enhancers propylene glycol and polyoxyethylene-23-lauryl ether on the penetration and permeation of procaine as a model substance was investigated. The drug content of skin samples that had been incubated with semisolid formulations containing one of these enhancers was examined after skin segmentation. The experiments showed that propylene glycol did not affect the procaine content that was delivered to the skin, whereas polyoxyethylene-23-lauryl ether led to higher procaine contents and deeper penetration. Neither substance was found to influence the permeation rate of procaine. It is thereby shown that CRM can provide additional information on drug penetration and permeation. Furthermore, the method was found to enhance the depth from which Raman spectra can be collected and to improve the depth resolution compared to previously proposed methods.

Lysine-based surfactants as chemical permeation enhancers for dermal delivery of local anesthetics

The aim of this study is to investigate the efficacy of new, biocompatible, lysine-based surfactants as chemical permeation enhancers for two different local anesthetics, tetracaine and ropivacaine hydrochloride, topically administered. Results show that this class of surfactants strongly influences permeation, especially in the case of the hydrophilic and ionized drug, ropivacaine hydrochloride, that is not easily administered through the stratum corneum. It is also seen that the selected permeation enhancers do not have significant deleterious effects on the skin structure. A cytotoxicity profile for each compound was established from cytotoxicity studies. Molecular dynamics simulation results provided a rationale for the experimental observations, introducing a mechanistic view of the action of the surfactants molecules upon lipid membranes.

Structure activity relationships in alkylammonium C12-gemini surfactants used as dermal permeation enhancers

The purpose of this study was to determine the ability and the safety of a series of alkylammonium C12-gemini surfactants to act as permeation enhancers for three model drugs, namely lidocaine HCl, caffeine, and ketoprofen. In vitro permeation studies across dermatomed porcine skin were performed over 24 h, after pretreating the skin for 1 h with an enhancer solution 0.16 M dissolved in propylene glycol. The highest enhancement ratio (enhancement ratio (ER)=5.1) was obtained using G12-6-12, resulting in a cumulative amount of permeated lidocaine HCl of 156.5 μg cm−2. The studies with caffeine and ketoprofen revealed that the most effective gemini surfactant was the one with the shorter spacer, G12-2-12. The use of the latter resulted in an ER of 2.4 and 2.2 in the passive permeation of caffeine and ketoprofen, respectively. However, Azone was found to be the most effective permeation enhancer for ketoprofen, attaining a total of 138.4 μg cm−2 permeated, 2.7-fold over controls. This work demonstrates that gemini surfactants are effective in terms of increasing the permeation of drugs, especially in the case of hydrophilic ionized compounds, that do not easily cross the stratum corneum. Skin integrity evaluation studies did not indicate the existence of relevant changes in the skin structure after the use of the permeation enhancers, while the cytotoxicity studies allowed establishing a relative cytotoxicity profile including this class of compounds, single chain surfactants, and Azone. A dependence of the toxicity to HEK and to HDF cell lines on the spacer length of the various gemini molecules was found.

Topical delivery of hyaluronic acid into skin using SPACE-peptide carriers

Topical penetration of macromolecules into the skin is limited by their low permeability. Here, we report the use of a skin penetrating peptide, SPACE peptide, to enhance topical delivery of a macromolecule, hyaluronic acid (HA, MW: 200-325kDa). The peptide was conjugated to phospholipids and used to prepare an ethosomal carrier system (~110nm diameter), encapsulating HA. The SPACE-ethosomal system (SES) enhanced HA penetration into porcine skin in vitro by 7.8+/-1.1-fold compared to PBS. The system also enhanced penetration of HA in human skin in vitro, penetrating deep into the epidermis and dermis in skin of both species. In vivo experiments performed using SKH1 hairless mice also confirmed increased dermal penetration of HA using the delivery system; a 5-fold enhancement in penetration was found compared to PBS control. Concentrations of HA in skin were about 1000-fold higher than those in blood; confirming the localized nature of HA delivery into skin. The SPACE-ethosomal delivery system provides a formulation for topical delivery of macromolecules that are otherwise difficult to deliver into the skin.

Predicting skin permeability from complex vehicles

It is now widely accepted that vehicle and formulation components influence the rate and extent of passive chemical absorption through skin. Significant progress, over the last decades, has been made in predicting dermal absorption from a single vehicle; however the effect of a complex, realistic mixture has not received its due attention. Recent studies have aimed to bridge this gap by extending the use of quantitative structure-permeation relationship (QSPR) models based on linear free energy relationships (LFER) to predict dermal absorption from complex mixtures with the inclusion of significant molecular descriptors such as a mixture factor that accounts for the physicochemical properties of the vehicle/mixture components. These models have been compiled and statistically validated using the data generated from in vitro or ex vivo experimental techniques. This review highlights the progress made in predicting skin permeability from complex vehicles.

Bacterial cellulose membranes applied in topical and transdermal delivery of lidocaine hydrochloride and ibuprofen: in vitro diffusion studies

Bacterial cellulose (BC) is a biomaterial with unique physical and mechanical properties that triggered considerable interest, but there are few studies addressing the use of such membranes for drug loading and controlled release. This study aimed to investigate the applicability of BC membranes in topical or transdermal drug delivery systems. To assess its therapeutic feasibility, the permeation through human epidermis of two model drugs (lidocaine hydrochloride and ibuprofen) in BC and other formulation systems was compared in vitro. A uniform distribution of both drugs in the BC membranes was achieved. Diffusion studies with Franz cells showed that the incorporation of lidocaine hydrochloride in BC membranes provided lower permeation rates than those obtained with the conventional formulations. However, the results obtained with the lipophilic drug were quite different, since permeation of ibuprofen in BC was almost three times higher than that of the drug in the gel or in a PEG400 solution. These results indicate that this technology can be successfully applied to modulate the bioavailability of drugs for percutaneous administration, which could be particularly advantageous in the design of delivery systems that have, simultaneously, the ability to absorb exudates and to adhere to irregular skin surfaces.

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.

Transdermal Permeability of N-Acetyl-D-Glucosamine

Transdermal permeation of N-acetyl-D-glucosamine (NAG), a metabolite of glucosamine was examined. Glucosamine salts are nutraceuticals used in the oral treatment of osteoarthritis. Sparse information is available regarding glucosamine and NAG transdermal or percutaneous transport and absorption. Permeability of NAG in various enhancer suspensions was evaluated by using shed snakeskin as a model membrane via Franz-type cell diffusion studies. Negligible permeability was observed for NAG in neat solutions of known membrane permeation enhancers ethanol, oleic acid, isopropyl myristate, and isopropyl palmitate, as well as from saturated solutions of NAG in water or phosphate buffer. Permeability measurements obtained from saturated solutions of NAG in DMSO and phosphate buffer solutions containing ethanol at 2%, 5%, 10%, 25%, and 50% demonstrated excellent permeation. Permeability coefficients of the phosphate buffer/ethanol solutions at 5%, 10%, and 25% were about threefold larger in value as those for saturated DMSO solution, whereas the 2% and 50% solution values were lower.

An alkylpolyglucoside surfactant as a prospective pharmaceutical excipient for topical formulations: The influence of oil polarity on the colloidal structure and hydrocortisone in vitro/in vivo permeation

There is a growing need for research into new skin- and environment-friendly surfactants. This paper focuses on a natural surfactant of an alkylpolyglucoside type, which can form both thermotropic and lyotropic liquid-crystalline phases. The aim of this study was to relate some physicochemical properties (characterised by polarisation and transmission electron microscopy, thermal analysis and rheology) of the three formulations based on cetearyl glucoside and cetearyl alcohol, to the results of in vitro and in vivo bioavailability of hydrocortisone (HC). The three formulations contained oils of different polarity (medium chain triglycerides: MG, isopropyl myristate: IPM and light liquid paraffin: LP), respectively. In vitro permeation was followed through the artificial skin constructs (ASC), while the parameters measured in vivo were erythema index: EI (using instrumental human skin blanching assay), transepidermal water loss (TEWL) and stratum corneum hydration (SCH). The vehicles based on cetearyl glucoside and cetearyl alcohol showed a complex colloidal structure of lamellar liquid-crystalline and lamellar gel-crystalline type, depending on oil polarity. Rheological profile of the vehicle was directly related to the in vitro profile of the HC permeation. In vivo results suggested that the vehicle with MG retarded the HC permeation, whereas less polar IPM and non-polar LP enhanced it. It is suggested that the enhancement is achieved either by a direct interaction with lipid lamellae of the SC or indirectly by improving skin hydration. There were no adverse effects during in vivo study, which indicates a good safety profile of this alkylpolyglucoside surfactant.

Effect of physicochemical properties of cyclic terpenes on their ex vivo skin absorption and elimination kinetics

BACKGROUND

The terpenes disturb lipid arrangement in the intercellular region of the stratum corneum (SC) that leads to the increased permeability of the skin. This effect is used in technology of transdermal drug forms and depends on physicochemical properties of terpenes and their amounts penetrated to the stratum corneum; however terpenes do not need penetrate into viable skin tissue and this event is not even desired.

OBJECTIVE

To correlate skin absorption and elimination kinetics of four cyclic terpenes, namely alpha-pinene, beta-pinene, eucalyptol and terpinen-4-ol, applied as neat substance with their physicochemical properties.

METHODS

The terpenes were applied onto the human skin in vitro, and after 1-4 h their content in the separated by a tape-stripping method stratum corneum layers and in the epidermis/dermis was determined using GC. Similarly, the amounts of terpenes in the skin were analysed during 4 h following 1 h absorption.

RESULTS

The fastest and progressive penetration into all skin layers was observed for terpinen-4-ol. All studied terpenes are absorbed in the viable epidermis/dermis, however penetration into this layers is time-dependent process, constantly increasing during 4 h. Like for stratum corneum, the largest cumulation in epidermis/dermis was observed for terpinen-4-ol. The elimination of terpenes from the stratum corneum was fast, especially in deeper layers, and much faster if the initial cumulation was small.

CONCLUSION

Investigated cyclic terpenes represent different penetration and elimination characteristics and do not permeate across the skin to the acceptor medium due to large cumulation in the skin tissue. The penetration of terpenes into stratum corneum is greater if their log P-value is close to 3.

How does the type of vehicle influence the in vitro skin absorption and elimination kinetics of terpenes?

Terpenes are widely used in the topical dermal preparations, cosmetics and toiletries and also in the experimental dermopharmacy, as penetration enhancers. Terpenes do not need to penetrate into viable skin tissue and this event is not even desired. The aim of this study was to investigate skin absorption and elimination kinetics of two terpenes, namely linalool and terpinen-4-ol, incorporated in three different dermatological vehicles: oily solution, hydrogel and o/w emulsion. The preparations were applied onto the human skin in vitro, and after 1-4 h the content of terpenes in the stratum corneum layers and in the epidermis/dermis was determined using GC. Similarly, the amounts of terpenes in the skin were analysed during 4 h elimination process following 1 h absorption. The highest skin absorption was observed when terpenes were applied in hydrogel--their total content in the skin after 4 h was 385 and 705 microg/cm2 for linalool and terpinen-4-ol, respectively. After 1 h of the elimination process about 10-20% drop of the total content of both terpenes in the skin was noted for all formulations. The skin penetration of both terpenes from the vehicles is increasing in the following order: emulsion < oily solution < hydrogel, while the elimination phase is relatively slower for terpenes applied in hydrogel.

Feasibility studies of dermal delivery of paclitaxel with binary combinations of ethanol and isopropyl myristate: role of solubility, partitioning and lipid bilayer perturbation

In the current investigation, paclitaxel (PCL) delivery into the different layers of skin, vehicle optimization and relationship between vehicle composition and the relative contribution of solubility, partition and diffusion towards drug transport has been outlined. Saturation solubility of PCL was determined in ethanol (EtOH), isopropyl myristate (IPM) and their binary combinations, and partition studies performed to study the probability of skin depot formation. Epidermal and dermal partitioning was carried from PCL saturated vehicles. Skin permeation of PCL was studied using the rat skin. FT-IR has been utilized to study the skin barrier perturbation, and the localization of PCL and isopropyl myristate (IPM) in epidermis. High K(app) value in mineral oil/buffer indicated the tendency of PCL to form a reservoir in skin, and an inverse relationship between PCL solubility in different solvent systems and partitioning into epidermis was found. Maximum K(epidermis) for PCL was observed with IPM, while PCL in EtOH/IPM (1:1) showed high partitioning into dermis. Maximum flux of PCL was observed with EtOH/IPM (1:1). For lipophilic drug like PCL modulation of vehicle seems to be effective approach to increase the permeability across the skin. With a binary combination of EtOH/IPM (1:1) higher concentration of PCL can be delivered to deeper layer of skin whereas with IPM higher concentration of PCL could be localized in the epidermis. While engineering the delivery vehicle selection of solvents should be such that one of them is miscible in both hydrophilic and lipophilic phase like ethanol and another should be lipophilic in nature (IPM in this case) so that an optimum balance between 'push-pull' and 'blending' effect can be achieved.

Exploration of the global antioxidant capacity of the stratum corneum by cyclic voltammetry

Cyclic voltammetry is proposed as a new method for evaluating the antioxidant capacity of skin based on the reducing properties of low molecular weight antioxidants (LMWA). Experiments were performed simply by recording the anodic current at 0.9 V/SCE of a platinum microelectrode placed directly on the epidermis surface without any gel or water. This method ensured a direct, rapid (less than 1 min), reliable (accuracy 12%) and non-invasive measurement of the global antioxidant capacity of the stratum corneum with a high spatiotemporal resolution. At the same time, the pH of the skin surface was determined by recording the cathodic current at 0 V/SCE. Based on an exploratory study involving nine volunteer subjects, the evolution of the amperometric response of the microelectrode with time revealed a periodic modification of the redox properties.

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.

Enhanced transdermal delivery of atenolol from the ethylene–vinyl acetate matrix

To enhance transdermal delivery of atenolol, ethylene-vinyl acetate (EVA) matrix of drug containing penetration enhancer was fabricated. Effect of penetration enhancer on the permeation of atenolol through the excised rat skin was studied. Penetrating enhancers showed the increased flux probably due to the enhancing effect on the skin barrier, the stratum corneum. Among enhancers used such as glycols, fatty acids and non-ionic surfactants, polyoxyethylene 2-oleyl ether showed the best enhancement. For the controlling transdermal delivery of atenolol, the application of EVA matrix containing permeation enhancer could be useful in the development of transdermal drug delivery system.

Unexpected clobetasol propionate profile in human stratum corneum after topical application in vitro

PURPOSE

The validity of using drug amount-depth profiles in stratum corneum to predict uptake of clobetasol propionate into stratum corneum and its transport into deeper skin layers was investigated.

METHODS

In vitro diffusion experiments through human epidermis were carried out using Franz-type glass diffusion cells. A saturated solution of clobetasol propionate in 20% (V/V) aqueous propylene glycol was topically applied for 48 h. Steady state flux was calculated from the cumulative amount of drug permeated vs. time profile. Epidermal partitioning was conducted by applying a saturated drug solution to both sides of the epidermis and allowing time to equilibrate. The tape stripping technique was used to define drug concentration-depth profiles in stratum corneum for both the diffusion and equilibrium experiments.

RESULTS

The concentration-depth profile of clobetasol propionate in stratum corneum for the diffusion experiment is biphasic. A logarithmic decline of the drug concentration over the first four to five tape strips flattens to a relatively constant low concentration level in deeper layers. The drug concentration-depth profile for the equilibrium studies displays a similar shape.

CONCLUSIONS

The shape of the concentration-depth profile of clobetasol propionate is mainly because of the variable partitioning coefficient in different stratum corneum layers.

Effect of vehicle pretreatment on the flux, retention, and diffusion of topically applied penetrants in vitro

PURPOSE

The flux of a topically applied drug depends on the activity in the skin and the interaction between the vehicle and skin. Permeation of vehicle into the skin can alter the activity of drug and the properties of the skin barrier. The aim of this in vitro study was to separate and quantify these effects.

METHODS

The flux of four radiolabeled permeants (water, phenol, diflunisal, and diazepam) with log Koct/water values from 1.4 to 4.3 was measured over 4 h through heat-separated human epidermis pretreated for 30 min with vehicles having Hildebrand solubility parameters from 7.9 to 23.4 (cal/cm3)1/2.

RESULTS

Enhancement was greatest after pretreatment with the more lipophilic vehicles. A synergistic enhancement was observed using binary mixtures. The flux of diazepam was not enhanced to the same extent as the other permeants, possibly because its partitioning into the epidermis is close to optimal (log Koct 2.96).

CONCLUSION

An analysis of the permeant remaining in the epidermis revealed that the enhancement can be the result of either increased partitioning of permeant into the epidermis or an increasing diffusivity of permeants through the epidermis.

Use of an 8132Asymmetrical Factorial Design for the In Vitro Evaluation of Ondansetron Permeation Through Human Epidermis

The in vitro permeation of ondansetron through human cadaver epidermis, as a preliminary step toward the development of a transdermal therapeutic system, was investigated. In vitro release studies were carried out using modified Franz diffusion cells and human epidermis, taken from cadaver skin by heat separation technique. To estimate the effect of the type and concentration of the penetration enhancers and the skin from different donors, an 8(1)3(2) asymmetrical factorial design was used. Formulations containing lauric acid and oleic acid as penetration enhancers, showed the largest Q values [amounts of ondansetron permeated per unit area of epidermal membrane (microg/cm2)] at 24, 48, and 72 hr, as well as steady-state flux values, among all formulations tested. The other enhancers increased the flux in the following order: lauryl alcohol>glycerol monooleate>Azone >cineole>oleyl alcohol>1-methyl-2-pyrrolidinone. Moreover, the concentration of the penetration enhancer and the type of the skin were proved to significantly affect the permeation rate of ondansetron through human epidermis. From the results obtained, it was shown that the formulations containing lauric acid or oleic acid at 5% or 10% could increase sufficiently the permeation of ondansetron. Therefore, the transdermal administration of ondansetron seems feasible.

Amphiphilic star-like macromolecules as novel carriers for topical delivery of nonsteroidal anti-inflammatory drugs

The objective of this study was to evaluate amphiphilic star-like macromolecules (ASMs) as a topical drug delivery system. Indomethacin, piroxicam, and ketoprofen were individually encapsulated into the ASMs using coprecipitation. The effects of the ASMs on percutaneous permeation of nonsteroidal anti-inflammatory drugs (NSAIDs) across full thickness, hairless mouse skin were evaluated in vitro using modified Franz diffusion cells. In addition, solubility and in vitro release experiments were performed to characterize ASMs behavior in aqueous media. Poly(ethylene glycol) (PEG) and Pluronic P-85 were used as polymer controls to compare the role of PEG and amphiphilic behavior in the ASMs. In vitro release experiments indicated that ASMs can delay drug release (P <.05), whereas solubility measurements showed that ASMs can increase NSAIDs aqueous solubility (P <.05). Percutaneous permeation studies revealed that ASMs decreased both flux and Q24 of drugs compared with the control (P <.10). Skin pretreatment studies with ASM-containing solution before drug application demonstrated that pretreatment similarly influenced NSAID percutaneous permeation. In conclusion, ASMs likely slow drug permeation through 2 mechanisms, delayed drug diffusion from its core and skin dehydration by its shell. Thus, ASMs may be useful for delayed dermal delivery or prevention of compound permeation through the skin (eg, sunscreens, N,N-diethyl-m-toluamide [DEET]) from aqueous formulations.

Permeation, metabolism and site of action concentration of nicotinic acid derivatives in human skin

A novel methodology for establishing a pharmacological dose-effect relationship of methyl nicotinate, hexyl nicotinate and nicotinic acid acting as peripheral vasodilators in the skin following topical application is investigated. This methodology involves the estimation of the unbound drug concentration in the aqueous compartment at the site of action in tissue, termed C(*), which was evaluated as the pertinent concentration responsible for the pharmacological effect. Blood capillaries next to the epidermis-dermis boundary were postulated to be the relevant site of action. C(*) was estimated from drug transport parameters for different layers of human cadaver skin determined in vitro. Immunohistochemical studies showed that the plane of separation of skin achieved by heat treatment was between the basal cells of the epidermis and the lamina lucida, confirming the integrity of the epidermis and the dermis used in the experiments. The permeation rate for epidermis increased drastically with increasing lipophilicity of the drug. Dermis permeability was roughly the same for all three compounds. The epidermis represented the major transport barrier in vitro for methyl nicotinate and nicotinic acid but not for hexyl nicotinate. The esters were metabolised to nicotinic acid during tissue permeation to an extent that was rather limited for the epidermis but very pronounced for the dermis. Nonspecific alpha-naphthylacetate-esterase activity was predominantly located in the dermis, which was in agreement with the metabolism results. The drugs were applied each at three different concentrations in vivo to the ventral forearm of healthy human volunteers and vasodilation was evaluated based on skin erythema which was quantified by measuring colour change of reflected light. Area under the curve of the change of colour co-ordinates as a function of time was used as a measure of pharmacological effect. The pharmacological effect of all three drugs was comparable when similar C(*) values were considered, even though the concentrations applied to the skin differed by orders of magnitude. The effect showed a strong positive dependence on C(*). Methyl and hexyl nicotinate showed identical, nearly sigmoidal effect/C(*)-profiles, while the profile for nicotinic acid was linear, suggesting a possible difference in the intrinsic pharmacological potency between the esters and the acid. These results demonstrate the validity of C(*) as the relevant drug concentration for the cutaneous pharmacological effect of the topically applied drugs and underline the usefulness of the presented methodology for establishing dose-response relationships in dermal therapy and expressing bioavailability.

Enhanced transdermal delivery of triprolidine from the ethylene-vinyl acetate matrix

Triprolidine-containing matrix was fabricated with ethylene-vinyl acetate (EVA) copolymer to control the release of the drug. The permeation rate of triprolidine in the stripped skin was greatly larger than that in the whole skin. Thus it showed that the stratum corneum acts as a barrier of skin permeation. The effect of penetration enhancer and stripping of skin on the permeation of triprolidine through the excised mouse skin was studied. Penetrating enhancers showed increased flux probably due to the enhancing effect on the skin barrier, the stratum corneum. Among enhancers used such as glycols, fatty acids and non-ionic surfactants, polyoxyethylene-2-oleyl ether showed the best enhancement. The permeability of triprolidine was markedly increased with stripping of the mouse skin to remove the stratum corneum that acts as a barrier of skin permeation. For the controlling transdermal delivery of triprolidine, the application of EVA membrane containing permeation enhancer could be useful in the development of transdermal drug delivery system.

Benzophenone-3: rapid prediction and evaluation using non-invasive methods of in vivo human penetration

The study described in this paper constitutes a practical assay system to evaluate in vivo drug penetration using two complementary non-invasive methods. An electrical capacitance test was first applied to the skin on the forearm to evaluate the hydration of the skin, and check the integrity of the stratum corneum. In the first step, the percentage absorption was measured using an occlusive and difference method; following benzophenone-3 application any residual formulation was washed off and the amount removed analyzed. In the second step, the tape stripping method-a useful procedure for selectively removing the skin's outermost layer, the stratum corneum, and measuring the stratum corneum adsorption-was performed. Under these conditions the human skin permeation of this UV-filter over four hours was near to 35% of the applied dose with the occlusive method. The amount of topically applied benzophenone-3 found in the stratum corneum after 30 min exposure using the stripping procedure was evaluated at 4% to the applied dose.

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.

Conjugation of arginine oligomers to cyclosporin A facilitates topical delivery and inhibition of inflammation

Many systemically effective drugs such as cyclosporin A are ineffective topically because of their poor penetration into skin. To surmount this problem, we conjugated a heptamer of arginine to cyclosporin A through a pH-sensitive linker to produce R7-CsA. In contrast to unmodified cyclosporin A, which fails to penetrate skin, topically applied R7-CsA was efficiently transported into cells in mouse and human skin. R7-CsA reached dermal T lymphocytes and inhibited cutaneous inflammation. These data establish a general strategy for enhancing delivery of poorly absorbed drugs across tissue barriers and provide a new topical approach to the treatment of inflammatory skin disorders.

Controlled transdermal iontophoresis by ion-exchange fiber

The objective of this study was to assess the transdermal delivery of drugs using iontophoresis with cation- and anion-exchange fibers as controlled drug delivery vehicles. Complexation of charged model drugs with the ion-exchange fibers was studied as a method to achieve controlled transdermal drug delivery. Drug release from the cation-exchange fiber into a physiological saline was dependent on the lipophilicity of the drug. The release rates of lipophilic tacrine and propranolol were significantly slower than that of hydrophilic nadolol. Permeation of tacrine across the skin was directly related to the iontophoretic current density and drug concentration used. Anion-exchange fiber was tested with anionic sodium salicylate. The iontophoretic flux enhancement of sodium salicylate from the fiber was substantial. As the drug has to be released from the ion-exchange fiber before permeating across the skin, a clear reduction in the drug fluxes from the cationic and anionic fibers were observed compared to the respective fluxes of the drugs in solution. Overall, the ion-exchange fibers act as a drug reservoir, controlling the release and iontophoretic transdermal delivery of the drug.

The maximum pharmacodynamic effect as a response parameter: pharmacokinetic considerations

In previous human in-vivo studies measuring the maximum pharmacodynamic response to characterize cutaneously applied ointment preparations, it was observed that differences between various formulations caused by penetration enhancement led to different enhancement factors depending on the method used for determination of these factors from activity-response curves. To clarify this discrepancy, pharmacokinetic simulations have been performed based on an open one-compartment model with either first- or zero-order drug penetration kinetics and first-order elimination kinetics. Under the assumption that the maximum pharmacodynamic response corresponds to the maximum effective drug concentration in the receptor compartment, which represents the difference between the maximum drug concentration and the threshold concentration, drug concentration vs time profiles and dose-response curves were simulated. In addition, maximum effective concentrations were calculated and plotted against the logarithm of the thermodynamic drug activity to obtain activity-response curves. Relative bioavailability and enhancement factors were determined either from the horizontal distance between the curves of a standard and a test preparation, or as the ratio of the maximum effective concentration of test and standard formulations. A significant difference between the first-order and the zero-order input kinetics with regard to the evaluation of bioavailability and drug penetration enhancement was shown. Under finite dose conditions, i.e. first-order input kinetics from solution-type preparations, a misestimation of the factors usually occurs. Only under infinite dose conditions, i.e. if large preparation volumes are applied to achieve zero-order input kinetics, is the determination of bioavailability and enhancement factors from dose- and activity-response curves accurate.