Transdermal drug delivery: overcoming the skin's barrier function

Microneedle-based intradermal delivery enables rapid lymphatic uptake and distribution of protein drugs

PURPOSE

The purpose of this research was to examine the pharmacokinetics (PK) of drug uptake for microneedle-based intradermal (ID) delivery of several classes of protein drugs compared to standard subcutaneous (SC) administration.

METHODS

Systemic absorption kinetics of various proteins were analyzed following microneedle-based ID delivery and standard injection methods in the swine model. Comparative PK data were determined using standard non-compartmental techniques based on blood serum levels.

RESULTS

Delivery of proteins using microneedles resulted in faster systemic availability, measured via t(max,) and increased maximal drug concentration, C(max,) over SC delivery for all proteins tested. Some agents also exhibited increased bioavailability for the ID route. Imaging studies using reporter dyes showed rapid lymphatic-mediated uptake.

CONCLUSIONS

Microneedle delivery is applicable to a wide variety of protein drugs and is capable of effective parenteral administration of therapeutic drug dosages. This delivery route alters absorption kinetics via targeting a tissue bed better perfused with lymphatic and blood vessels than the SC space. Microneedle delivery may afford various advantages, including a robust method to increase the absorption rate and bioavailability of proteins that have been challenging to deliver at therapeutic levels or with physiologically relevant profiles.

Transdermal iontophoresis of insulin. Part 1: A study on the issues associated with the use of platinum electrodes on rat skin

We have studied the issues associated with the use of platinum electrodes for transdermal iontophoretic delivery of peptides, using insulin as a model peptide. Insulin permeation was studied using full-thickness rat skin by varying the donor solution pH as a function of electrode polarity. The stability of insulin under the iontophoretic conditions was studied using TLC, SDS-polyacrylamide gel electrophoresis and HPLC. Large pH shifts were observed during anodal iontophoresis (AI), when the donor solution pH was above the isoelectric point of insulin and in cathodal iontophoresis (CI), when the donor solution pH was below the isoelectric point of insulin. The direction and magnitude of electroosmotic flow was influenced by pH of the donor solution and the electrode polarity. On the other hand, the buffer used to maintain the pH governed the contribution of electrorepulsion to the overall transport of insulin. Electrochemical degradation of insulin was significant during Al at pH 7.4. Among the pH investigated, Al of insulin at pH 3.6 and Cl at pH 8.35 were better, as the pH shift was relatively less and electrochemically more stable during iontophoresis as compared with other pH. In summary, the pH shift caused by platinum electrodes had a significant influence on the permeation and stability of insulin.

Development and validation of an analytical method for the quantification of cytochrome c in skin transport studies

A simple isocratic HPLC method for the quantification of Cytochrome c in skin permeation samples was developed and validated. The mobile phase comprised a 41 : 59 mixture of an organic phase A (0.1% trifluoroacetic acid in a 90 : 10 mixture of MeCN-H(2)O) and an aqueous phase B (0.1% trifluoroacetic acid in H(2)O). The Cytochrome c retention and run times were 2.62 and 8.0 min, respectively--much shorter than those for existing gradient methods. The response was accurate, precise and linear from 2.5 to 25 microg/mL. The mean recoveries for intra-day and inter-day analysis ranged from 88.5 to 103.8% and the RSD varied from 0.05 to 1.55%. The assay was used to quantify transport of Cytochrome c across intact and laser-microporated porcine skin in vitro. Cytochrome c permeation and the amount of protein retained within the membrane over 24 h were quantified as a function of the number of micropores. Although no Cytochrome c permeation was observed across intact skin, laser microporation enabled delivery of 22.9 +/- 3.3 and 56.0 +/- 15.9 microg/cm(2) of the protein across skin samples with 300 and 1800 micropores, respectively. In conclusion, the HPLC method provided a fast, efficient means to quantify Cytochrome c in samples from skin transport studies.

Synthesis of methoxypoly(ethylene glycol) carbonate prodrugs of zidovudine and penetration through human skin in vitro

OBJECTIVES

The aim of this study was to synthesise a series of novel methoxypoly(ethylene glycol) carbonate prodrugs of the antiretroviral drug zidovudine (azidothymidine, AZT) in an attempt to enhance the physicochemical properties for transdermal delivery, which may reduce the severe side-effects and toxicity associated with high oral doses of AZT.

METHODS

Methoxypoly(ethylene glycol) carbonates of AZT were synthesised in two steps: activation of the relevant methoxypoly(ethylene glycol) with p-nitrophenyl chloroformate, followed by reaction with AZT. Analysis of the hydrolytic stability in phosphate buffer at pH 5.0 and 7.4 revealed that all the carbonates were markedly more stable at pH 5.0 than at pH 7.4 (0.01 M), with half-lives ranging from 15 to 44 days at pH 5.0 and from 6 to 24 days at pH 7.4. The potential of the series to penetrate the skin was evaluated in vitro by measuring diffusion through excised abdominal female human skin at pH 5.0.

KEY FINDINGS

Prodrugs with 1-3 or 8 oxyethylene units in the methoxypoly(ethylene glycol) moiety were found to permeate the skin whereas those with 12 or 17 units did not. The prodrug with eight oxyethylene units was the most effective penetrant, permeating the skin with a mean flux of 53.3 +/- 46.5 nmol/cm2 per h, which is 2.4-10.1 times that of AZT (8.55 +/- 5.3 nmol/cm2 per h).

CONCLUSIONS

The bioreversible conjugation of the methoxypoly(ethylene glycol) promoiety to AZT appears to be a promising strategy for the transdermal delivery of AZT at a therapeutic dose.

Sonophoresis: recent advancements and future trends

OBJECTIVES

Use of ultrasound in therapeutics and drug delivery has gained importance in recent years, evident by the increase in patents filed and new commercial devices launched. The present review discusses new advancements in sonophoretic drug delivery in the last two decades, and highlights important challenges still to be met to make this technology of more use in the alleviation of diseases.

KEY FINDINGS

Phonophoretic research often suffers from poor calibration in terms of the amount of ultrasound energy emitted, and therefore current research must focus on safety of exposure to ultrasound and miniaturization of devices in order to make this technology a commercial reality. More research is needed to identify the role of various parameters influencing sonophoresis so that the process can be optimized. Establishment of long-term safety issues, broadening the range of drugs that can be delivered through this system, and reduction in the cost of delivery are issues still to be addressed.

SUMMARY

Sonophoresis (phonophoresis) has been shown to increase skin permeability to various low and high molecular weight drugs, including insulin and heparin. However, its therapeutic value is still being evaluated. Some obstacles in transdermal sonophoresis can be overcome by combination with other physical and chemical enhancement techniques. This review describes recent advancements in equipment and devices for phonophoresis, new formulations tried in sonophoresis, synergistic effects with techniques such as chemical enhancers, iontophoresis and electroporation, as well as the growing use of ultrasound in areas such as cancer therapy, cardiovascular disorders, temporary modification of the blood-brain barrier for delivery of imaging and therapeutic agents, hormone replacement therapy, sports medicine, gene therapy and nanotechnology. This review also lists patents pertaining to the formulations and techniques used in sonophoretic drug delivery.

Microneedle-based automated therapy for diabetes mellitus

This article discusses the use of microneedles in automated diabetes therapy systems. Advanced bioengineered systems have the potential to close the loop between diagnostic and therapeutic elements of diabetes treatment, thus constituting a "smart" system. Prevalent insulin therapies, and most glucose sensing techniques, involve the transfer of physical entities through the skin. Micromachined needles (microneedles) can achieve this in a noninvasive or minimally invasive manner while contributing various other technological merits. The dynamics of autonomous diabetes therapy systems include highly complex interdependencies between the various physical and biological entities involved, thus warranting multidisciplinary research initiatives. The iterative development of a noninvasive, bioengineered interface such as microneedles necessitates a better understanding of the human skin, its molecular architecture as a polymer film, and its role as a functional biological unit. This review addresses application-specific requirements of a microneedle-based interface system specifically for autonomous diabetes therapy. Key design issues and related parametric interdependencies specific to this application are discussed.

Evaluation of the Transdermal Permeation Behavior of Proterguride from Drug in Adhesive Matrix Patches Through Hairless Mouse Skin

The transdermal in vitro permeation behavior of the highly potent dopamine agonist Proterguride was investigated using hairless mouse skin as a model membrane. Drug in adhesive matrix formulations based on different types of pressure-sensitive adhesives (Eudragit E 100 and Gelva7883 as acrylates, Oppanol B 15 SFN as polyisobutylene, and BioPSA 7-4202 as silicone) with a drug load of 3% by weight were manufactured. All patches were examined for drug crystallization by polarized microscopy immediately after the manufacturing process and after storage for 30 days in sealed aluminium laminate bags at ambient temperature and at 40 degrees C, respectively. Furthermore, the influence of the drug load in acrylate-based formulations onto the steady-state flux of Proterguride was examined. The Eudragit E 100 system delivered a significantly higher steady-state flux than the systems based on Oppanol B 15 SFN and also a somewhat higher steady-state flux than the Gelva-based patch. An addition of 10% by weight of the crystallization inhibitor povidone 25 did not significantly influence the steady-state flux of Proterguride from acrylate matrices. The lipophilic silicone and polyisobutylene adhesives facilitated drug crystallization within the short storage periods at both conditions, probably due to the absence of povidone 25, which was incompatible with these polymers. Varying the drug load in acrylate-based formulations led to a linear increase of the steady-state flux until the steady-state flux of Proterguride leveled off and the patches tended to drug crystallization. It was found that Gelva-based patches show good physical stability, good skin adhesion, and moderate flux values and, thus, can be evaluated as a basis for a suitable formulation for the transdermal administration of Proterguride.

Sumatriptan succinate transdermal delivery systems for the treatment of migraine

We have successfully obtained sumatriptan transdermal systems with different polymer compositions: methyl cellulose (MC), polyvinyl pyrrolidone (PVP) and a polyvinyl pyrrolidone (PVP)-polyvinyl alcohol (PVA) mixture. The systems contained 1,2-propylenglycol (MC) or sorbitol as a plasticizer (PVP and PVP-PVA), methacrylate copolymer as an adhesive agent, and an occlusive liner. Azone (5%, w/w) was incorporated into all the systems as a percutaneous enhancer. Transdermal systems are thin, transparent and non-adhesive when in a dry state. The permeation of sumatriptan succinate across pig ear skin was studied using the systems prepared. The formulation with MC polymer produced a statistically significant increment with respect to the PVP and PVP-PVA formulations (p < 0.05). Azone incorporation into the systems produced an increment in the sumatriptan flux values of all three transdermal systems with respect to those of the controls (p < 0.05). In addition, the application of iontophoresis to the wet methyl cellulose-Azone formulation produced a much higher increase of sumatriptan transdermal flux.

Modelling transdermal delivery of high molecular weight drugs from microneedle systems

In the past few years, a number of microneedle designs have been proposed for transdermal drug delivery of high molecular weight drugs. However, most of them do not increase the drug permeability in skin significantly. In other cases, designs developed based on certain criteria (e.g. strength of the microneedles) have failed to meet other criteria (e.g. drug permeability in skin, throughputs of the drugs, etc.). It is obvious therefore that in order to determine the 'optimum' design of these microneedles, the effect of different factors (e.g. length of the microneedle, surface area of the patch, etc.) along with various transport properties of drug transport behaviour using microneedles should be determined accurately. Appropriate mathematical models for drug transport from these systems into skin have the potential to resolve some of these issues. To address this, a parametric analysis for transdermal delivery of a high molecular weight drug from a microneedle is presented in this paper. The simulations have allowed us to identify the significance of various factors that influence the drug delivery while designing microneedle arrays. A scaling analysis is also done which shows the functional dependence of drug concentration on other variables of skin and microneedle arrays.

Controlled release in transdermal pressure sensitive adhesives using organosilicate nanocomposites

Polydimethyl siloxane (PDMS) based pressure sensitive adhesives (PSA) incorporating organo-clays at different loadings were fabricated via solution casting. Partially exfoliated nanocomposites were obtained for the hydroxyl terminated PDMS in ethyl acetate solvent as determined by X-ray diffraction and atomic force microscopy. Drug release studies showed that the initial burst release was substantially reduced and the drug release could be controlled by the addition of organo-clay. Shear strength and shear adhesion failure temperature (SAFT) measurements indicated substantial improvement in adhesive properties of the PSA nanocomposite adhesives. Shear strength showed more than 200% improvement at the lower clay loadings and the SAFT increased by about 21% due to the reinforcement provided by the nano-dispersed clay platelets. It was found that by optimizing the level of the organosilicate additive to the polymer matrix, superior control over drug release kinetics and simultaneous improvements in adhesive properties could be attained for a transdermal PSA formulation.

Optimization of a jet-propelled particle injection system for the uniform transdermal delivery of drug/vaccine

A jet-propelled particle injection system, the biolistics, has been developed and employed to accelerate micro-particles for transdermal drug delivery. We have examined a prototype biolistic device employing a converging-diverging supersonic nozzle (CDSN), and found that the micro-particles were delivered with a wide velocity range (200-800 m/s) and spatial distribution. To provide a controllable system for transdermal drug delivery, we present a contoured shock-tube (CST) concept and its embodiment device. The CST configuration utilizes a quasi-steady, quasi-one dimensional and shock-free supersonic flow to deliver the micro-particles with an almost uniform velocity (the mean velocity and the standard deviation, 699 +/- 4.7 m/s) and spatial distribution. The transient gas and particle dynamics in both prototype devices are interrogated with the validated computational fluid dynamics (CFD) approach. The predicted results for static pressure and Mach number histories, gas flow structures, particle velocity distributions and gas-particle interactions are presented and interpreted. The implications for clinical uses are discussed.

Thermosensitive polymers: Synthesis, characterization, and delivery of proteins

Three triblock copolymers based on the poly(lactide) or poly(lactide-co-glycolide) and poly(ethylene glycol) or poly(ethylene oxide) blocks were synthesized and characterized. The weight average molecular weight and number average molecular weight were determined by gel permeation chromatography and proton nuclear magnetic resonance spectroscopy, respectively. Fourier transform infrared spectroscopy was used to determine the completion of synthesis of polymers. Thermoreversible sol-gel transition temperature and concentration were determined by an inverted tube method. Two formulations each of three synthesized polymers containing 5% (w/v) of lysozyme or bromelain but differing in polymer concentrations (20-30%, w/v) were prepared and studied for in vitro release of the incorporated protein. In vitro biocompatibility of the delivery systems was studied by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell viability assay. Biological activities of lysozyme and bromelain were determined by enzyme activity assays. Critical gelling concentrations were found in the range of 20-30% (w/v). In vitro biocompatibility study showed that all the formulations were biocompatible. Increasing the polymer concentration led to a decrease in burst release and extended the in vitro release of proteins. Furthermore, biological activities of lysozyme and bromelain in released samples were found to be significantly (p<0.05) greater in comparison to the control. Thus, the above thermosensitive polymers were able to deliver proteins in biologically active forms at a controlled rate for 2-8 weeks.

Hydrogel-thickened microemulsion for topical administration of drug molecule at an extremely low concentration

In this work, a hydrogel-thickened microemulsion (HTM) was investigated for delivering an extremely low concentration of drug molecule. The pseudo-ternary phase diagrams were constructed using isopropyl myristate (IPM), Tween 80, propylene glycol and water. The various HTM were prepared and characterized. We described that HTM has the combination of o/w microstructure of microemulsion and the three-dimensional gel network of hydrogel in continuous phase using transmission electron microscope. The stability tests showed that HTM had good stability. The influence of the addition of hydrogel into microemulsions on the viscosity and permeation ability is investigated. The abilities of HTM to deliver an extremely low concentration of triptolide as a model drug were evaluated using the in vitro permeation studies. The permeation rates of triptolide from various HTM were 2.2-3.6 times over that from the control hydrogel. The addition of 2% menthol into HTM consisting of 3% IPM, 30% Tween 80, 15% propylene glycol, 0.75% carbomer 940 resulted in the highest permeation rate of 0.105+/-0.006microcm(-2)h(-1), which was 5.8 times over control gel. The powerful permeation enhancing ability of HTM with a suitable viscosity makes it promising alternative carrier for transdermal administration of drug molecule at an extremely low concentration.

Poly(2-hydroxy-3-phenoxypropylacrylate, 4-hydroxybutyl acrylate, dibutyl maleate) membrane controlled clonidine zero-order release

Poly(2-hydroxy-3-phenoxypropylacrylate, 4-hydroxybutyl acrylate, dibutyl maleate) membrane was synthesized by UV curing method in our laboratory for the first time. When above-mentioned monomers were in the weight ratio of 4:4:2, the membrane not only had perfect permeation property but also had excellent plasticity, so the membrane made from monomers in the ratio of 4:4:2 was chosen as an optimized membrane. The optimized membrane provided perfect linear permeation properties in clonidine transdermal drug delivery system. The permeation rate decreased in proportion to the thickness of membrane. When the concentrations of clonidine were in the range of 0.5-7.0mg/ml, the permeation rate was proportional to the square root of clonidine concentrations. The optimized membrane was characterized by FTIR, DSC and SEM.

Design of Original Bioactive Formulations Based on Sugar–Surfactant/Non-steroidal Anti-inflammatory Catanionic Self-Assemblies: A New Way of Dermal Drug Delivery

A new kind of catanionic assembly was developed that associates a sugar-based surfactant with a non-steroidal anti-inflammatory drug (NSAID). Three different assemblies using indomethacin, ibuprofen and ketoprofen as NSAIDs were easily obtained in water by an acid-base reaction. These assemblies formed new amphiphilic entities because of electrostatic and hydrophobic effects in water and led to the spontaneous formation of vesicles. These catanionic vesicles were then tested as potential NSAID delivery systems for dermatological application. The anti-inflammatory activity was evaluated in vivo, and this study clearly showed an improved therapeutic effect for NSAIDs that were formulated as catanionic vesicles. These vesicles ensured a slower diffusion of the NSAID through the skin. This release probably increased the time of retention of the NSAID in the targeted strata of the skin. Thus, the present study suggests that this catanionic bioactive formulation could be a promising dermal delivery system for NSAIDs in the course of skin inflammation treatment.

A new copolymer membrane controlling clonidine linear release in a transdermal drug delivery system

A new type of copolymer membranes was prepared through photosynthesis of mixtures of three different monomers. The membranes present a linear permeation property in clonidine transdermal drug delivery system. Monomers used in the photosynthesis were 2-hydroxy-3-phenoxypropylacrylate, 4-hydroxybutyl acrylate and sec-butyl tiglate. Permeation property of the membranes with different monomer ratios and thickness were investigated. When clonidine concentrations were in 3.0 - 5.0 mg/ml range, membranes showed near zero order permeation rates. An optimized membrane was characterized by FTIR, DSC and SEM.

Transdermal drug delivery: Basic principles for the veterinarian

The use of topical pharmaceutical formulations is increasingly popular in veterinary medicine. A potential concern is that not all formulations are registered for the intended species, yet current knowledge strongly suggests that simple extrapolation of transdermal drug pharmacokinetics and pharmacodynamics between species, including humans, cannot be done. In this review, an overview is provided of the underlying basic principles determining the movement of topically applied molecules into and through the skin. Various factors that may affect transdermal drug penetration between species, between individuals of a particular species and regional differences in an individual are also discussed. A good understanding of the basic principles of transdermal drug delivery is critical to avoid adverse effects or lack of efficacy when applying topical formulations in veterinary medicine.

Facilitated skin penetration of lidocaine: Combination of a short-term iontophoresis and microemulsion formulation

The objective of this study was to demonstrate the potential of the application of a short-term iontophoresis on the topical delivery of lidocaine hydrochloride from a microemulsion-based system. Five- and 10-min durations of anodal iontophoresis applied onto porcine skin were examined in combination with a microemulsion containing 2.5% lidocaine hydrochloride. A similar combination (10-min iontophoresis with microemulsion in the anodal electrode) was also examined in vivo in a rat model. It was shown in vitro that by combining microemulsion application with a 10-min iontophoresis of 1.13 mA/cm2 electric current density, a significantly increased flux was obtained compared with a combination of aqueous drug solution with the same iontophoresis protocol. In vivo studies revealed that 57.71 +/- 18.65 and 18.43 +/- 9.17 microg cm(-2) were reached in the epidermis and dermis, respectively, at t = 30 min of microemulsion application, when iontophoresis was applied for 10 min. In contrast, the application of aqueous solution-iontophoresis resulted in a relatively lower drug accumulation (21.44 +/- 10.42 and 5.30 +/- 2.25 microg cm(-2) in the epidermis and dermis, respectively, at t = 30) with more rapid clearance of the drug from the skin. Ten-minute application of a low-current electric field on a new topical microemulsion appears to make significant changes in skin permeability. The potential advantages of this procedure include significantly increased flux, accumulation of a large skin drug depot, short lag times, reduced irritation (compared to long-term iontophoresis), simplicity and ease of compliance.

Iontophoretic transdermal delivery of sumatriptan: effect of current density and ionic strength

Iontophoretic transdermal delivery of sumatriptan was investigated in vitro. Among the conditions tested, 0.25 mA/cm2 and low ionic strength (NaCl 25 mM) was the best experimental condition to increase its transport across the skin. The flux increased 385-fold respective to passive diffusion, thus resulting in a transdermal flux of sumatriptan of 1273+/-83 nmol/cm2 h.

Contributions of electromigration and electroosmosis to peptide iontophoresis across intact and impaired skin

d-(Arg)-Kyotorphin iontophoresis was investigated across intact and impaired skins in vitro. Iontophoretic flux increased from 68+/-12 to 538+/-116 nmol cm(-2) h(-1) when the peptide concentration in the anodal compartment was raised from 5 to 40 mM. Electromigration was the principal transport mechanism, accounting for approximately 70% of total peptide delivery. Reducing the number of competing ions in the formulation significantly increased iontophoretic flux but did not affect convective solvent flow. The latter was independent of peptide concentration indicating that skin permselectivity was not modified by kyotorphin transport. Total iontophoretic flux was unaffected when the stratum corneum was removed by tape-stripping (146+/-34 versus 150+/-26 nmol cm(-2) h(-1)). However, the contributions of the different transport mechanisms were significantly altered: (i) electromigration decreased, as more of the charge was carried by anions from the sub-dermal milieu; (ii) electroosmosis was absent; and (iii) passive permeation increased significantly. Transport rates across intact and impaired skin barriers were statistically indistinguishable when the donor electrolyte composition was modified; increased competition from anions was mitigated by the decreased Na+ levels in the formulation. Removal of Cl- ions from the receiver phase further increased peptide delivery, and also increased anodal electroosmosis.

Slow internal release of bioactive compounds under the effect of skin enzymes

A new strategy for the skin delivery of bioactive compounds has been developed, using enzymes involved in the maintenance of the epidermal barrier function and the enzymatic transformation of corresponding precursors. This new strategy has been tested with regard to two enzymatic activities of the skin barrier: extracellular glucosidase and esterase/lipase. An analysis of the requirements for the glycosidic bond hydrolysis of any glycoconjugate by beta-glucocerebrosidase indicates that the release of the moiety linked to the glucose unit is obtained as long as the glycosidic bond being broken is not hindered, and as long as the leaving group property of the released moiety is good enough. This strategy was first applied to the release of the antioxidant delta-tocopherol. It was then extended to retinoic acid by introducing a spacer between the glucose unit and the bioactive moiety. This spacer was either a good leaving group such as hydroquinone, or a structure akin to a ceramide, namely glycerol. In these conditions, beta-glucocerebrosidase releases the complex spacer-active compound that is cleaved by an esterase. One of the advantages of this strategy lies in the slow release of the bioactive compound, extending in time its effect and most likely its tolerance, as is the case for retinoic acid.

Switching from reservoir to matrix systems for the transdermal delivery of fentanyl: a prospective, multicenter pilot study in outpatients with chronic pain

Matrix systems for transdermal fentanyl will replace the reservoir systems in Europe. In an industry-independent, prospective, multicenter pilot study, 46 outpatients with chronic pain were asked to assess pain intensity, sleep interference, adverse events, and multiple secondary parameters during administration of the last reservoir and the first two matrix patches. There was no difference in pain intensity, sleep interference, and the rate of adverse events between both systems. Self assessment on a 6-step numeric rating scale (1 = very good, 6 = insufficient) comparing the two systems (reservoir vs. matrix) showed that skin compatibility (2.6 vs. 1.5), adhesive properties (3.2 vs. 1.8), wearability/comfort (2.8 vs. 1.5), and general satisfaction (2.5 vs. 1.8) improved significantly with the new matrix technology. At study endpoint, 91% of patients preferred the matrix system for future use. The new fentanyl matrix system is characterized by a high level of general satisfaction, ease of use, patient acceptance, and improved skin compatibility. Reservoir and matrix systems appear to have comparable efficacy and safety so that outpatients can be switched directly from the reservoir to the matrix system without difficulties and new dose titration.

Synthesis and transdermal penetration of NSAID glycoside esters

The glucoside and mannoside derivatives of the NSAIDs flurbiprofen, ibuprofen, ketoprofen and naproxen were synthesised and their penetration through human skin was determined. Experimental transdermal flux data showed that the parent NSAIDs penetrated human skin to a much higher extent than the glycosides.

In Vitro/in Vivo Correlation of Transdermal Naltrexone Prodrugs in Hairless Guinea Pigs

PURPOSE

The purpose of this investigation was to evaluate the in vitro and in vivo percutaneous absorption of the following prodrugs of naltrexone (NTX): 2'-ethylbutyryl-3-O-ester-NTX (ETBUT-ester), methyl-3-O-carbonate-NTX (ME-carbonate), ethyl-3-O-carbamate-NTX (ET-carbamate), and N,N-dimethyl-3-O-carbamate-NTX (DME-carbamate) in hairless guinea pigs.

METHODS

In vitro fluxes of NTX and its prodrugs through guinea pig skin were determined using a flow-through diffusion cell system. The pharmacokinetics of NTX prodrugs were determined after topical application of transdermal patches in guinea pigs.

RESULTS

All the prodrugs hydrolyzed to NTX on passing through the skin, and ME-carbonate provided the highest NTX flux and had the highest apparent permeability coefficient (K(p)). ME-carbonate and ET-carbamate underwent the highest extent of bioconversion to NTX upon passing through the skin as compared to ETBUT-ester and DME-carbamate. The results of the in vivo studies indicated that a significant amount of NTX was delivered after the application of transdermal patches of NTX prodrugs. A mean steady-state plasma concentration of 7.1 ng/ml was obtained after the application of transdermal patches of ME-carbonate. A good correlation was obtained between the in vitro and in vivo results.

CONCLUSIONS

The results of the in vivo studies indicated that the ME-carbonate prodrug of NTX was the most promising drug candidate for transdermal delivery.

Emerging strategies for the transdermal delivery of peptide and protein drugs

Transdermal delivery has been at the forefront of research addressing the development of non-invasive methods for the systemic administration of peptide and protein therapeutics generated by the biotechnology revolution. Numerous approaches have been suggested for overcoming the skin's formidable barrier function; whereas certain strategies simply act on the drug formulation or transiently increase the skin permeability, others are designed to bypass or even remove the outermost skin layer. This article reviews the technologies currently under investigation, ranging from those in their early-stage of development, such as laser-assisted delivery to others, where feasibility has already been demonstrated, such as microneedle systems, and finally more mature techniques that have already led to commercialisation (e.g., velocity-based technologies). The principles, mechanisms involved, potential applications, limitations and safety considerations are discussed for each approach, and the most advanced devices in each field are described.

Transdermal Drug Delivery: Clinical Considerations for the Obstetrician–Gynecologist

INTRODUCTION

The first transdermal drug delivery system was introduced in the United States over 20 years ago. Created as an alternative route of administration to improve patient compliance as well as to reduce side effects, the transdermal delivery of drugs now represents a $1.5 to $2 billion market and is growing rapidly.

DATA SOURCES

The medical literature from 1980 to 2005 was searched using the PubMed search engine. The search term was "transdermal," limited to human clinical trials. Abstracts were used to identify clinical trials that compared transdermal preparations and their oral counterparts in the same study.

TABULATION, INTEGRATION, AND RESULTS

In this article, we review the rationale for the transdermal administration of drugs; discuss aspects of the anatomy and physiology of the skin relevant to drug transport; examine the qualities required for a drug to be a good candidate for transdermal delivery; and consider key principles for the evaluation of the relative benefits of transdermal delivery, using studies of hormone replacement therapy and oral contraceptive as examples.

CONCLUSION

The data reviewed here suggest that certain clinical situations support the use of the transdermal administration of drugs over their oral counterparts.

A study of microemulsion systems for transdermal delivery of triptolide

Triptolide possesses immunosuppressive, anti-fertility and anti-cancer activities. Due to its severe toxicity, microemulsions with controlled, sustained and prolonged delivery of triptolide via a transdermal route are expected to reduce its adverse side effects. The purpose of the present study was to investigate the microemulsions for transdermal delivery of triptolide. The pseudo-ternary phase diagrams were developed and various microemulsion formulations were prepared using oleic acid as an oil, Tween 80 as a surfactant and propylene glycol as a cosurfactant. The droplet size of microemulsions was characterized by photocorrelation spectroscopy. The transdermal ability of triptolide from microemulsions was evaluated in vitro using Franz diffusion cells fitted with mouse skins and triptolide was analyzed by high-performance liquid chromatography. The effect of menthol as a permeation enhancer, and the loading dose of triptolide in microemulsions on the permeation rate were also evaluated. The triptolide-loaded microemulsions showed an enhanced in vitro permeation through mouse skins compared to an aqueous solution of 20% propylene glycol containing 0.025% triptolide. The permeation of microemulsions accorded with the Fick's first diffusion law. No obvious skin irritation was observed for the studied microemulsion ME6, but the aqueous solution of 20% propylene glycol containing 0.025% triptolide revealed the significant skin irritation. The results indicate that the studied microemulsion systems, especially ME6, may be promising vehicles for the transdermal delivery of triptolide.

In vivo evaluation of 3-O-alkyl ester transdermal prodrugs of naltrexone in hairless guinea pigs

Naltrexone (NTX) is a potent competitive antagonist with high affinity for the mu-opioid receptor. Therapeutically, NTX is used for the treatment of alcohol dependence and opioid addiction; however, it does not have the ideal physicochemical properties necessary to achieve therapeutic plasma concentrations via the transdermal route. The aim of the present investigation was to evaluate the in vivo transdermal delivery of three 3-O-alkyl ester prodrugs of NTX, including NTX-3-O-acetate (ACE-NTX), NTX-3-O-propionate (PROP-NTX), and NTX-3-O-hexanoate (HEX-NTX) in hairless guinea pigs. The pharmacokinetic parameters for NTX and the 3-O-alkyl ester prodrugs of NTX were determined after intravenous drug administration and topical drug application of transdermal therapeutic systems (TTS) in guinea pigs. The results of the in vivo studies showed mean steady-state plasma concentrations of NTX from NTX, ACE-NTX, PROP-NTX and HEX-NTX at 4.2, 25.2, 16.0, and 8.3 ng/mL, respectively. These NTX plasma concentrations were maintained for 48 h. The results of these in vivo studies demonstrated that ACE-NTX and PROP-NTX prodrugs of NTX were the most promising drug candidates for transdermal delivery.

A protocol for iontophoresis of acetylcholine and sodium nitroprusside that minimises nonspecific vasodilatory effects

Iontophoresis of vasoactive substances is a promising tool for studying pharmacological aspects of the (patho)physiology of the microvasculature. However, nonspecific microvascular responses are a common problem in most protocols used. We studied the effect of current density (mA/cm2), charge density (mC/cm2), drug concentration (mass %) and vehicle concentration (M) on the nonspecific vasodilatation during iontophoresis of sodium chloride, acetylcholine (ACh) and sodium nitroprusside (SNP). We found that nonspecific vasodilatation depended on current density and charge density in both anodal and cathodal iontophoresis. The responses to ACh and SNP were dependent on current density, charge density and drug concentration. We found that by limiting current density (<0.01 mA/cm2) and charge density (<7.8 mC/cm2) and with adjusted concentrations for drugs and vehicles, it is possible to prevent nonspecific effects during iontophoresis of ACh and SNP, while maximum drug effects (plateaus in the dose-response curves) are still obtained. These new findings are important for future iontophoresis studies in which vasoactive drugs are used to assess microvascular function because the presented approach has advantages compared to older techniques, which mainly have attempted to suppress or compensate for the nonspecific responses during iontophoresis by the use of local anaesthetics or the measurement of drug-minus-vehicle responses, both of which present well-known experimental shortcomings.

Formulation and In Vitro Evaluation of Transdermal Patches of Melatonin

The present study was undertaken to prepare and evaluate monolithic drug-inadhesive type transdermal patches of melatonin containing penetration enhancers such as fatty alcohols, fatty acids, and terpenes. The patches were prepared using Eudragit E 100 as the adhesive polymer. The release profile of melatonin from control as well as enhancer-containing patches showed an initial burst of melatonin release for up to 4 hours and then a plateau after 8 hours. The release profiles of melatonin from patches containing various enhancers were similar to the control patch. However, the addition of enhancers in the patch increased the permeation of melatonin through hairless rat skin. The flux values of patches containing octanol, nonanoic acid, and myristic acid were higher than the control patch (no enhancer), but the differences were not statistically significant (P>0.05). Decanol, myristyl alcohol, and undecanoic acid at 5% concentrations showed significantly higher flux values through hairless rat skin (enhancement ratios 1.7, 1.5, and 1.6 for decanol, myristyl alcohol, and undecanoic acid, respectively) (P<0.05). Menthol and limonene at 5% w/w showed maximum permeation of melatonin among all enhancers studied (enhancement ratios=2.1 and 2.0 for menthol and limonene, respectively) (P<0.001). In general, there was about 4-6 hours of lag time observed before a steady state flux of melatonin was achieved. Though the flux of melatonin observed in the present study is 5-10 times higher than the required delivery rate in humans, it must be noted that the present study was performed using hairless rat skin, which is generally more permeable compared to human skin. Further studies using human skin would prove the usefulness of these patches.

Transdermal Iontophoresis of Insulin

The delivery of large peptides through the skin poses a significant challenge, and various strategies are under active investigation for enhancing the transdermal permeation. For large peptides, it is difficult to achieve significant permeation using iontophoresis alone. Hence a combination of fatty acids with iontophoresis was hypothesized to result in higher enhancement than achieved with either of them alone. Saturated fatty acids and cis unsaturated fatty acids were studied in combination with iontophoresis using excised rat skin. The skin was pretreated for 2 h with an ethanolic (EtOH) solution of 5% w/v or v/v fatty acids, namely lauric acid (LA), oleic acid (OA), linoleic acid (LOA) and linolenic acid (LLA), followed by either passive or iontophoretic permeation (0.5 mA/cm2 for 6 h). Fourier transform infrared spectroscopy (FT-IR) was used to investigate the biophysical changes on treatment with fatty acid/EtOH or neat fatty acid, mainly focusing on the infrared region at 2,920, 1,710 and 1,720 cm(-1). Unsaturated fatty acids showed higher enhancement than LA, and the enhancement increased with the number of double bonds. On the other hand, in the presence of iontophoresis, LA/EtOH showed the highest enhancement. Neat LOA did not show any significant difference (p > 0.05) compared to the LOA/EtOH combination. FT-IR studies revealed that fatty acids act by interacting with the skin lipids. All the fatty acids showed synergistic enhancement when combined with iontophoresis. The flux enhancement was highest with LA, which in the presence of iontophoresis showed 20 times enhancement of insulin flux in comparison to passive flux and 9 times enhancement as compared to iontophoresis alone. Flux enhancement of unsaturated fatty acids was in the following decreasing order LOA > OA > LLA.

Effect of chemical enhancers and iontophoresis on thiocolchicoside permeation across rabbit and human skin in vitro

The aim of this work was to study the permeation of thiocolchicoside across the skin in vitro. The effect of the chemical enhancer lauric acid and the physical technique of iontophoresis was investigated. Permeation experiments were performed in vitro using rabbit ear skin as barrier. The effect of lauric acid at different concentrations (2% and 4%) and of the vehicle (water, ethanol, or ethanol/water) was investigated. The primary effect of lauric acid was on the partitioning parameter, whereas the diffusive parameter did not change significantly. When human epidermis was used, the permeation parameters were generally lower, although not significantly different from rabbit ear skin. The data obtained with full-thickness human skin indicate that, despite the hydrophilic nature of thiocolchicoside, the resistance to drug transport is not limited to the stratum corneum, but that the underlying dermal tissue can also contribute. Iontophoresis enhanced the flux of thiocolchicoside compared with the passive control. The mechanism by which iontophoresis enhanced thiocolchicoside transport across the skin was electroosmosis. The permeation of thiocolchicoside across the skin can be enhanced using chemical or physical penetration enhancers.

Emerging technologies that overcome biological barriers for therapeutic protein delivery

In the past decade, genomic research and the nascent field of proteomics have exponentially increased the number of potential protein therapeutic molecules for treating medical needs that were previously unmet. To realise the full clinical potential of many of the novel protein drug entities arising from these intense research efforts, emerging protein delivery technologies may be required. Advanced delivery technologies may offer the ability to overcome biochemical and anatomical barriers to protein drug transport, without incurring adverse events, to deliver the agent(s) at a certain desired rate and duration, to protect therapeutic macromolecules from in situ or systemic degradation, as well as increase their therapeutic index by targeting the drug action to a specific site. This review will cover a myriad of novel and emerging technologies that are directed at bypassing biological barriers and that have shown promise in advancing the therapeutic potential of protein drugs.