Current status and future potential of transdermal drug delivery

Controlled transdermal delivery of model drug compounds by MEMS microneedle array

This article reports an in vitro study of microneedle-array-enhanced transdermal transport of model drug compounds dispersed in chitosan films. Each microneedle array has 400 out-of-plane, needle-shaped microstructures fabricated using micro-electro-mechanical systems (MEMS) technology to ensure adequate mechanical strength and high precision, and consistency. A nanometer coating on the microneedles ensured the biocompatibility that is important in the application of transdermal drug delivery. Model drugs selected to investigate skin permeation in vitro were calcein, a small molecule (molecular weight, 623 d) that has little skin penetration, and bovine serum albumin (BSA) (molecular weight, 66,000 d), a hydrophilic biological macromolecule. A Franz permeation cell was used to characterize the permeation rate of calcein and BSA through the rat skin. The transdermal transport behavior of BSA was investigated from solid films coated on the surface of microneedle arrays with various chitosan concentrations, film thicknesses, and BSA contents. The BSA permeation rate decreased with the increase of the chitosan concentration; the thicker the film, the slower the permeation rate. In addition, the permeation rate increased with the increase of BSA loading dose. A linear relationship existed between the permeation rate and the square root of the BSA loading dose. Results showed that the chitosan hydrophilic polymer film acts as a matrix that can regulate the BSA release rate. The controlled delivery of BSA can be achieved using the BSA-containing chitosan matrix film incorporated with the microneedle arrays. This will provide a possible way for the transdermal delivery of macromolecular therapeutic agents such as proteins and vaccines.

Comparative Study of the Skin Penetration of Protein Transduction Domains and a Conjugated Peptide

PURPOSE

We examined the ability of a protein transduction domain (PTD), YARA, to penetrate in the skin and carry a conjugated peptide, P20. The results with YARA were compared to those of a well-known PTD (TAT) and a control, nontransducing peptide (YKAc). The combined action of PTDs and lipid penetration enhancers was also tested.

METHODS

YARA, TAT, YKAc, P20, YARA-P20, and TAT-P20 were synthesized by Fmoc chemistry. Porcine ear skin mounted in a Franz diffusion cell was used to assess the topical and transdermal delivery of fluorescently tagged peptides in the presence or absence of lipid penetration enhancers (monoolein or oleic acid). The peptide concentrations in the skin (topical delivery) and receptor phase (transdermal delivery) were assessed by spectrofluorimetry. Fluorescence microscopy was used to visualize the peptides in different skin layers.

RESULTS

YARA and TAT, but not YKAc, penetrated abundantly in the skin and permeated modestly across this tissue. Monoolein and oleic acid did not enhance the topical and transdermal delivery of TAT or YARA but increased the topical delivery of YKAc. Importantly, YARA and TAT carried a conjugated peptide, P20, into the skin, but the transdermal delivery was very small. Fluorescence microscopy confirmed that free and conjugated PTDs reached viable layers of the skin.

CONCLUSIONS

YARA and TAT penetrate in the porcine ear skin in vitro and carry a conjugated model peptide, P20, with them. Thus, the use of PTDs can be a useful strategy to increase topical delivery of peptides for treatment of cutaneous diseases.

Biodegradable polymer microneedles: fabrication, mechanics and transdermal drug delivery

To overcome the skin's barrier properties that block transdermal delivery of most drugs, arrays of microscopic needles have been microfabricated primarily out of silicon or metal. This study addresses microneedles made of biocompatible and biodegradable polymers, which are expected to improve safety and manufacturability. To make biodegradable polymer microneedles with sharp tips, micro-electromechanical masking and etching were adapted to produce beveled- and chisel-tip microneedles and a new fabrication method was developed to produce tapered-cone microneedles using an in situ lens-based lithographic approach. To replicate microfabricated master structures, PDMS micromolds were generated and a novel vacuum-based method was developed to fill the molds with polylactic acid, polyglycolic acid, and their co-polymers. Mechanical testing of the resulting needles measured the force at which needles broke during axial loading and found that this failure force increased with Young's modulus of the material and needle base diameter and decreased with needle length. Failure forces were generally much larger than the forces needed to insert microneedles into skin, indicating that biodegradable polymers can have satisfactory mechanical properties for microneedles. Finally, arrays of polymer microneedles were shown to increase permeability of human cadaver skin to a low-molecular weight tracer, calcein, and a macromolecular protein, bovine serum albumin, by up to three orders of magnitude. Altogether, these results indicate that biodegradable polymer microneedles can be fabricated with an appropriate geometry and sufficient strength to insert into skin, and thereby dramatically increase transdermal transport of molecules.

Novel powder formulations for controlled delivery of poorly soluble anticancer drug: Application and investigation of TPGS and PEG in spray-dried particulate system

Biodegradable poly (lactic-co-glycolic acid) (PLGA), D-alpha-tocopheryl polyethylene glycol 1000 succinate (TPGS) and/or polyethylene glycol (PEG) were combined as pharmaceutical excipient to fabricate microparticles containing sparingly soluble drug paclitaxel by spray-drying technique with successful achievement. The effect of formulation variety on particle morphology, surface composition, thermal property, drug entrapped capability, and drug release profile was investigated. The result indicated that the use of the appropriate mixtures of PLGA, TPGS and/or PEG produced paclitaxel-loaded microparticles characterised by acceptable pharmaceutical properties. Atomic force microcopy (AFM) and scanning electron microscopy (SEM) showed that the produced microparticles were spherical in shape with dimples or pores. The particle size ranged from 0.88 to 2.44 microm with narrow distribution. The combination of TPGS and PEG in the formulation resulted in a narrow particle size distribution in general although the influence of the formulation on the particle size was not significant. Differential scanning calorimetry (DSC) study implied that all those components in consideration were compatible well in the blend formulation systems. The paclitaxel entrapped in the particles existed in an amorphous or disordered-crystalline status in the matrices and was independent of the PLGA/TPGS/PEG ratio. X-ray photoelectron spectroscope (XPS) analysis revealed that after incorporation the particle's surface was dominated with PLGA due to its hydrophobic property. The formulation variety had an important impact on the drug release that was reduced with the presence of large fraction of TPGS resulting from a strong hydrophobic interaction between various matrix materials and the drug inside the particle. A zero order release could be yielded by optimising the ratio of PLGA/TPGS/PEG. The combination of PLGA/TPGS/PEG as safe pharmaceutical excipient to formulate particulate delivery system is beneficial in improving the pharmaceutical properties for further powder dosage application.

The transdermal revolution

Historically, developments in transdermal drug delivery have been incremental, focusing on overcoming problems associated with the barrier properties of the skin, reducing skin irritation rates and improving the aesthetics associated with passive patch systems. More-recent advances have concentrated on the development of non-passive systems to aid delivery of larger drug molecules, such as proteins and nucleotides, as the trend for discovering and designing biopharmaceuticals continues. Fundamentally, improvements in transdermal delivery will remain incremental until there is wider acceptance of this route of administration within the pharmaceutical industry. Only then will the transdermal revolution live up to its true potential.

Vesicles as a tool for transdermal and dermal delivery

Transdermal and dermal drug delivery is problematic because the skin, as a natural barrier, has a very low permeation rate. Therefore several methods have been assessed to increase this rate locally and temporarily. One approach is the use of vesicle formulations. In this paper the effectiveness of conventional and deformable vesicles as drug delivery systems as well as their possible mode of action as permeation enhancers or transdermal drug carriers will be discussed.:

Effect of penetration enhancers on the release and skin permeation of bupranolol from reservoir-type transdermal delivery systems

A reservoir-type transdermal delivery system (TDS) of bupranolol (BPL) was designed and evaluated for different formulation variables like gel reservoirs (made with anionic and nonionic polymers), rate controlling membranes and penetration enhancers on the drug release and in vitro skin permeation kinetics of the devices. Keshary-Chien type diffusion cells and pH 7.4 phosphate buffered saline (PBS) were used for drug release studies and excised rat skin was used as a barrier for permeation experiments. The release rate of BPL from nonionic polymer gel reservoirs [hydroxypropyl methyl cellulose (HPMC), hydroxypropyl cellulose (HPC)] was much higher than anionic polymer gel reservoirs [carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (Na CMC) and sodium alginate)]. Among different rate controlling membranes, Cotran-polyethylene microporous membrane demonstrated highest release rate for BPL than all other membranes. An optimized TDS formulation with HPC gel and Cotran-polyethylene microporous membrane was used to study the effect of penetration enhancers on the release and skin permeation rate of BPL from the TDS. Permeation rates of the devices containing 5% (w/v) pyrrolidone (PY) or 1-methyl-2-pyrrolidone (MPY) were about 3- and 1.5-fold higher than control (no enhancer, P<0.01) indicating PY to be better penetration enhancer for BPL than MPY. The permeation rates of devices containing partially methylated beta-cyclodextrin (PMbetaCD) and PMbetaCD-BPL complex were about 2.5- and 1.4-fold higher than control (P<0.01). Inclusion of 10 and 30% w/v propylene glycol (PG) in the devices increased the permeation rate by 1.4- and 1.8-fold higher than control (P<0.05). In conclusion, reservoir-type TDS of BPL was developed and penetration enhancers increased the skin permeation of BPL at 4-5 times higher levels than the desired target delivery rate.

BOTOXR delivery by iontophoresis

We report two patients with severe palmar hyperhidrosis who responded to BOTOX delivered not by injection, the usual method of delivery, but by iontophoresis. The Botulinum molecule has been considered too large for delivery into the skin this way. However, other large peptides, both non-ionic and cationic, have been delivered successfully by this method, so we suspected that BOTOX could in fact be iontophoresed. Our saline-controlled treatment of these two patients with a small iontophoresis unit (Iomed Phoresor II) allowed small volumes of standard BOTOX dilutions to be used, and demonstrates that iontophoresis can indeed deliver BOTOX successfully. This has important therapeutic potential for the large number of patients with focal hyperhidrosis. They may be spared painful injections, and in more severe cases, invasive surgery.