Microneedle mediated delivery of nanoparticles into human skin.

The development of novel cutaneous delivery technologies that can produce micron-sized channels within the outermost skin layers has stimulated interest in the skin as an interface for localised and systemic delivery of macromolecular and nanoparticulate therapeutics. This investigation assesses the contribution of physicochemical factors to the rate and extent of nanoparticle delivery through microchannels created in a biological tissue, the skin, by novel delivery technologies such as the microneedle array. The hydrodynamic diameter, zeta potential and surface morphology of a representative fluorescent nanoparticle formulation were characterised. Permeation studies using static Franz-type diffusion cells assessed (i) the diffusion of nanoparticle formulations through a model membrane containing uniform cylindrical microchannels of variable diameter and (ii) nanoparticle penetration across microneedle treated human skin. Wet-etch microneedle array devices can be used to significantly enhance the intra/transdermal delivery of nanoparticle formulations. However the physicochemical factors, microchannel size and particle surface charge, have a significant influence on the permeation and subsequent distribution of a nanoparticle formulation within the skin. Further work is required to understand the behaviour of nanoparticle formulations within the biological environment and their interaction with the skin layers following disruption of the skin barrier with novel delivery devices such as the microneedle array.

Cutaneous gene expression of plasmid DNA in excised human skin following delivery via microchannels created by radio frequency ablation.

The skin is a valuable organ for the development and exploitation of gene medicines. Delivering genes to skin is restricted however by the physico-chemical properties of DNA and the stratum corneum (SC) barrier. In this study, we demonstrate the utility of an innovative technology that creates transient microconduits in human skin, allowing DNA delivery and resultant gene expression within the epidermis and dermis layers. The radio frequency (RF)-generated microchannels were of sufficient morphology and depth to permit the epidermal delivery of 100 nm diameter nanoparticles. Model fluorescent nanoparticles were used to confirm the capacity of the channels for augmenting diffusion of macromolecules through the SC. An ex vivo human organ culture model was used to establish the gene expression efficiency of a beta-galactosidase reporter plasmid DNA applied to ViaDerm treated skin. Skin treated with ViaDerm using 50 microm electrode arrays promoted intense levels of gene expression in the viable epidermis. The intensity and extent of gene expression was superior when ViaDerm was used following a prior surface application of the DNA formulation. In conclusion, the RF-microchannel generator (ViaDerm) creates microchannels amenable for delivery of nanoparticles and gene therapy vectors to the viable region of skin.

Minimally invasive cutaneous delivery of macromolecules and plasmid DNA via microneedles.

The stratum corneum (SC) represents a significant barrier to the delivery of gene therapy formulations. In order to realise the potential of therapeutic cutaneous gene transfer, delivery strategies are required to overcome this exclusion effect. This study investigates the ability of microfabricated silicon microneedle arrays to create micron-sized channels through the SC of ex vivo human skin and the resulting ability of the conduits to facilitate localised delivery of charged macromolecules and plasmid DNA (pDNA). Microscopic studies of microneedle-treated human epidermal membrane revealed the presence of microconduits (10-20 microm diameter). The delivery of a macromolecule, beta-galactosidase, and of a 'non-viral gene vector mimicking' charged fluorescent nanoparticle to the viable epidermis of microneedle-treated tissue was demonstrated using light and fluorescent microscopy. Track etched permeation profiles, generated using 'Franz-type' diffusion cell methodology and a model synthetic membrane showed that >50% of a colloidal particle suspension permeated through membrane pores in approximately 2 hours. On the basis of these results, it is probable that microneedle treatment of the skin surface would facilitate the cutaneous delivery of lipid:polycation:pDNA (LPD) gene vectors, and other related vectors, to the viable epidermis. Preliminary gene expression studies confirmed that naked pDNA can be expressed in excised human skin following microneedle disruption of the SC barrier. The presence of a limited number of microchannels, positive for gene expression, indicates that further studies to optimise the microneedle device morphology, its method of application and the pDNA formulation are warranted to facilitate more reproducible cutaneous gene delivery.

Cutaneous DNA delivery and gene expression in ex vivo human skin explants via wet-etch micro-fabricated micro-needles.

Micro-needle arrays increase skin permeability by forming channels through the outer physical barrier, without stimulating pain receptors populating the underlying dermis. It was postulated that micro-needle arrays could facilitate transfer of DNA to human skin epidermis for cutaneous gene therapy applications. Platinum-coated "wet-etch" silicon micro-needles were shown to be of appropriate dimensions to create micro-conduits, approximately 50 microm in diameter, extending through the stratum corneum (SC) and viable epidermis. Following optimisation of skin explant culturing techniques and confirmation of tissue viability, the ability of the micro-needles to mediate gene expression was demonstrated using the beta-galactosidase reporter gene. Preliminary studies confirmed localised delivery, cellular internalisation and subsequent gene expression of pDNA following micro-needle disruption of skin. A combination of this innovative gene delivery platform and the ex vivo skin culture model will be further exploited to optimise cutaneous DNA delivery and address fundamental questions regarding gene expression in skin.

Microneedle delivery of plasmid DNA to living human skin: Formulation coating, skin insertion and gene expression.

Microneedle delivery of nucleic acids, in particular plasmid DNA (pDNA), to the skin represents a potential new approach for the clinical management of genetic skin diseases and cutaneous cancers, and for intracutaneous genetic immunisation. In this study excised human skin explants were used to investigate and optimise key parameters that will determine stable and effective microneedle-facilitated pDNA delivery. These include (i) high dose-loading of pDNA onto microneedle surfaces, (ii) stability and functionality of the coated pDNA, (iii) skin penetration capability of pDNA-coated microneedles, and (iv) efficient gene expression in human skin. Optimisation of a dip-coating method enabled significant increases in the loading capacity, up to 100µg of pDNA per 5-microneedle array. Coated microneedles were able to reproducibly perforate human skin at low (<1N) insertion forces. The physical stability of the coated pDNA was partially compromised on storage, although this was improved through the addition of saccharide excipients without detriment to the biological functionality of pDNA. The pDNA-coated microneedles facilitated reporter gene expression in viable human skin. The efficiency of gene expression from coated microneedles will depend upon suitable DNA loading, efficient and reproducible skin puncture and rapid in situ dissolution of the plasmid at the site of delivery.

Gene delivery to the epidermal cells of human skin explants using microfabricated microneedles and hydrogel formulations.

PURPOSE: Microneedles disrupt the stratum corneum barrier layer of skin creating transient pathways for the enhanced permeation of therapeutics into viable skin regions without stimulating pain receptors or causing vascular damage. The cutaneous delivery of nucleic acids has a number of therapeutic applications; most notably genetic vaccination. Unfortunately non-viral gene expression in skin is generally inefficient and transient. This study investigated the potential for improved delivery of plasmid DNA (pDNA) in skin by combining the microneedle delivery system with sustained release pDNA hydrogel formulations. MATERIALS AND METHODS: Microneedles were fabricated by wet etching silicon in potassium hydroxide. Hydrogels based on Carbopol polymers and thermosensitive PLGA-PEG-PLGA triblock copolymers were prepared. Freshly excised human skin was used to characterise microneedle penetration (microscopy and skin water loss), gel residence in microchannels, pDNA diffusion and reporter gene (beta-galactosidase) expression. RESULTS: Following microneedle treatment, channels of approximately 150-200 microm depth increased trans-epidermal water loss in skin. pDNA hydrogels were shown to harbour and gradually release pDNA. Following microneedle-assisted delivery of pDNA hydrogels to human skin expression of the pCMVbeta reporter gene was demonstrated in the viable epidermis proximal to microchannels. CONCLUSIONS: pDNA hydrogels can be successfully targeted to the viable epidermis to potentially provide sustained gene expression therein.