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 mu m 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 pCMV beta 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.