Skin permeation, biodistribution, and expression of topically applied plasmid DNA

Background. Topical application is emerging as a new route of gene delivery. However, the extent of skin permeation and the in vivo fate of topically applied plasmid DNA are not fully understood. Methods. In vitro permeation of plasmid DNA across human skin and keratinocyte layers was tested using Franz diffusion cells. In vivo absorption and biodistribution of topically applied plasmid in mice were determined using quantitative polymerase chain reaction (PCR). The expression levels of plasmid DNA in various tissues were measured by semiquantitative reverse transcription PCR. Results. In vitro, topically applied DNA was capable of penetrating human skin and keratinocyte layers. Following topical application of plasmid DNA onto murine skin, the levels of plasmid DNA in the serum peaked at 4 hr. At 24 hr post-dose, topically applied DNA existed at higher levels than intravenously administered DNA in almost all tissues, and induced 11.4- and 22-fold higher mRNA expression in muscle and skin, respectively. Moreover, the topical route showed sustained expression of plasmid DNA in the regional lymph nodes over 5 days, whereas the intravenous route did not. Conclusions. Taken together, our results show that topically applied plasmid DNA is capable of permeating the skin and being expressed for prolonged periods in various tissues including lymph nodes. This suggests that skin may provide an appealing, noninvasive route of delivery for DNA vaccines and other therapeutic genes.