Overcoming challenges for skin deposition of drugs

Delivery of macromolecules such as siRNA into cells that reside in the basal epidermis of the skin is a major challenge due to the transport barriers that need to be overcome. siRNAs have potential therapeutic applications in various dermatological diseases such as psoriasis, atopic dermatitis, and cancer. Unfortunately, thelow permeability of siRNA through the stratum corneum and epidermis has significantly limited its use for topical application. There are two important factors that specify permeation and deposition of molecules into the skin. One is the membrane properties, and the second is the molecule properties. The objective of this study was to develop a topical siRNA delivery system that can permeate through the stratum corneum and viable epidermis and efficiently deposit therapeutic levels of siRNA to the basal epidermis/upper dermis where melanoma cells reside. To achieve this objective, we first compared permeation rate variation of different hydrophilic and lipophilic compounds through the human cadaver skin samples and two synthetic membranes. Afterward, in order to specify molecular properties that can deposit into the skin, we formulated two lipophilic antimicrobial agents and compared their skin deposition and permeation rate. At the end, we formulated series of liposome compositions that contained various concentrations of edge activator in their structures and then complexed with siRNA at different ratios to generate a small library of liposome-siRNA complexes (lipoplexes) with different physicochemical properties. In this study we used melanoma as a disease model. Through use of quantitative imaging analysis, we identified the necessary design parameters for effective permeation of lipoplexes through the skin layers and deposition at the upper dermis. The ability of the formulated lipoplexes to internalize into melanoma cells, knockdown the expression of the BRAF protein and induce cell death in melanoma cells was studied by fluorescent microscopy, in-cell immunofluorescence assay and WST-1 cell proliferation assay. By providing direct quantitative and qualitative microscopy evidence, the results of this study demonstrate for the first time that the passive delivery of an edge-activated liposomal formulation can effectively carry siRNA through the stratum corneum and deposit it at the lower epidermis/upper dermis.