Organotypic cell cultures and two-photon imaging: tools for in vitro and in vivo assessment of percutaneous drug delivery and skin toxicity

The outermost protective layer of the skin, the stratum corneum, is responsible for skin impermeability toward external medications and potentially harmful chemicals. Stratum corneum is the target for physical and chemical approaches to enhance drug permeation. These approaches are commonly investigated in the field of drug delivery, but the drug absorption enhancement is often linked with local toxicity. In this review we are discussing two emerging technologies for drug and chemical studies in the skin: organotypic cell cultures and non-invasive two-photon microscopic imaging. Even though several cell culture based 'skin equivalents' have been introduced and validated for skin irritation testing, they are usually leaky and inadequately characterized in terms of permeation. Rat epidermal culture model (ROC) has been thoroughly characterized and it shows comparable barrier properties with the human skin thereby being useful in drug permeation and toxicity studies. In vitro and in vivo visualizations of permeants and skin structures are now feasible due to the rapid development of two-photon microscopy that allows improved depth scanning and direct in vivo visualization of the permeating compounds and adverse reactions in the skin structures. In summary, the new tools in percutaneous drug delivery studies will provide new insights to the permeation process and local toxicity. These tools may facilitate development of effective and safe transdermal drug delivery methods.