Ex vivo dermal studies are used to assess the feasibility of transdermal administration, to rank-order compounds in terms of permeability or accumulation, and to optimize formulations.
Such studies are performed with human or porcine skin in Franz cells. Test formulations (gels, creams, ointments, patches, etc.) are applied to the upper (external) surface, and samples are removed at pre-determined time points from the reservoir containing buffer that is in contact with the lower (serosal) surface. Reference compounds are either co-dosed (if the test compound is in solution) or run in parallel (if the test compound is in some other type of formulation) for quality control. The skin is often extracted at the end of the study to quantify accumulation of the test compound.
It is possible to determine accumulation in different layers of the skin by tape stripping or heat separation. Transepidermal water loss (TEWL) may be measured as a proxy for skin dryness. Test formulations may also be incubated with tissue homogenates to assess the metabolic stability of the test article.