How to optimize drug penetration through the skin.

The main problem of the therapy with drugs applied to the skin is the high diffusional resistance of the intact stratum corneum. To increase the flux of a given drug the selection of the vehicle is of utmost importance. Incorporation of the drug at its maximal thermodynamic activity leads to the maximal possible flux, as in vivo studies show with different drugs. The formation of supersaturated solutions and dissociation equilibria as well as drug-vehicle interactions and drug depletion also result from the vehicle selection and influence the flux. The resistance of the stratum corneum is not a constant parameter. It may be reduced by specific vehicle effects, penetration enhancers and hydration. Examples for the increase of drug fluxes are given.