Accumulation of sunscreens and other compounds in keratinous substrates

Several cosmetic ingredients, especially sunscreens, should be substantive, which means they are to be adsorbed to specific binding sites within the upper skin layers, particularly keratinized structures of the stratum corneum, and thus show resistance to washing off. We investigated the affinity of 10 non-ionic compounds, among these UV-absorbing chemicals, antioxidants, antimicrobial compounds and a repellent to animal keratin and human callus. In each case a linear relationship between the drug amount, which has accumulated in the respective keratin, and the remaining free concentration of the applied solution could be established. Moreover, drug affinities to keratinous substrates are in direct proportion to the octanol/vehicle partition coefficients, pointing to the fact, that drug enrichment in keratinic substrates is clearly governed by lipophilicity, while specific adsorption, i.e. genuine substantivity, does not seem to occur. After application of a saturated solution non-ionic compounds with a pronounced keratin/vehicle partition coefficient will build up the highest concentration within the stratum corneum. If these compounds show, at the same time, a high solubility in the vehicle, they will penetrate the skin most easily. The used callous tissue seems to be a suitable substrate to simulate and quantify solute uptake into human skin.