Research Summary:
Human skin has unique properties, with one of the most apparent being its role as a physicochemical barrier. The human integument is capable of resisting the penetration of many molecules. However, smaller molecules, particularly, can surpass the skin transcutaneously. These molecules are able to bypass the corneal layer, which is thought to serve as the main deterrent.
We argue that the molecular weight (MW) of a compound must be under 500 Dalton to allow for skin absorption. Larger molecules are unable to penetrate the corneal layer. Arguments for this "500 Dalton rule" include:
1) Virtually all common contact allergens are under 500 Dalton, as larger molecules are not known to be contact sensitizers. They cannot penetrate the skin and, thus, cannot act as allergens in humans.
2) The most commonly used pharmacological agents applied in topical dermatotherapy are all under 500 Dalton.
Key words: drug design – skin – topical drugs.
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