[Mechanisms of transcutaneous absorption. Pharmacologic and biochemical aspects]

Known and potential mechanisms of transcutaneous permeation or resorption of drugs are discussed. The relatively well documented transcellular route is passable for compounds with a low molecular mass (up to about 2000 Da) and sufficient lipid as well as water solubility. Because of the limited absorption capacity of the skin and the slow progress of absorption systemic effects of cutaneously applied drugs with the mentioned properties may be expected only, if they are acting in low concentrations and if a retarded onset of action is tolerable. Further compounds which do not fulfil these conditions sufficiently may be transported through the skin under the aid of exogenic factors such as permeation enhancers. Advantages, disadvantages and limitations of transdermal drug therapy as compared with other routes of application are discussed. Finally the question is dealt with, if the transcutaneous route can be opened also for polar compounds with a higher molecular mass. The permeation of such compounds could take place through intercellular clefts, which are reported to have dimensions permitting the passage of compounds with a molecular mass greater than 2000 Da. The transfer through the masses filling the clefts may require, however, the aid of sorption enhancers.