Pharmacological differences between drug transporters

For transdermal drug transport the active substance should have appropriate physico-chemical and pharmacological properties. Although different requirements are described in literature, in general the molecular weight of the active substance should not exceed about 500 Da, the partition coefficient (log P(octanol/water)) should be between 1.0 and 4.0 and the dose should be low (less than 20 mg/day). Recent research focusses on transdermal delivery of active substances that do not meet these requirements, such as relatively large molecules. 

The transport of substances between different phases is of immense importance in such diverse fields as pharmacology and environmental science. For example, if a drug is to pass from the aqueous phase with the stomach into the bloodstream, it must pass through the lipid (oil-like) phase of the epithelial cells that line the digestive tract. Similarly, a pollutant such as a pesticide residue that is more soluble in oil than in water will be preferentially taken up and retained by marine organism, especially fish, whose bodies contain more oil-... 

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