Direct influence at site B: A number of enhancers can also act directly in the aqueous domain (Figure1, Site B) by increasing the solubility of this site (aquous domain) for the permeant. Solvents such as propylene glycol PG and ethanol are believed to act in this way.21 High concentration of solvents such as dimethylsulphoxide, propylene glycol or ethanol in a vehicle or device penetrates into aqueous regionof the tissue becoming a better solvent for molecules such as hydrocortisone and oestradiol. Various reports show that the underlying mechanism of B site interaction of these solventsis that they modify the skin permeability by altering the solubility parameter of the skin.21 As a result, the partitioning of the drug from the vehicle into the SC increases because operational partition coefficient now favors an elevated drug concentration in the skin. The solvent then diffuses out into the dermis followed by the drug diffusing down its concentration gradient
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