Biopharmaceutics of topical and transdermal formulations

Topical pharmaceutical products are usually liquids or semisolids, whereas transdermal products are often formulated as patches that deliver drugs to or through the skin to provide therapy. The pharmacokinetic response to a topical/transdermal formulation depends on the drug release, then penetration of the drug through the skin then distribution to the site of action. The relative efficiency of the processes will determine the exposure of the drug at the site of action, hence the biopharmaceutics. The rate of drug release can depend upon several factors including active pharmaceutical ingredient particle size. Drug release from topical and transdermal products is measured using a modified dissolution apparatus. Tape stripping is also used as a tool to measure the concentration of drug in the stratum corneum over time, often termed dermatopharmacokinetic methods. Dermal microdialysis enables in situ measurement of drug concentration in the dermis using an extremely thin dialysis probe that is located within the dermal layer.