Topical Drug Bioavailability, Bioequivalence and Penetration.

Bioavailability defines the rate and extent of absorption of a drug substance. Blood levels are generally measured to assess bioavailability, based on the assumption that drug molecules equilibrate between the blood and the site of action, which is usually a tissue that is not easily sampled.

Bioequivalence describes the relationship between two similar products in terms of relative bioavailability. They are bioequivalent if they contain the same drug in the same concentration and either produce the same clinical effect or, based on an accepted in vivo test, exhibit the same bioavailability.

Demonstration of bioequivalence is a required step when a manufacturer develops a generic drug to replace a prescription drug product going off patent. To gain regulatory approval, the generic product must first demonstrate bioequivalence to the product it attempts to mimic.

This book treats drug delivery from both transdermal and dermatological drug products. While both product types involve application to the skin, they differ in terms of target tissue and design. Transdermals target the blood and use the skin as a route for getting there. For dermatologicals, the skin is itself the desired focus of drug administration while uptake into the blood represents a loss from the desired site. Additionally, transdermal patches are closed systems that adhere to the skin for an extended period of time, up to seven days. Most …