Bioadhesives in Drug Delivery

The use of bioadhesives in drug delivery systems is by no means new, although increased interest in its unique applications in therapy is evidenced by the recent spate of publications. Bioadhesives complement drug delivery systems through increased residence time in the various routes of administration. Prolonged contact time can offer very substantial improvements in local drug therapy as well as significant increases in bioavailability for some drugs. Indeed, for a number of drugs that can only be administered by injection, because of either poor membrane absorption or excessive drug degradation, prolonged residence time at a particular site can obviate the need for an injectable mode of drug administration. A bioadhesive can be defined as any substance that can adhere to a biological substrate and is capable of being retained on that surface for an extended period of time [1,2]. Drug delivery systems using bioadhesives usually adhere to membrane surfaces or the mucin layer coating such surfaces. The majority of the targeted areas used in drug delivery have a coating of mucus, and bioadhesive polymers that attach to this mucus coating are generally called mucoadhesives. Their residence times on these surfaces are controlled by whether the bioadhesive is water soluble or insoluble. In the case of water-soluble bioadhesives, contact time is generally only a few hours, depending on the adhesive and flow of biological fluid at the site of drug administration. Water-insoluble polymers, in contrast, remain in place until the mucin or tissue replaces itself, typically a period of about 4 to 72 h. Contact between the adhesive and the mucosal membrane or its coating can be seen as a two-step process, the initial contact between the bioadhesive and substrate and the subsequent formation of bonds between the two surfaces. Success of the initial contact appears dependent on similarity of physicochemical properties between the adhesive and substrate and is often associated with ‘‘wetting’’ of the substrate surface. Formation of bonds, which can be electrostatic, hydrophobic, or hydrogen bonds, permit the bioadhesive (and drug delivery system) to attach to the substrate. To better understand the use