From sticky stuff to sweet receptors--achievements, limits and novel approaches to bioadhesion.

About 10 years ago, the concept of bioadhesion was introduced into the pharmaceutical literature and has since stimulated much research and development both in academia and in industry. The first generation of bioadhesive drug delivery systems (BBDS) were based on so-called mucoadhesive polymers, i.e. natural or synthetic macromolecules, often already well accepted and used as pharmaceutical excipients for other purposes, which show the remarkable ability to 'stick' to humid or wet mucosal tissue surfaces. While these novel dosage forms were mainly expected to allow for a possible prolongation, better localization or intensified contact to mucosal tissue surfaces, it had to be realized that these goals were often not so easily accomplished, at least not by means of such relatively straightforward technology. However, although not always convincing as a 'pharmaceutical glue', some of the mucoadhesive polymers were found to display other, possibly even more important biological activities, namely to inhibit proteolytic enzymes and/or to modulate the permeability of usually tight epithelial tissue barriers. Such features were found to be particularly useful in the context of peptide and protein drug delivery. But still, the interest in realizing 'true' bioadhesion continues: instead of mucoadhesive polymers, plant or bacterial lectins, i.e. adhesion molecules which specifically bind to sugar moieties of the epithelial cell membrane, are now widely being investigated as drug delivery adjuvants. These second-generation bioadhesives not only provide for cellular binding, but also for subsequent endo- and transcytosis. This makes the novel, specifically bioadhesive molecules particularly interesting for the controlled delivery of DNA/RNA molecules in the context of antisense or gene therapy.