Mucoadhesives exhibit high adhesiveness at their interface with the mucus layer, but should exhibit sufficient cohesiveness as well in order to prevent internal fracture of the (swollen) mucoadhesive polymers. Solid forms of mucoadhesives show in general satisfying cohesiveness. Another aspect of the correlation between cohesiveness and mucoadhesive ability has been pointed out by Hagerstrom et al. (2000). Together with his coworkers he investigated the mucoadhesiveness of common polymers to several kinds of mucins. The results show that a too high interaction between the polymer and the mucins led to weakening instead of strengthening of the internal gel structure, since the increased interaction at the interface disturbed the internal cohesive structure of the polymer network.
In a recent investigation, Accili and collaborators (2004) showed that the poly-acrylates Carbopol 974P and Pharmacoat 606 showed different mucoadhesive properties depending on which type of mucus (sublingual, esophageal, and duodenal bovine) they were brought in contact to. The significantly different behavior of the two polymers was correlated with the desquamation layer thickness and the differential sialic acid and fucose expositions in the targeted mucosae.
A number of solid mucoadhesive dosage forms for drug delivery in the gastrointestinal tract, such as tablets, micro- and nanoparticles, granules, pellets, and capsules have been already studied in vitro and in vivo in 1992 (Duchene and Ponchel, 1992). They showed satisfying mucoadhesive properties, although a correlation between in vitro and in vivo performance of the delivery systems cannot always be made. Until today no reliable mucoadhesive drug delivery system for intestinal application is available. This is mainly due to the fact that the mucoadhe-sive polymers have to first swell in order to obtain their mucoadhesive properties. Second, the particulate or single unit dose delivery systems have to get attached to the mucous linings of the gut, which is a randomly occurring process during which most of the delivery systems already come in contact with soluble mucus (fragments), amply available in the gut fluids, and which adhere also to the mucoad-hesive surface of the drug delivery systems, hence deactivating the mucoadhesive properties of the drug delivery systems before they are able to reach the mucosal surface of the gut. In this case, most of the drug is delivered into the intestinal liquids rather than to the mucosal gut surface and most of the (peptide) drug is degraded and lost for drug absorption.
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