Mucins drug binding

Mucin, a viscous mucopolysaccharide that lines and protects the intestinal epithelium, has been thought to bind certain drugs nonspecifically (e.g., quarternary ammonium compounds) and thereby prevent or reduce absorption. This behavior may partially account for the erratic and incomplete absorption of such charged... 

These include absorption by adjacent palpebral and bulbar conjunctiva, with concomitant rapid removal from ocular-tissues by peripheral blood flow. For example, the extensive vascularity of the uvea underlies the bulbar conjunctiva, a mucous membrane, and the sclera, a white tissue providing a tough outer covering [177]. Binding of drug to either external sites, like the tear polymers such as mucins or lysozyme, or internal tissues like the sclera can be detrimental to efficacy. 

Gu, J. M., Robinson, J. R., and Leung, S. H. Binding of acrylic polymers to mucin/epithelial surfaces Structure-property relationships. Crit. Rev. Ther. Drug Carrier Syst. 1988, 5(1), 21-67. 

Apart from being a diffusional barrier, mucin can also interact with drugs to decrease their bioavailability, as has been shown with tetracycline [106], phenylbutazone, and warfarin [107]. On the other hand, studies in rats showed that binding of some water-soluble drugs to intestinal mucus was essential for their absorption and that damage to the mucus significantly reduced absorption [108], The acidic mucus is essential for lipid absorption and could be important for the diffusion of lipophilic drugs (see below). 

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