Figure 3.6 The gingival margin
Figure 3.6 The gingival margin or non-existent, and its base is formed by junctional epithelium, which extends from the base of the sulcus. The intercellular spaces in the junction epithelium are considerably larger than in either the oral sulcular epithelium or the oral epithelium, and desmosome attachment is four times less common. The sulcus produces a fluid which is composed of an inflammatory exudate24. Mild mechanical agitation of the surface of the sulcular epithelium increases the flow of gingival fluid, and it is believed that no fluid flow occurs if the tissue is undisturbed, both in healthy and inflamed states.
Studies which have been carried out to examine the penetration of materials into the body via this route have rarely distinguished between junctional epithelium and oral sulcular epithelium. In addition many animals show differences in the way the epithelium is attached to the tooth in the sulcular region. Substances which have been claimed at various times to penetrate the sulcular epithelium are albumin, endotoxins, thymidine, histamine and horseradish peroxidase, which indicates a permeability of substances up to a molecular weight of 1 million16. Particulate material such as polystyrene microspheres with a 1-3 pm range of diameters have been reported to penetrate the epithelium25. It is possible that substances entering the gingiva do so through the intercellular spaces26 27. Topically applied peroxidase was found in the intercellular spaces after 10 minutes and application of hyaluronidase, which increases intercellular space, causes increased tracer uptake28.
Gingival disease and ageing are likely to influence drug absorption through the buccal cavity, since the gum margin may recede or become inflamed. This may allow more access to the underlying connective tissues which have little barrier function to small molecules.
Improving penetration through the mucosa
There are three methods by which penetration of compounds through the oral mucosa can be achieved:
a) increasing the metabolic stability of the drug either by the use of pro-drugs or coadministration of enzyme inhibitors. For example, ketobemidone absorption was greatly increased when the phenolic hydroxyl group of the drug was derivatised into a carboxylic acid or carbonate ester29. This improved lipophilicity and resistance to saliva catalysed hydrolysis.
b) penetration enhancers, including chelators such as ethylenediaminepentaacetic acid or salicylates, surfactants (e.g sodium lauryl sulphate), bile salts (e.g sodium deoxycholate), fatty acids (e.g. oleic acid) and membrane fluidizers (e.g. Azone®).
c) physical enhancement, e.g. stripping layers from the epithelium using an adhesive strip, scraping the mucosa, or the application of an electric field across the epithelium (iontophoresis, see Chapter 8).
The use of penetration enhancers may be necessary to achieve adequate absorption of large molecules. However, their action is non-specific and care must be taken to ensure that toxins and bacteria are not allowed to enter the body in addition to drug. Currently no marketed buccal or sublingual products contain excipients registered as absorption enhancers.
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