Creams and ointments cannot be used successfully in the oral cavity since they will not adhere well and may be washed away by saliva, although the original mucosal delivery system Orabase® (E.R.Squibb and Sons Inc.) consisted of finely ground pectin, gelatin and sodium carboxymethylcellulose dispersed in a poly (ethylene) and mineral oil gel base. Generally, oral drug delivery devices have been adapted from traditional technology, for example tablets, but these do not adequately address the problems unique to the mouth. Formulations which have been specifically designed for oral delivery include gums, fast-dissolving dosage forms and mucoadhesive patches.
It is important that neither the drug nor the excipients stimulate saliva secretion since this will increase the amount of drug swallowed. Taste, irritancy and texture problems may discourage patients from taking dosage forms which are designed to reside in the oral cavity. In order to enhance patient compliance, the dosage form has to be
Figure 3.7 Release of a radiolabelled marker from a chewable formulations when (a)
sucked and (b) chewed unobtrusive and pleasant to take: a maximum dimension might be no larger than 3 cm2. Some formulations are keratinolytic and hence cannot be placed over the same site without the risk of ulceration.
Chewing gum was first patented in 186936 and medicated chewing gum containing aspirin (Aspergum®) was marketed in the USA in 1924. It was the discovery that smokers self-titrate the amount of nicotine which they are absorbing which led to the development of a nicotine gum to help people trying to withdraw from the habit. In theory the gum could be chewed until the correct amount of nicotine was absorbed, then the formulation could be discarded. It has been of some value as a tobacco substitute for people attempting to give up smoking37.
Patients who have difficulty in swallowing tablets or capsules may prefer chewable systems, which also have the great advantage that they can be taken without water. The most important physiological variable which will markedly affect the release characteristics of a drug is whether a person sucks or chews the formulation since systems designed to be chewed will invariably be sucked and vice versa by a proportion of patients (Figure 3.7). The abuse potential of narcotic drugs is reduced in this type of formulation since it is much harder to extract the active ingredient from the base for subsequent intravenous administration.
The release of a drug from chewing gum is dependent upon its water solubility. Water-soluble substances are released rapidly and completely from chewing gum but it is possible to retard and extend their release. Slightly water-soluble drugs are released slowly and incompletely from chewing gum and require special formulation techniques to produce a satisfactory release profile38. The release of 99mTc-HIDA, a hydrophobic marker, was prolonged from a chewing gum compared to a sublingual tablet or lozenge39. Gums can be used to deliver drugs for the treatment of dental health and antifungal therapy e.g. nystatin40 and miconazole41. The absorption of some substances such as vitamin C can be increased when administered in a gum compared to a conventional tablet42. Recently, mucin containing chewing gum has been used in the treatment of dry mouth43.
Chewable formulations are used for the delivery of antacids where the flavouring agents give the sensation of relief from indigestion. Chewing antacid tablets prolongs the effect when compared to liquid antacids. Antacid tablets will react more slowly with gastric acid than liquids, even when thoroughly chewed, since the particle size will still be greater44, but mixing the tablets with saliva also contributes to the prolonged duration of effect45.
Fast dissolving dose forms for analgesics are well established as convenient systems for patient dosing, e.g. Solmin® (Reckitt and Colman Pharmaceuticals). These are solid dose forms which can be taken without water since they are designed to disperse on the tongue. They are also potentially useful where swallowing is difficult or oesophageal clearance is impaired.
Recently a new type of dosage form, Zydis™ (Scherer DDS), based on a freeze-dried mixture of drug and fast-dissolving excipients has been introduced to deliver sedative drugs such as benzodiazepines. Incorporation of 99mTc labelled micronised "Amberlite" CG400 resin during manufacture enabled the deposition and clearance of these formulations to be followed by gamma scintigraphy46. Two marker loadings were used, 2.5 mg and 10 mg, and the effect of incorporating the salivary stimulants talin and saccharin, and citrate, was investigated. Buccal clearance of the formulation containing the 10 mg resin was significantly faster than that containing 2.5 mg resin (Figure 3.8); however, calculation of the total activity remaining after dissolution showed that the amount remaining on the tongue was approximately 1 mg in each case. This probably represents the amount of resin
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