Enemas vehicle

Olive oil has been used in enemas, liniments, ointments, plasters, and soap. It has also been used in oral capsules and solutions, and as a vehicle for oily injections. 

Information is very limited and the interaction is not firmly established, nevertheless its seriousness indicates that sodium polystyrene sulfonate should not be given as an enema in aqueous vehicles containing sorbitol. More study is needed. Note that the manufacturer advises against the concurrent use of both oral and rectal sorbitol with sodium polystyrene sulfonate, because of the risk of colonic necrosis. ... 

The volume of enemas may vary from a few millilitres (micro-enema) to more than 100 mL, mainly depending on the intended effect systemic or local. For large-volume enemas water is commonly used and a water-soluble form of the active substance is preferred. The solubility may be increased by addition of co-solvents, to be applied in small volume enemas. If a soluble active substance or an adequate co-solvent cannot be found, a suspension may be prepared. If this is also not an option, a lipophilic vehicle may be chosen. Choice of pH depends on the chosen form of the active substance and is important for the absorption. Excipients may be added to correct the osmotic value, to increase the viscosity, to prevent oxidation or for preservation. 

For enemas with an oily vehicle, the considerations regarding solubility of active substance and choice of particle size resemble those for fatty suppositories (see Sect. 11.4.1). The process of release and absorption of the active substance is also largely comparable to that of fatty suppositories just the melting step is not necessary. 

In enemas for a systemic effect, water as a vehicle has the great advantage of presenting only one liquid phase in the rectum, so no transition of the active substance from fat into rectal fluid is needed. The addition of co-solvents (see Sects. 

In all enemas having a volume of more than 20 mL, water is used as the single vehicle. These enemas are intended for a local effect. The volume of these enemas usually exceeds 40 mL, see Sect. 11.9.3. 

Choice of the Vehicle for a Chloral Hydrate (Micro) Enema... 

Two vehicles have been proposed for diazepam enemas. Both are mixtures of water and co-solvents propylene glycol - ethanol 96 % - water (4+1+5, parts by volume, pH 4.8) and glycofurol - ethanol 96 % - water (5 + 1+4, parts by volume, pH 3.6). No significant difference in irritation score was observed with healthy volunteers [52]. 

Both vehicles are also usable for some other benzodiazepines. Temazepam 10 mg in 2 mL of the glycofurol mixture, administrated to healthy volunteers, had a bioavailability equivalent to an orally administered capsule of 10 mg temazepam [4]. In this study the enema was preserved with methyl parahydroxybenzoate (0.15 %). However, a vehicle containing such high percentages of glycofurol and ethanol does not need this supplementary preservation. 

Rectal solutions have water or oil as a vehicle. If necessary to enhance the solubility of poorly soluble active substances, aqueous rectal solutirms may contain cosolvents, such as ethanol and propylene glycol. However, cosolvents and surfactants should only be used in limited amounts because of the potential irritation and the defecation reflex they may cause. For the rectal absorption of active substances from enemas the same mechanisms as for suppositories apply. A major advantage of a rectal solution over a suppository may be the fact that the active substance is already in a dissolved state which may increase the absorption rate. Increasing the volume of a rectal solution to dissolve a poorly water-soluble active substance will enhance the dissolution rate and thereby increase the absorption rate. Because of the higher volume more active substance will be dissolved and the membrane surface over which absorption occurs, is increased as well. 

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