The most extensive application of a formulation strategy for colonic delivery and generally for controlled release has been the employment of enteric coatings on solid substrates. This is a natural development of conventional coating technologies employed to avoid gastric release thus preventing stability problems such as degradation, or unwanted pharmacological effects including gastric irritation and nausea. The underlying principle in this approach has been the selection of polymers which are insoluble at the lower pH values of the stomach, but which disintegrate and release the drug as the pH increases as the formulation enters the small bowel. The principal discrimination is in the functional use of enteric polymers for delayed release in the mid gut and beyond as compared to a general application for colonic drug delivery. For delayed release, the disintegration and drug release from enteric coated products should be targeted to the proximal small bowel, whilst for colonic delivery release from the enteric coated system should take place closer to the ileo-cecal junction. Since the time of exposure to gastric

acid may affect the subsequent release profile further down the tract and gastric emptying is a variable dependent on feeding, idiosyncratic and temporal differences, this is a fairly challenging task.