ORAL DRUG ADMINISTRATION RELEASE AND ABSORPTION

The coated tablet contains a drug within a core that is covered by a shell, e. g., wax coating, that serves

1.  To protect perishable drugs from decomposing.

2.  To mask a disagreeable taste or odor.

3.  To facilitate passage on swallowing.

4.  To permit color coding.

Capsules usually consist of an oblong casing— generally made of gelatin—that contains the drug in powder or granulated form. In the matrix-type tablet, the drug is embedded in an inert meshwork, fromwhich it is released by diffusion upon being moistened. In contrast to solutions, which permit direct absorption of drug (A, track 3), the use of solid dosage forms initially requires tablets to break up and capsules to open (disintegration), before the drug can be dissolved (dissolution) and pass through the gastrointestinal mucosal lining (absorption). Because disintegration of the tablet and dissolution of the drug take time, absorption will occur mainly in the intestine (A, track 2). In the case of a solution, absorption already starts in the stomach (A, track 3). For acid-labile drugs, a coating ofwax or of a cellulose acetate polymer is used to prevent disintegration of solid dosage forms in the stomach. Accordingly, disintegration and dissolution will take place in the duodenum at normal rate (A, track 1) and drug liberation per se is not retarded. The liberation of drug, and hence the site and time-course of absorption, are subject to modification by appropriate production methods for matrix-type tablets, coated tablets, and capsules. In the case of the matrix tablet, this is done by incorporating the drug into a lattice from which it can be slowly leached out by gastrointestinal fluids. As the matrix tablet undergoes enteral transit, drug liberation and absorption proceed en route (A, track 4). In the case of coated tablets, coat thickness can be designed such that release and absorption of drug occur either in the proximal (A, track 1) or distal (A, track 5) bowel. Thus, by matching dissolution time with small-bowel transit time, drug release can be timed to occur in the colon. Drug liberation and, hence, absorption can also be spread out when the drug is presented in the form of a granulate consisting of pellets coated with a waxy film of graded thickness. Depending on film thickness, gradual dissolution occurs during enteral transit, releasing drug at variable rates for absorption. The principle illustrated for a capsule can also be applied to tablets. In this case, either drug pellets coated with films of various thicknesses are compressed into a tablet or the drug is incorporated into a matrix- type tablet. In contrast to timed-release capsules slow-release tablets have the advantage of being divisible ad libitum; thus fractions of the dose contained within the entire tablet may be administered. This kind of retarded drug release is employed when a rapid rise in blood levels of drug is undesirable, or when absorption is being slowed in order to prolong the action of drugs that have a short sojourn in the body.