Oral administration of solid formulations has been the most common route of administration for almost a century. However, the importance of dissolution processes in the oral drug absorption was only recognized about 50 years ago when Nelson published his finding that showed a relationship between the blood
1 The opinions expressed in this chapter by the authors do not necessarily reflect the views or policies of the Food and Drug Administration (FDA).
levels of orally administered theophylline salts and their in vitro dissolution rate (Nelson, 1957). The need for dissolution testing can be understood easily by considering the importance of dissolution on oral drug absorption, which is described below.
When a systemically acting drug is administered in solid dosage forms, such as a tablet or capsule, its absorption into the systemic circulation can be generally described by four consecutive steps (Fig. 3.1). The first step involves delivery of the drug into its absorption site through gastric emptying and intestinal transit flow. It is followed by the second step in which dissolution takes place in the stomach and/or in the small intestine. It should be noted that the first two steps need not to be sequential and that lymphatic absorption is not considered. The third step is characterized by the permeation of the dissolved drug across the gastrointestinal (GI) membrane. Finally, the absorbed drug passes through the liver (first pass metabolism) and reaches the systemic circulation. Although this is a simplified description of the drug absorption process, it shows that transit (gastric emptying), dissolution, absorption across intestinal membrane, and metabolism constitute the fundamental processes of oral drug absorption. If the dissolution process is slow relative to the other three processes, which is usually the case for most poorly soluble drugs formulated in a conventional dosage form, dissolution will be the rate limiting step. As a result, the dissolution rate will determine the overall rate and extent of drug absorption into a systemic circulation, and hence bioavailability.
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