Lakhasly

There are three potential rate-limiting steps in the bioavailabil- ity of drug from a dosage form: 1.

Before an extended-release dosage form is devel- oped, the suitability of the drug in question should be considered. The solubility of a drug in aqueous media and the intestinal permeability of the drug are key considerations when assessing whether a drug may be suitable for modi ed release. There are three potential rate-limiting steps in the bioavailabil- ity of drug from a dosage form:

1. release from the dosage form


2. dissolution of drug


3. absorption through the gastrointestinal mucosa.
   Drugs are categorized according to steps 2 and 3. The Biopharmaceutics Classi cation System of drugs (see Chapter 21 for details) classi es drugs into four categories:
   • Type I: high solubility, high permeability
   • Type II: high solubility, low permeability
   • Type III: low solubility, high permeability
   • Type IV: low solubility, low permeability.
   High solubility and high permeability drugs (Class I) are most suitable for extended-release delivery (ideally by passive diffusion). These properties mean that drug release from dosage forms can be the rate- limiting step in the process and this can then be tailored by the dosage form design. For drugs with low solubility (