For solid dosage forms, the process of dissolution is the first step of the oral absorption process. Dissolution is an important factor that can influence both the rate and extent of oral absorption. The Noyes-Whitney equation (see Eq. 3.4) as shown below, or modifications of it, is commonly used to describe the dissolution of compounds.
Dissolution Rate = dX = AD (■S - Xdissolved^ (3.4) dt h V V J
where X is the amount of solid, Xdissolved is the amount of dissolved drug, A is the surface area available for dissolution, D is the diffusion coefficient of the drug in the dissolution medium, h is the thickness of the diffusion layer, S is the solubility of the drug in relevant media, and V is the volume of the dissolution medium.
Based on this equation, the main properties governing drug dissolution are:
1. The solubility of the drug in gastrointestinal fluid
2. The surface area available for dissolution
3. The concentration of the drug already in solution
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