Several physicochemical processes need to be considered along with the drug substance dissolution process to determine the overall dissolution rate of drugs from solid dosage forms under standardized conditions. The dissolution process for a solid dosage form (or a drug product) in solution starts with the wetting and the penetration of the dissolution medium into the solid formulation. It is generally followed by disintegration and/or deaggregation into granules or fine particles. However, this step is not a prerequisite for dissolution. The final step involves solubilization (or dissolution) of the drug substance into the dissolution medium. A schematic diagram illustrating the processes involved in the dissolution of solid dosage forms is shown in Fig. 3.3. It should be noted that these steps can also occur simultaneously during the dissolution process. For most poorly soluble drugs, dissolution is considered to be dissolution controlled, since solubilization of drug particles is slow relative to disintegration or deaggregation of the dosage form. If the step of disintegration or deaggregation is rate-limiting, dissolution is considered to be disintegration controlled. The factors that affect the dissolution rate of solid dosage forms can be classified under four main categories: (1) factors related to the physicochemical properties of the drug substance, (2) factors related to drug product formulations, (3) factors related to manufacturing processes, and (4) factors related to dissolution testing conditions.
Solid Dosage Form
Drug in vitro or in vivo
Absorption (in vivo)
Drug in Blood, Other Fluids and Tissues
Figure 3.3. Schematic illustration of a dissolution process of a solid dosage form (modified from Wagner 1970)
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