Dissolution, the process by which drug particles dissolve in a solvent, is explained by the diffusion layer model, a theoretical framework that simulates the absorption of oral drugs and allows us to analyze experimental data.
This process starts with a thin layer, saturated with the drug, forming at the interface between the solid and liquid. The solute then diffuses from this layer into the main solution. The Noyes-Whitney equation suggests that the rate of dissolution relies on the diffusion rate, surface area, concentration, and thickness of the stagnant layer. The concentration gradient between the stagnant layer and the bulk solution fuels the dissolution, with a steeper gradient typically resulting in a higher diffusion rate..
To create a strong correlation between in vitro (in the lab) and in vivo (in the body) dissolution rates, tests are conducted at 37oC with a consistent stirring rate. These conditions replicate the body temperature and peristaltic movements in the gastrointestinal tract. Sink conditions ensure that the concentration of the drug remains low enough to drive the dissolution process. This can be done by adsorbing the dissolved drugs, increasing the solution volume, regularly using fresh solvent, or separating the drug from the water-based medium with an organic phase.
Dissolution, the process of drug particles dissolving in a solvent, is explained by the diffusion layer model.
According to this model, initially, a thin layer saturated with the drug forms at the solid-liquid interface. Next, the soluble solute diffuses from the stagnant layer into the bulk solution.
According to Noyes-Whitney's equation, the dissolution rate depends upon the stagnant layer's diffusion rate, surface area, concentration, and thickness. The driving force for dissolution is the concentration gradient between the stagnant layer and the bulk solution.
Experimentally, to establish a robust in vitro to in vivo dissolution rate correlation, in vitro testing is performed at 37 degrees Celsius at a constant stirring rate to mimic body temperature and peristaltic movements in the GI tract.
In addition, sink conditions are maintained to avoid drug build-up in the bulk solution by adsorbing the dissolved drugs, increasing the volume of the solution, using fresh solvent at regular intervals, or partitioning the drug from the aqueous medium using an organic phase.
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