absorption of drug

Topics: Pharmacology, Chemistry, Solubility Pages: 3 (816 words) Published: September 21, 2014
Dissolution[edit]
In the most common situation, a tablet is ingested and passes through the esophagus to the stomach.

The rate of dissolution is a key target for controlling the duration of a drug's effect, and as such, several dosage forms that contain the same active ingredient may be available, differing only in the rate of dissolution. If a drug is supplied in a form that is not readily dissolved, the drug may be released more gradually over time with a longer duration of action. Having a longer duration of action may improve compliance since the medication will not have to be taken as often. Additionally, slow-release dosage forms may maintain concentrations within an acceptable therapeutic range over a long period of time, as opposed is quick-release dosage forms which may result in sharper peaks and troughs in serum concentrations.

The rate of dissolution is described by the Noyes–Whitney equation as shown below:

\frac{dW}{dt} = \frac{DA(C_{s}-C)}{L}
Where:

\frac{dW}{dt} is the rate of dissolution.
A is the surface area of the solid.
C is the concentration of the solid in the bulk dissolution medium. C_{s} is the concentration of the solid in the diffusion layer surrounding the solid. D is the diffusion coefficient.
L is the diffusion layer thickness.
As can be inferred by the Noyes-Whitney equation, the rate of dissolution may be modified primarily by altering the surface area of the solid. The surface area may be adjusted by altering the particle size (e.g. micronization). For many drugs, reducing the particle size leads to a reduction in the dose that is required to achieve the same therapeutic effect. However, it should be noted that although the reduction of particle size increases the specific surface area and the dissolution rate, it does not affect solubility.

The rate of dissolution may also be altered by choosing a suitable polymorph of a compound. Different polymorphs exhibit different solubility and dissolution rate...
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