Dissolution

Dissolution
Dissolution is defined as the process by which solid substances enters in solvent to yield a solution. Stated simply, dissolution is the process by which a solid substance dissolves.
Fundamentally, it is controlled by the affinity between the solid substance and the solvent. The physical characteristics of the dosage form, the wettability of the dosage unit, the penetration ability of the dissolution medium, the swelling process, the disintegration, and the deaggregation of the dosage forms are a few of the factors that influence the dissolution characteristics of drugs, figure below show the processes involved in the dissolution of solid dosage forms.



A number of studies conducted, especially in the United States, confirmed the significance of dissolution- bioavailability relationship in the pharmaceutical product development.

Although considerable efforts were made to establish in vitro/in vivo correlations between release of drug from the formulation and drug absorption, the limited knowledge of the complex composition and hydrodynamics of the gastrointestinal fluids remains a real barrier.
In spite of the reported success of several in vitro/in vivo correlation studies, dissolution cannot be relied upon as a predictor of therapeutic efficiency. Rather, it is a qualitative tool that can provide valuable information about the biological availability of a drug, as well as batch-to-batch consistency. Another area of difficulty is the accuracy and precision of the testing procedure, which is dependent, to a large extent, on the strict observance of so many subtle parameters and detailed operational controls.

Dissolution is considered, today, as one of the most important quality control procedures performed on pharmaceutical dosage forms, and dissolution studies have become an essential part of drug applications to regulatory bodies worldwide. Whether or not it has been correlated with biological effectiveness, the standard dissolution test is a simple and inexpensive indicator of a product’s physical consistency.
If one batch differs from the other in its dissolution characteristics or if the dissolution profiles of the production batches show a consistent trend upwards or downwards, it sounds a sure warning that some factor in the raw material, formulation, or process is out of control.
Additionally, dissolution data seems to be a useful tool in the early stages of drug development and molecular manipulation.

Carstensen proposed a scheme incorporating the following sequence:
1. Initial mechanical lag.
2. Wetting of the dosage form.
3. Penetration of the dissolution medium into the dosage form.
4. Disintegration.
5. Deaggregation of the dosage form and dislodgement of the granules.
6. Dissolution and occlusion of some particles of the drug.

Carstensen explained that the wetting of the solid dosage form surface controls the liquid access to the solid surface and, many times, is the limiting factor in the dissolution process.
The speed of wetting directly depends on the surface tension at the interface (interfacial tension) and upon the contact angle between the solid surface and the liquid.