Formulation factors affecting drug dissolution2

2.Formulation factors

Any drug it is possible to alter its bioavailability considerably by formulation modification, a drug must be in solution to be absorbed from the GIT, and you may expect the bioavailability of drug to decrease in the order solution› suspension › capsule › tablet › coated tablet this order may not always be followed but it is a useful guide.

Solutions

Formulation factors affecting drug dissolution

1. Excipients                                                                           

are added to a formulation to provide certain function properties to the drug & dosage form. Some of these functional properties of the excipients are used to improve the compressibility of the active drug. Stabilize the drug from degradation, decrease gastric irritation, control the rate of drug absorption from the absorption site, increase drug bioavailability……………etc.

·    Tablet lubricants such as magnesium stearate may repel water and reduce dissolution when used in large quantities.

·    Low conc. of surfactants decrease the surface tension  and increase the rate of drug dissolution, whereas higher surfactants conc. Tend to form micelles with the drug cause ↓the dissolution rate

·     Large drug particles have a smaller surface area and

Physiological factors influence gastrointestinal absorption

1.    Surface area of the gastrointestinal absorption sites:

The biological environments and the areas of membrane available for absorption are the stomach, small intestine & large intestine.

The presence of

Transport cross cell wall

Mechanism of GIT absorption of drug

The cell membranes of GI barrier are lipoprotein structure which act as semipermeable membranes allowing rapid transit of lipid soluble molecules, the passage of water & hydrophilic molecules through aqueous pore.

There are two main mechanisms of drug transport across the GI epithelium

Transport cross cell wall1